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Film Speed Calculator: ISO/ASA Equivalences and Exposure Adjustment

Reviewed by the CalculatorHub team, edited by James Graham. Formula based on the standard logarithmic ISO exposure relationship. Last verified 14 June 2026.

ISO (or ASA on older film) measures how sensitive a camera sensor or film is to light. The ISO scale is linear and logarithmic in stops: doubling the ISO value doubles the sensitivity and gains exactly one stop of exposure. If you change from ISO 100 to ISO 400, you gain two stops, meaning you can use a shutter speed four times as fast, or an aperture two stops narrower, while achieving the same exposure. This calculator finds the number of stops difference between any two ISO values and shows what new shutter speed or aperture to use if you want to keep the overall exposure constant. For example, switching from ISO 100 at 1/100 s (f/8) to ISO 400 means you need only 1/400 s at the same f/8 to achieve the same exposure (or you could close down to f/16 at 1/100 s instead). The ISO and ASA numbers are numerically identical for exposure purposes; this calculator works for digital and film cameras alike. A reference table of common ISO values and their typical uses is included below.

Stops difference: --

Enter base and new ISO values below to see the exposure adjustment needed. Source: Wikipedia: Film speed, as at 14 June 2026.

Starting ISO (your current film or setting)
New ISO you want to use
e.g. 0.01 for 1/100s, 0.5 for 1/2s, 2 for 2s
e.g. 2.8, 4, 8, 11
+2.00 stops
ISO increased: reduce exposure by 2 stops
1/400 s (0.0025 s)
f/16.00

Film speed formula

Stops difference = log2(new ISO / base ISO)
Positive stops: higher ISO, reduce exposure by that many stops
Negative stops: lower ISO, increase exposure by that many stops

New shutter speed (keep aperture) = base shutter / (new ISO / base ISO)
New aperture (keep shutter) = base aperture * sqrt(new ISO / base ISO)

Worked example

Base: ISO 100 at 1/100 s (0.01 s) and f/8. Switching to ISO 400:

  1. Stops = log2(400 / 100) = log2(4) = 2 stops (higher ISO)
  2. New shutter (same f/8) = 0.01 / (400/100) = 0.01 / 4 = 0.0025 s (1/400 s)
  3. New aperture (same 1/100 s) = 8 * sqrt(400/100) = 8 * 2 = f/16

Note: ASA and ISO numbers are numerically identical for exposure calculation. A roll of ASA 400 film and an ISO 400 digital setting require the same exposure.

ISO sensitivity reference guide

ISO value Typical use Noise level Light required
25, 50 Studio, bright sunlight, tripod work Minimal (film: very fine grain) Bright
100 Bright daylight, outdoor photography Very low (film: fine grain) Bright
200 Bright outdoors, slightly overcast Low Bright to moderate
400 Overcast, indoor with good light, action Moderate (film: noticeable grain) Moderate
800 Indoors, dim natural light, fast action Noticeable on older sensors Low to moderate
1,600 Dim indoor light, concerts, sports Visible on most sensors Low
3,200 Very low light, night interiors High (film: very coarse grain) Very low
6,400+ Night photography, astrophotography Very high; modern sensors handle better Minimal

Digital cameras handle high ISO far better than film. A modern full-frame sensor at ISO 6,400 may produce cleaner images than ISO 400 film from the 1970s. Always test your specific camera at high ISO settings to understand its noise floor.

Film speed calculator: frequently asked questions

What is ISO in photography?

ISO is a measure of a camera sensor's (or film's) sensitivity to light. A higher ISO value means the sensor is more sensitive and can capture an image with less light, but typically introduces more noise (or grain in film). ISO 100 is a standard daylight sensitivity; ISO 3200 is very high sensitivity for low-light situations. The ISO scale is linear: ISO 200 is twice as sensitive as ISO 100.

What is the difference between ISO and ASA?

ASA (American Standards Association) was the original standard for film sensitivity used in the United States. ISO (International Organization for Standardization) later unified the ASA and DIN (German) standards into a single international number. The ASA and ISO numbers are numerically equivalent for exposure purposes: ASA 400 film has exactly the same sensitivity as ISO 400. Modern cameras and digital sensors use ISO. You will still see ASA on older film packaging, but you can treat the values as identical when calculating exposure.

What is base ISO?

Base ISO is the native sensitivity of a digital camera sensor at which it produces the best image quality, lowest noise, and the widest dynamic range. It is determined by the sensor's physical design and is typically ISO 100 or ISO 200 on most cameras, though some cameras have a dual base ISO (for example ISO 800 as a second base). Shooting at base ISO gives the cleanest image. Higher ISO values amplify the signal electronically, which increases noise.

How does ISO affect image quality?

Higher ISO values amplify the signal from the sensor, which also amplifies electronic noise. This produces a grainy or speckled appearance in the image, particularly in shadow areas. On film, higher ISO (faster film) produces more visible silver grain. On digital sensors, noise manifests as luminance noise (brightness variation) and chroma noise (colour speckling). Modern sensors handle high ISO much better than older ones, but there is always a trade-off: higher ISO means more noise.

What is native ISO?

Native ISO, sometimes used interchangeably with base ISO, refers to the ISO value at which the sensor works without any analogue gain amplification. Some cameras have two native ISO settings (dual native ISO), where the second setting is also a low-noise point achieved by switching to a different amplification circuit rather than boosting digitally. Shooting at a native ISO gives the cleanest results; any other value applies gain that can increase noise.

Sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.