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Old June 18th, 2005, 02:54 AM   #1
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Which condenser to use

I thought I'd share my experience with condensers. I'm using an Optosigma GG (part no 099-0160) and the grain on it is better than what I was getting with a Beattie BTF100 (from Adorama). So I'm happy with the GG.

Next is a condenser to correct the vignette, this is just as critical as the GG IMO, cause otherwise the adapter is no use.

I've tried an aspheric lens from Optosigma, it had a focal length of 49mm, gave a lot of barrel disortion and colour aberration. Putting the condenser between the GG and 35mm lens gave blurring towards the edges, so between the GG and camcorder seems to be the best place for it (flat side towards GG). The barrel distortion was fixed by moving the condenser further away from the GG, but this narrowed the field of view too much. Too narrow a field of view and you may as well zoom in without an adapter IMO. The colour aberration was also still there.

So next I tried two 80mm focal length BK-7 PCX lenses 011-2880 from
Optosigma. One on its own has lots of vignetting and some barrel distortion.
One either side of the GG is better for vignette but has edge blurring. The
best arrangement is GG then PCX with flat side of PCX against GG, then the second PCX facing the first with flat side to camcorder like this |)(

This has very little barrel distortion, the image is brighter, thereís only a small
amount of colour aberration and virtually no vignetting. This is what Iím going with.

The GG and condensers are 50mm dia, maybe a larger diameter condenser gives better results??? The main issue for me now is the considerably narrower field of view with the adapter on the camera... is there any way around it (other than going wide angle, then you lose your shallow DOF).
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Old June 18th, 2005, 04:09 AM   #2
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If you set camera to a wider angle, don't lose any DOF. DOF determined by 35 mm lens.

Try larger diameter lenses with 2x focal length. Vary camera to condenser distance for best result.

What happens if this arrengement:

I)), that means both lenses are with flat surface towards GG. How does it affect picture?

Radek
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Old June 18th, 2005, 04:42 AM   #3
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When I said wide angle I was referring to using a wide angle 35mm lens - it has a deeper depth of field. The object is to have a shallow depth of field so that is not an option. My problem now is losing field of view. With the adapter on the camera you have a considerably narrower field of view, you have to move back from the subject to get the same stuff in the picture.

With this arrangement )) you get a bit more barrel distortion.
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Old June 18th, 2005, 05:13 AM   #4
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You have to use larger condenser lenses. Try 60 mm at GG and 80 mm for other. If condenser touches GG, then 50 and 70 mm should probably do. I never did this but imagining how light travels, this is what I would do.

With two 80 mm lenses you get effect of 40 mm lens. Maybe you should try use bigger focal length, then see what is best camera to GG distance. I would start with trying removing one of lenses, that could make it better.

Radek
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Old June 18th, 2005, 07:59 PM   #5
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Well I feel like I've already spent too much on this thing :). I don't want to keep buying condensers blindly to see if they'll work... I did try to have one condenser only but it has a lot of distortion and vignetting. I was going to have one either side of the GG but then I found the field of view with |)( is no worse than |) and is in fact better in every way.

All my parts are inside a 50mm dia tube so they have to be 50mm dia... I just wondered if anyone had already solved the problem and I missed it on here.
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