 Linda Kessler |
are scratched lenses okay I am interested in purchasing a used lens and the owners says it is scratched. He also says it will not affect the printed image. Is this true? Linda
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Alan N. Marcus |
Hi Linda, The job of the camera lens is to project an image of the outside world onto a tiny screen inside the camera (image plane). The screen I am talking about is actually the film or the surface of the digital chip. The film or chip records a replica of the image. The lens works by bending (refracting) light. Each ray is targeted to specific spot at the image plane. The design of the front surface of the lens generally resembles a portion of a sphere. The sphere shape of the lens is actually a comprise shape and sometimes we try to improve the figure by slightly changing the steepness of the curve particularly at the edges to a more complex ellipsoid or conic surface (aspheric). Such a design can improve the ability of the lens to cause the rays to hit their targets. No matter what we do, we can’t get every ray to hit at the image plane correctly. Think of a line of target shooters aiming at a bull’s-eye target. They fire a volley and in a perfect world, when we inspect the target, we would get one tiny bullet hole. In reality we get a scalloped hole with some random misses. That’s exactly how a lens targets light rays. The spot of light is called the circle of confusion. If the circle of confusion is tiny and there are few misses, the lens will be said to be sharp. Also the lens will produce a nicely contrasty image. More misses, we get flare and flare is devastating as it reduces contrast. Now surface defects like oil – grease – scratches – chips – below surface bubbles – cause random misses. All add to the ambient flare. All flare degrades the image. You won’t believe this but you can paint over scratches and chips with opaque black paint. This prevents these blemishes from causing flare. Spots of paint on the lens surface have little effect on sharpness. Instead they reduce the lens’s surface area thus slowing the lens. Painting a lens’s surface is kind of like adding a neutral density filter. This technique reduces exposure without changing dept-of-field as the aperture is not changed. The bottom line is: Unless making big prints, most tiny surface defects are mainly cosmetic and have little effect on optical performance. If you want a particular lens and can’t afford a “perfect” one, why not borrow and test before you buy. Regards,
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John H. Siskin |
Hi Linda, Alan’s comments are accurate as far as I know, but I wanted to add a couple of comments. Scratches and marks on the real element of a lens are a much bigger problem than marks on the front element or one of the internal elements. This is because marks at the rear element of the lens can actually throw a shadow on the sensor. I also wanted to mention the Leitz Thambar, an incredibly rare soft focus lens for the Leica Camera. There was a filter for this lens that was a clear glass filter with a large black spot in the middle. The spot kept light from going through the middle of the lens, the best-corrected part of the lens. Instead the picture is made from light rays from the edge of the lens, which contain more random misses. This made for more aberration in the lens; this can be a pleasing effect. You can try this yourself; all it takes is an extra UV filter. It has a pleasing soft focus effect with many lenses, particularly older lenses. I’ve been mentioning articles I’ve written a lot lately and I’ll do it again. I worte an article on building soft focus lenses, you can download it from the magazine articles page n my website: www.siskinphoto.com. Thanks, John
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David A. Bliss |
I have an older Canon lens that has a pretty good ding in the front element. It still produces better photos than some of my other "lesser quality" lenses, even with the chip. You cannot look at a photo produced with that lens and tell there is a defect. Of course, there will be a point at which there is enough damage that it effects the photo. That was something I was taught early on in my photography. Since I bought mostly used gear (couldn't afford new), I could talk down the price at pawn shops because of "defects" that I new wouldn't effect the photo, but the salesman at the pawn shop had no idea. Of course, that has all changed with E-bay, since nobody sells camera gear in the pawn shops anymore :( Rear element is a different story, like John said. I would shy away from a lens with a damaged rear element.
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David A. Bliss |
[url]http://www.betterphoto.com/gallery/dynoGallDetail.php?photoID=1065428&catID=&style=&rowNumber=7&memberID=119007[/url] This is a photo that was shot with the damaged lens.
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