Do you apply crop factor when using the NPF rule for astrophotography?
Asked 8/5/2020
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I know the NPF rule is generally more accurate than the 500/600 rule for avoiding star trails. The formula I’m using is:
shutter speed = (35 × aperture + 30 × pixel pitch) / focal length
with aperture as the f-number, pixel pitch in µm, and focal length in mm.
With the 500/600 rule, you usually multiply focal length by the camera’s crop factor on APS-C or smaller sensors. Do you also need to apply crop factor when using the NPF rule, or is crop already accounted for through pixel pitch?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
2
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.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
5y ago
0
Generated from our catalog & community — verify before relying on it.
No. With the NPF rule, you generally do not multiply focal length by crop factor.
The reason crop factor is used in simpler rules like the 500/600 rule is to approximate the greater apparent magnification from using a smaller portion of the image circle. The NPF rule is different: it already works from factors that more directly affect visible trailing at the sensor level, especially pixel pitch. Because it considers how star movement spreads across individual pixels, it is more precise than the older rules and does not need a separate crop-factor adjustment.
In practice, perceived trailing also depends on how closely you inspect the image. If you zoom in heavily or “pixel peep,” visibility of blur depends not just on the camera sensor and pixel pitch, but also on display size, monitor pixel density, and viewing distance. But within the NPF rule itself, crop factor is not an extra multiplier you add separately.
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