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Old May 1st, 2008, 05:10 PM   #1
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Reducing minimum focus distance.

I'm doing some experementing with a 2/3" camera head. Had trouble finding some nice c-mount 16mm lenses to use, ran across a "Zeiss VarioTevidon 2/18-90mm" zoom on ebay.

I thought it would be perfect for my needs, but there is a slight problem. At 18mm I can't focus on anything near me. If I zoom in to almost 90mm I can focus on something about ~8 feet away from me. This is not what I had in mind.


I did notice that when the lens was not screwed on all the way I could focus on things that are close at 18mm. If I was to use a spacer, I would have a versitile lens that could adjust through most of the focal lengths I would need.

By moving the lens away from the sensor the projected image is different and the marked focal length values are no longer valid?

I was wondering about the potential side effects of using the lens this way?

Should I expect to see a considerable reduction in resolving power?
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Old May 1st, 2008, 05:24 PM   #2
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Resolving power won't change. You'e talking about an extension tube really. It will cut the amount of light a little depending upon how much extension you have (with 18mm lens you won't need much), you'll also lose the ability to focus at distance. You can buy c mount extension tubes, but don't know where off-hand.
Other alternative is diopters for the front of the lens (aka close up filters). Some expensive ones are really good quality, not so sure about some of the cheaper ones, but even with the best ones I'm not that keen on putting things in front of the lens.
Good luck.
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Old May 1st, 2008, 06:14 PM   #3
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Great, thanks.

I'm only increasing the distance from the sensor by maybe 1mm or so.
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Old May 5th, 2008, 04:30 PM   #4
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i assume youre talking about the sumix camera? anyway, do the focus distances match the lens barrel markings? are you sure the camera has its flane focal distance set correctly? do other lenses have correct focus distances? i know my old sumix m73 seemed a little off in its FFD, especially since sumix shipped it with a behind the lens screw in ir-cut filter (totally messes with FFD). anyway, sumix or whoever makes the camera might be able to supply you with spacers or stoppers, something you can screw in behind the lens to stop the lens so it doesnt screw in so deep or rings you can put around the male cmount that have a similar effect. its not the most convenient thing but it should work if you arent dealing with really big changes in FFD.

i looked up that lens on ebay and the pictures certainly seem to show focus markings that go as low as at least 2 (feet or meters, should be closer than 8feet...).
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Old May 5th, 2008, 10:24 PM   #5
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I'm talking blind here on two fronts so do not take my comments seriously. It is just a hint on a few things to look for.

The camera head? If it is a compact JVC KY-F50 or similar type, there might be a backfocus screw which is accessable by a pull-out rubber plug about 100mm from front on right side of body.

The C-mount itself may be an adjustable threaded sleeve inside another thread in the camera body.

In this arrangement there may be a single small grubscrew which locks it from turning once the correct flange to focal plane distance has been set or its distance may be set by adding or taking away shims between the rear of a shoulder and a face in the camera body..

The C-mount on the rear of the lens may itself be a sleeve which attaches over the lens body by screwing along a thread until it butts up tight against an internal face.

There may be tiny lockscrews on radial centres to secure it from turning.

It may be fastened by several screws on centres parallel to the optical centre axis around a shoulder.

Many lenses arranged like this have a facility where shims or spacer washers can be added between the mount and the lens body to achieve correct backfocus or collimation to an individual camera.

If the lens has been matched to a particular camera and is not adjustable, there is not much you can do except add spacers or shims between the lens flange face and camera flange face.

The only other thing I can think of is that your lens may be a CS-Mount type. The CS and C mounts are identical except for flange to focal plane distance. C-mount flange to focal is I think 5mm longer than CS-mount.

Vendors of CCTV security camera equipment may have small black nylon or metal spacers which have a c-mount threaded tail on one end and a c-mount threaded hole on the other.

I am not confident the available wall thickness in this spacer would support the weight of a film-camera lens if that is what you have. Added support would be necessary. They are made to support small lightweight security camera lenses.

Last edited by Bob Hart; May 5th, 2008 at 10:26 PM. Reason: error
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Old May 7th, 2008, 08:50 AM   #6
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Yes it is the Sumix camera, and as far as I can tell there is no backfocus adjustment. Every other lens seems to work fine, no problem at all. I have a Zeiss prime that works without any problems at all too.

The Zeiss zoom will not focus on anything set at 18mm. I was trying to do a timelapse video of a sunset and could not focus on clouds, trees, houses, or anything up close. I did not try focusing on anything inches from the lens.

It may be that the lens can be adjusted itself. I have examined it closely but was unable to find anything like that. I will have a closer look later on.

As far as I can tell the only solution is using spacers. I did try a CS mount converter but it was too long. Could not focus on anything at all. The spacers I need will probably be plastic/metal thin sheets to stack up in between the lens and the camera head. The overall adjustment only needs to be about ~1mm.

Furthermore, i'm not sure this is a very good lens at all. The resolution test charts I shot were on a less then ideal setup, but I find them disturbing.
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Old May 7th, 2008, 12:47 PM   #7
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Sounds like the lens uses a completely different focal flange distance. Lets assume you screw it in 1 mm less, does the c-mount screw mount still hold it?

Seems I will have to revise my judgement regarding this specific Zeiss glass.
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