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01-22-2014, 02:52 PM   #1
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Compression distortion

I've been unable to get a real answer to this question, so I'm hoping someone can set me straight. We all know that an 18mm, 28mm, and 35mm lens on the same camera will result in different angles of view, DOF, and perspective distortion.

But what if you had the 18mm on an APS-C, the 28mm on a FF, and the 35MM on MF? Will the use of different focal lengths on different cameras, which produce equal angles of view, produce equal amount of perspective distortion? Since the distance to the subject has not changed, will the perspective distortion be the same too? That is to say will the 35mm on the MF camera have less elongation of the perspective compared to the 18mm on the APS-C camera?

I've always thought putting a 150mm on an 4X5 would result in equal angle of view as a 50mm on full frame/35mm camera. However, since the distance to the subject is equal, then the perspective compression would be greater with the 4x5 than the 35. Thus resulting in the backgrounds appearing closer and larger in the 4x5 image vs the ff shot.

If this is true, then a 50mm on an APS-C vs an 80mm on a FF would result in equal angles of view, but the distortion would be different?

Thanks for your input.

Frank

01-22-2014, 03:03 PM   #2
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QuoteOriginally posted by FrankC Quote
If this is true, then a 50mm on an APS-C vs an 80mm on a FF would result in equal angles of view, but the distortion would be different?
If the field of view is the same, the only thing you have to worry about is the lens itself. In other words, it only becomes an issue at really wide focal lengths, since for instance a 18mm is harder to correct than a 28mm. So you'd most likely get less barrel distortion with the 28mm on FF than a 18mm on APS-C thanks to the simpler lens design, etc. But the perspective wouldn't change as long as the distance to the subject & the angle is the same.

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01-22-2014, 03:04 PM   #3
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QuoteOriginally posted by FrankC Quote
I've been unable to get a real answer to this question, so I'm hoping someone can set me straight. We all know that an 18mm, 28mm, and 35mm lens on the same camera will result in different angles of view, DOF, and perspective distortion.

But what if you had the 18mm on an APS-C, the 28mm on a FF, and the 35MM on MF? Will the use of different focal lengths on different cameras, which produce equal angles of view, produce equal amount of perspective distortion? Since the distance to the subject has not changed, will the perspective distortion be the same too? That is to say will the 35mm on the MF camera have less elongation of the perspective compared to the 18mm on the APS-C camera?

I've always thought putting a 150mm on an 4X5 would result in equal angle of view as a 50mm on full frame/35mm camera. However, since the distance to the subject is equal, then the perspective compression would be greater with the 4x5 than the 35. Thus resulting in the backgrounds appearing closer and larger in the 4x5 image vs the ff shot.

If this is true, then a 50mm on an APS-C vs an 80mm on a FF would result in equal angles of view, but the distortion would be different?

Thanks for your input.

Frank
The magic issue here is you need to shoot all shots from the same shooting location and print with the same sensor to paper magnification

By this I mean the subject, on paper is the same size

Assuming aperture is the same, perspective and DOF should be the same
01-22-2014, 03:59 PM   #4
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QuoteOriginally posted by FrankC Quote
then the perspective compression
There is truly no such thing.

The short explanation is that perspective is determined by the position of the camera relative to the subject. Focal length only determines FOV. You can demonstrate this by taking a photo of a subject from a given position using both short and long focal length lenses and cropping the wide-angle shot to the same field of view as the long lens shot. Notice that the perspective is the same and that the resulting images should be almost identical. This concept is pretty fundamental and understanding it is part of learning how to make a view camera work. It is also important when evaluating concepts like "zooming with one's feet" as it pertains to primes vs. zooms.*

DOF is determined by absolute aperture, final viewing distance, and final image magnification. Notice that focal length is not a factor, at least not directly. This can also be demonstrated using example images, but not quite as easily as the perspective example above.


Steve

* You can't. Moving closer to frame the subject changes the perspective and hence the composition while a zoom lens allows you to retain the perspective.

01-22-2014, 09:11 PM   #5
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Steve's got it correct. If you shoot from exactly the same distance, the perspective is identical. A cropped image will show this. Take your DA 12-24 at 12 and your DA 560. If you crop the 12 mm image to 1/47 in the middle, the image will be the same as to perspective and everything else except for some rather more apparent grain.
01-22-2014, 11:26 PM   #6
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In re reading the original post and the title, I wonder if what the OP is getting at, along with a bunch of other things mixed in, is the concept that longer lenses "compress" the image, this was a favourite topic in the photo books published in the late 1970's and early 80's.

The truth of this concept is really magnification

Image size = subject size x focal length / distance

The compression resulted for the different magnification ratios applied due to the change from foreground to background distance, relative to the fixed shooting position. In reality this is what gives perspective distortion

Then you have wide angle distortion, especially at the edge of the image, which is also driven by magnification on a rectalinear lens, because the lateral and vertical "stretching" is due to objects away from the center of the field of view being further away, hence smaller (this is what gives barrel distortion) and correcting for this such that you get straight lines results in increasing the magnification at the edge of the frame
01-25-2014, 12:41 PM   #7
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Thanks for all your replies. Lowel has an interesting point which makes me want to do an experiment for myself.
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