Making a scanner with a DSLR

[rdenney]

Getting back to the telecentric idea. Wouldn't this only be helpful if one employs focus stacking, since the whole point seems to be keeping the subject the same size when changing focus? If we don't change focus during the scan, then my very cursory understanding of the idea is that there wouldn't be any benefit. Is that correct?

I think it would also be helpful to correct any subtle misalignments in the negative. If one side of the negative is slightly further away than the other side, it will be subject to less magnification than when it's on the other end for the next tile. Those misalignments might result from negative curvature.

Let's do some math to see if it's an issue:

I have measure a maximum curvature in my holder of 0.3mm in the center of the frame. That is actually not bad, according to what others have said. Assuming (conservatively, since the scan frame doesn't at all cover the whole range of curvature) that this is the maximum error, one end might be 0.3mm further from the camera than the other end. Let's assume 1:1 target magnification with a 100mm focal length. At 1:1, the film should be 100mm in front of the lens's rear node (I think I have that right). So, a 0.3mm error means that one end of the negative being scanned will be 0.3% farther from the node than the other end. And the project size of the negative will therefore be 0.3% smaller, using similar triangles. That's 8.7 pixels across the width of my Canon's sensor, and it has big pixels. A telecentric lens, because the magnification does not change, would eliminate that problem altogether.

Or, let's say the negative stage is out of square with the camera by 2 thousandths across the width of the 4x5 frame. 0.002" = 0.05mm, and one 36mm-wide frame might encompass half of that. That error is smaller than film curvature. If one side of the frame is 100.03mm from the node (along the parallel line, of course) while the axial distance is 100.00mm, then that's a 0.03% error. That means the negative on that end will be 0.03% smaller than when it's on the other end. 0.03% of my 2900 pixels is about a pixel. In that case, it's okay. Getting the negative square within 0.002" of the camera is no minor task, however.

Since I don't have a telecentric lens, I'm hoping my calculation above is conservative enough to render it not important, or that it can be corrected easily in practice. I haven't gotten there yet.

Rick "correcting focus but moving the camera for each frame might solve the problem, but create others" Denney

[Struan Gray]

I've been tinkering. I'll post in the lenses thread when I have some real numbers, but FWIW and IMHO, here are some qualitative experiences.

1) Dan Fromm knows what he's talking about when it comes to photomacrography. I'm surprised he hadn't yet told us all to stop messing about and read the Lefkowitz book.

2) There are no free lunches. And you can't make a silk purse out of a sow's ear. If you want real quality, suck it up and buy a real macro lens.

3) Many of the manual focus 70s-80s era primes I have at my disposal have their entrance pupils quite deeply recessed behind the front of the lens barrel. Quite often a simple stacking arrangement is already close to telecentric on the object side.

4) In particular, two stacked Pentax 50 mm f1.7 lenses work very well at 1:1. (but my DSLR is only 6 mpix, so I'm aiming for 2:1 :-).

5) Stacked MF lenses give a lovely even illumination. Distortion can be surprisingly high though, even with Zeiss/Hasselblad primes. [a side discovery: my orphaned Kowa lenses might make good candidates for telecentric Kohler illumination, even if they are not worth the hassle as taking lenses].

6) An arrangement which is telecentric on both the object and the image sides is going to have a long tube length.

7) Telecentric arrangements need careful alignment, particularly for angles. Any working setup will need to be locked down and left alone.

That seems like a bunch of negatives, but it's really a narrowing of parameter space to the point where I can ask some real questions. I've been cobbling together ways of mounting lenses, cameras and apertures on my Sinar's rail, so if I'm not hiding eggs round the garden the entire time, I'll see if I can take some test shots over the Easter weekend.

[Dan Fromm]

Dan Fromm knows what he's talking about when it comes to photomacrography. I'm surprised he hadn't yet told us all to stop messing about and read the Lefkowitz book.

Y'all are having too much fun to be bothered. If you look at Lefkowitz, you might also want to look at:

Gibson, H. Lou. Close-Up Photography and Photomacrography. 1970. Publication N-16. Eastman Kodak Co. Rochester, NY. 98+95+6 pp. The two sections were published separately as Kodak Publications N-12A and N-12B respectively. Republished in 1977 with changes and without the 6 page analytic supplement, which was published separately as Kodak Publication N-15. 1977 edition is ISBN 0-87985-206-2.

Gibson is very strong on lighting, exposure, and on what can and cannot be accomplished. His books, although relatively weak on getting the magnification with lenses made for modern SLR cameras, provide a very useful foundation for thinking about working at magnifications above 1:10 and especially above 1:1. Extensive bibliography.

H. Lou Gibson is one of the Englishmen that Kingslake brought to EKCo. He's best known for IR photography. His Photomacrography pamphlet is the most frightening photography book I've ever read.

[Nathan Potter]

Dan, thanks for reminding me. I see I have that 1977 Kodak publication which is quite good but with some weaknesses, as you point out.

Another useful Kodak publication was their "Photography Through the Microscope", ISBN 0-87985-019-1, 1974. It of course deals with high magnification and is pretty informative about some sophisticated techniques such as interference contrast and the Nomarski version of such.

Nate Potter, Austin TX.

[Serge S]

Can any of you recommend a good lightbox for shooting negs with my 5D. My Bretford Acculight has hot spots....there must be something better out there and I don't have any local shops so I need to mail order.

Thanks,

Serge

[Peter De Smidt]

Struan, what Lefkowitz book?

[Struan Gray]

Lester Lefkowitz, "The Manual of Close-up Photography"

On Amazon here: http://www.amazon.com/Manual-Close-u...dp/0817421300/

[Peter De Smidt]

Here's and article on DSLR film scanning: http://www.thedambook.com/downloads/...ning_Krogh.pdf

[Peter De Smidt]

You might try modifying your box a bit. For instance, you could get some white foam core. Make a flat-topped pyramid, with the bigger opening sized to fit over your light source. At the smaller end, put a piece of diffused plexiglas. You will lose light, of course, but you should be able to make a pretty even source.

[Serge S]

You might try modifying your box a bit. For instance, you could get some white foamcore. Make and inverted pyramid, with the bigger opening sized to fit over your light source. At the smaller end, put a piece of diffused plexiglas. You will lose light, of course, but you should be able to make a pretty even source.

Thanks Peter I'll give it a shot