why are my images blurry on the sides... some one explain this flat field thing

[AnselAvedon]

guys its focus coverage the is an issue



not an issue with vignette

I do not get the dark areas on the corner of my frames

but the corners are a bit less out of focus.

just like someone made a circle around the center with perfect sharpness and the perfection fades diagonally in maybe one inch in at each of the four corners.

I do not see how this could be from rise fall swing shift or tilt all of the movements lock strongly into their detent and all the knobs are well tightened down.

[blue4130]

It really just sounds like field curvature. And it souldn't be unexpected with a 300 f2. I have a 216mm f3 that covers 4x5 but not sharp to the edges, if I want a sharp image, I am limited to 6x9 or so. Your lens is likely similar. Try it on a smaller format.

Do you have a photo of this lens? I am curious to see it.

[Nodda Duma]

It’s field curvature, a residual aberration due to shooting onto a flat piece of film and not a curved surface.

Stop the lens down and the corners will sharpen up.

[Dan Fromm]

Folks, so far the OP has made four posts in this discussion.

First post, #1 (no surprise, that), introduced the idea of a 300/2 process lens and completely misled us. It also listed the OP's lenses and said that it was having coverage problems (edges/corners less sharp than the center of the frame) on 8x10. Three of the lenses don't come close to covering 8x10. The post flunks for incoherence.

Second post, #2, tried to clarify what the OP was asking about. It introduced that fuzzy concept circle of confusion. It flunks for irrelevance.

Third post, #7, tried to clarify more, revealed that the OP hasn't used the 300/2 monster lens. It is a classic red herring. Posts after #7 that address that lens flunk for poor reading comprehension. #7 flunks because it states the OP shot a 90/4.5 and 180/5.6 (Fujinon or Nikkor, which makes no difference) on 8x10. They don't cover the format, the 90 probably doesn't even illuminate all of it. The clarification mentions that the OP shot his problem lens(es) at f/22 and smaller apertures. Subsequent posts that suggest stopping down to improve image quality off-axis flunk for poor reading comprehension.

Fourth post, #11, tried to clarify still more. Whichever lenses the OP is talking about (not clear because not stated) don't cover at the apertures the OP is using.

As always, the people who replied to the OP and each other clearly tried to be helpful. That's what we do here, mainly. Try to help.

If the OP's longer lenses (600, 800, 1200) are Nikkor telephotos, well, all three barely cover 8x10 so the OP's bad news isn't much of a surprise.

OP, please spell out exactly which lenses you have. If I understand correctly what you have, your expectations for most of them are unrealistic.

[Tin Can]

Good analysis Dan!

I mean that.

[Vaughn]

The OP certainly shifts the goal posts around with every post...

[AnselAvedon]

field curvature and stopping down thank you Blue4130 and Nodda Duma thats all I needed to know.



final note

most of you people should try things instead of making pessimistic claims that they are impossible

[ic-racer]

Try changing the lens spacing.

[Havoc]

I cannot help it but I'm still thinking about this. Is this really a "flat field" thing or is it just that the lens is being judged outside its designed image circle? I mean lens design always optimises for a given image circle. Once you go outside it the lens is worse than inside it. It may give light and a discernible image outside that circle but it was never intended to be used that way.

[Nodda Duma]

Yes, Havoc. It’s not an “either / or” situation and it’s not a matter of “reading comprehension” as Dan erroneously concluded above (I got a chuckle out of that). The concepts are fundamentally related and closely coupled.


Outside of a design’s fully corrected image circle, the image quality blows up primarily because the tangential & sagittal fields are blowing up. These also push up astigmatism. The other aberrations comtinue to increase as well, but the field curvature components create the most displeasing aspects of the mess. Stopping down — even beyond f/22 — helps to pull tangential and sagittal back onto the Petzval field (the image surface — not flat — of best focus for a given lens), enlarging the usefully corrected image circle as much as the Petzval field and increasing vignetting allows.

Sorry if that’s not very clear.. It’s about as good as I can explain it in layman’s terms. To go further would require a deep dive in aberration theory and ray fan plots. You can google information on optical aberrations, and the presentations you’ll find online by Dr. Jose Sasian will probably be the most mathematically succinct.