Hiding In The Snow - Cold Lake, Alberta
Let's see where we've been. So far we've started our trail on learning about exposure by introducing the whole concept. Along the way you've learned what the sunny-16 rule of thumb is. We've learned there are three variables to adjust on your camera to set your exposure. We've figured out that a subject's color will change the exposure without the light changing at all. We learned a couple of tricks when dealing with those situations.
We've learned about what to consider if you keep some of the variables constant. What do you think about if you keep the shutter speed constant--I.e. Shutter priority. What if the ISO is variable?
So far we've covered a lot of ground. Now we're in the home stretch for considering exposure if you keep the aperture the parameter you choose to set based on what you’re trying to achieve for your imagery.
I've saved the best for last. Talking about exposure when the aperture as the creative parameter can be by far the most import discussion. The bottom-line reason for this is the aperture setting as part of the lens’ optical formula directly contributes to the quality of the imagery your sensor sees when you pull the trigger.
Let me explain this a little more. Camera lenses are made up of a complicated configuration of highly manufactured glass intended to overcome the difficulties of physics to display the best possible image on your sensor. While this sounds all well and good, a tremendous part of this optical equation is the quantity of light passing through the lens. The quantity gets metered by a little iris in the lens called the aperture.
By opening and closing the aperture you increase and decrease the quantity of light passing through the lens. Lens opening has a direct affect on the lens design equation and the resulting image quality. Several image quality issues are at stake like color aberrations and most importantly sharpness dependent on what aperture setting you choose.
We’re going to discuss how changing the aperture affects first the image quality then talk about how it affects exposure.
Let's get down to it.
Depth Of Field
Say this to yourself out loud; "there is only one single focus point".
Why would I make a point of saying this? For those that know a little bit about photography seem to not know this little fact. There is only one point that can be in focus at one time. Why is this important? It speaks to the whole misunderstanding about this optical characteristic called Depth Of Field (DOF). DOF is the range where things appear to be in focus. Note the key word here, appear. Let's talk about this thing called DOF.
DOF is an optical characteristic where when you focus on a single point there's a range forward and aft of this point that appears to be in focus. This phenomenon is a byproduct of the physics of optics. Even though you're focused on a single point/distance, based on DOF there's a range that appears to you the viewer that's in focus in front of and behind your focus point. This is pretty much an illusion. What's not at your focus point that appears to be in focus is not in focus. What's happening here is you're seeing things sharp because they're in the resolving ability of your eyes.
Things beyond the focus point appear to be in focus because these items are within the resolving power of your eyes to see things out of focus. What's considered sharp--which is not what's in focus, btw--is a function of many things including how far your eyes are from the image when viewing it, the size of the image capture sensor, the size of the final image. Lots of things factor into how much DOF you have. Included in this list is the lens aperture setting.
Let’s run an example. Let’s say you’re looking at a relatively normal sized 8x10 inch image from way across the room. Unless you have telephoto lenses for eyes, you probably can only tell you have an image and it looks as sharp as anything you’ve seen. Everything in the image appears sharp because you’re not close enough to see detail in the image. Now, let’s get closer. You begin to resolve more detail in the image including whether the components are sharp or not. What was previously sharp at a distance is now looking less sharp as you get closer. Viewing distance is one factor in determining what’s sharp or not on an image. The size of the print is part of that equation. Even the size of the capturing sensor is a variable in that function. Sure, the human eye has a part and that’s quantified by the experts so you don’t have to figure out that component your self. Finally, there’s the lens aperture.
There’s physics involved that allows what appears to be in focus to change with the changing aperture size. We won’t get into the technical detail of why this is but we’ll conclude it’s a factor. Generally the smaller the aperture the more DOF you’ll get.
I’ll talk more about DOF and sensor sizes in the sidebar.
What does this mean to mean that DOF changes based on the aperture. What matters is by changing the aperture or the optical formula of the lens, you’re changing how your image appears. If you’re trying to completely isolate your subject and want to have a very non-de-script background you’ll what to set a wide aperture. If you want lots of DOF you’ll need a small aperture. If the light that’s falling on your subject is cooperative, you have no other challenges. But, if it’s not... you have problems.
For example, let’s say you need to blur the background and want to set a wide aperture. You’ll need a very fast shutter or slow ISO to set the right exposure. For the other example where you need a wide DOF, you’ll need just the opposite. Of course, the slower the shutter the more likely you’ll need a tripod, which is how I shoot 95% of my landscape photographs anyway. As noted in my article on Exposure-The Shutter Speed, a slow shutter speed may cause image sharpness issues if the speed is particularly slow.
As you can see while everything is related, you can set priorities of what settings to keep fixed based on what you’re trying to achieve. If you want to keep DOF a certain size then you’ll likely set the aperture setting first, the ISO next to retain image quality, then finally deal with the challenges of a fast or slow shutter speed last. By setting the aperture first, you’ve taken more control of how much DOF you have in your image.
This is a complicated problem. As I usually say, there’s no free lunch in photography, setting the aperture will eventually affect the overall optical goodness of your lens. What I mean is at some aperture settings, most lenses perform better under some aperture settings than others.
Using that widest possible aperture might not result in the sharpest possible subject when compared to shooting the subject at a slightly smaller aperture. The other end of the spectrum is true as well. In your zeal to increase DOF to have as much of your scene to appear in focus as possible, you might want to use your lens’ smallest aperture setting little knowing that optical physics is becoming involved causing the resulting image to degrade with such a small opening.
Let’s talk about that stuff.
As a matter of course, you don’t ever want to count on the sharpness of your lens by using either its widest or smallest aperture settings. For most high-quality lenses, a slightly smaller aperture setting results in a better performing optical formula. How do people measure these kinds of things? In the most antiseptic way possible. What they do is put an eye-chart on the wall at certain distances from the camera and shoot the it at various lens settings. From that they’re able to determine the resolving power. How does this apply to real world applications? Not very much. Rarely is your subject a flat eye-chart. You need results for non-linear, non-man-made, natural subjects. How do you figure how your lens works in those situations? Trial and error.
While I use data from those antiseptic resolution charts as a starting point for how to set my aperture. I never let them drive how I want to use my lenses. I use them as a starting point. Most folks will say pro lenses start being really effective when the aperture is about stop less than its maximum aperture. A f/2.8 maximum lens starts being really effective at less than f/4.0. Other folks say f/8.0 and f/11 are where lenses really start showing their stuff. Well, if I was to only believe what these folks say, I’d be stuck at f/8.0 and f/11 all the time. When I’m trying to control DOF and only have two apertures to choose from, this seems a bit limiting. Just because someone said the lens performs best at so-and-so aperture setting doesn’t mean you should set your lens there all the time. It only means among other compromises, you probably sacrifice less by having your aperture at one of these better performing settings. If you’re creative vision says you need to have a more isolated background and therefore a larger aperture, you just do it comfortable in knowing the compromises you made to get the image.
Let’s talk about this color aberration thing I mentioned earlier. With really small aperture settings--i.e. iris openings--the hole the light passes through becomes extremely small. Because of the physics of the light passing through such a small hole, dependent on the color of the light, it will “bend” around the tiny aperture hole at different angles. The red will bend at one angle. The blue at another. The green light another angle yet. This results in the image have a color fringe or other non-sharp characteristics because the colors aren’t lining up. With the larger aperture size--wide hole--all the colors pass through the lens’ aperture the same way. There’s no divergent color thing going on and less of an issue with this color fringing. This color fringing is called an chromatic aberration. Typically, most lenses exhibit this characteristic in high contrast situations with the aperture set to f/16 or smaller. As a result, I hardly ever set my aperture to f/22 or smaller unless I really need the DOF and don’t mind the chromatic aberration.
Clearly there’s a lot to think about when figuring out the scene’s exposure. Most times, it’s perfectly fine to set the camera on automatic and let’r rip without thinking about aperture or shutter settings. However, if you want to have more control over your final image, it’s very simple to set an aperture based on how you want your final image to look, cross-check the shutter speed is barely acceptable, then shoot the image. It’s pretty simple stuff.
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