diffraction in macro lenses

[KenC]

Can anyone tell me the smallest aperture one would expect to give optimum results for macro shots with a 100 mm? I recently got a Canon EF 100 f2.8 macro (used on an XTi) and all of the shots from f2.8 to 11 were nice and sharp, but I also took a bunch at f25 and they are a little soft. The f25 shots were mostly taken very close up, but one taken at infinity is sharper than the others - would diffraction be worse at closer distances? I had a 105 macro for my film camera and seem to remember using f16 and f22 without any problem, although I wasn't blowing the images up on a screen the way I am now. I read somewhere that diffraction effects somehow are worse in digital, or perhaps just more noticeable?

 

[Dao]

From what I read, quality of the image degrade quite a bit after f/16 due to diffraction. So I usually shoot between f/11 to f/16 for macro shots.

I have the older EF 100mm f/2.8

 

[NateS]

I personally can even see a drop off at f/16 in my lens. F/13 is the sharpest point on my macro and I think most macros are sharpest at f/11 to f/13...after f/13 sharpness starts dropping off. I will hardly ever go past f/16 as any increase in DOF is negated by the fact that I won't ever print the shot due to the sharpness dropoff. f/13 is plenty of DOF I think even at 2:1 and if you need more focus..you can always do a focus stack.

In regards to whether it's worse closeup...I have no proof, but I think it probably is. Everything seems to be amplified when doing close up shots.

 

[Overread20]

This gets a little confusing so lets deal with one bit at a time:

1) generally speaking fullframe camera sensors/films get about 1 stops worth more aperture they can use before diffraction starts to take effect with most lenses. That is somewhat offset by the fact that (in comparison to 1.6crop) you gain about 1 stops worth of depth of field with a crop sensor camera. So whilst you can't close down as much you can still aquire the same depth of field to the shot.

2) Most (all?) modern macro lenses achive 1:1 by adjusting their aperture and focal length as you approach 1:1. Canon camera bodies don't report this change in effective aperture, so you keep f2.8 on the camera settings even though the aperture itself (including the depth of field and diffraction) have closed down (I think to around f5.6 when wide open).
Nikon camera bodies do however report this change in effective aperture to the photographer which makes comparing apertures between canon and nikon a little fiddly when in macro areas.

Generally speaking (as a canon shooter) I keep f13 as my main working aperture and I generally don't stop down any further - f16 I will sometimes use, but most of the time I either stick to f13 or use focus stacking to stack shots to help preserve clarity and sharpness whilst also allowing for a deep depth of field.

 

[KenC]

Thanks to all for the advice. I guess I'll be shooting at f16 max from now on, but it makes one wonder why manufacturers of most macro lenses bother to provide f32.

 

[Josh66]

Sometimes you just need it, even though it will affect sharpness.






edit
(Sometimes sharpness isn't the ultimate goal.)

 

[Overread20]

exactly - I know some people who happily shoot at f22 even though the shots are softer - their main output is to the internet where after resizing and sharpening that loss of sharpness isn't really a factor. This is part of what complicates the diffraction limit because different people will have different standards and also different requirements for their shots.

Having more options is always good as opposed to having things get too "specialist" and thus limiting your overall options.