BetterPhoto Q&A
Category: New Answers

Flash/Focal Length Relationship?

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January 17, 2007

Good luck!

Ray

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January 17, 2007

Your question is a good one! Maybe I should have said a good two! That’s because there are two different phenomena involved.

First is the Inverse Square Law:
When the subject is illuminated by a point source (the on camera flash), the intensity of the light reaching the subject changes with distance, and this change off is astonishingly quick. Imagine this set-up. A slide projector is projecting on a white wall; the image is a square slide. The projector is 4 feet away from the wall. With a meter you measure the brightness of the image at the wall, it reads 100 foot candles. Additionally you measure the size of the image and it is 1 ft by 1 ft.. Now you move the side projector further away from the wall, doubling the distance. It is now at the 8 feet mark. You re-focus, the image on the wall, which has enlarged and now measures 2 ft. by 2 ft. The revised image size is twice as big (2X) as before. Using the meter you find the enlarged image now reads 25 foot candles. Why? Answer: The lamp in the projector did not change however, previously it was 1 sq. ft. whereas the revised image area is now 4 sq. ft. Stated another way, by backing away twice the distance; you enlarged the area of the projected image by a factor of 4X. Now the same lamp is forced to spread out its energy over 4 sq. ft. instead of 1 sq. ft. Note this results in an image brightness change equal to ¼ of the original brightness.

Factoid: Each time you back away with your camera mounted flash, doubling the distance between light source and subject, you reduce the light brilliance at the subject plane by a factor of 4X. That’s two f/stops worth requiring you to open up, otherwise under exposure will result. Each time you move forward, halving the camera to subject distance, you increase the light energy on the subject by a factor of 4X thus you must close down two f/stops to compensate or over exposure results. One f/stop reduction at the subject is realized by a distance change calculated by multiplying subject distance by 1.4. One f/stop increase is accomplished by a distance change using a multiplier of 0.77.

Second is what happens when you zoom:
The normal prospective view for any camera is achieved when the focal length used approximately matches the diagonal measure of the chip or film. In your case this is probably 33mm. Zooming to longer, enters the realm of telephoto, shorter is wide angle. When using shorter than 33mm, you realize a much smaller head size and, because you are composing in the viewfinder, human nature abhors wasted space around the subject. You step closer to fill the viewfinder. As your distance to subject closes, the light energy at the subject plane bumps up higher than you realize. Your camera must stop down to compensate, you plumb run out of tiny apertures.

Solution: Stay back using your lens at 60mm. This will force you to step back yielding a prospective that your subject will most appreciate as it is the view he/she sees in their make-up/shaving mirror. Otherwise use a short lens and a neutral density filter on the lens (4x) or cover the flash with two or more layers of a white handkerchief.

Best of luck,
Alan Marcus
ammarcus@earthlink.net

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January 17, 2007

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January 29, 2007