How do extension tubes affect depth of field on a 105mm macro lens?
Asked 8/6/2014
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I’m using a Nikon 105mm f/2.8 macro lens and want to know how adding extension tubes (for example 12mm, 20mm, or 36mm) changes close-focus distance and depth of field.
At macro distances, can I still use a normal depth-of-field calculator, or do I need a different method? I’m looking for a quick way to estimate the new DOF when extension tubes increase magnification.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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The DoF calculators won't work well here, unless they are specifically designed to handle these extremely short distances.
To calculate DoF for macro distances you can use magnification-based formula
DOF = 2*N*c*((m/P+1)/m*m)
Where N is f-number, c is circle of confusion (typically 0.025 mm or 0.001 in), m is magnification and P is pupil magnification. f-number you know, circle of confusion is above, magnification you can figure out. The problem is the P - pupil magnification. For symmetric lenses it should be 1, but your lens is most likely a telephoto construction, for which P should be less than 1, possibly 0.5. I am not sure if you can figure out P for your lens exactly.
There is a detailed article Depth of Field on Wikipedia.
It is a good idea to consider these DoF calculations only as a starting point, because there are many factors like lens construction, size of the print etc. that play important role in the final perception.
Originally by user27944. Source · Licensed CC BY-SA 4.0
user27944
12y ago
0
Generated from our catalog & community — verify before relying on it.
At macro distances, ordinary DOF calculators become unreliable unless they are specifically made for close-up/macro work. With extension tubes, the important change is increased magnification, and DOF is better estimated from magnification rather than just focal length and focus distance.
A macro DOF formula is:
DOF = 2Nc*((m/P + 1) / m²)
where:
- N = f-number
- c = circle of confusion (often about 0.025 mm)
- m = magnification
- P = pupil magnification
The hard part is P. For a symmetric lens, P is about 1, but many telephoto-style lenses have P below 1. That means exact results for your 105mm may be difficult without lens-specific data.
So your proposed method is not the best approach for macro work:
- New close-focus distance does change with extension tubes.
- Focal length does not meaningfully become a “new focal length” for DOF calculation.
- A standard DOF calculator usually won’t give accurate macro results.
The practical fast method is to estimate the new magnification from the added extension and then use a macro DOF formula or a calculator designed for macro photography. Also treat any calculated value as an approximation.
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