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Optics Theory - This forum is aimed towards the TECHNICAL side of photographic OPTICS THEORY. There will be some overlap by camera/manufacturer, but this forum is for the heavy duty tech discussions. This is NOT the place to discuss a specific lens or lens line, do that in the appropriate forum. This is the forum to discuss optics or lenses in general, to learn about the tech behind the lenses and images. IF you have a question about a specific lens, post it in the forum about that type of camera, NOT HERE.

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Old 07-15-2017   #41
Erik van Straten
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Quote:
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This lens has a small amount of barrel distortion which can be corrected by setting the distortion slider in Lightroom to about 4.
Yes, correction in Lightroom or Photoshop is possible, but not if someone (like me) uses the whole negative with a "natural" black border around it. For those people the distortion of a lens can be an issue.

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Old 07-15-2017   #42
willie_901
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You shouldn't confuse camera tilt (spirit level) with optical distortion. Those are two completely different things.

Erik.
Absolutely.

Lines that are parallel in reality converge in a photograph when the film/sensor is not square (i.e. parallel on both axes) to the subject. This is not an optical error in rectilinear projection. It is an error in camera alignment.

Human perception seems to notice converging vertical lines more than horizontal lines.

Often it is impractical to locate the camera where it can be square to the subject. One remedy is using a tilt-shift lens or bellows. The other is to tilt the image during post-production. In both cases the life is much easier when the camera is level on it's horizontal axis. The post-production results always have to be cropped to maintain a rectangular or square frame. So you have to throw pixels (or paper surface area) away.

I often photographed tall homes with a shorter focal length than required so the corrected, cropped frame would contain all of the subject. Once I photographed a sales brochure for a large aircraft hanger. By chance the building owner had a tall bucket crane on site. This was the only time I could locate my camera high enough to photograph a tall building where the tripod had no tilt whatsoever.
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Old 07-15-2017   #43
David Hughes
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Hmmm, but still no mention of Scheimpflug's Rule, which I've been waiting for.

Anyway, I agree about cropping etc but my old M9 churns out a lot of pixels and I reckon I only need about 8 megapixels to do a 12" x 8" at exhibition quality of 300 dpi. So room to crop there.

And, btw, printing at 300 dpi is something no one seems to notice; just like distortion and so on.

I read a lot on forums that people seem to believe, this is not aimed at any one on RFF btw, we seem pretty sensible here. Anyway, I have printed on photocopy paper 5 megapixel shots as a 4 sheet poster, carefully glued it together and mounted it under glass. No one seems to notice anything odd about it and so I wonder... Often they don't believe me until I point out the joins.

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Old 07-15-2017   #44
sebastel
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Hmmm, but still no mention of Scheimpflug's Rule, which I've been waiting for.
why wait while you can throw it in yourself?
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Old 07-16-2017   #45
David Hughes
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Yes, many people believe that reality and a photograph are identical. But they are wrong.

We don't know what we see, that is the problem.

Erik.
Hi,

I think we see what we expect and/or were taught to see.

When I did what was called art at school we were taught about horizontal vanishing point but not vertical ones and moving on to carefully draw "artist's impressions" that were a branch of technical drawing years ago, we are told to do verticals upright with a set square and horizontals as projections from the measured drawing.

Moving on to photography we then get taught that verticals converging are bad wicked evil un-natural things. But as this old fool sees it they are true and we are wrong but we go along with fashion, what the customer wants, etc, etc.

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Old 07-16-2017   #46
Ko.Fe.
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Quote:
Originally Posted by Erik van Straten View Post
Yes, correction in Lightroom or Photoshop is possible, but not if someone (like me) uses the whole negative with a "natural" black border around it. For those people the distortion of a lens can be an issue.

Erik.
I"m not correcting my darkroom prints, either, this is how I get pictures from 50 lenses most.
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Old 07-16-2017   #47
willie_901
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I"m not correcting my darkroom prints, either, this is how I get pictures from 50 lenses most.
A completely analog workflow changes everything!
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Old 07-16-2017   #48
Doug
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...My Beseler enlarger had a tilting negative stage. I used the feature occasionally to "correct" convergence and propped up one side of the easel to match. I don't know any way to deal with barrel/pincushion distortion though, in the analog darkroom.
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Old 07-17-2017   #49
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...My Beseler enlarger had a tilting negative stage. I used the feature occasionally to "correct" convergence and propped up one side of the easel to match. I don't know any way to deal with barrel/pincushion distortion though, in the analog darkroom.
Thanks for this.

Now I 'm wondering why easel tilting alone wouldn't be enough and if such a device ever existed.

Unless the tilting angle was very high, would light fall off be the reason the negative is tilted as well?
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Old 07-17-2017   #50
Erik van Straten
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You can tilt an easel as much as you like, but you won't correct barrel/pincusion distortion. Barrel/pincushion distortion can not be corrected by analoge means. That is why in the analoge era the wideangles were always free of these distortions. See the Super Angulons, Hypergons etc.

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Old 07-17-2017   #51
Doug
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Thanks for this.

Now I 'm wondering why easel tilting alone wouldn't be enough and if such a device ever existed.

Unless the tilting angle was very high, would light fall off be the reason the negative is tilted as well?
Coordinating the tilt of the easel with the opposite tilt of the negative stage allows focus to be correct across the field, while dealing with the convergence issue...
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