DIFFRACTION WITH 10-22mm F/3.5-4.5 LENS

[digiquisitive]

I have used this lens for several years, with excellent results, on my 50D and 7D.  Now, shooting Fall landscapes, I find what may be diffraction problems when shooting at minimum apertures (f/22 to f/29).  Shooting conditions were overcast/cloudy.  Shooting at ISO100, shutter 1/4 sec to 1 sec, tripod, self-timer.  Results show much less resolution than ever previously experienced with this lens -- even at aperture as large as f/16.  Does anyone know at what fstop this lens (depending on focal length in use) starts losing resolving power due to diffraction?  Could mirror slap be the issue instead of diffraction, or a contributing factor?  Counsel sought, based on your experience.  Thanks.

[Skirball]

Well sure, it could be mirror slap too, as well as camera movement, at those shutter speeds.  But it doesn't surprise me in the least that it's softer at those apertures.  The lens only goes to f/29, and even then only at longer FL.  All lenses get softer as they're stopped down.  I can't tell you where the turning point is, but I'm sure someone has done the analysis online.  Personally I've never taken it past F/13 or so.  I usually use mine on the wider side, and the DoF is huge by the time you get to f/8.  I'd recommend some good quality ND filters, don't stop down so much, and bring up your shutter speed.

[digiquisitive · Accepted solution]

Thanks very much.  I was quite surprised to see such small apertures available (was shooting in Av, so they came up on the Q screen), and thought they might be useful in my situation.  Now I know not to ever use them again -- probably not past f/8 -- the alleged extra DOF is not worth soft images.  But it cost me some wonderful Fall color shots to find this out.  Live and learn, even after 60 years of shooting.

[Skirball]

Indeed.  There's really not much use for the upper apertures in an UWA, since the DoF is so large.  As opposed to something like the 100mm macro, which I'll frequently take up to f/22 to get a larger DoF.

[digiquisitive]

Thanks to encouragement of Skirball's responses, I tested my 10-22mm lens last night, at all apertures and focal lengths.  Without going into excruciating detail regarding the thoroughness of the testing, let me summarize and say that this lens is wonderfully and superbly sharp up through f/8, but my tests showed me to never stop it down past that.

To validate my testing last night, I went back to the location of my prior debacle with shooting this lens between f/22 and f/29 (two days ago), and re-shot the same scenes at apertures between f/5.6 and f/8 and all focal lengths.  Today's results, viewed at 100% with DPP, showed that this lens is amazingly sharp at those apertures, as I have noted over the past few years of owning it.

I also validated the use of Auto Exposure Bracketing in conjunction with the self-timer, and using Live View to begin framing my shots, even being able to maximize the use of the polarizer through the Live View screen, then going to the optical viewfinder for placement of the AF points and taking the shots with the self-timer and AEB.  Hope this helps someone.

Yes, I would absolutely recommend this EF-S 10-22mm f/3.5-4.5 lens, even though it does not have IS. I shot everything on a tripod today.

[TCampbell]

You can use this website: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

They have a diffraction calculator you can use and termine when your camera becomes diffraction limited based on sensor resolution (pixel size).

But also keep in mind that even if an image exceeds diffraction limits, you may not notice depending on how large you plan to exhibit the image. In smallish sizes, the effect is not noticeable.

[digiquisitive]

Thank you, Tim -- I was unaware of this excellent site. 

Best regards