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Testing c-mount lenses


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#1 Kristian Schumacher

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Posted 17 December 2008 - 07:43 AM

Hi everyone!

I have shot some footage for a music video on 16mm and s16. One roll I shot on r16 with my Milliken high speed camera and a Schneider 10mm. The footage looked fine when I just used that lens. But I also tried using a Schneider wide angle adapter with the 10mm - I checked looking into the gate for vignetting before shooting, but the footage shot with the adapter is really blurry in large parts of the image. I couldn't tell just from looking in the gate before shooting..... It will have to do for this time, but I want to solve the problem for next time, So:

What is the easiest way to test my c-mount lenses with different adapters etc? I have a Nikon DSLR that would be great to use if I could lock up the mirror and somehow mount the c-mount lenses far enough back into the camera. Or a surveilance video camera with a chip large enough to check sharpness in the corners. Alternatively if there is some still photo camera using film that will accept c-mount lenses.

Any ideas how I can shoot stills (ideally digital) with c-mounts? Or another convenient, not too expensive way to experiment with finding the best lens/adapter combination?

Thanks,

Kristian
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#2 Kristian Schumacher

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Posted 21 December 2008 - 02:23 PM

Anybody? :rolleyes:

Kristian
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#3 Charles MacDonald

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Posted 21 December 2008 - 10:13 PM

What is the easiest way to test my c-mount lenses with different adapters etc? Or a surveilance video camera with a chip large enough to check sharpness in the corners.


Lens techs will sometimes set up a projector with a test chart at the right back distance and use the lens ot project the chart on the wall. Light travels in both directions the same.

Another tool is an autocolminator (and I probalyspelled that wrong) which lets them see if the lens is actually focuing on the film, although that works best in the center of the frame as I understand it.

IF (and it is a big if) you can find an ancient working video camera using a 1 inch vidicon, you could see what the 16mm film would see fairly closely in real time. I don't know if anyone made CCD cameras with that large a target, and many of the later model Vidicon Cameras used the samlelr 2/3 inch vidicons.

Trick is that the cameras were probaly left in service until they failed and then repleced with CCD units and it might be hard to find old stock vidicons. as an old survalence camera will probaly have some parts of what it was looking at for years burned into the target on the vidicon.
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#4 Kristian Schumacher

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Posted 22 December 2008 - 05:29 AM

Thanks for that Charles,

I was hoping to do the fiddling myself without the help of an expensive tech, but like you said - finding a working camera with a large enough sensor is probably not easy. I think I read somewhere that someone had managed to attach a c-mount to a dslr, but I can't find it again. I guess the lens would need to fit almost 2cm further back than my nikon lenses, so that would be a tricky mount, though excellent for testing lenses.

Kristian



Lens techs will sometimes set up a projector with a test chart at the right back distance and use the lens ot project the chart on the wall. Light travels in both directions the same.

Another tool is an autocolminator (and I probalyspelled that wrong) which lets them see if the lens is actually focuing on the film, although that works best in the center of the frame as I understand it.

IF (and it is a big if) you can find an ancient working video camera using a 1 inch vidicon, you could see what the 16mm film would see fairly closely in real time. I don't know if anyone made CCD cameras with that large a target, and many of the later model Vidicon Cameras used the samlelr 2/3 inch vidicons.

Trick is that the cameras were probaly left in service until they failed and then repleced with CCD units and it might be hard to find old stock vidicons. as an old survalence camera will probaly have some parts of what it was looking at for years burned into the target on the vidicon.


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#5 Bengt Freden

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Posted 22 December 2008 - 02:28 PM

Hej Kristian,

Putting a wide angle adapter lens on a zoom, or a prime lens, is VERY demanding on the exact focal flange distance to the camera gate. The long zooms and also wide angle primes are of retrofocus construction, i. e. the distance to the camera gate is LONGER than the focal length of the lens, for example 4 or 6 for Super-8 or 6 or 9 for 16mm, because of the distance occupied by the sector shutter and mirror, and in some cases, a viewfinder prism.

Normally, such a wide lens is focused using the macro lever on the zoom lens, at least with Schneider or Angénieux zooms on the Beaulieu R16 (por the 4008 Super8) camera. You have to be very careful, though (make sure that the viewfinder eyepiece is exactly adjusted for your own eyesight) - perhaps taping it down, when you have arrived at a perfect focus.

The wider the lens (the shorter the focal length), the narrower the depth of field at the camera fate - that´s why it is especially important to collimate (check the focal flange distance with a collimeter) wide angle and super wide angle lenses. Even very small differences will reveal them selves as an unsharp image, and sometimes the image can be sharp on the ground glass while it is slightly off focus in the camera gate. This is something that only an experienced movie camera technician can determine and do something about. The universal norm for the focal flange distance from the focal point in the lens to the camera gate on C-mount lenses is 17,52mm, by the way. You may find a further explanation of this here: http://www.schneider...ustrial.htm#qu5

A small tip: Putting a small bit of matted transparent tape flush against the camera gate, and looking at it with a good lupe or magnifier (sometimes you also need a small dentist´s mirror), can be one relatively easy way to do a first check. Always check the longest distance first - at infinity, with the aperture fully opened (full stop). This is where it is really critical - it HAS to be completely sharp at the infinity mark (where the focal ring ends). At near distances, you can adjust for slight differemces btw the scale of the lens and the exact, physical distance measured with a measuring tape to the mark on the camera where the camera gate is situated (if there is such a mark on your camera).

Hope this helps.

Best regards,
/Bengt ;)

Edited by Bengt Freden, 22 December 2008 - 02:29 PM.

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#6 Kristian Schumacher

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Posted 22 December 2008 - 02:50 PM

Hjertelig takk, Bengt!


I used a piece of tape in the gate, but no magnifying glass/lupe - maybe that would have done the trick as far as spotting the blurry areas of my image. My camera is non-reflex, so I figured this was the only way to check - in the gate. Actually, my main subject was reasonably well focused when using the adapter, but a large area about 1/3 of the frame on the left of the image was severely blurred. The whole frame looked good in the shots with the 10mm by itself, but maybe the FFD problem didn't show until it had the adapter on...? This was my first and only time using this adapter, so it is not so easy to tell if the problem is with the adapter or with the FFD. That's why I'd like to learn some more about this lens and adapter the not-so-hard way :huh:

Thanks again, Bengt. I'll try it again tomorrow with tape in gate, at infinity with a magnifying glass.

Kristian



Hej Kristian,

Putting a wide angle adapter lens on a zoom, or a prime lens, is VERY demanding on the exact focal flange distance to the camera gate. The long zooms and also wide angle primes are of retrofocus construction, i. e. the distance to the camera gate is LONGER than the focal length of the lens, for example 4 or 6 for Super-8 or 6 or 9 for 16mm, because of the distance occupied by the sector shutter and mirror, and in some cases, a viewfinder prism.

Normally, such a wide lens is focused using the macro lever on the zoom lens, at least with Schneider or Angénieux zooms on the Beaulieu R16 (por the 4008 Super8) camera. You have to be very careful, though (make sure that the viewfinder eyepiece is exactly adjusted for your own eyesight) - perhaps taping it down, when you have arrived at a perfect focus.

The wider the lens (the shorter the focal length), the narrower the depth of field at the camera fate - that´s why it is especially important to collimate (check the focal flange distance with a collimeter) wide angle and super wide angle lenses. Even very small differences will reveal them selves as an unsharp image, and sometimes the image can be sharp on the ground glass while it is slightly off focus in the camera gate. This is something that only an experienced movie camera technician can determine and do something about. The universal norm for the focal flange distance from the focal point in the lens to the camera gate on C-mount lenses is 17,52mm, by the way. You may find a further explanation of this here: http://www.schneider...ustrial.htm#qu5

A small tip: Putting a small bit of matted transparent tape flush against the camera gate, and looking at it with a good lupe or magnifier (sometimes you also need a small dentist´s mirror), can be one relatively easy way to do a first check. Always check the longest distance first - at infinity, with the aperture fully opened (full stop). This is where it is really critical - it HAS to be completely sharp at the infinity mark (where the focal ring ends). At near distances, you can adjust for slight differemces btw the scale of the lens and the exact, physical distance measured with a measuring tape to the mark on the camera where the camera gate is situated (if there is such a mark on your camera).

Hope this helps.

Best regards,
/Bengt ;)


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#7 Bengt Freden

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Posted 22 December 2008 - 03:12 PM

Hi Kristian,

Was this lens adapter especially made for your lens?
In the case of Schneider, the ASPHERIC acryllic glass UWL super wide angle lenses are deigned especially for the Schneider zooms (for example in Super8, the 6-66 or the 6-70 zooms). The aspheric design is developed to address issues with sharpness, contrast and color shift in the corners of the image. As with all lenses, image quality improves when stopped down two or three steps. If you write to Schneider Optical (or any other optical glass manucaturer), they can probably give you the optimum f-stop for each lens or lens adapter.

No filters in between the lens adapter and the lens itself? And, are you sure that the piece of tape was applied to the gate in a perfectly flat fashion, and not slightly curved? That might influence the test image that you see. If you try the crude and simple tape test again, and get the same result, there MIGHT be something wrong with how the lens is put together.

The C-mount flange focal distance is of course measured from the flange of the C-mount on the lens to the camera gate, not from the focal point as I happened to write (which is even farther away from the gate) - sorry about that mistake.

Best,
/Bengt ;)
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#8 Kristian Schumacher

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Posted 22 December 2008 - 05:52 PM

Thanks again Bengt,

The adapter is made by Schneider, it has the 55mm thread, but is probably not made for that lens, no... It will not screw all the way in the thread, or the optical elements will actually touch. So it may very well be a bad solution. That is why I would like to do some experimenting to see what combination would be best to get a wider angle than the current 10mm. Alternatively, I will keep my eyes out for a wider c-mount lens. The lens gives good results on it's own, though and I am happy with its performance - just not with the adapter. I don't actually recall the f-stop for the shots with the adapter, but I think they were at least not all open - probably 5.6-8-ish So that should not be the problem.
I will give the tape trick another go, and see if I can see any of the same problems on the tape as I saw on the footage.
No worries about the FFD-mistake....I am good with the theoretical stuff - It is the real world of getting good results that is my problem :lol:


Kristian


Hi Kristian,

Was this lens adapter especially made for your lens?
In the case of Schneider, the ASPHERIC acryllic glass UWL super wide angle lenses are deigned especially for the Schneider zooms (for example in Super8, the 6-66 or the 6-70 zooms). The aspheric design is developed to address issues with sharpness, contrast and color shift in the corners of the image. As with all lenses, image quality improves when stopped down two or three steps. If you write to Schneider Optical (or any other optical glass manucaturer), they can probably give you the optimum f-stop for each lens or lens adapter.

No filters in between the lens adapter and the lens itself? And, are you sure that the piece of tape was applied to the gate in a perfectly flat fashion, and not slightly curved? That might influence the test image that you see. If you try the crude and simple tape test again, and get the same result, there MIGHT be something wrong with how the lens is put together.

The C-mount flange focal distance is of course measured from the flange of the C-mount on the lens to the camera gate, not from the focal point as I happened to write (which is even farther away from the gate) - sorry about that mistake.

Best,
/Bengt ;)


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