Is Perspective distortion dependent on Format or only on focal length ?

Perspective distortion has nothing whatsoever to do with optics. It's a simple matter of geometry.

If the print as reproduced and viewed subtends a smaller angle than the scene originally did, you will get wind-angle perspective distortion. If it subtends a larger angle, you will get telephoto compression. It makes no difference what lens or format was used.

Perspective depends only on where your camera is in relation to the subject. Longer lenses do not cause compression--moving further away from the subject (which a longer lens allows you to do while keeping the same magnification) is what causes "compression."

Perspective only depends on the distance from the subject. It appears to be different, because with a wide angle lens you get close to the subject, and with a tele lens you shoot from farther away. But if you shoot from the same position with a wide lens and then with a telephoto lens, you will see that the perspective is actually identical (you have to crop the wide-angle shot to match the field of view of the telephoto lens).

To give a slightly more complete answer, the important factors are camera-to-subject distance, angle of view of the lens and film/sensor combination (factor in cropping if the print is not full frame), size of the print, and distance from the viewer to the print.

If the distance from camera to subject is the same as the distance from viewer to print, and the angle of view captured is the same as the angle of view viewed, then there is no perspective distortion. If the print is enlarged, it can be moved further from the viewer to mitigate the resulting perspective distortion.

Hope that helps more than it hurts...

If the distance from camera to subject is the same as the distance from viewer to print, and the angle of view captured is the same as the angle of view viewed, then there is no perspective distortion.

That doesn't even make any sense. It's perfectly possible to take a photo of something at infinity without perspective distortion without the print being at infinity.

As I explained previously, what matters is that the angle when viewed is the same as the angle when taken. Everything else is completely irrelevent and needlessly confusing, if not outright incorrect.

The FOV of a lens doesn't change with the size of the film/sensor. The image circle that a lens produces makes more or less suitable for certain film/sensor sizes and not others.

Thus, a 15mm 180 degree, full-frame fisheye will produce a full 180 image on a 35 mm film but, if the film/sensor were smaller it would be cropped accordingly. The same would happen if you cropped the print. While the crop may hide the distortion characteristic of a given lens at the edges it would NOT change the lens behaviour. Same is true for telephoto lenses.

As the lenght of a lens increases you go from "expansion" to "compression" with the "normal" lens being the one that gives a FOV appx equal to the human eye for the given film/sensor size.

Mark Ci: For a shot at infinity, the print only needs to be at infinity if the angle of views match (which is what you said, but in different words).

The print can be viewed at arms length if it's the right size. If you stand where the camera was with print in hand, and lift the print up with your arms extended and it perfectly matches the scene, then you're at the perfect viewing distance for that print size.

At any other viewing distance for that print size, there will be perspective distortion. Or at that viewing distance with any other print size, there will be perspective distortion.

...I should be able to get the equivalent of a single image using a 200mm lens on an 8x10 camera. I would therefore end up with the equivalent of an 8x10 image with a focal length considered close to normal for an 8x10.

Yes. Except that a normal lens for 8X10 is closer to 325mm.

However unlike an image taken on 35mm with a 50mm lens this would have a different feel altogether because of the apparent compression being caused by the longer lens.

No.

If you stitch together a series of pictures taken with a 200mm lens, they will result in an image which will be the equivalent of the resulting format, taken with a 200mm lens. So if you stitch together enough 35mm frames to result in an 8X10 image, you'll have what you would have got by taking a single picture with a 200mm lens... mostly.

The "apparent compression" to which you refer is only an effect of cropping to a narrow angle of view, and then viewing it from a distance that causes the image to comprise a much wider viewing angle than the original scene. The panoramic stitching you plan to do obviously negates the narrow angle of view, since you will then view the resulting image from further away, or you will print it smaller. (which effectively is the same thing).

You will introduce a certain amount of distortion because you will be turning the camera in order to pan. This means that your resulting composited film plane will actually comprise a curved surface (as you tilt and pan the camera to effect different angles).

Of course, since your film plane will be a semisphere, and your images rectangular, the edges won't fit together correctly... unless you use panoramic stitching software, which will then distort the images as required to get them to fit together.

Hi,

Thank you all for taking the time to answer this question.

Regards, Hemendra Chonkar