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Minimum Focal Distance Experiment Findings


In this thread:

The question came up about minimum focal distance. So I decided to do an experiment. I used my T6S and EF 50/1.8 II. I picked this lens since it is a prime, it removes a lot of variables and it also has a "hard stop" for minimum focus.

I set up a 45 degree card and put the lens in manual focus mode. The first image was a bust:


I had looked online for the minimum focus distance and found 13.75 inches. After I finished this series I realized I had an older lens, and the online specs did not apply. Doh! I looked up the correct specs and found the MFD was 18 inches. Here is 18 inches at f/1.8:


Ah, much better. I switched to f/16 and took another image:


As you can see, the area in front of the minimum focus distance is following the standard depth of field you would expect at f/16.

The conclusion is that MFD is simply a mechanical stop of the lens, and the standard DOF rules apply, even when focused at the mfd.



I get what you are saying, and appreciate the experiment and the effort you made to go there.
One of the things I have always stated is that DoF is impacted by distance to the subject, aperture and focal length.  Essentially, I think we are expressing the same thing in two ways, so it's down to semantics.  As I understand it you are suggesting it is purely a Depth of Field issue, whereas I look at it as a set value that is defined within the lens specs that are themselves dependent on the focal length, and the structure of the elements that make up the lens optical path. 

Thus two lenses of the same focal length may have a minimum focus distance that is different and even a different DoF profile (e.g. a Canon EF 100mm USM macro has a MFD of 0.31m, while a non-macro EF 100mm f/2 lens has a MFD of 0.9m!)   

I think we arrive at  the same point, however I use a different definition of MFD than you that embraces the structure of the optics as demonstrated, but in the end I think we both come to the conclusion that when something gets close enough, the lens will not longer project it to the sensor as a viable image.  If I misunderstand your position, I hope you will enlighten me.

cheers, TREVOR

"The Amount of Misery expands to fill the space available"
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris


I do think we are saying the same thing, but that is not what the diagram you posted shows. I am simply saying that you can get acceptable focus at distances *shorter* than the MFD if your aperture number is high enough.

As for different lenses having different MFD's it is not so much the optics as the physical design of the lens. A macro lens *has* to have a short MFD since for a given focal length the magnification is a function of distance, so a macro lens has to rack way out. (Think extension tube). Other lenses have different requirements so the MFD is relaxed as a constraint.