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



AnselAvedon
5-Feb-2019, 21:00
I have a 300 mm f2 process / flat field lens its prob 10 lbs. cool can't carry that in the field. . . yet well maybe. the whole Packard shutter ( Sorry side question how do you guess shutter speed for less than one second with these) no stopping down with this fixed barrel ehhhhhh Yes I have tried to get a shutter created for it... it lead me to having to learn to create them because even if someone would do it well I prob don't have enough to have it done even with maxing my credit cards. so learn. I got some good people teaching me so that will all eventually happen...


however lets get to the original question.

Is the reason that my images are slightly out of focus on the edges have to do with the lens not being flat field?
They are super sharp in the center.


I used to think oh maybe its camera movement during my long exposures - no way tripod is tight no movement and a heavy majestic beast + sandbags thats not the problem.

I used to think its maybe its the expired film


but this last image I got has both those issues cancelled out

1 I shot at an 8th of a second with the heaviest tripod on earth
2 The center of the film is crystal clear

clearly it has to do with the lenses ...not being clear...

When Im shooting landscapes at obviously infinity I see this issue.
Portraits not so much.
I can figure that I am not using the part of the lens when I shoot close and thats why its sharp all around.

I hear about lens coverage. and ... its not vignetting but ... the sharpness isn't there either. Is there sharpness coverage?



some people say oh you just need better lenses and your get sharper images but from what I read nearly all lenses are pretty damn sharp. especially large format lenses
I mostly shoot fujinon / nikkor lenses

90mm f4.5
180mm f5.6
240mm f9
250mm f6.7
600mm f9?
800mm f12?
1200mm f18?

because I can buy those. They have been exceptional but now Im finding the side sharpness issue


sooooo? Are Schneider or Rodenstock really much better? Are they going to be sharp on the corners?
Nothing is Leica glass but theres also no way to put that on an 8x10 so yo whats up


I have some ideas about what the problems are can someone confirm some of this in a more organized way than my brain can ramble out its questions. or not organized I just have questions any thoughts

AnselAvedon
5-Feb-2019, 21:10
UHHHHHH my research says this sis going to have to do with with circle of confusion awesome answer for a exactly what its done to me

Can increasing f stop widen the circle of confusion?

lucaas
5-Feb-2019, 21:36
Are you shooting with the lens wide open? Normally the process lens is optimized for 1:1 when stopping down to f16 or smaller apertures. Try stopping the lens to f22 or smaller you will get a better result.
This has nothing to do with the CoC's.

Vaughn
5-Feb-2019, 21:44
What size negative are you using? Fixed, very fast aperture, 300mm -- hmmm, may not be designed to cover a whole 8x10 sheet sharply. Sounds more like a magic lantern lens (for projection), but that is just a hunch.

Dan Fromm
6-Feb-2019, 05:51
Ain't no f/2 process lenses. Is "300 mm f2" correct? I'm with Vaughn.

How do you get timed exposures with the monster?

Andrew O'Neill
6-Feb-2019, 10:12
If you're shooting wide open and are unable to stop down, wouldn't that be the problem? Take a black piece of opaque paper and punch a hole in it (hole puncher). Tape it to the rear element with the hole centred. Under the dark clothe, take a look at the GG with a loupe. Should be pretty sharp, edge to edge. If not...

AnselAvedon
6-Feb-2019, 10:55
alright everyone yes it is a monster f2 300mm lens
please disregard the fact that I own this lens because
I haven't shot with it ( I plan to use it to print ) and further more
it is not giving me the problem in question of what I am guessing is referred to as circle of confusion.

I am shooting 8x10
I am shooting with all of the other lenses mentioned above
I this shows up in my images when I shoot landscapes / infinity. They are shot at f22 or further closed down but I always see that the sides of the image do not have the same sharpness as the center.

Is this an issue another lens such as Schneider or Rodenstock would eliminate? Is this just an issue that has to do with landscape images creating a need for a flat field lens?

Vaughn
6-Feb-2019, 11:00
Might be an alignment issue with your camera, then. If the lenses cover 8x10 and you are at F22 or smaller aperture, there will be slight differences in sharpness across the field, but should not be significant. You might have some swing coming into play.

Jac@stafford.net
6-Feb-2019, 11:19
If it is a projection lens then the original device might have had a condenser (or pair) behind the subject (slide) plane. If it did and it is missing it could wreak havoc with its outcome, especially with 8x10 film.

Dan Fromm
6-Feb-2019, 11:55
AA, please be more specific. Your 90 and 180 won't cover 8x10, your 240 might not either. Which lenses are you having this problem with?

AnselAvedon
7-Feb-2019, 02:07
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
7-Feb-2019, 02:32
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
7-Feb-2019, 03:31
Itís 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
7-Feb-2019, 07:19
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
7-Feb-2019, 08:31
Good analysis Dan!

I mean that.

Vaughn
7-Feb-2019, 12:24
The OP certainly shifts the goal posts around with every post...

AnselAvedon
7-Feb-2019, 23:47
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
8-Feb-2019, 13:02
Try changing the lens spacing.

Havoc
9-Feb-2019, 02:25
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
9-Feb-2019, 04:31
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.

Dan Fromm
9-Feb-2019, 08:28
Thinking of reading comprehension, I missed the import of something the OP posted. He seems to be using the long Nikkor teles (600/9, 800/12, and 1200/18) at f/22. Stopping the 1200 down two stops wants f/36, not f/22.

Bob Salomon
9-Feb-2019, 10:48
Thinking of reading comprehension, I missed the import of something the OP posted. He seems to be using the long Nikkor teles (600/9, 800/12, and 1200/18) at f/22. Stopping the 1200 down two stops wants f/36, not f/22.

36, not 32?

Dan Fromm
9-Feb-2019, 11:59
36, not 32?

18*2 = 36. The 1200/18 Nikkor tele's maximum aperture is f/18, not f/16.