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Forum: Pentax SLR Lens Discussion
08-14-2017, 04:21 AM
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You need to go back to basics.
Depth of field is based upon the definition of what is acceptably sharp. This all goes back to the film era where the common print format was an 8" X 10" print.
Acceptably sharp was considered when a point source still appeared to be less than 1/100 of an inch. When looking at a 35mm negative blown up to fill the 8" X 10" format.
It all relates to this definition, and to the amount of magnification in your processing. If you enlarge beyond the 8x10 print size depth of field goes down.
Overall you need to consider the full magnification subject through lens, to sensor then enlarged to print.
Play with any of the DOF calculators on line and you will see the relationships form.
Subject distance , lens and sensor size are all in the equation, (but they seem to always omit the print size) but if you brake it down, you will soon see that in the end focal length, distance and sensor size (assuming you fill the sensor with the image) really only come back to the magnification of the system.
What depth of field calculators do not do, is give you any idea of the rendering of the out of focus area or bokeh. This is far more important that depth of field on its own, and generally is "nicer" with longer focal lengths, because the out of focus points appear as big blobs, compared to small circles (or what ever shape your aperture is)
Also consider when shooting portraits, it is as important to control the subject to background distance and background pattern as it is to control depth of field
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