Photography Calculators

Astrophotography

Milky Way Exposure Calculator

Untracked Milky Way: shutter from the 500 rule, lens wide, ISO high enough that the histogram leaves the left wall. Suggested ISO scales from a common starting point — 20 s, f/2.8, ISO 3200 — so a faster lens or a longer 500-rule shutter pulls ISO down. Light pollution and moon will still toast the sky; take a test shot and read the histogram, not the LCD.

Inputs

Units
mm

Wide primes (14–24 mm) are the usual untracked Milky Way glass.

×

1 full frame, about 1.5 APS-C, 2 Micro Four Thirds. Skip it if the focal length is already an equivalent.

f/

Shoot near wide open. Coma at the corners may want a half stop down — check a test shot.

Working ISO you plan to set. Suggested ISO is a separate output.

Results

500-rule shutter
35.71s

Untracked ceiling before stars streak, not the exposure that brightens the sky.

Suggested ISO
1,792

Scaled from 20 s, f/2.8, ISO 3200. Not rounded to camera ISO steps.

Night EV (ISO 100)
-7.19

EV100 your settings expose for: log2(N²/t) − log2(ISO/100). The 20 s, f/2.8, ISO 3200 start is about −6.4.

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Calculation method

Shutter comes from the 500 rule: 500 divided by crop times focal length. Suggested ISO starts from a working dark-sky trio — 20 seconds, f/2.8, ISO 3200 — and scales with the square of your f-number over 2.8 and with 20 over your shutter, so a faster lens or a longer shutter pulls ISO down. Night EV is the EV100 of your settings: log2 of N²/t minus log2 of ISO/100 at the ISO you typed, not the suggested one. That is the scene level they expose correctly, referred to ISO 100 like a meter reading; the 20 s, f/2.8, ISO 3200 start sits at about −6.4.

Exact formula

shutter = 500/(crop×f). suggestedISO = 3200 × (N/2.8)² × (20/shutter). evNight = log2(N²/shutter) − log2(ISO/100), the EV100 of the settings (about −6.4 at 20 s, f/2.8, ISO 3200). Limits: Light pollution, moon, tracking and true NPF time omitted; ISO is not clamped, so slow glass on a long lens can suggest an ISO your body does not have.

White balance

Set a fixed Kelvin (around 3800–4800) or daylight and correct in RAW. Auto WB on the Milky Way is a lottery.

Focus

Infinity marks lie. Magnified live view on a bright star, then tape the ring. Recheck when the temperature drops.

Frequently asked questions

Why is suggested ISO not a camera step?

It is a continuous scale. Set the nearest native ISO (2500/3200/4000). Dual-gain bodies often prefer 500 or 800 as a floor — do not go below that just because the formula said 200.

Foreground too dark?

Blend a longer, lower-ISO foreground frame. Do not lengthen the sky shutter past the 500/NPF limit or the stars dash.

Bortle 8 backyard?

Skyfog will fill the histogram before the core does. Lower ISO, shorter total integration, or drive darker. This formula assumes a dark sky.

NPF instead of 500?

Yes, once you know your pixel pitch. At 14 mm the NPF time on a modern sensor is often about half the 500-rule time. The suggested ISO here assumes the longer 500-rule shutter, so raise it to match if you shoot the NPF time.

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