Assuming your copy setup can hold the paintings in a fixed plane easily and reliably, you can substitute a mirror for the painting and fiddle around with the camera position until you see the camera lens centered on the ground glass. That will get your film plane and subject plane about as parallel as possible.
I second the recommendation for cross-polarized lights for oils and acrylics. My first attempt with that was very successful -- until the gels in front of the lights started to melt. Sort of argues for working fast! I've lately been using softboxes and daylight color temperature compact fluorescents. That takes care of the heat, but I don't have (and wonder if I can afford) polarizing gels big enough. Most of this sort of work I do is as a volunteer for an art club I belong to -- hard to justify buying hundreds of bucks worth of equipment for non-paying work, especially when I have little other use for it. (Note to self: "Buy lottery tickets ....")
I bought my polarizing gels from Calumet, but I'm sure most major suppliers carry them. For the frames to hold them, I had a frame shop make me a couple of wooden frames, held together with wing nuts. Yes, it is important to mark the top of each frame so that they are both oriented correctly.
level the camera on all 3 axis', level the painting on all 3 axis', then measure the distance from the top right corner of the painting to the top right corner of the front standard (assuming you are using LF, 4X5 presumably). measure the distances from each of the corners, and they should be all the same. use a color checker chart (if you have one, but at least a grey card). use a slightly longish lens (say 210-240mm on 4x5), preferably a macro lens, due to the flat field. I've used an APO Rodagon on 4x5 before with EPY 64t 4X5, and it worked out totally fine. I shot a sheet with my 210(non macro), and the Rodagon looked much better IMO, much more accurate!
Do as all the others have recommended, and use lighting from both sides, and depending on the size, I prefer to use 4 lights minimum (on paintings smaller that 16x20), and at least 6-8 on larger ones. I don't do this for a living by a long shot, but I have some friends who paint regularly, and its a nice way to make some extra cash .
Many prefer to use continuous lighting (even the work clamp lights that you can get at Home Depot, and then diffuse them with some gels. I use up to 8 of them, and i point them at the opposite corner of the painting (so the light on the bottom right points at the top left corner of the painting, and the such. All the lights are at the same distance from the painting, and each is measured individually, one at a time.
Its a meticulous process, and usually not all that "fun", but it can be a rewarding experience to see someone's art entrusted in YOUR hands to reproduce it faithfully. Just put on some good music while you're doing it, and it won't get that boring .
If its possible, shoot a polaroid, or better yet, shoot a test sheet, process it(or get it processed), and then examine it for accuracy.
If no tungsten film is available, then I'd recommend the Fuji Astia, Provia, or preferably, the Kodak E100G, or ektachrome 64(EPR). of course, since these are all daylight balanced films, you will need to compensate accordingly with filtration.
Let me add one other thought: when reproducing some art for a colleague, I included a colour sep chart in the slide. You could incorporate it into the actual frame of your photograph... or you could take it separately, giving it the identical exposure that you give the artwork. Then you have the sep chart to guide any white balance amendments that are needed, if necessary.
Rosco makes polarized gels in sheets and rolls. Sheets are about 20x20" and are not too expensive. Any large professional photo retailer will carry them. Be sure not to get them too close to a hot modeling light or tungsten source, they will melt or burn through.
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Originally Posted by pgomena
Alas, I guess it depends on the definition of too expensive. A 17 x 20 inch sheet which might work over my softboxes appears to be around $40 -- and I would need two. The 17 inch by ten foot rolls are around $200. My original attempt used smaller gels that were $14 apiece and nearly destroyed on first attempted use even though spaced an inch or two from the glass of a pair of halogen worklights. I'd love to be set up with some pols, but since it affects about five shots of the several thousand I take each year, it's pretty low priority in my retirement budget.
Some of it depends on use/intent; for low-res display on a website, I can usually fake my way through with the softbox diffusion, attention to light angles and a polarizer on the camera, but I know it doesn't achieve "fine art reproduction" standards.
Dave ("Cheapo") T
One thing not mentioned here is that some dye's can have a diferent color on the photo than seen by the eye.
You can get to a point where you photographed 5 colors perfectly and one is off: don't wory, it can happen and you can't do anything to it.
That's why the color-chart is so important, just at the edge of your photo, outside the real painting.
Good shooting !!!!!