Odd distortion in images

[Farkle-Mpls]

<p>Hello. I just got back from a two week vacation where I ran a bunch of rolls of (new) Portra 400 through my M7. I brought a few lenses with but ended up favoring my CV 21/4 because of the wide field of view. I'd used this lens before without any issues.</p>

<p>When I got the developed negs back yesterday, I noticed a very frustrating artifact in many images. It seems the very center of the image loses sharpness. I'd also risk saying it seems like some vignetting too. </p>

<p>Because the daylight was pretty strong and I desired a great DOF, I shot around F11-F16 very often. In my prior uses of this lens, I don't recall if I used such a small aperture with any regularity. Also, ALL shots I took at F11+ displayed distortion in the center (I can post more examples if it would be helpful).</p>

<p>The image below shows the issue. The M7 was on a tripod and I used a cable release. You can tell the rest of the image is sharp so the camera wasn't moving but look at the horrible quality of light around the windows ... it looks like there is Vaseline on the lens. It's terrible! (And the lens is clean -- I cleaned it daily.)</p>

<p>The images which were shot around F5.6/F8 don't seem to show this. What is this effect? Diffraction? Any ideas?</p>

<p>Thank you!</p><div></div>

[tjfuss]

<p>Does it do this without such a strong highlight right in the middle of the frame? That looks like flare to me and doesn't seem too surprising given the amount of light coming in through those windows.</p>

<p>If it does happen with other (more evenly lit) scenes, then check out the glass for scratches, pits or internal haze. Use magnification and a strong light source.</p>

[Robin Smith]

<p>It's flare and halation. Sometimes no film can contain the dynamic range you require of it. Neither will a lens. The only way to cope is to bracket and pick the one with a contrast range you like. Or you can illuminate the interior to match the light outside (you'd need a good lighting setup-impossible in this case), or combine two or more bracketed exposures when digitally processing. This is one of the few times when these HDR processes could be useful. Next time you should not have so many large windows in the shot (shoot down the nave) and/or make sure that the windows you pick have the least amount of illumination. Shoot when the outside light levels are lower (as in the early morning or evening) is another approach.</p>

[Robin Smith]

<p>Also do you have a filter on the lens? If so remove it!</p>

[SCL]

<p>Definitely looks like flare to me. If you're shooting in non-contre jour situations and you don't see this phenomenon, that confirms it. I would call it "Leica glow", but you were using a CV lens :)</p>

[John Seaman]

<p>Look carefully at the rear element of the lens for any scuff marks or smearing. I has a Minolta 28mm lens with a graze on the back which caused a similar effect - wide angle lenses seem very sensitive to damage to the rear element.</p>

[tedms gallery]

<p>There is flare there, but you may also be suffering from diffraction. Stick with f8, if that covers the DOF for you. Diffraction calculations often assume the aperture is circular, with the 21/4 CV lens, it may not be, and in any case, with certail wavelengths (colors) you will be diffraction limited at F16, probably F11 as well, depending on film.</p>

[robert_hooper1]

<p>Check the lens for haze or fog. Haze or fog within the lens will act like a diffusion filter. Shine a flashlight through one end of the lens and look through the other. Any fog should be very apparent.</p>

<p>Another possibility is condensation. If you quickly moved from a cold area into a warm area, condensation might have formed within your lens. That would cause a similar effect to haze or fog.</p>

<p>Make sure the front and rear lens elements are free from fingerprints.</p>

[Farkle-Mpls]

<p>Cr*p! Examining the lens in greater detail revealed a finger print smack in the middle of the rear element! I did not clean it off because I want to shoot a test roll, before and after. This is really frustrating -- I bet there are 200 pictures I shot with that lens and although I cleaned it, I only cleaned the front element because it was misting out every day.</p>

<p>Lesson learned the hard way.</p>

<p>I'll post results of my cleaning this weekend but I bet that's it ... particularly since I hadn't noticed any particular issues with this lens before, except some vignetting at F4 (which is easily worked around).</p>

<p>Thank you everyone for you comments -- looks like you nailed it Robert!</p>

<p>(I'm still intrigued by the diffraction test so I may try that this weekend too.)</p>

[frederick_muller]

<p>If it proves out, it also indicates how important the rear element is. The worst place to take any degradation of the lens surface ...</p>