Colors need to be accurately rendered. Lighting needs to be evenly distributed. Reflections need to be minimized if not totally eliminated. Whites have to be whites and blacks have to be blacks.
I made a set of photographs of some of the art of Frannie Schwab using a full frame digital Canon 5D with a 135 mm lens set to f/8 and ISO 100. Why 135mm? Because the farther back I got from the picture the less problematic the “family of angles” associated with reflections from the flashes that were placed near the edges of the paintings. The shutter was set to 1/100 sec, but anything from 1/160 on down would have worked fine. What really mattered was that the shutter curtain was synched with the flash. Since the duration of the strobe is what really controlled the duration of the light hitting the sensor, the real variable that mattered was the f stop. So it only took a few test shots to see that the histogram was where I wanted it to be at these settings.
Here’s the final result of one of the images.
This was a re-shoot of the same art I photographed a number of weeks earlier.
I thought I had corrected for reflections by putting a telephoto lens on the camera and shooting from 15 feet away. Setting up two flashes in umbrellas placed on either side of the painting should have been enough to eliminate the glare when shooting at such a distance from the subject. But we discovered a problem when we enlarged the image to 16x20 inches. There was still too much reflection off the varnished surface of the oil painting.
For the reshoot, I attached a polarizing filter to the lens in an effort to kill the glare. The polarizer certainly altered the reflections, as is evidenced by what happened to the light reflecting off the gold frame. However, the filter accentuated a drop-off of light across the plane of the picture. Adding a third flash corrected this to a large extent.
Here’s a shot of the setup, showing the positioning of the three umbrellas.
I would be remiss if I didn’t mention color calibration. I shoot a frame with a Color Checker Passport by X-rite placed in it. With its accompanying software I’m able to assure that the colors I ultimately output are as true as possible to the original art.
Here’s an image of the Passport at work.
If any of my readers have had experience photographing oil paintings, I'd love to hear from them. There's scant little information out there about the nuances of the craft.