Re: Primer on photographic exposure, etc.
Re: Primer on photographic exposure, etc.
- Subject: Re: Primer on photographic exposure, etc.
- From: Ben Goren <email@hidden>
- Date: Tue, 21 May 2013 10:22:04 -0700
On May 21, 2013, at 9:27 AM, José Ángel Bueno García <email@hidden> wrote:
> As you might know, the angle of view is not a problem but the direction of light yes.
What it took me a while to figure out is that the two are very closely related.
A 180° fisheye will have a direct angle of view to the lights.
With specular reflections, the angle of incidence equals the angle of reflection. Imagine a standard 45°/0 geometry, and a lens with a 90° field of view. If you were photographing a mirror, you'd be able to see the light source at the edge of the field of view.
But we're rarely photographing perfectly flat, textureless features. That's even the case with matte surfaces, which are actually very, very finely textured glossy surfaces that throw specular reflections in all directions (thereby adding white light and thus decreasing saturation and increasing lightness). But unless the surface is perfectly Lambertian, there's going to be some residual specularity. And who amongst us is using Spectralon targets and imaging nothing but Spectralon? Also, the less-fine texture contributes to the problem -- think of oils heavily layered on, almost creating a bas-relief. Many of those will have at least small surfaces that will throw specular highlights almost no matter what you do. Or, imagine something with a pitted or a pebbled surface -- same problem.
In practice, a 24mm lens is unsalvageable for copy-type work in a 45°/0 geometry. I'm pretty sure I've fought the problem with a 50mm lens, but I haven't yet critically examined examples since I figured this out to know for sure. I think 70mm is "good enough." I still might see if I can figure out a setup using a 180mm lens, which should be perfect for all practical purposes...but it requires a *lot* of working distance.
I might also have had some success with increasing the angle of the lights (away from the camera axis / towards the plane of the art). Something closer to 60°/0 might be better, but I again need to do more experimentation in this matter. I'm also a bit concerned about the effects of deviating significantly from the geometry of my spectrophotometer...again, I need to do more experimentation.
Cheers,
b&
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