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 06:50:45 -0700
On May 20, 2013, at 8:02 PM, Iliah Borg <email@hidden> wrote:
> BTW, shooting targets filling the frame with the target may be not the best approach.
This is most emphatically the case, especially with shorter focal lengths.
Since I put up that post that started this thread, I've figured out where my problems with specular reflections on the target were coming from.
My worst problems came when I was shooting with a 24mm lens (and mostly filling the frame with the target). I was using a standard 45°/0 geometry but still getting lots of specular highlights. It was through experimentation that I figured out the geometry that I described in the primer -- an offset 45°/0. After I wrote the document and before I had done enough research to know the standard notation for spectrophotometer geometries, I had tentatively decided that a 0/45° geometry might work better and was thinking of making a new chart as a parallelogram that would correct for the perspective distortion.
But a bit more playing around with an (on-screen) protractor finally helped me figure out what the real source of the problem was.
A normal lens has more than a 45° angle of view on the diagonal. Even with a perfect 45°/0 setup, there are going to be specular reflections on glossy surfaces from the lights within its field of view. My shots with the 24mm lens had very prominent hotspots corresponding with each light -- and those were even on matte surfaces.
I need to do some more practical investigation, probably by photographing a bunch of ball bearings, to get an intuitive real-world grip on the problem...but the short version is that the longer the focal length, the better. If I had the space, I'd only use my 180mm macro from here on out. In practice, I think 70mm is probably okay...but I haven't done those experiments yet.
Or, you could always build a giant integrating sphere and use that to illuminate the subject....
Note that, because of the way the geometry works out, the distance to the subject is irrelevant; only the field of view matters (and thus focal length). The distance only controls how much of the subject fills the frame, but the specular reflections will always appear in the same spot in the frame, regardless of where that falls on the subject. (Assuming, of course, fixed 45°/0 geometry and all you're doing is moving the camera on the central axis.)
I'm still planning on building profiles for wide lenses that I use in the studio, especially the 24. I'll just use the central portion of the image and crop heavily. How much to crop...I'll know after I play around with some BBs. Note that you'll want a background that doesn't throw specular reflections into the lens causing flare.
Cheers,
b&
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