Yes, you are right. I guess the light source needs to be sufficiently large to avoid light falloff. I was thinking of an array of white LED's behind frosted glass acting as a diffuser. I suppose if the stage was solid (no aperture) and of frosted/milk glass, the negative would have a better chance of being flat and the light could still be stationary.
Here is a link to the sketchup model.
My flickr stream
Could you save that again for a previous version, I can't open it as it's newer than my Sketchup version, 2013.
As for the fixed lamp approach, if you can get a consistent exposure and 1 or 0 points out of 255 variation across the capture zone you might get away with it. The acid test would be to create a scan with plenty of sky, stitch the images and reduce the result image size to 1" in height and look for a grid pattern artifact. That's pushing the system very hard but will reveal a weakness in the illumination method very well.
Film flatness will make or break the scan, DOF (at 1:1 at least) is that shallow. I've gotten great resolution from a Rodenstock Magnagon f/5.6 but the DOF is ~.006", less than the film's thickness requiring the emulsion be face down and runout of the travel over the film area held to very nearly nothing.
Are you planing on scanning black and white negatives, color negatives or slides? If it's the latter two, you might consider an rgb led setup.
“You often feel tired, not because you've done too much, but because you've done too little of what sparks a light in you.”
― Alexander Den Heijer, Nothing You Don't Already Know
I am planning on using a 100mm macro lens (Canon EF 100mm f/2.8 Macro USM Lens) with my Canon Rebel for starters. The specs say the lens is capable of 1:1 but I'm not sure if that is with an APS-C sensor or full frame. I'm hoping that the lens will result in minimum light falloff given that the lens is meant for a full frame sensor but will be used with an APS-C sensor. Given the testing of the lens at Photozone, it looks like f5.6 or f8 will be the best place to start for testing.
Now I'm worried about DOF if you say that it is 0.006 at 1:1. I suppose that the film and sensor need to be absolutely parallel over the capture area of the negative. How difficult was that set up? It would seem that everything is critical in alignment here and even variations in thickness of the stage material would impact this. Lots of variables to control. What gotcha's did you guys encounter?
How critical is wet scanning to this? What fluid did Dan use in his video? Does Peter also do wet scanning? It didn't look like it from his YouTube video, but that was some time ago and maybe he has moved on to that. I have not done wet scanning.
I plan on most scanning B&W negatives but may scan color negs as well. Why would the RGB light source aid in scanning color material? I'll use this almost exclusively for large format film as I use a Nikon LS-8000 for 120. If the Nikon breaks, I might use the DSLR scanner for 120 also.
My flickr stream
Some general notes, the DOF I stated was an estimate based on the specific lens and it's fixed f5/6 aperture I'm using. At f/8 and 100mm you will have a lot more in relative terms. It's such a nebulous subject it's hard to say exactly, too many factors to consider. I leave it to the experts to clarify further. One of the best ways to see what's happening I've found is to pick up a set of .002" plastic shims and cut and stagger them. Then carefully photograph the edge of the stack and see for yourself what sort of acceptable range you're getting.
I can't speak to other systems but in my it wasn't as hard as I anticipated since I made as perfect a lens mounting ring as I know how on high end machinery, the rest was maintaining extremely consistent stage thicknesses. Finally, allowing for shimming the glass carrier for final tuning with a dial indicator.
In my opinion, wet mounting is important for best results. Film curl can easily exceed DOF. I considered it a black art before I started doing it. As you saw in the video, it's very straight forward. I did get some bubbles in my first attempts but gauging the speed of laying down the film and mylar and how much fluid to use is a simple matter with a little practice and I only get bubbles on trouble film like that with bends due to sprocket hole bending or clip dents on 4x5 sheets, which I cut off now. I'm using Lumina from ScanScience. Also, let's not forget that wet mounting results in higher contrast and less spotting, scratch removal so it's worth the extra time IMO.
The RGB LED's we've been using allow to adjust the light color to establish a more neutral capture for each color film type, leaving only to invert and optimize the color channels to arrive at exceptionally good color starting points. Not doing so meant a more work trying to get to a good starting point. I'm going to be testing another LED source, noted for it's high CRI at some point.
Also, I'd worry about vignetting last as it's easily corrected in Raw processing.
I'd be remiss if I didn't mention one big 'gotcha'. Peter discovered there was an unfavorable interaction between the sensor and the LED PWM pulse frequency which resulted in a rolling banding of uneven color on the result image. This artifact is visible at faster shutter speeds and not at slower ones. Regardless, it's undesirable and the solution is to use a technique proposed by Ludvig, turn on each color for a specific duration. This allows to eliminate the PWM frequency of the color controller entirely. Duration of on time for each color determines the color balance relative to the exposure itself. That new circuit is ready to go and the code ironed out, it just needs squirting into the Scanduino code base which I don't expect to be much trouble.
I'm having a hard time visualizing how turning on each color for a specific duration works relative to PWM? Did you try different PWM frequencies? What about using a low-pass filter after the PWM output to reduce ripple or using a DAC output? How many effective bits do you need to acheive the color range? Are you using a custom LED setup of your own design or a commercial one? Are you just looking for a change in color temperture of the light source?
Sorry about the rapid-fire questions. I design and program embedded systems for a living for consumer products so I'm hoping to be able to lend a hand here. Although, given that you are designing your own circuits, I'm guessing you don't need any help.
Regards,
Rob
My flickr stream
Regarding wet-mounting, I agree with Daniel that it is needed for the best results, but the results without it can be very good. Wet-mounting eliminates Newton's rings, helps with film flatness, increases contrast, minimizes some physical problems on the negative. Note, though, that if you film is really curly, wet-mounting here might not solve the flatness issue. In a drum scan, the negative is pulled down, since as the cover sheet is pulled, it exerts downward pressure along the curved drum. In out situation the mounting platform is very flat. The cover sheet can be pulled tight, but it's like a drum skin, with a minimal amount of downward pull. In addition, the extra surfaces of the mylar sheet, which is a dust magnet, can add to the dust problems. I've used Lumina, Kami, and Prazio Oil. Lumina is my favorite.
There's much more info about our light sources thread: http://www.largeformatphotography.in...er+lightsource
“You often feel tired, not because you've done too much, but because you've done too little of what sparks a light in you.”
― Alexander Den Heijer, Nothing You Don't Already Know
Rob, let me quote your questions and answer them in the light sources thread Peter linked to.
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Originally Posted by Daniel Moore
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