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Deep DOF and diffraction artifacts

From: Foundations of Photography: Lenses

Video: Deep DOF and diffraction artifacts

To capture a really deep depth of field you use a smaller aperture, bigger-up number. This should be basic exposure theory that you're already comfortable with as well as an understanding that where you focus when you're trying to shoot deep depth of field matters. We covered this earlier. I want to talk to you now about how after a point closing your aperture down might actually soften your image a little bit due to an optical phenomena and called diffraction. Basically it works like this. As you close the aperture down to a smaller hole, when the light gets squeezed through that hole, it gets diffractive, knocked around a little bit, and it ends up getting softened.

Deep DOF and diffraction artifacts

To capture a really deep depth of field you use a smaller aperture, bigger-up number. This should be basic exposure theory that you're already comfortable with as well as an understanding that where you focus when you're trying to shoot deep depth of field matters. We covered this earlier. I want to talk to you now about how after a point closing your aperture down might actually soften your image a little bit due to an optical phenomena and called diffraction. Basically it works like this. As you close the aperture down to a smaller hole, when the light gets squeezed through that hole, it gets diffractive, knocked around a little bit, and it ends up getting softened.

It doesn't focus as sharply as it should. Now, what we're about to demonstrate and show is a pretty finicky level of sharpness. Sharpness is a very subjective thing. Opticians typically refer to it as acceptable sharpness, meaning everyone has a different idea of what's sharp and what's not. Sharpness and how much it matters also depends on how you're outputting. If you're printing something small like a 4x6 from a 16 megabyte image, sharpness is probably not going to be that critical in-camera because you're going to pick up so much sharpness when you sized down.

Nevertheless I think it's interesting for you to see how this works. I have a test target down here. it's just a simple resolution target. I put my camera in Aperture Priority mode and I'm going to shoot it on several whole-stop apertures, making the aperture smaller each time. I've connected a remote control to my camera because I want to be sure that I'm not getting any camera shake. So right now I'm set in on a 5.6. I'm going to just take that picture. And just frame kind of tightly on that subject there. I'm going to go up to 8 and take another picture. I'm waiting a moment after handling the camera to be sure that it's got time to stop vibrating. There is 11.

And the reason I'm concerned about vibration is we're talking about sharpness differences here that could be impacted by a camera movement. I'm up to f16 and I'm going to do one more at f22. Now a lot of times when you trying to shoot deep depth of field, you think, "Oh, I'm just going to close my aperture down as far as it will go and that will give me really deep depth of field" and it will. But look at this. Here's the image shot at f5.6. Notice the edge, one of those black & white edges. I've zoomed in here to 100%. Now look at it at f22.

You should see there's a little bit of softness in the f22 image that's not there in the f5.6 image. That is diffraction artifacts. That is not going to be true with every lens. That's this particular lens, which is the kit lens that came with this camera, but if I put better lens on there I may not see that. Another question is whether that level of softening matters in the real world. And again, this type of sharpness discussion is a really subjective thing and it's kind of up to you to decide whether you think that that's a level of softening that matters.

Before you can make that decision though, you need to know how your lens responds. There is no problem to Google around and find yourself a test chart, print it out on your printer, and do just what I've done here. Set it up in some nice light. We're not having to be that careful about how we're taking a picture. I've over exposed by about a stop to get my brightness levels up to where they need to be, and you saw that I'm just controlling aperture and taking these shots, taking them into my image editor, and looking at them at 100% to see how sharp they are. Next do the same thing with a real world example. Go and take a landscape shot or something that has some fine detail in it and see if the level of sharpness changes in a way that really matters.

For really sturdy test you might then want to do some output. Maybe print some pictures and see if you can tell a difference. You may find that this is not a factor at all in your shooting. The important thing to know is that, it is a potential weakness in your lens and you want to test your lens to find out if your lens possibly has a diffraction issue that could cause you some sharpness that you're not willing to lose.

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Foundations of Photography: Lenses

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Ben Long
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