After much fun and games trying to photograph my painting, including outside on an overcast day and in the studio with my desk lamps, and failing to achieve good results, I decided to brush up on my photography skills and invest in some new equipment.
The main issue I had was how to eliminate glare, which ruined the fine detail across the painting. After much research I found the solution: cross-polarisation photography.
This process works by filtering the light from a light source so that the waves light vibrate in only one direction. The polarised light bounces off the painting into the camera lens via another polarisation filter, which rotates the light waves 90 degrees to extinguish all reflected light, thereby eliminating glare.
First I needed to buy some new equipment: a couple of entry-level continuous tungsten lamps, lighting stands, a 50mm prime lens and linear polarisation filter for my DSLR camera, and a sheet of linear polarisation filter for the lights.
You can use any powerful lamps (300w minimum), and I looked at work lamps and red tops as cheaper alternatives, but I wanted a compact kit with a bag for easy storage, which I could also use for other studio photography work. These Interfits were £200 on Amazon including stands and umbrellas.
I needed a prime lens because entry-level zooms aren't as sharp, tend to distort flat images and have a small aperture range. So I bought the Canon EF 50mm 1.8 STM, a classic budget prime lens, with a Hoya PL49 linear polarisation filter, for about £120. You can use a circular polarisation filter, but the effect is apparently not as great.
The filter for the lights was a bit tricker to get hold of. It's easy and cheap to buy online in the US, but they wouldn't ship to the UK, and for some reason in the UK it costs 3 times the price. I finally managed to buy some reasonably priced from Israel via eBay. The quality wasn't amazing, it was a bit scratched, but it did work, and cost around £30.
As you can see below, I set the lights up at a shallow angle to the painting, with the framed polarisation filters clipped to the stands in front of them. I had to be careful not to put the gels too close, as the lights get very hot and they did start to blister at one point, so I kept them switched off when I wasn't shooting. You could of course use strobes instead which wouldn't have this problem. but they're much more expensive.
As the filters dim the lights considerably, a long exposure is required so a camera tripod is essential. I tethered my camera to Adobe Lightroom, which you can then use to trigger the shots and reduce all camera shake. I shot at night and switched off all other lights, to avoid light contamination, and used RAW / ISO100 / 4 seconds / F18, with a custom white balance using a grey card.
Some cameras have a problem auto-focusing with a linear polarisation filter, but my Canon EOS 100D worked fine. Once the background lights are off and the spots are on, you turn the camera filter round clockwise, and as if by magic the glare disappears.
The process worked brilliantly for me and completely eliminated the glare, however there was one issue: I used gold paint for the coat detailing, and the polarisation filter killed the colour dead. I got round this by taking further pictures without the filters, and composited the images in Photoshop.
The other thing which really helped was shooting RAW and editing in Adobe Lightroom. With the painting set up next to my monitor I found it much easier than I would have done using Photoshop alone to edit the photos to match the original image as faithfully as possible.
A costly exercise to be sure, but well worth the time and money, and I'd highly recommend for photographing your artwork on a budget.