Macro photography - Part 13: Depth of field and reproduction ratios are strongly correlated.

Rose weevil on wild rose flower.  Nikon 7200, 105 macro, f/45 @ 1/250 and ISO 200 

We know that depth of field is affected by numerous factors including focal length, aperture, and point of focus (there are others as well, but these are the ones the photographer has direct control over in camera).  We also know that reproduction ratios correlate to depth of field in that the greater the magnification, the less depth of field there is.  What's cool though is that at any magnification, regardless of what lens is involved, and regardless of what accessories are used with it, depth of field is a constant for that magnification or reproduction ratio (given a constant aperture between all values).

Let's look at three different macro lenses; a 60 mm, 105 mm, and 200 mm macro.  If each lens is focused on a subject giving the exact same field of view (same area at point of focus), the reproduction ratio for each lens would be the same.  The depth of field is also the same.  What is different is the compressive or distortive view of the subject due to the focal length.  For each lens, a 1:1 reproduction ratio will result in different subject-focal plane distances, but the resulting depth of field will be the same in each case.

The pros and cons of each lens has nothing to do with the depth of field it can produce at any given reproduction ratio.  Shorter focal length lenses are less expensive but have less working distance between the end of the lens and subject.  The reverse is true for longer focal length lenses.  Another difference is the effect extension tubes have on the image with shorter focal length lenses producing greater reproduction ratios relative to longer ones for any given length of extension. 

This doesn't have any real practical value, but it is helpful in understanding the relationship between reproduction ratio and depth of field.  None of the lenses will produce a superior depth of field over the other.  Extension tubes, closeup filters, and other such add-ons will have the exact same impact as long as the reproduction ratio is the same.  In searching for equipment then, it is best to consider the desirable advantages for individual circumstances.

Thanks for reading.

Eric Svendsen     www.ericspix.com

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