# Relationship between shutter speed and amount of light?

With all else remaining the same (ISO, aperture etc.) is there a way to compensate for changes in the lighting of the scene by changing the shutter speed of the camera such that the intensity of the captured image remains the same? If yes, then what's the relation between shutter speed and light?

Basically, I have a light sensor measuring the light of a scene and based on that I would like to set the shutter speed of my camera such that each image has a nearly the same average intensity.

• This is de facto what aperture priority mode on your camera does. Is there a reason you can't just use that? Commented Nov 29, 2016 at 17:00
• @PhilipKendall No I want to change shutter speed manually Commented Nov 29, 2016 at 17:44

The relationship is linear. Keep the shutter open for twice the time, get twice the light. This is known as "one stop", and you can use that in conjunction with your light meter to get the desired results.

The built-in light meter in DSLRs usually gives the value in stops — basically, the number of halvings-or-doublings from what the meter thinks is correct. (See What is one "stop"? for more on this concept.) So if it says "-1", simply double the exposure to bring it to 0. If it says -2, double twice (going from, say, ¹⁄₆₀th of a second to ¹⁄₁₅th). Or, the other way around, if the meter says you're overexposed by a stop, cut the shutter speed in half.

If you have a fancy external light meter, it may give you the option to fix the aperture and ISO and literally tell you what shutter speed to use. In that case, of course, do that.

You note in a comment above that you want to shoot manually rather than using aperture priority mode. There are plenty of good reasons for that, but whatever yours is, you might want to look at What is the relationship between ISO, aperture, and shutter speed? as you figure it out — answers to that question cover this and more.

• This is not a built-in light meter so the values are not in stops. Anyway, so if the light in the scene (which I cannot control) were to get doubled, I would just have to half the shutter speed to achieve the same image brightness? Commented Nov 29, 2016 at 17:52
• @simplename Yes. That's all there is to it. Commented Nov 29, 2016 at 17:53
• The values that you mentioned -2 -1 0 and so on - what are they indicative of - do those numbers represent the light in the scene? Are they like lux or some light measurement? Commented Nov 29, 2016 at 18:28
• @simplename I've edited to elaborate. They are indicative of how far the settings you have are from what the meter has decided is optimal exposure, and each whole number is either half or double, depending on the direction. +1 means the camera thinks you are overexposing; -1 means it thinks you are underexposing. Of course, the meter isn't always right — it will try to make both a polar bear in a snowstorm and a crow in a coalmine be the same middle gray. Commented Nov 29, 2016 at 18:45
• @mattdm isn't the scale logarithmic?As you write, "if it says -1, double the time to make it 0". Adding "science" to photography is sometimes hazardous. :) Commented Dec 1, 2016 at 17:04