Diffraction Limit Discussion Continuation

Started Feb 21, 2014 | Discussions thread
richarddd Veteran Member • Posts: 3,213
Re: Diffraction Limit Discussion Continuation

From prior thread

Great Bustard wrote:

thk0 wrote:

I have developed a reluctance to shoot at small apertures, largely as a result of reading posts about diffraction limits. However when I look at a large number of pictures in my library I fairly often wish I had shot at a smaller aperture for greater DOF. I rarely wish I had shot at a larger aperture, the exception being motion blur. My conclusion is controlling DOF trumps any consideration of diffraction.

There's always a balance at play. Assuming you want the entire frame to be as sharp as possible, you want to stop down for a greater DOF, as elements of the scene outside the DOF will, by definition, be soft.

It's relatively easy to see DOF differences - this part of the photo is sharper than that part of the photo. It's relatively harder to see diffraction differences, because they cause an overall decrease in sharpness, so we don't have a this and that to compare when looking at the photo. It's easier to discern comparative differences.

That's why I tend to agree with thk0.

We also want to stop down since this reduces the aberrations in the lens. This is why, for example, a lens is sharper at f/2.8 than f/1.4. However, at some point, the increasing diffraction softening outweighs the lessening lens aberrations (usually by f/4 - f/5.6, with the edges typically lagging behind the center by a stop).

Aside from diffraction softening, stopping down also results in a lower shutter speed (at the risk of more motion blur and/or camera shake) or less light falling on the sensor, which increases noise. So, for example, while someone might shoot a scene at f/5.6 in good light, they would shoot the same scene at f/2.8 in poor light, even though f/5.6 will give the deeper DOF as well as being at a sharper aperture.

Joe, meet tripod. Tripod, meet Joe.

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