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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”
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.
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3 Responses

  1. This morning I was working on a new BLOG entry on Hi Dynamic Range Imagery, and it occurred to me that I left out an important thought in this article. Modern cameras allow adjustment of these “F stops” to only partial stops. As I note in the upcoming HDRI Blog entry, perhaps it is more modern and technically correct to refer to these “F stops” as Exposure Values (“EV”). In any event, today’s DSLR bodies allow adjustments in either 1/3 or 1/2 stop values (or both). While this may not be as valuable in digital imagery, because the the greater latitude of exposure in general, it is worth noting that you can “bracket” exposures in less than full stops.

  2. […] of the tutorials here have been technical in nature, discussing things like exposure, “F-stops,” histograms, and the like. They are geared toward the budding enthusiast who wants to take […]

  3. […] my first tutorial, “How F-stops Work,” I detail (hopefully in a readable and understandable way) how the aperture and shutter […]

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