Do you have to account for crop factor in the NPF rule?

Question

The NPF rule is somewhat more "advanced" and more accurate than the 600 or 500 rule for astrophotography - both the latter tend to still produce star trails as camera resolution increased in recent years.
The NPF rule is

Shutter speed = (35* Aperture + 30* pixel pitch) / focal lenght

Shutter speed is in seconds, Aperture in f-Stops (you can just insert the value, e.g. 2.8 for f/2.8), pixel pitch (the distance between two pixels on the camera sensor) in µm and focal lenght in mm.
In the 500 or 600 rule, you have to multiply the focal lenghts by the crop factor of your camera (e.g. 1.5 for APS-C). For the NPF rule, I found no information whether this is necessary.
I assume it is not necessary because for a full frame camera with 24mp, the pixel pitch is larger than for a 24mp APS-C camera, which could possibly account for the crop factor.
So: Do I have to multiply the focal lenght by my camera's crop factor in the NPF rule?

Steven Kersting · Accepted Answer

No, you do not have to account for crop factor with the NPF rule.
Accounting for crop factors with the other rules is a method of accounting for movement across sensor area due to apparent increased magnification (image circle crop). Instead the NPF rule accounts for movement across individual pixels based on their size; which is exactly why it is considered to be more accurate/advanced.

Michael C · Answer

When you are pixel peeping, the only things that matter are the size of the camera's pixel pitch, the size of your monitor's pixel pitch, and if your eyes are good enough to discriminate a single pixel on your monitor at the distance from which you are viewing the monitor.
All "rules of thumb" based on standard viewing conditions, e.g. an 8x10 inch print viewed by a person with 20/20 vision from a distance of 12 inches, go out the window when enlarging the image, or a portion thereof, to such large ratios. When one is looking at, say, a 24 MP image on a 24" monitor with a pixel pitch of around 96 ppi one is looking at a portion of an approximately 60 x 40 inch display size!

Do you have to account for crop factor in the NPF rule?

2 Answers

Add your own answers!

Ask a Question