DSLR Scanner: Light Sources

[Peter De Smidt]

The Rosco LitePad looks interesting. A 3"x3" version is about $50. The main thing will be if you get enough light. With a continuous light exposure, we probably want to avoid shutter speeds from about 1/15th of a second down to a second, as these are the most susceptible to vibration.

Near the beginning of the main thread, I showed a stepwedge photographed with a dslr+macro lens made with one exposure, and one with blended exposures. There are certainly gains to be had there, if needed. While most bw negatives would fit within the dynamic range of one exposure, I just scanned a tech pan negative with no problems, digital images are noisiest in the shadows, and with bw negatives this means that the noisiest areas will be the highlights when the image is inverted. As a result, some type of multi-exposure blending could be very useful, especially in images with lots of very delicate highlights.

[AJSJones]

I use mirror lock-up and 10 sec timer - my camera stand is not the greatest and exposures are indeed down in that range where I'd normally do that for taking pictures. I blended a 1 sec exposure with a 1/15 sec exposure and that dramatically reduced the shadow noise on the test (from a Provia 4x5 slide where the highlights were just not blown in the DSLR and were almost, but not quite blown on the slide). The result was way better dynamic range than a single scan from my AgfaScan Duo T2500 that broke down, and the detail captured depends on how many shots I stitch - I can get it all with about a 12 shot stitch, possibly fewer. That was the next stage in the project...

[Peter De Smidt]

Sounds good. I haven't down any work with slides yet.

[rdenney]

Well, I had a "duh!" moment, or two.

I reported last week that I had had no luck with the slide projector as a light source. I was giving a presentation in another city this week, and looked into the projector light, and, well, DUH!, it was a pinpoint of light. So, I think to use the projector as a source, it needs to be a good distance away (to make it as much a pinpoint as possible), and then a condenser needs to be close to the film, maybe right behind it. Well, DUH!, that's the way condenser enlargers do it. I had thought the condenser in the slide projector would be adequate to achieve that, but (1) it's not nearly enough of a condenser and (2) it's small and close to the bulb. When looking into the front of the (turned-off) projector, I can see the ELM bulb reflector in its entirety. And the condenser is too weak. It really does need to be a point source with enough of a condenser to collimate the light.

So, in response to the above, I looked into the Rosco Litepads. For $50, the 3"x3" panel looked interesting. Then, when perusing reviews of it, someone commented that it looked like the standard LED back-light arrangement for computer monitor.

Well, DUH! I have computer monitors hanging from the ceiling. So, I took my work laptop, which has a very bright LED backlight, and set it on edge behind my negative carrier, opened Notepad and maximized it to get a white display (or white enough for testing), and made some tests. And the tests look a lot better. I'm strongly tempted to task an old computer to displaying a fall-off correction pattern on the screen of an old monitor.

At the f/11 that works best with my best lens, I get about 3 or 4% falloff with an exposure of about half a second. I suspect the fall-off is sensor-related--the falloff pattern looks the same on all lenses, being a little off-center no matter how I position the monitor panel.

I learned a little about my lenses, too--in the lens thread. And I learned a little about sharpening and the effects of the anti-aliasing filter, also in the lens thread.

Rick "not quite ready to attempt stitching just yet" Denney

[Peter De Smidt]

Sounds like you're making some good progress.

[peter ramm]

Rick, have you considered trying this type of LED back light?

http://www.edmundoptics.com/products...productid=3358

Can't remember if it has come up here or not. They work pretty well imaging film, if you can live with the wavelength emission. We played with these and with fiber optic plates trying to get even lighting. In the end, we used a custom fluorescent that we had on hand anyway.

Otherwise, looks like you are heading towards Kohler with the projection system.

[Peter De Smidt]

Those are nice but a little pricy, and I wish they listed a CRI rating. Daniel has been investigating a similar, and much cheaper, approach. His current panel should work well for BW, and a high cri panel is in the works.

With continuous lighting, we really want to avoid exposure times in the 2-1/15th of a second range, as those are most susceptible to vibration.

Note that a very experienced coin photographer has found that using a canon model with an electronic first curtain shutter significantly helps with image sharpness even at a low 1x magnification. For more on Canon's low vibration shutter, see: http://krebsmicro.com/Canon_EFSC/index.html

[rdenney]

Rick, have you considered trying this type of LED back light?

A little pricey for me, too.

But I'm sorta liking the idea of using a computer display, because of the possibility of projecting a correction pattern on it. And the correction pattern can be made by inverting a photo with no negative in the stage and displaying it at the appropriate size on the display used for illumination. Should be able to dial in perfect illumination.

It does not solve the problem of shutter speeds in the danger range. My Canon doesn't have the electronic first curtain, but the opening curtain makes no discernible movement that I can detect. It's a custom function on my camera to turn the self-timer into a timer that raises the mirror, waits three seconds, and then releases the shutter. I see no indication of camera movement as a cause of fuzziness.

Instead of setting a laptop on edge, I need to take a spare monitor and do something a little more stable and repeatable, so that I can experiment further. For a diffused light source, though, I think this is the way to go.

For a collimated source, that's another matter. There, I think an enlarger head is the cheap way to go. But I'm not yet motivated to dig that deeply in my storage room--I know how many "might as wells" will be attached to that effort, unless I do it on the sly, which isn't easy.

I also need to experiment with exposure levels. I've been photographing to put the peak in the middle of the histogram, which is fine for negatives, but which may exaggerate lighting variability when photographing the light source without a negative. I will eventually need to set the exposure to just sit under 255 (on an 8-bit scale) with no negative, or, at most, through film base+fog. That will provide the maximum dynamic range, I think.

Rick "whose experiments are continuing" Denney

[ludvig friberg]

Perhaps this is making it more complicated that it needs to be, but if we use a flying spot perhaps with a white laser or a fast smooth wipe with a line laser we would get rid of flaring. I have no idea how difficult or expensive that would be. But In a way it would turn the areasensor in the dslr to a linesensor.

Ludvig

[rdenney]

Perhaps this is making it more complicated that it needs to be, but if we use a flying spot perhaps with a white laser or a fast smooth wipe with a line laser we would get rid of flaring. I have no idea how difficult or expensive that would be. But In a way it would turn the areasensor in the dslr to a linesensor.

Ludvig

Yes, I think that would be best. But now we'd have to have two movement apparatuses, one for the film and another for the laser. And the laser apparatus would have to be tightly controlled at the resolution of the scan, and also constrained by an aperture. Very soon, we are at the mechanical complexity of a drum scanner, which works similarly.

We need to be sure that our diffusion sources are truly limiting before it would seem worth trying something else, and then we should try to concoct a proper collimated source. Those are much cheaper and easier alternatives, it seems to me, and cheap and easy are objectives given that people will probably have to build their own.

Rick "back in travel mode and having to delay further progress for a while" Denney