View Full Version : lens distortion
Im having problems with barrel/lens distortion with my super angulon 90mmxl. If i am shooting a building straight on (with moderate shift)i notice the top horizontals are curving from about half way up. It looks like curvature of the lens. You would probaly really only notice this on shooting straight on, without sky. would a lens of lesser image circle reduce this? Is this just a trait of the 90mm? Could it be something with the camera?
It is most likely that your film plane is NOT perpendicular to the ground. In other words, you are pointed up or down ( most likely UP). Use a level ( available at Sears for about $4 in the form of a round bubble level) placed on the back standard. Level the bubble, check the image, relevel, and shoot. That should clear up the problem you are having..... Remember that you must be perpendicular to the ground, and the levels on your camera may not be correctly calibrated.
I am far from being an expert on these matters so take this with a grain of salt. Are you saying only part of the picture shows distortion? If so, it sounds like you may have an issue with film flatness. Did this happen on just one exposure?
Make this test. Find something which presents an array of squares. Shoot it head on so that it fills the film frame. Make sure the lens, film, and subject are exactly parallel. The resulting negative should show all the squares as square throughout the image. If it shows curvature it indicates the lens has geometrical distortion. This is unlikely but perhaps possible. Geometrical distortion is common in telephoto and retrofocus lenses because they have no symmetry to cancel it. Symmetrical lenses are automatically corrected for geometrical distortion; this lens is relatively symmetrical so should have good geometry. Most lenses (fisheye lenses are an exception) are made to be orthographic, that is, they will reproduce a scene in the way the eye would see it from the same position. If you photograph an array of squares they will be recorded as square and of the same size throughout the image. If you photograph an array of spherical objects the ones away from the center will be reproduced as egg or tear-drop shaped. If the eye is the correct distance from the print they will look round and the squares will seem to diminish in size. This is as the eye would see them from the viewpoint of the camera, i.e., with diminishing perspective. When a print is viewed from the wrong distance, as is the usual case for pictures taken with wide angle lenses, the effect of the orthographic lens will become apparent as "distortion" although it really isn't. Fisheye lenses are not orthographic and display the image as it would look on the surface of a sphere (severe pincussion distortion). This can happen in a smaller degree for reverse telephoto lenses, especially wide angle ones. Check your lens on a grid pattern as suggested above to find out whether it actually has distotion or if you are seeing a common optical illusion.
The super angulon 90mm xl is a very good lens indeed when looking at the specifications. http://www.schneideroptics.com/photography/large_format_lenses/super-angulon/ Only 0.5 % at most distortion. This means that perhaps something else is happening. First of all let me state the following: if a lens is free of distortion, any straight line, no matter wat orientation it has relative to the filmplane, will be projected as a straight line on the film. So it does not matter that you point the camera up to a building, if the building is straight, it should be straight on the photograph. However, there is one more often forgotten element that influences this: When the film is not flat, the line will not be straight, espcially at the edges of the film and especially in wide angle lenses, since the rays at the edge of the film make an angle with the film plane.
I did some experiments with normal film holders (Elite) and with the Sinar film holder using a Schneider Super Angulon 47XL and found this curvature effect in the normal film holder.
When you measure the bulging of the film in an Elite film holder, you find up to 0.5 mm of bulging away from the aluminium plate.
So to see if this explains it, do the following: take a straight line on your photograph and measure its length. Now make your ruler coinside with the outer tips of the straight line and measure with a second ruler the distance the line has to you first ruler in the middle of the line. Divide this distance to the length of the line. Measure the distance of the middle of the line to the centre of the print. Finally establish the magnification of the print and the original film format. With these data I can backcalculate whether film bulging is the origin of you problem.
My S SA 47XL seems to give slight pincushion distortion, but it was a a very old building, and it was not straight!
David A. Goldfarb
Doesn't barrel/pincushion distortion depend to some degree on subject distance? I think for that reason, Bert's test might not be entirely definitive. One would hope that a fancy lens like a new 90mm SA XL would be well corrected for distortion over a wide range, but a test involving rulers would be more a test of the lens's suitability for tabletop work than for architecture.
It is possible that the distortion is caused by faulty lens mounting. If the rear section of the lens is not seated correctly, then distortion is possible in this otherwise excellent lens. If the lens is usually stopped down to f/22 or smaller, the intrusion of other aberrations might go unnoticed.
David misinterpreted the test I described with rulers to measure distortion. This shows how hard it is to explain something in words, so let me clarify: the rulers are used to measure in the prints. The prints can be photographs from buildings with straight lines. I did not mean to say that rulers are photographed. So the test also applies to architecture. David is right in pointing out that distortion depends on subject distance. This can be seen in the Schneider data, but it is not much, everything remains inside that 0.5%.
The lesson I've learned from my experiments with wide angle lenses and film holders is: if you do not want to correct in PhotoShop (since you are working in a wet darkroom or if you do not like changing the photograph), use a very good filmholder when using wide angle lenses and straight objects. Alternatively you can stick the film to the baseplate of the filmholder as long as you do not create new bulges. The Sinar holder with its spring plate is really the best solution I found, but are horribly heavy and expensive.
David A. Goldfarb
That's clearer. Indeed, at first I thought you were photographing rulers to compare them to their image projected on film.
Powered by vBulletin® Version 4.2.2 Copyright © 2013 vBulletin Solutions, Inc. All rights reserved.