When shooting with wide angle lens and a camera on a tripod, i.e. without motorized equatorial mount, we have two choices, whether long lasting exposures to see the stars trailing in the sky, or short exposures to avoid the star trails. The NPF rule is a new way to calculate the exposure time to avoid star trails.
There are some "rules of the thumb" to calculate the exposure time, such as the "rule of 500" (or 450 or 600), taking into account the crop factor or not, the declination or not... but though these methods work sometime, they fail in several occasions.
I developped a new rule which is more consistent to modern sensors and gives results that are reliable with any sensors. I called it "the NPF rule", N standing for aperture, P for pixel size (in µm) and F for focal length (in mm) ... and delta the declination angle.
There are two formulas, the easy one that gives a quick result and correct time most of the times and a second one, a little bit more complex, but really good.
Here is the "easy" one :
And the complex one :
I developped an online tool to calculate the exposure time for many DSLRs. You just choose your DSLR among the list (406 listed from 19 brands : this list is almost exhaustive), then you enter the focal length and aperture of your lens, and some details on the field you intend to photograph (direction, elevation above horizon, your latitude).
Then you just press "Calculer" and you get the result for 9 areas of your picture so that you can choose the exposure you want, depending on the scenery you shoot :
This is in french but you can let Google translate it for you.
The link (in french) : http://www.sahavre.f...-file-d-etoiles
The link (in googlish) : https://translate.go...t-text=&act=url
The maths behind the NPF rule is explained after the online tool.
Clear sky to all !
Edited by Fred76, 09 April 2016 - 05:09 AM.