UV Filters
While writing the previous post I was tempted to make the analogy that sudo is similar to putting a UV blocking filter on the front of a camera lens, but the analogy breaks down so fast it really isn't accurate. Since a great many people use UV filters indiscriminately, it seemed appropriate to point out when they are useful, when they are not, and why that analogy is not accurate.
A UV filter is a piece of glass that has been treated to block UV light. The actual cutoff wavelength for a UV filter depends on the manufacturer, so I won't go into that here. Originally the purpose was to get rid of the bluish UV "haze" that shows up on some photo emulsions that are particularly sensitive out past the blue.
Most UV filters are not used for this purpose. They're stuck on the end of an expensive piece of glass (known from this point on as "the lens") in order to protect it from fingerprints, dust, mist, salt spray, etc. And protect they do! Take a good look at the last element in a 35mm camera lens some time and see what it looks like. Considering how pristine they look when they start life, they can look pretty bad after a few trips into the field.
But any time you stick a transparent piece of something into an optical system, you change the resulting image quality. This is a fact of nature. It's not something that can be avoided, it simply is. Unfortunately with filters, the change is rarely one for the better. There are two effects that are well-known and much-discussed, which I'll touch on next. But there is a third effect that few ever mention. I'll touch on that last.
Any optical element is made to the best tolerances money can buy. The catch is, not every element has the same amount of money dropped on it. If you use a high end professional lens costing upwards of $10,000, expect it to perform a little better than a consumer level mass-produced lens costing $95. At the same time, expect a 50mm lens (optically quite simple) to cost less than an 18-350mm image stabilized zoom lens of the same quality by the same manufacturer (trust me... optically quite un-simple).
But we're talking about filters! These are flat pieces of glass! Exactly. And how flat they are depends on how they were made, which is directly reflected in their price. A high-end camera filter may be flat lapped on both sides for good parallelism and optical quality. Such a filter will also likely be treated with anti-reflection coatings (or AR coatings for short). A low-end filter won't have the AR coatings and will likely be made from common float glass.
Comparing the cheaper filter to the more expensive one, the optical system would suffer from more internal reflections (no AR coating) and more optical aberrations (the glass won't be quite as flat or quite as parallel). As a result, optical quality degrades.
Simple! Buy expensive filters!
Aaaaah, but that's not the whole story. ANY time a flat optical element is introduced into an optical system, spherical aberration is introduced. ANY time. Again, this is a fact of nature and is not something you can just "design out". The act of screwing a filter onto the end of the lens introduces spherical aberration into the optical system.
Before talking about image degradation, I need to take a step back and say that spherical aberration does not necessarily equate to bad image quality. Almost every optical element will introduce spherical aberration. The trick is to balance the spherical aberration, some positive, some negative, so that the entire system, taken as a whole, will have minimal spherical aberration. So you can design a lens to have negative spherical so that it will perform best with a filter of a given thickness and index of refraction stuck on the front of it.
But if you think that's what Nikon, Pentax, and Canon are doing... You have another thing coming.
Even so, this trick has been used before. Schneider and Rodenstock used to make these gawdawful wide angle lenses for 8"x10" view cameras that suffered from horrid light falloff toward the outer corners. No flaw in their design work; when you go past a certain point you wind up with light falloff. You just do. So to fix it they designed the lens with negative spherical and stuck a filter on the rear element that was darker in the middle than on the sides. This evened out the exposure level across the frame, and because of the design choice the system taken as a whole had very little spherical aberration.
I don't personally use UV filters on my lenses. I used to, but I learned how to keep fingers and dirt off my glass. When I do use a filter, I have a very good reason to. Otherwise the filter stays in my bag.
The filters I tote around these days are a circular polarizer, for cutting reflections on vegetation and water, and split neutral density filters for darkening overbright skies so they fall in the same exposure range as the rest of the landscape. I do my black and white photography with a 4x5 camera. For that I have a set of Wratten gels (which, because of their thin cross-section, don't introduce nearly as much spherical as a glass filter.)
-- Pencil
P.S. The analogy! I almost forgot! sudo is a lot like using a UV filter to protect your glass. It'll stop a fingerprint, and help to teach you how not to smudge up your lenses. But if you manage to run "sudo rm -rf /" it'll still erase every file on your computer, just as if you were logged in as root.
P.P.S. sudo doesn't cause spherical aberration, as far as I know.
A UV filter is a piece of glass that has been treated to block UV light. The actual cutoff wavelength for a UV filter depends on the manufacturer, so I won't go into that here. Originally the purpose was to get rid of the bluish UV "haze" that shows up on some photo emulsions that are particularly sensitive out past the blue.
Most UV filters are not used for this purpose. They're stuck on the end of an expensive piece of glass (known from this point on as "the lens") in order to protect it from fingerprints, dust, mist, salt spray, etc. And protect they do! Take a good look at the last element in a 35mm camera lens some time and see what it looks like. Considering how pristine they look when they start life, they can look pretty bad after a few trips into the field.
But any time you stick a transparent piece of something into an optical system, you change the resulting image quality. This is a fact of nature. It's not something that can be avoided, it simply is. Unfortunately with filters, the change is rarely one for the better. There are two effects that are well-known and much-discussed, which I'll touch on next. But there is a third effect that few ever mention. I'll touch on that last.
Any optical element is made to the best tolerances money can buy. The catch is, not every element has the same amount of money dropped on it. If you use a high end professional lens costing upwards of $10,000, expect it to perform a little better than a consumer level mass-produced lens costing $95. At the same time, expect a 50mm lens (optically quite simple) to cost less than an 18-350mm image stabilized zoom lens of the same quality by the same manufacturer (trust me... optically quite un-simple).
But we're talking about filters! These are flat pieces of glass! Exactly. And how flat they are depends on how they were made, which is directly reflected in their price. A high-end camera filter may be flat lapped on both sides for good parallelism and optical quality. Such a filter will also likely be treated with anti-reflection coatings (or AR coatings for short). A low-end filter won't have the AR coatings and will likely be made from common float glass.
Comparing the cheaper filter to the more expensive one, the optical system would suffer from more internal reflections (no AR coating) and more optical aberrations (the glass won't be quite as flat or quite as parallel). As a result, optical quality degrades.
Simple! Buy expensive filters!
Aaaaah, but that's not the whole story. ANY time a flat optical element is introduced into an optical system, spherical aberration is introduced. ANY time. Again, this is a fact of nature and is not something you can just "design out". The act of screwing a filter onto the end of the lens introduces spherical aberration into the optical system.
Before talking about image degradation, I need to take a step back and say that spherical aberration does not necessarily equate to bad image quality. Almost every optical element will introduce spherical aberration. The trick is to balance the spherical aberration, some positive, some negative, so that the entire system, taken as a whole, will have minimal spherical aberration. So you can design a lens to have negative spherical so that it will perform best with a filter of a given thickness and index of refraction stuck on the front of it.
But if you think that's what Nikon, Pentax, and Canon are doing... You have another thing coming.
Even so, this trick has been used before. Schneider and Rodenstock used to make these gawdawful wide angle lenses for 8"x10" view cameras that suffered from horrid light falloff toward the outer corners. No flaw in their design work; when you go past a certain point you wind up with light falloff. You just do. So to fix it they designed the lens with negative spherical and stuck a filter on the rear element that was darker in the middle than on the sides. This evened out the exposure level across the frame, and because of the design choice the system taken as a whole had very little spherical aberration.
I don't personally use UV filters on my lenses. I used to, but I learned how to keep fingers and dirt off my glass. When I do use a filter, I have a very good reason to. Otherwise the filter stays in my bag.
The filters I tote around these days are a circular polarizer, for cutting reflections on vegetation and water, and split neutral density filters for darkening overbright skies so they fall in the same exposure range as the rest of the landscape. I do my black and white photography with a 4x5 camera. For that I have a set of Wratten gels (which, because of their thin cross-section, don't introduce nearly as much spherical as a glass filter.)
-- Pencil
P.S. The analogy! I almost forgot! sudo is a lot like using a UV filter to protect your glass. It'll stop a fingerprint, and help to teach you how not to smudge up your lenses. But if you manage to run "sudo rm -rf /" it'll still erase every file on your computer, just as if you were logged in as root.
P.P.S. sudo doesn't cause spherical aberration, as far as I know.
posted by pencil at 2:38 PM
0 Comments:
Post a Comment
<< Home