I’ve been trying to understand if crop sensors can perform about as well as FF sensors for portrait photography. Assume that means we want depth of field to be as narrow as possible.

I feel like I keep arriving back at the conclusion that sensor size doesn’t matter all that much on a fundamental level (that is, ignoring factors like “FF lenses tend to be higher quality because that’s what professionals tend to use”).

Here’s the distilled logic. Please correct me if anything seems incorrect. For a given scene and composition (same distance away, same field of view):

• Total amount of incident light on a sensor depends only on pupil diameter (absolute aperture size, not f-stop).
• Depth of field depends only on pupil diameter.

So for example, let’s say we want to shoot a scene at f/2 on a FF camera with an 75mm lens. The pupil diameter will be 75mm/2=37.5mm. It seems like we can shoot the exact same scene on a 1.5x crop sensor using a 50mm lens at f/1.3, which will have the same field of view and a pupil diameter of 50mm/1.3=38.4mm. Same amount of light on the sensor, same bokeh.

The only differences I can see here would be in possible increased cost/difficulties in manufacturing an f1.3 lens for a 1.5x crop sensor vs. f/2 for a FF sensor. I don’t know enough about lens manufacturing to have a sense for this. Is it harder to make a lens with the same absolute largest aperture for one sensor size vs. another?

Does this make sense? Any obvious inaccuracies?