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06-27-2013, 01:09 PM   #1
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Real odd lens correction/Bokeh

Hi, I have recently started using my 6X7 lenses with an adapter on DSLR K7.
The lens correction looks really odd on the Super-Takumar 4/200, when used on the digital?
What´s going on? Is it the difference in format or digital vs rollfilm or what?
Here are two samples and 100% close-ups
First On K7
Second on 67 II

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06-27-2013, 01:57 PM   #2
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These are 2 very different shots and I think it would be difficult to make any meaningful comparisons between the two. The first one has the sun just out of frame which is obliterating the contrast, shining onto the far side of the actual lens elements which causes internal reflections and has the bright sky as the background (causing CA from the difference in contrast). The second shot has a far less contrasty scene. There are dark bushes for backgrounds (instead of sky) and no lens flare because the sun is well out of frame
06-27-2013, 02:40 PM   #3
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QuoteOriginally posted by enoeske Quote
These are 2 very different shots and I think it would be difficult to make any meaningful comparisons between the two. The first one has the sun just out of frame which is obliterating the contrast, shining onto the far side of the actual lens elements which causes internal reflections and has the bright sky as the background (causing CA from the difference in contrast). The second shot has a far less contrasty scene. There are dark bushes for backgrounds (instead of sky) and no lens flare because the sun is well out of frame
Thank´s

But the point is not to compare.
My question is: Why does the lens correction look like it does? Ive never seen such ‘onion rings‘ as seen most clearly in crop of the K7 (first) image. There are no such rings in the 67 II (second) image. You are right about the amount of light, the bright sun near the edge in the first one and it might make the odd correction more visible?

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06-27-2013, 03:09 PM   #4
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First, I think you need to define "correction".

From what I can tell you are seeing the effects of using the lens on a large format camera (therefore having a 'more defocused' background for the focal length) and on APSC (with a 'less defocused' background w/ regard to focal length).

Also you get a lot of CA because you're using a LF film lens on APSC digital.

06-27-2013, 04:29 PM   #5
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more importantly the focus is different.
But the defocus in the left lower corner of the 6x7 comes close though.
06-28-2013, 01:26 AM   #6
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QuoteOriginally posted by Giklab Quote
First, I think you need to define "correction".

From what I can tell you are seeing the effects of using the lens on a large format camera (therefore having a 'more defocused' background for the focal length) and on APSC (with a 'less defocused' background w/ regard to focal length).

Also you get a lot of CA because you're using a LF film lens on APSC digital.
I think, you are right: It is the difference in focal length, the shorter depth of field on the medium format, that is at play here. The lens was corrected for the 6x7 format and not APS-C. On the 67 II image we see discs with slightly bright edges and otherwise quite even distribution of the light.

Ideally, a completely corrected lens would show even discs (undistorted anywhere beyond the focus plane in the image) without brighter centers or edges at any distance and wide open or closed down alike.

But in this case there are multible rings seen on the DSLR image. So has the lens really been corrected (designed/shaped/polished) this way? No.

Why (in optics) do we get the waves of light (the ‘onion rings‘ must be curved waves if they could be seen from the side)? The lens elements are not polished like that (right?) and the answer would have to be, something about, how the light passes through the lenses.

hmm
06-28-2013, 02:31 AM   #7
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Bokeh is different throughout the range.
it could be that the "rings" peaked at that certain setting/range and that you missed it with the 6x7 image. When you look at the 6x7 image you will see 3 kind of situations.

The flowers in the background left looks good, the bushes on the left that are slightl further away you see a harsher bokeh, and the tree in the back ground are smooth again.

You certainly see the rings with the 6x7 image as well but there seems to be a differnce of resolution between the two.
06-28-2013, 03:22 AM   #8
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JT: Same reason why when you push on a rubber eraser, the sides bulge out. Usually, solution sets to the wave equation are coupled across dimensions. By that I mean changes along one axis, say the one parrallel to the focal length, affect the axes perpendicular to it, i.e. the ones in the image plane.

06-28-2013, 08:41 AM   #9
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Thank´s for your thoughts and sharing of knowledge guys.

Im sure, that no lens producer, would market a lens with the visible caracteristics seen in the K7 image. And that it only occurs like that, because they could not possibly foresee the DSLR APS-Cs at the time and how their lenses works on them. Thus Im sure too, that whatever setting and distance I use on my 67 system, it will never produce such odd (and ugly) bokeh. And I will test the 4/200 on K7 to learn where this happens, so that I will be able to avoid it. It would be a hell of a job in PP to fix it, would it not.
06-28-2013, 11:01 AM   #10
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You shouldn't look at the bokeh of the DA*60-250 then...

Anyway the lens preforms the same on both formats though...
07-08-2013, 12:34 PM   #11
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Newton rings? Newton's rings - Wikipedia, the free encyclopedia
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