Friday, July 29, 2011

Photographing Paintings

UPDATE August 11: I just got my "First Studio" lighting kit from B&H Photo and am getting it set up. I'll write a post about what happens. (I'm never quite sure what will happen when I start tangling with things that attach to the power grid.)

The faint of heart need go no farther. Here's a treatise by Robert Ward from a DPChallenge forum on the how to set up your equipment to photograph paintings. It's too long to tack onto the Digitizing Paintings post (which I've added more info to tonight as well), so I'm giving it a post of it's own.

Here goes:
We used to record exhibitions for major museums in California, in San Diego, Los Angeles, and San Francisco. This was back in the 70's. The work was done with large-format cameras on 4x5 film.

The key question here is, "How many of these do you have to do?" In other words, is it worth your while to engineer a setup for repeatable results?

As others have noted in earlier posts, the most important thing is to have the sensor plane (back of the camera, roughly) parallel to the image plane. I'd add to that; the axis of the lens, projected forward, should point to the geometrical center of the painting being reproduced. By doiung these things, you remove distortion from the equation; no "keystoning" in either the vertical or horizontal direction.

The easiest way to accomplish this is to hang the art on a vertical wall and square your tripod up so that the center column is a true vertical. Ideally you'd draw a vertical line on the wall and extend that at right angles on the floor to our past where the tripod will be standing. Then you'd attach a plumb bob to your center column and be sure that the tripod was "directly in front of" the line on the wall. With a dSLR a small horizontal shift will be required if the lens is not aligned with the tripod screw, which they typically are not. If the lens were offset by 3/4 of an inch to the left, then you'd want to plumb to a point 3/4 of an inch to the right of the line on the floor.

Then you'd set the camera up at eye level and measure the height of the optical axis of the lens above the floor. You'd measure up along the line on the wall and mark that height ohnt he wall, and use a level to draw a true horizontal on the wall passing through that point and extending far enough on each side to be wider than the largest painting you will photograph. You'd use a high-quality zoom lens (this would be easiest, for photographing art of wildly varying sizes) and set up so the tripod were at a position to cover the largest painting you're gonna shoot at the wider end of the zoom. For smaller painings, you'd zoom in.

If you need to shoot vertical and horizontal paintings both, it's best to do them as two separate groups, because rotating the tripod head to portrait orientation will move the offset of the lens sufficiently to one side to require repositioning the tripod, plumbing it to a different offset from the line.

Anyway, using this setup you can position paintings on the wall so they are centered on the intersection that defines the projected optical center of the lens, and you never need to adjust the tripod settings, just the zoom. It's also desirable to have a "quick change" system for hanging paintings on the wall. An acceptable approach is to use a true "artist's easel" (the kind with two parallel vertical bars and adjustable "tray" to hold the painting and clamp to snug it in place) as long as the easel can be set up to a true vertical. A better approach would be to replicate the easel directly on the wall, but that's a lot of work.

When photographing exhibitions in museums, we had a system for measuring the degree-off-vertical a painting was hanging at (some leaned out from the wall a tad) and transfering that angular deviation to the fore/aft tilt on the tripod, but this gets complicated.

Your next issue is lighting. It's generally better to shoot art with floodlights than with strobes, because you can more easily see the results you're getting. Your standard setup would be to draw on the floor two lines coming out at 45-degree andles from where the tripod-axis line intersects the wall. You'd position 4 floodlights, one high and one low on each axis, measuring the same distance out from the wall along each axis. You'd then use an incident light meter to measure the light falling at the center and on each corner of the largest painting you had to shoot, and angle your lights so the measured illumination was even across the entire surface of the painting. If it's even on the largest painting, it will be even on any smaller painting subsequently hung in that space.

You now have a geometrically consistent setup and a constant light level across the entire working space. All you have to do is move art on and off the wall, shooting each piece as you go. Use a cable release, and ideally mirror lockup as well. You should also photograph a "color target" (available at camera stores and online) under your controlled setup. You will process the color target image to eliminate any color cast and add slight exposure corrections needed, and then set up an action you can use to batch process all the other images to correct color balance and exposure.

That, in a nutshell, is a bare-bones "professional" approach to copying largish art that's too big to fit within the confines of a copy stand.

Thank you, Robert!

No comments: