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Why is My Image Blurry?

Those who read my Blog know that I am a Tripod-Zealot. I know there are circumstances where a tripod is just not going to work (my friend, sometime mentor, and talented professional photographer, Ray Laskowitz, often points out how important good hand-holding technique is if you want to get some images). But in most of the shooting I do, there is no good reason not to use a tripod. And too often, a handheld shot will look “good” on screen, but will simply not hold up when enlarged, because of even very minute camera movement. But now that I have once again led (misled?) you down that path, it is not really what this blog is about.

I am a Tripod-Zealot

Here is an example of one of those times when the lack of a tripod is not the culprit. It is, however, a case where using an “automatic” feature of the camera may have waylaid an otherwise good image. A friend sent me a night time image of a small harbor. From her vantage point, she could see shoreline and boats in the fore and middle ground and water and then the distant shore, with lit buildings in the background. Her shot was “framed” with some low-hanging branches from trees on the shore where she was standing. She had set up on her tripod and considered the far shore as her focusing point. But the buildings in the distance were blurry. My friend’s question involved whether the quality of her inexpensive tripod caused camera movement or vibration.

Maybe. But in this case, I don’t think camera movement was the problem. I think the issue was that the image was simply not in focus. Why? It appears to me that the focus point that the camera chose was the leaves/branches in the immediate foreground! Hmnn. Think about that for a minute. “The focus point that the camera chose.” More in a minute…..

Camera movement, once mounted on the tripod, can only really be caused by external factors like wind, or vibration (i.e., if you are set up on a bridge and cars keep driving over it, or if, after carefully setting up the shot with a long lens, you wait until everything is perfectly still, and then touch the shutter release). So my friend was correct in questioning whether an “inexpensive” (really, one that is not structurally built to be sturdy and vibration-free, regardless of cost) tripod can contribute to poor image quality. Long focal lengths can “magnify” this error. It is a concept that is often hard for us to accept. It has frequently struck me as ironic that folks will go out and spend $2-500 on a tripod and then not use a remote release (and then, ironically, there are times when putting a “hand” on the tripod-mounted camera can actually help damp vibrations — tripod technique is a topic of importance, but not really the focus here).

In order to properly use automatic settings like AF, we must control them

But looking at this image, I have assumed those things away. The image was shot at 50mm — certainly not a long telephoto. It was also shot at f-11, so DOF shouldn’t be a serious problem here. Modern cameras (and “old eyes” in my case) have taught us to rely on AF. And AF is a wonderful technology. When used properly it is “failsafe.” But I have to re-emphasize the phrase, “when used properly.” AF only works when you understand what it is doing. It is mechanical. Like all automatic features, it does not think for you. You must set a mechanical point on the camera to determine what object the camera will focus on. When the camera indicator says focus is locked, trust me, it is locked and in focus (obviously, like all things mechanical, either the camera or the lens me have a defect which will affect this). But focused on what? That is the critical factor, and it relates back to the phrase I repeated above: “the focus point that the camera chose.” If we are going to truly control our image-making, we must not let the camera make any choices! This is why I advocate, learning rules and fundamentals first and then letting the automatic stuff make our lives easier. In order to properly use automatic settings like AF, we must control them. In this case, she set up the image, and pre-focused on the distant buildings. Let us assume that she focused on a well-defined part of a building and the AF indicator gave her a true reading of locked in focus.

What happens next is critical. With the default setting on most modern automatic DSLRs (SLRs, too), the camera re-focuses every time you re-actuate the shutter release. Not realizing that she had the AF bracket (focusing point) floating and that it chose to focus on the near, the camera took charge and made the arbitrary decision to focus on the low hanging branches! Here’s my comment to her:

It is important to understand how focus and DOF work together. The important parts of this image are not in sharp focus, which means we need to think very carefully about what was the focus point. If you were using an AF focus point and it moved, it tried to find something if could focus on (that would probably be the boats in the foreground, or the tree branches–which appear to me to be the most “in-focus” in the image). So, in short, it looks to me like your camera (note it was your camera and not you) focused on those leaves. Maybe you did focus on either infinity or the far buildings. But if your AF was still on, the second you touched the shutter release, it tried to re-focus.

If we maintain focus on the far away buildings @f/11 with a 50mm length we obtain acceptable focus throughout the image. This is simply a matter of optics. The optical Depth of Field (DOF) would ensure that in this case (in theory, finding a focus point about 2/3 of the way into the image will obtain maximum “acceptable sharpness” from front to back. The topics of DOF and “hyperfocal” distance are well beyond the scope of this blog, but worth some study, for the serious shooter). The best way to do that is for us—the photographer—to control the focus point.

Think about that for a minute. “The focus point that the camera chose.”

I will often try to find a point 2/3 into the photograph. But sometimes, particularly in a night shot, that is simply not possible. Take some time to figure out how to be sure the lens is focused on infinity (the furthest focal point out) before you go out next time. If you can see (or if the AF can see) well enough to focus on one of the far objects, go ahead and do that (the little confirmation light will tell you if you have been successful). But if you cannot, then set the lens on infinity. Once you have achieved proper focus, Turn AF off.

On most modern SLR/DSLR cameras, those little red brackets in the viewfinder are not only indicative of the focus point, but also can signify where the center of the light-metering system is reading. So, I may want to focus on one point of an image, and then meter the light on another point. It is a bit of a nuisance to have the camera try to re-focus every time I move the bracket. One way to avoid this is to use the AF only as a focus-check. In other words, focus using AF, then switch it off. This is a bit of a nuisance, too, in my opinion. Another method is that most camera bodies will also allow you to set a focus-hold button which is on the back or top of the body. You can focus and then depress and hold the focus-hold button and move the bracket or set up around (the same button can generally be reversed to do the same with exposure, making it an exposure-hold button). Both of these approaches come within the “PIA” definition to me. I would much rather simply switch to manual mode and do things the “old-fashioned way.” But as I have said here a number of times, the automatic settings on the camera are there for our convenience. Once we understand the fundamental exposure equation and how the camera measures and treats it, why not use the convenience tools? My camera (and I will bet most others do, also), has a setting which allows me to de-couple the AF from the shutter button. With that setting, AF is ONLY triggered by the little button on the back of the camera and NOT by the shutter release. That way, I can focus and then not worry about that getting changed when I do any other action, including trip the shutter. While this may not be the optimum setting for handheld “action” shooting, I find it works well for static landscape shooting from a tripod.

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2 Responses

  1. Andy, you are right about removing focus from the shutter release button. I’ve set up a custom function on my camera so that focus is controlled from the button on the back.

    One other thing that most people don’t even know is on their cameras let alone how to use it: the depth of field preview button. It stops the camera down to the selected aperture so you can see through the lens what will be in focus. I use it ALL the time.

    Al

  2. Hi Al: Thanks for reading, and for your reply. I agree on the DOF Preview button. Over the years, it has been one of the “must have” features for me when I purchase a camera. I hope to blog about understanding DOF sometime this winter. When I do, I’ll certainly mention this valuable tool on the “higher end” bodies.

    There was an old trick for bodies that did not have the DOF preview feature, which involved partially dismounting your lens, by releasing it and giving it a small turn as if removing it. This would cause the lens to “stop down” (which is in effect what the DOF preview button does) to the smallest aperture. HOWEVER, it was a risky procedure, as it could result in the disaster of dropping the lens.

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