Three calculations every night photographer reaches for - the longest shutter before stars trail, the stricter NPF exposure, and depth of field with the hyperfocal distance - worked out live for your exact camera, lens and sensor, entirely on your device.
The Earth turns. Leave the shutter open too long and every star becomes a streak. Two rules tell you where the line is.
The classic. Divide 500 by your effective focal length - focal length times crop factor - and you get the longest exposure, in seconds, before star trailing becomes obvious. At 24mm on full-frame that works out to about 21 seconds. It is fast, it is memorable, and it needs nothing but your lens, which is why it has earned a place in every camera bag.
Its weakness is its age. The 500 rule was tuned for an era of film grain and modest enlargements. On a modern 45-megapixel sensor, a 500-rule exposure will often show short trails the moment you zoom to full resolution.
The NPF rule is the stricter, more accurate answer for high-resolution sensors. The name is simply its three variables: N for aperture, P for pixel pitch, F for focal length. Alongside focal length it weighs your aperture and the sensor's pixel size, because how far a star can drift before it smears across a neighbouring pixel depends on exactly those things. The result is almost always a shorter exposure than the 500 rule - for that same 24mm lens it might be closer to 9 seconds. Use it when you want the stars truly sharp.
| At 24mm, full-frame | 500 rule | NPF rule |
|---|---|---|
| Longest exposure | About 21 seconds | Closer to 9 seconds |
| What it weighs | Focal length and crop factor | Focal length, aperture and pixel size |
| How stars render | Sharp at modest sizes, trailed at 100% | Pinpoint at full resolution |
| Reach for it | As a fast field estimate | When the shot has to hold up large |
Shorter exposure, same light budget. When NPF cuts your shutter time in half, make it back with a wider aperture, a higher ISO, or by stacking several frames. The rule tells you the ceiling; how you reach proper exposure under it is your call.
A great nightscape has a foreground. This calculator makes sure the rock and the stars are both sharp.
For a chosen focal length, aperture, sensor and subject distance, Almanox gives you the near and far limits of acceptable sharpness - the zone your scene has to sit inside. Focus on a boulder three metres away at 24mm and f/2.8 and you can read exactly where sharpness begins and where it falls off.
The number that matters most at night is the hyperfocal distance. Focus at or beyond it and the far limit reaches to infinity - everything from roughly half that distance out to the stars sits inside acceptable sharpness. For a wide lens at a night-friendly aperture the hyperfocal distance is usually only a few metres away, which is why the classic move is to focus on a foreground feature near that mark rather than cranking the ring to the infinity stop.
In practice: check the hyperfocal distance for your lens and aperture before you head out, find a feature at about that range, focus there once, and stop touching the ring. Foreground and Milky Way, both sharp, in one frame.
Generic tables assume a camera you do not own. These calculators run on your numbers.
Every calculation is driven by the same small panel of inputs, and every number on screen updates the instant you change one:
Pixel size is on your camera's spec sheet, or you can derive it: sensor width in millimetres, divided by horizontal pixel count, times one thousand, gives microns. Set it once for your body and the NPF figures are computed for your sensor, not an industry average.
Like everything in Almanox, the maths runs entirely on your device - no network, no lookup service, no waiting. Change the aperture on a ridge with zero signal and the numbers move with you. Photo calculators are part of Almanox Pro, alongside the Milky Way planner, the shot planner and the dark-sky finder.
Almanox is a one-time $12.99 purchase - no subscription, no ads, no account. Every calculation runs on your device.