Originally posted by traderdrew I've been playing around with this link:
Online Depth of Field Calculator
For example, when I calculate DOF with the Nikon D7000, I have less DOF than I do with a Nikon FF with the same settings. I thought I have been reading around here that FFs have a narrower DOF. Please explain.
If you're leaving the focal length, aperture, and focusing distance the same then this is what you'd expect. This calculator is assuming that the final print size is the same, so the acceptable circle of confusion on the sensor changes (since the image on the crop sensor is magnified more to get the print than the full frame sensor, we use a smaller acceptable circle of confusion), which affects the DoF (see
The DOF equations if you'd like the full gory details).
Where these things pretty much
always go wrong is not clearly defining what parameters are staying the same, and what are changing. In the calculator you've linked to, you're changing the framing of the subject if all you change is the format. Usually, when people say "FF has shallower DoF than crop" they're implying that the framing of the subject stays the same in one of two ways:
1) Change the distance to subject to keep the framing of the subject the same. Focal length and aperture remain the same. (this method is for people with a closet of film cameras and lenses
)
or
2) Change focal length to keep FoV the same. Distance to subject and aperture stay the same. (this method is for DA lens apologists
)
In both situations, the magnification of the subjects image on the Full Frame sensor is higher than on the Crop sensor, and this more than counters the effect of the differing circles of confusion for the two formats (see the simplified equation 13 in the DoF equation link above). Try a few numbers in this calculator:
Similaar DoF and FoV calculator - www.similaar.com
Keep the focal length and aperture constant. Note the Field of View, Height/Width and the DoF. Change the format, but now change the focus distance until the Field of View, Height/Width is the same as before to represent the same framing of your subject (the perspective has changed of course).
Do a similar experiment with changing the focal length so the FoV is the same on the two formats and keep the distance the same so the Field of View, Height/Width is also the same.