Redrock M2 and XL2: MY first impressions

[Bill Porter]

The image projected onto the GG is not 36mm x 24mm; it's a circle of course, and tends to be around 43mm.

If you attached an SLR lens directly to the camcorder body, the lens would be projecting its 43mm circle onto a tiny CCD rated at 1/3" (it's not really 1/3"; dimensions vary between manufacturers but tend to be around 4.8mm x 3.6mm, so, 6mm diagonal). Therefore, the CCD would only be "seeing" a small section of that image. This would give the illusion of a very narrow field of view. It will NOT turn the SLR lens into a telephoto however.

An analogy may help if anybody reading this is lost. Imagine printing a 43mm (~1.7") round image onto a piece of paper, then using scissors to cut a tiny rectangle out of the circle's center that's only 4.8mm x 3.6mm (.19" x .14"). That rectangle is what the CCD would be picking up.

You could use a lens that has a 6.0mm image circle (like the camcorder's stock lens) to solve this problem, but then you wouldn't have shallow depth of field.

[Bill Porter]

I just re-read the first line in your post, Matthew. What we were just talking about wasn't what Bob meant.

Bob was talking about how you can increase the distance between, for example, a lens and its focal plane and shift its close-focus range closer.

For example, say you have a normal SLR camera. Its lens can be focused as close as, say, one foot, or as far as infinity of course. Now place a "macro tube" or "spacer tube" (search eBay for these) between the camera lens and camera body, and you'll shift that lens' range down to, for example, 2" to 40 feet. You'll lose infinity focus. This is how macro photography is done and that's how people get those super-close-up shots of insects on flowers and such.

[Matthew Nayman]

Oh wait... a tube between the SLR lens and XL2 body? I was confused.


Basically, what I am gunning to know is, is it possible to find an SLR lens, that could fit fairly snugly (macro tube or not), to the XL2 body (With proper adapter), and could work as a relay lens.

If so... what focal length would that lens have to be? I understand that the 7.6 is not REALLY a magnification, and that we are basically seeing only a small chunk of the real 35mm image... but for our discussion, lets use the 7.6 magnification.

So what focal length, plus 7.6 mag, with proper spacer tube to narrow focal distance, would relay the image to a 1/3 CCD chip in the XL2?




I think we're making progress here guys!

[Bill Porter]

Originally Posted by Matthew Nayman

Oh wait... a tube between the SLR lens and XL2 body?

No. I said, "a normal SLR camera." Anyway that topic has to do with focal planes and what happens when you move a lens out of its correct registration distance.

Originally Posted by Matthew Nayman

Basically, what I am gunning to know is, is it possible to find an SLR lens, that could fit fairly snugly (macro tube or not), to the XL2 body (With proper adapter), and could work as a relay lens.

Yes. Adapters exist and they work. Quyen Le for example, makes such an adapter.

Originally Posted by Matthew Nayman

If so... what focal length would that lens have to be?

Have to be to do what? Work without a 35mm adapter and retain shallow DOF? No such SLR lens exists. Or did you mean, work in conjunction with an adapter? People are already doing this and you should look at the Letus subforum.

Originally Posted by Matthew Nayman

I understand that the 7.6 is not REALLY a magnification, and that we are basically seeing only a small chunk of the real 35mm image... but for our discussion, lets use the 7.6 magnification.

You can't use magnification just for the sake of discussion. It's cropping, not magnification, and the two are totally different.

Originally Posted by Matthew Nayman

So what focal length, plus 7.6 mag, with proper spacer tube to narrow focal distance, would relay the image to a 1/3 CCD chip in the XL2?

I think we're making progress here guys!

The spacer tube was in an example about an SLR camera, so strike that one. You do not want to give up infinity focus.

Also, this train of thought has actually already been thought out here on dvinfo (two years ago! check the archives). Bottom line is, SLR lenses put out way too big of an image circle for our CCD's. That's why we focus on a piece of GG, focusing screen, or what have you.


Sucks, don't it!

:-)

[Justine Haupt]

I'd still like to see that footage with the M2 and XL2 ;)...

...but I don't see what the confusion is with the SLR lens-as-relay thing.

I think Quyen spelled it out quite clearly (quoting):

Originally Posted by Quyen Le

24mm lens need 195mm
28mm lens need 229mm
50mm lens need 400mm
85mm lens need 697mm
Hope this info helps.

Quyen

Getting proper cropping for the 35mm frame is a matter of changing the flange focal distance between the relay lens and CCD array.

Those are the proper numbers to project 36x24 frame on to a 1/3" CCD. For example, a 28mm lens would have to be positioned 202mm (that's a typo above) behind the GG for the proper scaling, and the lens would have to be as far in front of the CCDs as necessary to bring the image into focus (GG/CCD flange distance). The flange distance will NOT be the same as the published distance for that particular lens... we're talking about changing the effective magnification of the lens by modifying flange focal distance here.

[Matthew Nayman]

BIll, I understand about that, as I own an M2 and am not an idiot :P

I have no interest in putting an SLR lens directly on the body to use as a normal lens, only as a Relay lens.

For example, a 28mm lens would have to be positioned 202mm (that's a typo above) behind the GG for the proper scaling, and the lens would have to be as far in front of the CCDs as necessary to bring the image into focus (GG/CCD flange distance).

Okay, I have drawn an illustraition to show how I interpret what you just said, and how I see it working (Keep in mind, I am a dolt!)

Image 1: http://www.notomatofilms.com/Explination.jpg

The green lines represent how far Justin and Queyen say the Nikon lens would have to be to refocus the image onto the CCD block. 19.5 CM (or 7.5 inches) on either side doesnt seem like a space savings at all!

Image 2: http://www.notomatofilms.com/Explination2.jpg

This image shows how I would like to set up the rig, but the Green Question mark is an unkown focal length. Is it not possible to find the right focal length lens, place it directly on a Nikon to XL adapter (or EF adapter) and then that part of the image circle which shows through to the XL CCD's would be exactly the 36x24 image? I assume this is how Queyen's and P+S's relay works.

Are spacer tubes really nessicary? If it's a matter of not being able to focus close enough on the GG, then cant I use an Acrhomat?

[Ben Winter]

Interesting proposition. I'd be interested to see where this goes.

[Bob Hart]

Okay. Here's some "in air" tests of some Nikon mount SLR lenses as pseudo-relay lenses.



LENS. - FLANGE TO F/P - FLANGE TO OBJECT


28mm---------51mm--------------224mm

50mm---------51mm--------------354mm

20mm---------51mm--------------233mm

14mm---------51mm--------------151mm


THE METHOD.

36mm x 24mm rectangular target cut out of a cardboard sheet. Bright TV screen used as backlight. This target is referred to in the following as the object or object frame and for a relay device, represents the position of the groundglass relative to the flange face of the lens to be used as a relay lens.

The flange face is a constant. Measuring from the front element of the lens is not valid as no two lenses of the same focal length but different brands or styles will be of the same dimensions. So all measurements have been taken from the flange face on rear of the lenses.

1/3" rectangular target drawn on a groundglass to represent the camcorder CCD area. 1/3" is the corner to corner or diametrical dimension. Groundglass in this experiment is NOT the groundglass used in the 35mm relay device, merely a tool in this instance to enable seeing the image as if it were on the camera CCD.

51mm approximately seems to be the optimum forward offset of the flange face relative to the focal plane to keep the image of the object frame "scaled" to the size of the camcorder CCD. So for Nikon, this means 46.5mm + 4.5mm. The 4.5mm forward offset would apply to all lens mount types.

The 14mm focal length lens would appear to be the most practical in terms of the length of any relay lens 35mm adaptor combination. Shorter focal length lenses are not a practical option in 35mm SLR styles.

A 14mm SLR lens is an expensive way to go about setting up a relay path. C-Mount lenses of the same focal length might be a better option.

C-mount lenses in the 14mm ballpark are likely to have a rear optical element which protrudes up to 10mm behind the C-Mount flange and may well interfere with the front internal optics in the XL1, either prism face or an optical filter.

The focus of the SLR lenses tested was set about midway to permit adjustment either way for backfocus adjustment. Given this variation, my results are roughly consistent with those published by Quyen above. His results will be more valid.

The dimensions in the table above should not be regarded as precise, but more of a general guide only as the testing apparatus was very primitive, carboard boxes, books and lots of stickytape to make things stay put.

The general results were consistent with earlier tests I have done with C-Mount crimcam and Bolex lenses.

A 20mm - 25mm focal length C-Mount lens is probably the most practical option as a relay lens. In a non-erecting design, the structure is going to have to be fairly long.

However, less than 14mm focal length lenses will create a problem if a prismatic path is used as the flange to object distance will be too short for the lens + prisms to fit in between the camera and groundglass. I think the 14mm lens might be a bit too short anyway.

Anyway. That's my contribution. Consider the baton now handed for somebody to take it to the next level.

[Matthew Nayman]

Alright, did some more tests. Here is what I got using a Nikon 50mm shooting through Redrock Achromat and M2 (mounted is a nikon 28mm 2.8).

www.notomatofilms.com/pic.jpg

I used a card to reflect the projected light. It seems mostly infocus, and roughly 1/3 inch digaonal.

This is a crude test at best, but may warrant further testing.