[RobG]

From what I understand, any given lens will have an aperture sweet spot which provides the maximum sharpness and DOF before diffraction softens the image. Is there a way of determining what this ideal f-stop is for each lens? Has anyone figured this out for the Pentax DA lenses, for example?

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Last edited by RobG; 11-21-2012 at 07:01 AM. Reason: correction

[msatlas]

Most lenses this is usually f/8. I don't know if there's a good way to test for this at home, but you can look at the MTF charts at places like Photozone.de, SLRgear, photodo, etc and work from there.

[StephenHampshire]

On the K5, you have the option to choose MTF priority in P mode this sets the aperture to the optimum (which varies with focal length on a zoom) for any modern Pentax lens

[audiobomber]

Most lenses this is usually f/8.

I don't think that statement holds up very well. My general statement would be that most lenses are sharpest at two stops down, but there are exceptions; for the DA 16-45 f4, maximum resolution is f5.6, for the FA 50 1.4 it's f4, the DA*300mm f4 is sharpest at 5.6 (all according to Photozone tests).

[reeftool]

You really have to shoot for a while with a particular lens to get the best out of it. While there are general "rules" about diffraction often showing up at f/8 or more, I have a couple of lenses that are their best at f/11. My Sigma 70-300 is at its absolute best at that aperture. I have learned that after shooting wildlife and sports with it for 5 years. If I have strong light, f/11 is going to be the best although that may or may not be the sharpest resolution in a test shot. An important thing to remember is that there are a lot of other factors in a photo than a resolution chart shot. If you start thinking that you can never stop down more than f/8 because of diffraction, you might be missing out. My DA 10-17 is excellent at f/16 and I use that setting often when shooting outdoors.

[wtlwdwgn]

I don't think that statement holds up very well. My general statement would be that most lenses are sharpest at two stops down, but there are exceptions; for the DA 16-45 f4, maximum resolution is f5.6, for the FA 50 1.4 it's f4, the DA*300mm f4 is sharpest at 5.6 (all according to Photozone tests).

Although there are many exceptions to any rule the rule of thumb I was taught at photo school was 2 stops from the max f stop for sharpness and 2 stops from the min f stop because of diffraction. As said above Photozone does a thorough job on their lens tests. Just my ¢¢.

[audiobomber]

Although there are many exceptions to any rule the rule of thumb I was taught at photo school was 2 stops from the max f stop for sharpness and 2 stops from the min f stop because of diffraction. As said above Photozone does a thorough job on their lens tests. Just my ¢¢.

I think two stops from minimum aperture works better for diffraction on full-frame (35mm) bodies. For smaller APS-C cameras I suggest this would be modified to three stops.
Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

[RobG]

Thanks for all the responses folks - the variety of answers demonstrates that there's a difference between the measured point of optimal sharpness and the perceived optimal setting. That's not a criticism - it's an interested observation. I'll certainly check the figures on photozone and try the MTF setting, since I'm generally using DA lenses.