Technique tip: close up magnification and exposure

The magnification, or scale of an object in a close up image is:

Image size/object size.

1. To determine magnification, you can measure the size of the image directly on the processed slide or negative.

2. To determine magnification before actually taking a picture, place a ruler beside the object and note how much you can see across the narrow dimension of the frame in the viewfinder.

3. Exposure compensation is required if you use extension tubes or bellows between the camera lens and body, and if the magnification is 0.25x or larer (exposure compensation is not required if you achieve a close up by putting a supplementary lens in front of a normally mounted camera lens)

4. A through the lens meter will automatically indicate correct close up exposure. If your camera does not have a through the lens meter, use magnification to figure the required exposure compensation this way:
Exposure increase factor = (magnification + 1)pwr2

5. In order to use a small f-stop (for good depth of field), change exposure time to make the compensation. To find the correct close up exposure time, multiply the shutter speed for normal exposure by the exposreu increase factor.
Example: Magnification = 0.4 x, normal exposure is 1/60 at f/16.
Exposure factor = (0.4 + 1)pwr2 = 2x.
corrected exposure = 1/60 x 2 = 2/60 = 1/30 at f/16.


More about close up lenses

- Supplementary close up lenses
- extension tubes
- bellow lenses
- macro lenses
- hints for close up photography

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