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Old December 27th, 2007, 02:39 PM   #1
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Idea: replacing camcorders lens with fast single element lens

I have been reading much about these 35mm-adapters, and experimented a bit with them myself. One disadvantage I've noticed, and read about, is the light loss.

Now, I got this idea. How about replacing the camcorders relatively slow multi element zoom lens with just a single lens. You position things so that the camera always frames and focuses on the ground glass.

If you, for example, replace the multi element f/1.8 lens on a Canon HV20 with a single f/1.0 you probably will get close to a full stop brighter, since light has to pass fever elements (and the lower f-number of course). That may cancel out the light loss of the 35mm-adapter. Maybe even get brighter.

One disadvantage, of course, is that you can't controll exposure by stopping down the camcorder lens. So if you want to shoot with a big aperture on the 35mm-lens when it's sunny, you may have to use a neutral density filter or shortening exposure time.

I'm aware that this may not be a new idea. But i haven't seen this discussion before.

What do you think?
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Old December 27th, 2007, 06:27 PM   #2
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If you don't use a color-corrected focusing element like an achromat, you'll get color fringing. A simple single-element lens (i.e. made out of just one type of glass) focuses each color at a different point. An achromat, which looks like a single lens, is actually two elements made of different glass glued together. The two materials' focusing errors --mostly-- cancel each other out.

As for not being able to reduce aperature, all you need is a variable opening either right behind or in front of the lens. You can either slide cards with different-diameter holes in and out of the lens assembly (cheap), or spring for a variable iris (check Edmund Scientific).

Image quality will not be as good as a multi-element lens. There's a good reason camcorders come with all that exotic glass, rather than just one element. Check out some of the old black-and-white prints from early photographers to get an idea what you might get. Remember, though, that color fringing didn't show as badly in black and white -- it gave a "halo" effect and softened edges, which might be useful if you're planning on rendering down to monochrome.

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Old December 28th, 2007, 03:36 AM   #3
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Oh, I didn't know that. Read some about it now. I see now that you have to use a few elements to account for these aberrations.

The point was really just to use a faster lens than the standard part to cancel out the light loss from the adapter. Maybe you can use a C-mount f/1.0 lens with aspherical and achromatic or apochromatic elements.
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Old December 28th, 2007, 04:06 AM   #4
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According to some information the Drake designers gave (independent camera package, discussed here 2 years ago), the microlens structure of the CCD / CMOS interferes with wide appertures. You get a blurred picture at F 1.6 and an unusable picture on anything wider than F 1.4 opening.

That's why they used one of the few sensors without microlens.

But, microlens are good, as they are "pixel sized" bits of optical structure that focus the lightrays on the active elements of the sensor. Look up "fill factor" as a term. In short - they increase light sensitivity of the sensor.

Theoretically you could replace your camcorder optics (eg. 2.2 when zoomed on the GG) with a 1.6 c-mount lens. But is that F-Stop with unpredictable quality worth voiding the warranty / potentially destroying the cam?

As you can see, I thought about these things a lot in the past as well ;-)
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Old December 28th, 2007, 06:40 AM   #5
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Deciding to not have microlenses is a very hard decision. In small sensors, microlenses are not an issue because accurate software sharpening can get the mtf back to the original levels. In larger sensors, uniformity is a problem anyway and is corrected in hardware. It's not like we have a wide selection of affordable distortion free f1.0 lenses:)
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