How To Calculate Exposure Compensation For Large Format Macro Photography
In the second article in the Large Format Macro Photography series, I walked you through how to calculate the required bellows extension for any lens and magnification ratio and in this article, I am going to cover the steps for how you can calculate exposure compensation with the extended bellows.
In the first article, I shared how you can calculate the near focus limit for any of your shorter standard lenses.
Let's jump right in and get to the point.
Building off of the example I used in the second article, I wanted to achieve 2:1 (2X) magnification using a 150mm lens. I determined that I would need 450mm of bellows extension. If you need a refresher on how to calculate the required bellows extension, please review that article.
The last piece of the puzzle after determining the magnification ratio and bellows extension is to calculate your exposure time required for the extra bellows extension.
Here is my shortcut...
New exposure = base exposure x exposure compensation factor + filter compensation + correction for reciprocity if applicable.
Let's walk through a scenario so you know how all of this works.
Let's stay with our example of using a 150mm lens for our new macro exposure with a 2:1 magnification. This scenario requires 450mm of bellows extension and let's say that when we metered our subject, we decided we needed a 2-second exposure before any additional adjustments.
B = bellows, F = focal length
Exposure Compensation = (B / F) squared
Compensation = (450 / 150) squared or 3^2
Exposure Factor = 3 x 3 or 9 seconds exposure factor (3 stops)
New exposure time = 9 (compensation factor) x 2 (base exposure) = 18 seconds
Note: based on the film you are using, now lookup the reciprocity failure for 18 seconds and adjust as needed and that is all there is to it!
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For 2 to 1 (2:1) magnification for any lens, your bellows draw will be three times the focal length and the exposure compensation for this will be 3 stops.
For example, let's assume I want to use my 180mm Nikkor lens on my 4x5 view camera for some 1:1 close-up flower photos. I know my bellows extension will need to be 2X 180mm (360mm) and my exposure will be 2 additional stops plus what I metered, plus any filters. I will then apply reciprocity failure times for my film.
New exposure = base exposure x exposure compensation factor + filter compensation and then a correction for reciprocity if applicable.
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I have always used the dial light meter calculator. It doesn't require any math and is fast. Using the f# dial, align the focal length in inches with 1 second. Measure the distance in inches from the center of the lens to the focal plane. On the f scale read factor that is adjacent to the total extension. No math required. Then deal with the reciprocity.
Reciprocity on modern films i.e. E100, T-Max and Portra is small since the emulsions are doped to efficiently capture and hold electrons. This is done with no sacrifice in other film characteristics. Film with high reciprocity factors expose the grains and then the electrons are lost, the grains fail. Kodak and Fuji implemented this in the 1980s.
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