An eclectic selection of technology with some cookery
Trying to work out which lens in my collection is best for macro photography with extension tubes. Rather than try them I thought I’d see if I could calculate the answer. Then potentially use this to work out which lens to buy next.
There are other methods to use non-macro lenses for macro photography however in this article I’m just considering Extension tubes.
The following are the terms used in the calculations:
The measure of how a lens converges / diverges light.
Distance from the subject to a point in the lens. This is not the same a MFD.
Distance from the projected image to the to a point in the lens. This is not necessarily the same point U and depends on lens design.
To be considered “macro” this needs to be 1 or greater.
The physical length of the extension tube / tubes from flange to flange.
Magnification once a given length of extension tube has been applied to a given lens.
Size of the image projected on to the sensor of the camera
Size of the subject the camera is pointing at.
Minimum distance the subject can be from the lens, not the same as U / subject distance.
1 / f = ( 1 / U ) + ( 1 / V )
Mag = V / U
( LT / f ) + Mag= MagT
I = Mag x S
Fundamentally the smallest focal length lens will always win however minimum focus distance may also have a factor here.
For my 35mm sensor camera I can obtain a magnification factor of just over 1 using my 24mm and 28mm lenses with 26mm of extension tubes.
For my APS-C sensor camera I can obtain a factor of just over 1 with a 16mm lens and a 16mm extension tube. If however I use 26mm of tubes I can get a factor of around 1.7.
Of course 1.7 sounds better than 1 however it needs to be as a 35mm sensor is 1.5 times bigger than an APS-C. Also for a given number of pixels the 35mm sensor will have bigger pixels so will be less prone to noise.