"Help..! My super wideangle lens vignettes..!"
Inbuild limits of super wide angle lenses
(c) Frank Mechelhoff                                                           NEW 4. Okt. 2005 Update Feb. 2013                                                          diese Seite auf    Deutsch

In earlier days of photography, super wide angle lenses were very expensive, heavy to carry, seldom, not available, or all of them to the typical amateur 35mm film photographer. Luckily, this has changed within the last five years. But not all photographers who can pay the lowered prices for a super wide angle lens (say a 21 or 15mm focal length) know about the physics of wideangle photography like the professionals of previous times. So there are lots of complaints about "vignetting" of specific lenses and stuff like that, which - in fact - isn't the fault of the lens or the designer, but of the photograph who don't know what limitations of physics

First, we have to sperate two physical facts from another. Sorry for the technical expressions here, but first "vignetting" is a description of a phiscial fact. Then, optics is a physical science. So we have to look at some facts, when we try to find out why a lens vignettes.

Vignetting means the light stream coming through the lens to the film is somewhat compromised physically either by shading, either from a sun-shade too tight for the lens (shaded corners), fingers from the photograph or other thingsb near to the lens, the barrell of the lens itself or the shutter (i.e. focal plane shutter malfunction or too tight a leaf shutter opening and stuff like that). I know of no lenses sold in present with these mistakes. When you look at performance diagrams of wideangle lenses of companies well known in the whole world for its quality lenses, like LEICA or ZEISS or even other companies, you see often diagrams like this:

This diagram says that of of 100% light which comes to the film in the center, at the utmost corners of the negative (21mm away from center) less than 25% arrives at the fastest aperture of the lens. That means the corners are probably underdeveloped by 2 f-/stops. Bad thing if you have slide film used. Closing down the lens 3 stops reduces the light loss to one stop (upper curve) This diagram is from a newly, excellent (Retrofocus type) designed 21mm lens. This form of, in everyday speech "vignetting" is unevitable, and occurs even in the best designed wide angle lenses. If it would be vignetting it could be avoid in one or another way, probably by the lens designer. But the only thing one can do against it belongs to the photographer: Either avoid very wideangle lenses, stop down at least two stops, change background, or use tolerant film (i.e. B&W negative)

Light falloff is a complete other fact, as far as physics is concerned, but sometimes resulting in the same or similar, for the photographer, unpleasent results, like darkened corners in pictures. In the case discussed above, we are talking about light-falloff due to physics - inevitable to some extend. We're not talking about (avoidable) vignetting.

Cos4 law
States that light fall-off in peripheral areas of the image increases as the angle of view increases, even if the lens is completely free of vignetting. The peripheral image is formed by groups of light rays entering the lens at a certain angle with respect to the optical axis, and the amount of light fall-off is proportional to the cosine of that angle raised to the fourth power. It gets worse with each additional grade angle of view. I.e. light-falloff of a 15mm is generally stronger than of a 21mm. As this is a law of physics, it cannot be avoided. However, with wide-angle lenses having a large angle of view, decreases in peripheral illumination can be prevented by increasing the lens’ aperture efficiency (ratio of the area of the on-axis entrance pupil to the area of the off-axis entrance pupil).

What can a lens designer do to avoid light-falloff?
First, he can design the lens in a way to enlarge the distance between last element of the lens and film, thus making the light rays falling onto the film in a less oblique angle (even if the angle of entrance pupil is larger). This  is usually done with an SLR wideangle lens to gain room for the moving mirror and such lens is called Retrofocus-design (invented by Angenieux and Voigtlander about 1950). All 35mm film wideangle lens with focal lengths 35mm and shorter are Retrofocus designs, except some very few which only can be used with mirror-up (and seperate viewfinder). This usually results in more glass elements, a bigger lens and more weight.
Second, he can enlarge the front element(s) to catch more of the oblique light rays in the corners, to some extend - more than the actual highest f-/ stop requires. This is often combined with method one. Drawbacks: with the enlarged font-element(s) the lens is more prone to flare (backlight-sensible), becomes bigger, heavier and more expensive.
Summary for users of compact rangefinder type wideangle lenses: It means, as a user of a small, lightweight, and/or low-priced lens, I have to deal with that natural light falloff, or have to pay (and carry) another lens - period!

What is about illumination?
When you have avery wide angle of view very often the illumination isn't the same over the whole field. When i.e you have a situation outdoors with bright sun and blue sky and a few clouds the scenery isn't constant illuminated. As the human eye only looks at a relative small angle very sharp, our brain makes the work to put together "the whole picture" as we know from former observations "how it should look like", i.e. to balance strong contrasts. To make it worse: from the perspective of a human standing on the earth, the sky is a globe. As we already know from optics, the reflecting light rays of "blue sky color" are of different angles, lengths and therefore, intensity. Sorry to say, but it is our brain - or the visual center within it - what makes the sky looking "nice looking equally deep-blue". The film is a more impartial, less emotional observer than we are, resulting in unevenly illuminated sky when using lenses with angles of 90° or more (i.e. 12, 15 or 21mm focal lenth). This illumination light-falloff is added to the light falloff mentioned above, so use your wideangle lens with care at bright sunshine outdoors.

What is about "natural perspective"?
That means, the print of the neg. should be viewed from a distance like the original scene, which can be applied at landscape or archtitecture pictures or other big objects, to avoid unnatural deformation (not distortion) So please don't  tilt your camera when shooting big objects like towers to catch the steeple - or at least, don't blame the lens for "distortion". If a photographer is present he will blame you. Better say to your friends you did it consciously to show that special effect or "falling lines"...
This natural-perspective issue is important for another reason. See, a 15mm lens for 35mm film takes an angle of view of 11 standard lenses, 11x it's size! If you are used to 3x4 inch prints everything on it is very small, and pressed. Is this the angleof view you had when you shot the scenery? Of course not - the view was so big you couldn't see it at one solely moment with both eyes open, but had to turn your neck to get the whole picture completely. So it is "natural" top press it in a 3x4 inch print like a picture of a standard lens? Of course not. To keep natural perspective, the print should be 10x15 inch viewd from the same distance. My first ultrawide lens was a 15mm Heliar on a Bessa-L in 1999, my solely lens for that camera then, so the whole film looks the same. First thing I learned with that lens: Take bigger prints! It works.


What exactly is"distortion" in difference to perspective deformation?

Distortion is a lens error. The photographer can do nothing against it. This often occurs in bad designed, or early wideangles, or zoom lenses. This results that a horizontal line in front of the camera (i.e. a  brick wall 10 meters away from the camera and precise straight, not oblique photographed) looks not straight, but deformed concave like a cushion (cushion-like distortion) or convex like a barrel (barrel-like disturtion). The most extreme (volitonal) form of distortion is the barrel distortion of a fish-eye lens.

Left: Image of a brick wall laid by an absolute beginner. Original image (no distortion) above and heavily barrel-distorted image (beneath) Middle: perspective deformed but distortion-free picture (falling lines). If you enlarge this picture large enough and look at it from close distance, the falling lines dissapear! This is what "natural perspective" means (don't do it with your monitor, please)

Does a digital camera vignettes in the same way?
It can be thge same or worse. Film is nealy equal sensible for light anyway if the light rays coming in straight/plan, or oblique. This is not the same with digital chips. Most chips are less sensible for oblique light rays than for plan, resulting in stronger light-falloff to the corners than with film. It can be so bad that a lens isn't to use.

Link: How to use super wide angle lenses

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frank "at" taunusreiter-de
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