Expose Right To Expose Correctly

This is the second installment of my tutorial on using metering tools to obtain correct exposure.  After all that detail, the tutorial on how to obtain correct exposure using a light meter, can now–to some extent–be thrown out the window, thanks to the tools given us by most digital cameras!  I want to start out by saying that, like all of my tutorials here, I did not invent the wheel.  This is (hopefully) an understandable distillation of what a number of much more knowledgeable gurus have taught me through their writings and seminars.
Photoshop (and similar software) users are already familiar with the histogram, which depicts, in a graphical way, the number of pixels in an image ranging from pure black to pure white.  And, we know that if there are spikes at either end of the graph, we have problems with under, or over-exposure (known in digital terminology as “clipping”).  The left example shows pixels “stacked” on the left and indicates lost detail in the shadows (underexposure).  The right example, conversely, shows pixels stacked on the right and loss of detail in the highlights.  The middle example is close to an ideal histogram and is neutral.  There are no lost details.  The LCD readout on the back of the camera measures the same phenomena.
Using The LCD on the Back of The Camera

Today’s digital cameras give us this tool, built into the camera and we are foolish if we don’t take full advantage of it!  Digital capture gives us something we never had before–immediate feed back about our exposure.  We can now take the shot and look at the LCD on the camera back to see the result.  But be careful!  It is easy to be fooled by this great tool, if you do not understand how to set it up and use it.

  The actual image you see on the back of the camera, for example, is rarely useful as a tool to determine exposure.


 It can perhaps tell you something about composition and maybe about sharpness.  But it is not a good measure of exposure.  The image on the LCD screen is a jpg which is interpreted by the camera’s internal software and displayed on a screen that is not likely calibrated like your computer screen.  So any judgment you make about exposure and rely on is likely to be disappointing.
The tool you need to understand is the histogram to see whether your exposure is technically correct.  This is a wonderful, and time-saving tool, because it is a pretty accurate measure, immediately after-the-fact, of your exposure.  However, caveats apply.  First, notice that I say “technically” correct.  Recall from my tutorial on Using The Light Meter, that it is up to you, as the photographer, to determine the aesthetically correct exposure.  This remains true even when using the DSLR and the histogram (though the aesthetics can be more easily done as a matter of post processing now).  Second, the histogram is based on the camera’s interpretation of a jpeg image and only the “luminousity” channel (measuring mainly the black and whites in the image).  It is possible that one of the RGB channels might be overexposed (blown out) and you will not know it. This is probably a minor concern that in practice isn’t a serious issue.  And, some of the newer bodies now have an RGB histogram display.  Last, but certainly not least,


There is a danger in becoming so “married” to the histogram that we begin to shoot mechanically and technically

missing a crucial moment or artistic approach (this is why I still believe sound exposure knowledge covered in previous tutorials on exposure are necessary as an internalized fundamental understanding.  Indeed, this penchant we have developed to shoot, then check the LCD on the back of the camera has been given the name, “chimping,” for its similarity to a chimpanzee looking up and down.

In order to take full advantage of the features offered by your camera, you will need to go into the camera’s menu system and make sure the histogram feature is enabled.  While you are in there, you should also enable the camera’s feature (most DSLRs) that shows a flashing display on the LCD image for “blown highlights” (more below).
Reading the Histogram
think the world of Michael Reichman’s contribution to digital knowledge and his Luminous Landscape is a must-bookmark site for any serious DSLR  user.  His article on understanding histograms is clear and concise and I recommend reading it.   Reading your histogram is essentially similar to the Photoshop Levels histogram discussed above.  As a general rule, you want the histogram to show an even dispersion of pixels between the far left and right of the graph.  If the graph is “stacked” against either side, it is an indication of an exposure problem.  Stacked against the left side means you are likely underexposed.  Stacked against the right means you are likely overexposed.  As a general rule, particularly if you are shooting jpg images, having the histogram centered is going to give you a good exposure (if you are shooting raw, however, there is a better way–more below).  It is worth mentioning here again, that the LCD image on the back of the camera may even look pretty good. Do not believe it.  Believe the histogram!  It is also worth noting that the shape of the histogram is not really important.  My “neutral” histogram is the theoretical shape of an “ideal” histogram with an even dispersion of tones.  Many (if not most) images will not have that characteristic.  If your image has a lot of shadow or darker content, the histogram will show more pixels to the left and they will likely reach higher up on the graph (perhaps making it look more steep and spikey).  If there is a lot of bright areas in the photo (highlights of water, clouds, and bright skies are a good example), it will likely show more steepness to the right. The critical determination is whether it stacks flat against the right or left.  If that happens, you are clipping important digital data and not getting the most from your exposure.
The beauty of the histogram is that it allows us to make that measurement immediately after the shot and adjust exposure until we get it correct.  Obviously, there are instances when that is not going to work.  Action photos may only give us one chance.  If you can anticipate action, however, the histogram allows us to take some “pre” test shots and be ready when the action happens.

Professional nature photographer John Shaw has been an inspiration to me for 25 years.  I have most of his books.  In the film days, John (as did Bryan Peterson — mentioned in my earlier tutorial in his Understanding Exposure–and many other professional photographer-writers) spent a fair amount of time and text addressing proper exposure techniques.  Last winter, I had the pleasure of attending one of John’s weekend seminars.  I was taken aback by something he said (and a little abashed that such a simple concept had escaped me until I heard him say it).  After acknowledging how much careful time and study he had put into proper exposure and metering techniques (using spot-metering and never trusting the camera’s automatic features) in the old days, he unashamedly acknowledged that he pays no attention to it these days!  He takes a shot at the camera’s suggested exposure using the matrix meter–as a test shot– and then uses the histogram to adjust for proper exposure.  Wow!  How simple is that?

 In two full days of sessions, that was the most important and useful piece of information I brought home with me


There are caveats, again, however.  First, I shoot in the camera’s raw format 99.99% of the time.  This is a technique that really works best when you are using the most digital information the camera can capture.  To me, this means shooting in the camera’s native 12 bit raw format, and making post-processing conversions in a raw converter.  While using the histogram will work in any format, it shines brightest when shooting raw.  Second, when shooting raw, the “rules” of reading the histogram change, as we will see next.
Expose To The Right in RAW
What follows only applies if you shoot in the camera’s native raw format.  Jpg shooters (which, unfortunately, includes most Point & Shoot users), disregard this information, center your histogram as much as possible and take a break.
The idea here, contrary to the thought that you want to “center” your histogram, is that you will get the maximum potential from the capture image data if you expose so that your histogram is as far right as possible, without blowing out any highlights.  This means that for the most part, that we want to shift that centered histogram to the right as far as we can.  Keep in mind that the advantage to this technique will only be realized in the adjustments you make in the image raw converter prior to opening the image in Photo Shop or other Post Processing software.
Some Theory
It has been suggested that digital sensors are capable of capturing up to 6 “stops” of range.  For the most part, in my experience, 5 stops is more realistic (besides, most other commentators use 5 stops to demonstrate, and the math is a little easier to grasp).  It is worth mentioning here that the “Levels” histogram in Photoshop uses 0-255 for its measurement from pure black to pure white.  This is based on an 8-bit jpeg model.  Most of the images I work on in Photoshop are 12 bit (everything should double and it should be 16 bit, which is–again–what Photoshop says it is, but current DSLR cameras are only able to capture a maximum of 12 bits, so we work with 12 bits in a 16 bit space). I know, too technical for me too!  Not to beat it to death, however, but consider how much more range of pixels we have to work with (and hence, margin for error) in a 12 bit image which gives us a 0-2048 pixel range instead of 0-255.
Now, here is why to expose to the right.  Consider the graphical 5-stop diagram below.  Note that as we progress from 128 to 2048, the area under consideration continues to double.  What this means in very simple terms is that the last step contains 50% of all the pixels captured!  Note that that is also the highlight.  So we want to capture as much of that as possible.  We accomplish that by shifting our histogram as far right as we can without blowing any highlights.  Another way to say this is that if you do not fill the right side of the histogram you are effectively potentially wasting up to 50% of the available information that your camera is capable of capturing.

There are two excellent tools on the camera-back LCD to determine this.  The first is the histogram we have been discussing.  Check it to make sure you have shifted it as far right at you can without a spike on the right side, or stacking.  The second tool was alluded to above.  If you turned on your camera’s flashing highlight display (affectionately called “blinkies” by many of us), the LCD will show blown highlights as blinking, or flashing.  From there, simply back off the exposure until we eliminate them, or push it to the right until you get them.

Again, do not be concerned with the look of the image on the LCD display.  And, when you open the raw file in the raw converter, do not be surprised to see your image look light and overexposed. Just use the image adjustment tools in the raw converter to bring your image to the look you desire. And, trust me, your images will look better.  A couple week after my John Shaw epiphany, I went out and shot, using my newfound approach.  

Set the camera meter to matrix, shoot your test shot and check the histogram.  Adust the exposure until you have it as far right at possible with no blown highlights.  And trust it.

 It will be right.  Other than that, I did nothing different than I always have.  I sent some shots to a fellow photographer in a “what I captured yesterday” kind of email and he responded that my work just keeps getting better.  Nice compliment (deserved or not), but  the point is that I was able to get all I could out of the capture, with the confidence that my base exposure was dead-on.

Some weeks later, I was experimenting with an old Tilt and Shift lens which did not have metering capability on my camera.  I had no real concern about how to properly meter.  I did not have my hand-held meter with me.  No matter.  I took my best guess (based on my fundamental knowledge of light, exposure and metering), took a test shot, and adjusted until I got the histogram I desired.
The histogram measurement tool has freed us to concentrate on the aesthetic aspects of photography.

Getting Exposure Right

As Andy Rooney might famously say, “Have you ever wondered why sometimes your photos will come out just great, and other times they will either be way too dark, or bleached out to the point that they just look awful?”

I think most photographers understand that modern cameras have a light meter built into them and the camera’s internal computer uses the meter to determine how to “properly expose” the photo.  But how, exactly, does the light meter work?

You might conclude that this meter and in-camera computer system is pretty “smart.”  The reality is that a meter is actually “dumb” (in fairness, it really doesn’t make sense to refer to an inanimate object as “dumb” or “smart”).  In reality, the  internal exposure system in modern cameras is very sophisticated.

But a light meter doesn’t think–it measures!

For many years, photography was all about acetate-based film.  Today, photographers mainly use digital sensors to record images. Both of these systems are based on the same principal ingredient: Light. The technical capture of images is based on quantity of exposure to light.  As we covered in the tutorial on “F-stops,” the amount of light which strikes the sensor is based on time of exposure, size of lens opening and the sensitivity setting (ISO) of the sensor.

My Philosophy. So how do we use these variables to obtain the proper exposure?  Before we go into mechanics, lets make a couple of important points.  First,

Letting the camera (sophisticated as its system may be) choose these variables for us is the worst and least dependable way to obtain correct exposure

Second, “correct” exposure will always be, to some degree, a subjective judgment.  Sure, there is a point at which all would agree that an image is underexposed (too dark) or overexposed (too light).  But there is also a range of acceptable exposure, and very small differences may give nuanced results in color saturation and brightness of an image.  Slight “overexposure” will tend to give a more “pastel” color result, with slightly brighter image, while slight “underexposure” will tend to give slightly more saturated colors, but a darker image.  This is a part of the reason why modern cameras often allow exposure adjustment in fractional stops.  Understanding correct “base” exposure will help us use these “nuances” to our advantage.

The “Science” of the Light Meter. The primary measurement tool is a photo-sensitive “light meter.”  It is actually possible to get a reasonably accurate exposure without a light meter by knowing the reciprocity of a particular ISO, based on a standard set of conditions (the traditional one is the “sunny F16 rule,” which says that proper exposure for a particular ISO in bright, sunny conditions is 1/ISO, when the lens aperture is set at F16).  Using this standard, you can interpolate between other stops, and guess pretty accurately about other light conditions.  But it is still a guess, not a measurement.

Reflected Light Meters.  How does the light meter measure light?  There are two different ways light meters measure light.  The most common method, which is used by light meters built into cameras, measures the light which is reflected from an object.  This is a very important concept to understand.  Photographic images often have multiple objects.  This makes it paramount that the photographer understand which of those objects are the most important to be properly exposed!  Often this means that the photographer will have to make choices.  This type of meter is referred to as a “reflected light meter.”

Ambient Light Meters.  The second type of photo-sensitive meter reads the “ambient” light.  This type of meter is normally a hand-held meter which can  measure the intensity of light falling on a subject rather than being reflected from the subject.  This method requires the photographer to point the light meter toward the camera.  One of the principal disadvantages of ambient light meters is that often the subject is far enough away or sufficiently transient (e.g., wildlife) that it is difficult or impossible to truly measure the light that is actually falling on the subject.  Since the meter must be pointed at the camera, it should be obvious that such a metering system cannot be part of a camera’s built-in metering.  Nor is it always possible to position an ambient meter in the scene near the light we are tying to measure.

Limitations of the Sensor.  It is not uncommon for a scene to simply have  more variation from light to dark, than can be captured.  In this case, the photographer must make the important choice about which parts of the image must be properly exposed.  Understanding how the camera’s meter works is the key to getting this right.

Metering Methods.  Modern mid to high-end DSLR meters typically have 3 choices for measuring light: Balanced Matrix Metering, Center-weighted and Spot.  Traditionally, SLR cameras had “Center-weighted metering (some pro models added Spot metering).

Center-weighted metering assumed that the most important part of the photograph was always in the center of the photo (that area in the square brackets in the center of the viewfinder), and thus weighted its measurement, typically about 80% in the center and 20% in the rest of the image..  The problem with this approach was that most good photographs do not place the subject “bulls-eye” in the center of the image.  When available most experienced photographers did not rely on this method.

Spot-metering allows the camera to meter a narrow area in the viewfinder.  Advanced photographers use this method most often, metering those specific areas they want properly exposed, letting the other areas fall wherever they fall.

Balanced Matrix Metering has gained great popularity in current day cameras (note: this is what Nikon calls it, Other manufacturers may have different names, but the concept is the same).  The camera’s system has internally memorized thousands of lighting situations.  The meter samples several areas of the image in the viewfinder and inputs this data into the in-camera computer.  The computer compares it against its internal list of situations and suggests an exposure base on its comparison.  My experience is that this suggestion is dead-on about 90% of the time.  The other 10 percent, the meter can be completely fooled.  If you are intent on getting the image, and may never have the chance to repeat it, this may not be the best system to rely on.  Seasoned photographers more often than not use their spot-metering system to obtain the correct exposure (if you shoot digitally, and shoot the camera’s native “RAW” format, there is an even better way.  We’ll cover that in a future topic).

What the Eye Can See. The human eye is a rather amazing optical instrument.  The eye can see many “stops” (see “How F Stops Work”).  The digital sensor, however, is not the human eye (yet–technology continually amazes me.  I have read that there are sensors in development that may approach the technological sophistication of the human eye).  The digital sensor can “see” (record) a range of about 5 photographic stops.  The human eye, in contrast can see thousands off stops.  So, often what we “see” simply cannot be captured and displayed with current technology.

It is critical for the photographer to understand this fundamental limitation of the digital sensor (or film).  When using the light meter to measure the light in a scene, the photographer must make critical choices about which objects must be in proper exposure, and which will simply fall where they fall.  Obviously, the subject must be in proper exposure.  The range of exposure that the sensor is capable of is known as “exposure lattitude.”

Getting Exposure Scientifically “Correct.” In order for a light meter to measure light correctly, it must be calibrated to a “standard.”  This standard goes back to black and white film days, and is known as “neutral gray,” halfway between pure black and pure white.  Convention has this as 18% grey (technically, ANSI standards calibrate the light meter at 12%, but that is a story for another day — for our purposes, 18% grey is the standard reference).  Theoretically, if you properly expose an object that is 18% grey in a scene, the entire scene will be well exposed, within the limits of the sensor’s exposure latitude.

Here is the critical point: The Light Meter is calibrated to “See” anything it is pointed at as 18% grey, and thus Directs you to Adjust your settings to get that result.

So, if you meter something that should be very light, such as snow, and follow the suggestion of the light meter, your result will be grey (actually, in color, blue/grey) snow!  Likewise, if you are shooting something that should be black or very dark, like a black bird, or a bear, if you blindly follow the suggestion of your camera’s meter, you will end up with a grey bird or bear.

In a photographic scene, there may be many different elements.  If you point your spot meter at a light or dark element, you will have the exposure variances noted above.  What about if you use one of the “smart” metering methods, like center-weighted or matrix?  These are the circumstances responsible for those “Andy Rooney” shots we started out mentioning.  What happens is the bright or dark influences in the scene “fool” the light meter.  Snow is a great example.  Another example is shooting into the sun (known as back lighting).  The meter is unduly influenced by the snow, and you end up with an underexposed image.  Or, in the case of backlighting, the meter sees the bright sunlight only, and you end up with a silhouette.

Finding a “Neutral” Point to Meter.  If you point the spot meter at a neutral object in the scene, you will theoretically get properly exposed snow and black objects.  Unfortunately, many scenes do not have objects in them that are 18% grey.  You can purchase a cardboard, or plastic card that is 18% grey, which can be carried in your camera bag.  If possible, the card can be placed in the scene and metered.  Again, this is not always a practical solution.

In real life, certain objects and colors will work as a good substitute.  One of the things a photographer learns to do is look for something “neutral” in the scene to meter off of.  Light grey tree trunks and rocks, blue sky on a sunny day, and grass are all good approximate 18% grey substitutes.

Learning to “Adjust.” There are other references that can be used with an “adjustment.”  Caucasian skin, for example, is one stop brighter than 18% grey, so in a pinch, you can meter off your hand and open up one full stop from the setting, to arrive at the scientifically correct exposure.  Snow, is 1 1/2 to 2 stops brighter, so in a winter scene, you can meter off the snow and open up to compensate.  For this method to work, you must use your camera’s spot meter, and the longest lens (or lens setting in the case of a zoom lens) to “focus” the meter on the area you are trying to meter.  Using the matrix or center-weighted metering will defeat this, by trying to “average” the entire scene.  The best method is to learn how the meter measures the light being reflected from the object and learn to adjust accordingly.

A final point about reflected light metering:  The meter must be pointed at the light you are metering.  Be careful not to meter light in a different area that the light actually falling on the scene.

Getting the Exposure Esthetically Correct. As I mentioned in the beginning, true correct exposure will always be a matter of subjective judgement.  When I shot slide film, I would routinely “bracket” (the practice of shooting several exposures, one or more “over” the correct exposure and one or more “under” the correct exposure).  There were those who did that routinely as “insurance” that they got the technically correct exposure.  That is not why I did it.  I always preferred to measure the subject properly and rely on proper techniques to get the exposure right.

I bracketed because I would often find that I would find the “look” of slightly over or underexposure more esthetically pleasing.  Higher end DSLR cameras allow adjustments of f-stops and shutter speeds by 1/2 or 1/3 stop.  I usually would bracket 1/3 over and under for this purpose.  With digital cameras (especially if you shoot in the camera manufacturers’ RAW formats), you can simply adjust for these exposures in post processing. With digital exposure, it is more important to expose properly for the maximum capture of data highlights (again, a topic for another day).

Understanding the “science” of the light meter will be the most important thing you can do to make your exposures consistently good.  And, one you have internalized these concepts, you can begin to use them creatively to “make” photographs the way, as an artist, you want them to appear.  For more in depth coverage of exposure issues, I highly recommend, Bryan Peterson’s “Understanding Exposure,” which is listed on the Blog here under Recommended Reading.

How “F-stops” Work

If you are a seasoned photographer, you may want to skip this entry.  These tutorials are my attempt to teach some photographers new to the SLR / DSLR World.  (See “TUTORIALS” in the “On This Site” box).

The best way to understand F-stops is to use the camera “Manual” setting (M).  When you set the camera on the manual setting and look at the meter readout, you can see (by the + or – signs) whether the setting will theoretically render a dark (underexposed) or a too bright (overexposed) photo (I say “theoretically, because there are many factors that influence exposure, but that is another topic).

F-stops are the openings on the lens (referred to as apertures).  Every lens has a maximum opening and a minimum opening.  The numbering system is backward.  The smaller the number, the larger the opening, and vice-versa.

Every stop either doubles or halves in size.  F16 is twice as wide open as f22.  F8 is twice as wide open as f11.  F8 is twice as wide open and so on.  Of course, the opposite is true (f22 is half as wide open as f16).  Notice it isn’t mathematical (it would be easy, for example, to think that f11 is twice as wide open as f22.  It is not).  The reason for this is mathematical, has to do with lens geometry, and well beyond the scope of this tutorial –and my feeble intellect.

Shutter Speeds
Shutter speeds also halve and double.  So a setting of 1/125 second will keep the lens aperture open twice as long as 1/250 second (remember, as the denominator gets larger in a fraction, the number gets smaller, or in this case, faster).  1/500 second is twice as fast (or short) as 1/250.  Notice that in the case of shutter speed, this is mathematical!  Nobody ever said “logic” was easy.

On most digicams and all DSLR cameras, shutter speed and f stops are controlled by the camera body.  Some have faster maximum shutter speeds.  Most have very slow speeds and even a setting in which the speed is controlled by how long the user keeps the aperture open (for reasons I cannot explain, this setting is usually known as “bulb”).

F stops and Shutter Speeds Work Together
Note that  F-stops and shutter speeds behave similarly.  Both have the characteristic that everything either doubles or halves.

Now, think of the lens as a water pipe, with the camera being the valve.  If you open the valve to f2.8 for 1 second, a certain amount of water will come out.  If you open it for 1/2 second, 1/2 as much will come out.  If you open it for 2 seconds, twice as much water will come out.

But if you open it to f3.5 for 1 second, only half as much will come out too.  So you can see that to get the same amount of water in your bucket, you have two different ways to get there.

Film and Digital Sensors
There is one other variable.  It is known as ISO rating (“ISO” stands for International Standards Organization).  Some of us “old fogeys” will remember that in years gone by, film–for those who remember what that is–had an “ASA” (American Standards Association) rating.

ISO ratings have an interesting characteristic.  They halve and double.  Sound familiar?

Photographic Images have to be “represented” on some kind of media.  For years, we used film.  Film had crystals embedded which were sensitive to light.  The more they were “exposed” to light, the more they changed, creating a photographic image (recall that the linguistic origin of photographic means roughly, “painting with light”).  As technology evolved, the industry was able to make films that responded more quickly and subtly.

A film with a 100 ISO rating was twice as sensitive to light exposure as one with a 50 ISO rating, and so on.  The more sensitive (higher numbered) films were referred to as “faster” films.  When I started serious shooting, in color slides we had Kodachrome 25 and 50.  By the time I stopped shooting with film, 400 ISO color print film was widely popular and even faster were available.

Digital sensors respond similarly.  But one of the great advantages to digital is the ability to freely select among ISO ratings.  In digital cameras, the ISO can be changed, even from shot to shot (in film days, you either changed rolls, or some of us carried more than one body, loaded with different films).

Depth of Field
F-stops are important, because they effect “depth of field.”  Depth of field, oversimplified, is how much of the photograph is in focus from near to far.  Depth of field is also effected by the focal length of the lens.  Longer lenses appear to have less depth of field, even at very small apertures.  Very short lenses — i.e., “wide angle” lenses– appear to have a lot of depth of field, even at wide-open apertures).

If you want less depth of field, you will choose a wider aperture.  This is often desirable for a pleasing way to isolate your subject and make it the focus of the viewer’s attention.  Sometimes, it just helps take out “clutter” in a background, in order that it not be distracting.

For more depth of field, the general rule is to select a smaller aperture.  This is often important when you are shooting close up, or you want things from near to far to be in focus.  When you see those gorgeous landscape shots that have flowers right in your face, a lake or something in the middle and snowcapped mountain in the background, all tack sharp, that result is almost always from a combination of a wide angle lense and a small to medium aperture.

Lens Design
Wide Angle lenses (14mm – 35mm) generally have great depth of field, even at wide open apertures.  They do not normally produce nice out-of-focus backgrounds that are used in portrait-style photographs (whether human, or something else).  They are good for landscape shots which beg for sharp focus throughout.  Large aperture telephotos (often desired in low-light situations where you are shooting subjects with a risk of some movement (e.g., wildlife or windy conditions) require very large front elements and are very expensive to build — and to buy.  They can also be heavy and require very good shooting technique from a tripod (remember that their higher magnification can also magnify every error, including camera or user movement)!

Telephoto lenses (135mm – 600mm) generally have limited depth of field, even at their smallest apertures.  This means more attention to the in-focus and out of focus parts of the photo must be paid.  It also means that in many cases, you can create attractive, out of focus parts of the photograph.  Some photographers refer to the “look” of a particular lens, especially telephotos, as bokeh.

Lenses that have a small f-stop number (sometimes referred to as “fast” lenses) are generally more expensive to build (and therefore, to buy).  The longer the focal length, the larger the front glass element needs to be in order to achieve the small f-number.

Zoom Lenses are trickier.  Less expensive designs have what is known as variable aperture.  What that means is that the largest opening will vary, depending on whether you are at the short end, the longest end, or in between.  This may also mean that the particular exposure solution for a given situation will change as you zoom in or out.  Some of the higher end zooms have a constant aperture.

Camera Design
In the old days, SLR (Single Lens Reflex — DSLR denotes Digital SLR) cameras functioned using manual adjustment of F-stops and shutter speeds.  With proper use of a light meter, the photographer could select the proper exposure combination for the desired result.

Over time, so-called “automatic exposure” features have evolved.  At first, they simply involved setting one of the two variables (aperture or shutter speed) at a fixed point and letting the camera and its light meter “choose” the other variable.  This is a valuable and handy tool, but it is not really “automatic.”  If you don’t understand the basics of F-stops and shutter speed, these “auto settings” will fool you and you will not obtain the desired result.  It is also important to understand how the light meter works in all of this (see upcoming tutorial on the Light Meter).

Newer designs have evolved that allow an “all automatic” setting.  Essentially, the all-automatic setting measures the light conditions and sets an F-stop / aperture combination that theoretically properly exposes the image.  On some of the more “advanced” models, there are even sub-settings, like “action”, “portrait”, “landscape”, etc.  All they are doing is forcing either the F-stop or the shutter speed to be fixed.  In the “action” mode, for example, the shutter speed will be fixed at the fastest shutter speed allowed by the lens and light conditions, and the F-stop adjusted accordingly, to provide proper exposure.  For “landscape,” the camera will look for the smallest possible shutter stop.  You can do this yourself and if you do, you will control things, not the in-camera computer.

In my admittedly biased view, “automatic” settings completely defeat the purpose of the DSLR. The DSLR gives the photographer the ability to choose the best settings for the desired creative result.  Using the all-automatic setting is choosing to let an unintelligent electronic gadget “think” for you!  The “all-automatic” settings are really for casual users, and best for simple Point & Shoot digicams.

Limitations of Equipment
Sometimes your options are simply limited by your equipment.  Some equipment choices are driven by “need.”  Others, more obviously, are driven by economic considerations.  A few general observations:

If you are shooting non-moving objects from a tripod (e.g., landscape, architecture, etc.), the need for a “fast” lens (wide aperture) is less critical

If you are shooting wildlife (especially birds) you will probably eventually want a “fast” telephoto lens.  Most serious bird photographers prescribe at least 400mm.

If you need to handhold shots, you are going to need wide apertures, shorter focal lengths and faster shutter speeds.  An old rule of thumb says that you should not attempt to hand hold shots at shutter speeds any slower than 1/focal length.  I tend to be conservative and strive for even higher speeds.

The advent of the new “image stabilization” (“IS”-Canon) or “vibration reduction (“VR”-Nikon), or similar technology available in most brands and now in many Point & Shoot digicams, will allow you, in some cases, to shoot at slower speeds with longer length lenses.