Sizing down an image

[Eric MacIver]

I have been reading the threads dealing with the 35mm look being achieved by recording the image off of a surface that houses a correctly size 35mm image on it. A very good idea, however it is also very bulky. I've been trying to think of ways to size that down. Here's one thought - can anyone help me prove/disprove the logic in the following?

Using a Canon XL1s, one can remove the lens, so I am referring to this camera only for now. Canon makes a lens adapter to attach a 35mm EOS lens to your camera - I have also seen other hardware options to do that on the XL1s, which would be nice, however the major problem is that the magnification factor is 7.2x the lens you are using (in 35mm equivilent). It is my understanding that this is caused because the CCD is smaller than a 35mm film frame, so it is taking out a smaller "chunk" of the lens' image from the center of its image, which causes the magnification.

Also, to get correct DOF, a 35mm lens needs to be placed the apropriate distance from the recording surface (ccd or film).

So, if I could manufacture an intermediary lens that would have a mount on the front allowing a 35mm lens to be attached to it, setting the 35mm lens the appropriate distance from the front of its "converter" lens and the back of this "converter" lens mounted to the XL1s an appropriate amount of distance from the CCD - and, further, if this "converter" lens made the 35mm lens' image 7.2x SMALLER, would the XL1s be able to record the image as the 35mm lens intended it to appear in a 35mm camera?

And if it would, is such a converter possible without too much loss of light (stops)?

Maybe I'm missing something huge here, but this seems like it might be a good plan, aside from the hight cost of development.

Thoughts?

[Brett Erskine]

Nope. Wont work. Your talking about basically a lens that will down size the image to 1/3 CCD size. This is a much easier idea than what we and the professionally built mini35 is doing and theres a reason why both of us arent. It wont work. What you will end up with is a image with the proper FOV but not the same DOF. When you down size the image your change the focal length which basically changes the characteristics of the optics. It will still have the same DOF as DV. This idea has been talked about before on these threads if you want to learn more.

[Giroud Francois]

if by lens adapter you mean any device, a ground glass followed by a lens that will resize the ground glass picture to the size of your ccd it is ok, .....and then you got a aldu35.

[Jonathon Wilson]

Yes, I'm sorry, Eric - but you will lose the desireable (== short) depth of field most people are striving for in these adapters.

I actually built one exactly as you described because I was frustrated at the amount of light loss with the ground glass. I replaced my ground glass with a 'relay lens' which changes the magnification at the focal point down to the size of my ccd. It worked beautifully -- incredibly bright image... but very long Video-looking depth of field. I basically had the focal length/field of view of my SLR Lens, but depth of field of video.

This makes sense based on how depth of field works. Without painful details - its a factor of the size of the opening the light comes in - and the size if the image plane. With a relay lens, you don't really have an image plane until you hit the ccd. So the math goes from 35mm lens aperture to ccd, just like your video cam lens would do.

With the GG, this becomes a real imaging plane, so the depth of field math is from 35mm lens aperture to the ground glass. Then we just take a picture of that image plane.

[Eric MacIver]

Great Explanation of DOF - thanks so much for your replies... Back to the thinking board :)