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"4-crop" rule instead of rule of 500 for pin point stars

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#1 Fred76



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Posted 18 September 2020 - 10:16 AM


I introduced the NPF rule 10 years ago to get pin point stars for your nightscapes (landscape photographies with starry sky as background, without tracking device). The rule is precise but quite difficult to calculate, though it is now implemented in several apps, like PhotoPills, PlanIt!... Many of you asked a simpler formula, maybe less precise than the NPF rule, but better than the 500 rule.

I did it !

The simplest quite precise rule I found is what I call the "4-crop rule".

It is :

t = 100*(4-crop)/f

Where :
  • crop : is your sensors crop factor
  • f : is your lens focal length in mm
Crop factor is approximately :
  • 1.0 for Full Frames
  • 1.6 for Canon APS-C
  • 1.5 for other APS-C
  • 2.0 for m4/3

Say you have a Canon APS-C camera and want to use a 18 mm lens. The exposure time will be :

t = 100*(4-1.6)/18 = 13 seconds

This simple formula is quite good with f/2.8 lenses and this is the most common aperture for nightscapes.

The formula slightly overestimates the exposure time for wider apertures (at f/1.8 you can reduce the time by about 25%).

This table compares the exposure time calculated for 3 focal lengths (14, 24 and 50 mm) and 3 apertures (f/1.8, f/2.8 and f/4.5) for the m4/3, APS-C Nikon, APS-C Canon and Full Frame sensors. The comparison is made against the result of the simplified NPF rule, for the 4-crop rule (top), and the rule of 500 (bottom).


Remember the 500 rule : t=500/(crop*f)

Have a clear sky !


PS : more (but in french, though the online calc is french/english) http://sahavre.fr/wp/regle-npf-rule/

Edited by Fred76, 18 September 2020 - 02:06 PM.

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#2 Craig H

Craig H

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Posted 18 September 2020 - 12:29 PM



I have found the 500 rule to not be very good, thank you for this update.



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#3 limeyx



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Posted 18 September 2020 - 02:02 PM

Wow, thanks for this! I have used the original formula before I got my star tracker and it worked well for me, so thank you !  - I never expected to be able to converse with the person who actually came up with this rule !

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#4 BobE102330


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Posted 18 September 2020 - 09:09 PM

FWIW, with a Canon APSC and 16 mm lens I used someone's online calculator that suggested 11 seconds maximum for whatever the default eccentricity was.  I was happy with that until I started guided deep sky imaging and found out how round stars look.  I now limit exposures to 8 seconds for acceptable roundness. The 500 rule worked OK until pixels started getting smaller.  It's worth playing around when the moon is up to see what exposure length works for your camera and lens combo.  

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