Major confusion NPF rule vs 500 rule

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OutsideTheMatrix Veteran Member • Posts: 9,038
Major confusion NPF rule vs 500 rule

Since I mostly do untracked astrophotography on a normal tripod, I've been using both the 500 rule and the NPF rule and for some reason the NPF rule sometimes brings about weird results. Especially with camera sensors smaller than M43 it sometimes shows longer exposures before star trailing occurs which goes against logic.

I used this-- but for sensors smaller than M43, I do the calculation directly, using this:

The NPF Rule

A more complex formula for calculating shutter speed at night is called the NPF rule. Here’s the formula:

  • t = Recommended shutter speed
  • k = Multiplication factor
  • N = F-number
  • f = Lens focal length (millimeters)
  • p = Pixel pitch (micrometers)
  • δ = Minimum declination


that formula is a bear though, so I rounded 16.856 to 16.9, 0.0997 to 0.1 and 13.713 to 13.7 lol.

Anyway the part confuses me is when I input the variables directly, should I use actual lens focal length or equivalent? What about for F-number? If I use the actual focal length for the lens shouldn't I also use the actual F-number or equivalent?

I did it both ways and got some very weird numbers-- I calculated for the Nikon P900/950 at both 600mm EFL and 2000mm EFL using the actual focal length and also using the EFL. Nikon P950mm real focal length at 600mm EFL is 107mm and max f-number is f/5. At 2000mm EFL it's 357mm and f/6.5 respectively. At the former I got numbers slightly over 1 second (at 600mm EFL) and at the latter I got numbers of just less than half a second (at 2000mm EFL). I ran the numbers for the Oly 75-300mm lens at the long end using the EM10 Mk2 and came up with around 0.65 sec shutter speed, which makes a lot more sense. Makes me wonder why the numbers for the P900/950 were so far off. That was using the real focal length. Using the effective focal length instead the numbers were more reasonable for the P900/950-- just less than half a second at 600mm EFL and around 1/6th of a second at 200mm EFL. I still used the same f-numbers as listed in the EXIF though. But at the wide end of the lens, the numbers using effective focal length are ridiculous (like 2.8 seconds at 24mm EFL) vs using real focal length where they are much more reasonable (like around 15.4 seconds). I can't see why the exposure times before star trailing occurs using the NPF rule would exceed the 500 and even the 600 rule on the P900/950 using real focal length at the tele end but not at the wide end. I can see why they wouldn't ever be shorter than 1/10th of a second at the celestial equator though because of the way the formula is written. And the reason why I want to calculate streaking exposure at 2000mm is actually for taking pictures of the planets-- specifically-- Mars, Jupiter and Saturn-- I'm doing exposures between 1/10 to 1/50 of a second at 2000mm EFL and I want to know at 2000mm EFL (and even with dynamic fine zoom added, 4000mm and 8000mm) what is the longest exposure I can do and keep Mars, Jupiter and Saturn from blurring without tracking. Using something called Continuous L mode I can take 200 images in 48 seconds and then I'll stack them. I'm reducing the resolution down to 4 MP Fine to do this, but then scaling that back up with dynamic fine zoom (either 4000mm or 8000mm depending on how much zoom I use.) The slowest shutter speed allowed in this mode is 1/30th of a second (I suspect because it uses movie mode and the electronic shutter only in this mode?) But calculating the longest shutter speed I can use without blur at those focal lengths-- whether using "real" or effective focal length, has been difficult. I can see why so many prefer the 500 rule lol.

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