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Alternative Imaging Methods
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Old September 22nd, 2006, 08:35 AM   #16
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Oh i'm sorry i am messing here with terms. But anyway i don't quite follow how You eliminate vignetting with condenser. My point is if You try more even distribute light with codenser then what is image itself on the GG. It is just bunch of lightrays as any other. How can condenser give light of well lighted object in center of frame, to object that are in edge of the frame? The only way bring that light to edge is bring object desired place(sound logically enough?).
I think there is another misunderstanding with terms. All the talk here goes about hotspotting not about vignetting. Difference is source of lightwaves. Lightwaves which are part of vignetting start from GG. But lightwaves which create hotspot come out directly from photolens last lens. So can condenser distort hotspot differently from image on GG. But never differently vignetting.
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Old September 22nd, 2006, 09:06 AM   #17
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Frank,

Best way I can explain what the condenser is doing is by holding the GG in 1 hand and lens in the other, and project the image on the GG. Look at the image on the GG down the optical axis about a foot away from your eye.

Now, you notice the edges are darker then the centre. Look at the left dark edge, and move your head left so your eye is 45 degrees off the optical centre. What happens? That edge now appears brighter. Do the same for the right, top bottom.

The condenser lens is basically letting the camcorder view the GG from the best possible angle for any given point on the GG.

Ofcourse optically this is an incorrect explaination, but its one I like to give for visualizing.
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Old September 22nd, 2006, 01:32 PM   #18
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Exactly such experiment made me think a few years ago thats not vignetting. Because vignetting is part of the image thatsway You just can't avoid it with any light redistibution. For instance You can print out vignetted image(place it where goes GG), place Your eye and lens as You described. What You see? Really artifacts are gone? I guess no. You'll see exactly the same image! Where the difference then? Actually You can get same result as with GG. Just take flash light and place it behind(where should be Your lens back) Your image. Without condenser You'll see now spot in the centre of picture. Use condenser now and Voila! Spot is gone.

The point is that condenser just redistributes light what goes from distance(as magnifier lens) but lives the picture itself alone because it's close enough.

So the bottomline is it's not vignetting but hotspot what can be "hidden" with condenser. Just because vignetting is part of the image and needs different kind of compensation but hotspot is direct unprocessed(not diffused) lightrays coming directly from lens.
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Old September 22nd, 2006, 01:34 PM   #19
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Wayne's quite correct

Very approximately, something like the attached is happening.
http://www.physics.ox.ac.uk/users/sm...intensity.html
The further the camcorder lens is from the gg, the less light is incident on it's lens. (It subtends a smaller (solid) angle relative to a given point on the gg). This is more dramatic for the gg corners.

The condenser affects those light rays going away from the axis more than those closer to it. Net result, it redirects them closer to where we want them to be.

So, like Wayne said, with a condenser in place, the image will be brightest down the axis of the device, and fall off rapidly away from it.

I've been experimenting with two condensers, one each side of the gg and this seems to have an even more dramatic "condensing" effect.


Frank - I think I see your point. You're saying that if a film cell was where the gg is then a vignette is darkening of the corners on that image (were it to be developed etc). Any effect of the redistribution of light through interaction with the gg/condenser etc is a hotspot. I.e, if the adapter itself affects the image in such a way as to get dark edges that's a hotspot, and if the actual picture incident on the gg has dark edges, that's a
vignette.
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Old September 22nd, 2006, 01:45 PM   #20
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Martin, most lenses (unless spec'd otherwise) are not designed to utilize the outer ~5% of their periphery. The differential corner focus you are seeing is likely a result of spherical aberration at the edges of your achros.
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Old September 22nd, 2006, 03:16 PM   #21
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Dennis, I'm sure you know tons more about this than me. But if he is using an achromat and zooming in, wouldn't he also be zooming past that 5% periphery? Just a thought. I'm sure you know more than me on this subject, I just didn't quite understand so I thought I'd throw this out there.
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Old September 23rd, 2006, 12:28 AM   #22
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Yes, David. Just vignette and hotspot look in most cases so similar that it's easy to get confused.
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Old September 23rd, 2006, 12:46 AM   #23
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Mike, I'm not necessarily any kind of expert :-) However, if you CAD up the 72mm DVX lens, 58 mm achro rim, and then GG, I suspect you would find that the 58mm periphery is an issue. That 5% could very well be 10%. I believe that the same issue comes into play when shooting with many fast 35mm lenses with their aperture wide open.
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