A so-called “normal” lens refers to a lens with roughly the same angle of view and perspective as seen by your eyes. A “wide-angle” lens produces an image with a wider field of view or more distant image, a long (or telephoto) lens has a narrower field-of-view and produces a more close-up image. The focal length lenses below use a 35mm full-frame camera’s equivalents. The 4 examples below show a 30” x 48” smooth canvas that has very fine (i.e. flat) brush strokes, but is painted with a very glossy topcoat. NOTE: The focal length lenses below use a 35mm camera’s equivalents compared to our larger cameras lenses.
Example #1: 24mm
Very wide-angle lens at a distance of 5’. This lens would only be used by a vertical copy stand which is limited by the height of the ceiling.
Example #2: 50mm
This “normal” lens taken at a distance of 8.5’. This heavily varnished canvas still causes fits unless cross-polarization is used.
Example #3: 135mm
As long lenses go, this one isn't much, but has solved many of the problems at a distance of 15’. But some photoshop work remains for a willing masochist.
Example #4: 250mm
This very-long lens taken at a distance of 25’. This lens has solved 99% of the problems reflections. Very minor touch-up is needed.
Longer Lenses are Better
One can see that the specular reflection problems with this painting go away as we use longer and longer lenses. Why is this? Because the angle of incidence equals the angle of reflection as shown below. With a totally smooth surface, like a glossy photograph, the reflection of the lights are reflected harmlessly away from the lens. Canvas, however, is not flat but bumpy, and if the canvas has been painted with a glossy varnish, specular reflections occur in constantly varying angles as below. This also accounts for the fact that the reflections become more intense toward the sides of the painting. The further we are away from the artwork, the narrower the field of view, and the reflections no longer can be seen.
Solution #2: Double (Cross) Polarization
The reality is that a large percentage of the artwork we deal with comes in with such severe reflection problems that we need to turn to this process which eliminates ALL reflections. It requires large polarized acetate filters over the light source as well as a glass polarizing filter on the camera lens. The camera filter is rotated to eliminate all or most of the specular “kicks”. We prefer doing it by eye as you sometimes want to preserve a small amount of these reflections as to remove them all can produce a flat image.
If it works so well, why not use cross polarization all the time?
The process reduces the available light to the digital sensor by around 85%, making for some very long scans and a potentially noisy image. It also changes color, which requires the creation of a custom profile to correct these changes. But because the color changes are dramatic, this profile has to “work” a lot harder to correct them and some of the color changes that result are, to an artist’s eye, not acceptable. To fix these requires masking and time.
The good news is that most of our regular artists postpone varnishing their works until AFTER we've scanned them.