A simple perspective question

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I shot a photograph with a wide-angle lens of a magnificent theatre interior, from center of back balcony, with every seat occupied. Audience looks fine, theatre looks fine, but actors on stage are blurred, as would be expected with 20-second exposure. Client wants me to pose actors on stage, having them hold still for the 20-second exposure (don't worry, it's all standing and sitting--no ballet!). The client wants to digitally combine that stage(d) picture with the wide-angle, full-house photograph and use it for publicity purposes.

Digital controversies aside, if I use a long-focal-length lens for the stage shot so that I fill my frame with the stage and the "wings" (which are ornately decorated), will the perspective of that photograph seamlessly match the center of my wide-angle photograph? I've always believed that it will, but my lab guy says no. Remember, camera position is unchanged, and both pictures are on the same axis; the long-lens photograph will be the exact center of the wide-angle photograph. Hmmm?

-- Simon Gammelin (fourthpres@aol.com), May 25, 1999


I'd do what your lab guy says. Putting theory aside, whenever doing any sort of special effect or composite shots, if you want it to look seamless you should duplicate as many variables as you can. In addition to using the same lens, film, and camera, from the identical spot, I would take effort to use lighting that is consistent between the two shots.

-- Howard Slavitt (nverdesoto@earthlink.net), May 25, 1999.

Sorry if I didn't make myself clear. That I'll use the same camera position, film, etc. is a given. My question is solely about perspective: if camera position is constant and rectilinear lenses are used, does perspective remain constant (with the only change being field of view)?

-- Simon (fourthpres@aol.com), May 25, 1999.

If you use the same camera position and orientation, your two photographs will match seamlessly provided that you correct for the different magnification of your two lenses. If you keep the same position but change the orientation, add a projective transformation.

-- Quang-Tuan Luong (luong@ai.sri.com), May 26, 1999.

Yes, assuming all else is the same, no matter what lens length you use if the camera position remains the same you'll be able match with minimum fuss, even if you switch to 35mm. Having shot photos like this I would also recommend that you light the cast with studio power level strobes rather than hope everyone stays exactly still for 20 seconds. place your units where the stage lights are in the wings and gel them as if they are stage lights.

-- Ellis Vener (evphoto@insync.net), May 26, 1999.

It's probably obvious, but I'll say it anyway: you also have to ensure the film plane stays at the same angle. You may find setting up easier if you set up the camera with the same lens that you used for the first shot, and compare the GGS with the first photo. Then you can change the lens, if you wish, for the shoot. The only transformation required for the second picture will be a linear shrink.

However, one point to remember is that real-world lenses are not exactly rectilinear. Even if a lens only suffers 0.1% barrel or pincushion, this is enough to upset the digital merge. Shooting with the same lens will eliminate this problem.

The strobes sound like a good idea, but ensuring a match on the ornate wings might be hard. On the other hand, a digital merge can cover all kinds of defects.

-- Alan Gibson (Alan.Gibson@technologist.com), May 26, 1999.

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