What is one "stop"?

  • I always hear this term, e.g.,

    • I had to go down one stop
    • Increasing X by Y raises Z by one stop
    • I turned down the flash/the light two stops
    • This lens/sensor/strobe/Photoshop tweak raises X by around one stop

    That one too!: "Around one stop"... Is there a 0.85 or a 1.13 of a stop, to be exact?

    Is this (always) the same thing as an f-stop? I am so confused!

  • John Cavan

    John Cavan Correct answer

    10 years ago

    A stop will halve or double the amount of light, depending on the direction and that could mean the amount of light reaching the sensor or how sensitive the sensor is made to the light that is reaching it.

    So, for example, to reduce something a stop, I could go from ISO 800 to ISO 400, or I could go from a 1/500 to 1/1000 shutter speed, or I could change the aperture from f/2.8 to f/4. Going opposite direction on any of these would increase the light by a stop.

    An f-stop is the term used to describe the aperture positions on a lens. It's the basis for the more general term of a "stop" when describing the amount of light for the exposure.

    A small doubt here. Given other parameters are unchanged, varying the ISO doesn't change the amount of light entering. It's just an amplification of the available light. Am I right here?

    @surajck Yes, you are right. Varying ISO doesn't change the amount of light entering. But it does change the eventual brightness values, and influences the appearance of the photo like changing shutter speed or aperture does. In casual conversation the difference is sometimes glossed over, but it's good to remember what actually happens. Good catch.

    @surajck I disagree. The relationship includes the ISO in exactly the same way. Doubling or halving the ISO changes the exposure/amount of light/relationship by one "stop." One "stop" is a statement about relative amount

    @RoelSchroeven I believe your comment is confusing. It seems to contradict itself.

    @Stan How does doubling or halving the ISO change the amount of light?

    @RoelSchroeven It changes the exposure by (wait for it) one stop. : ^ )

    @Stan The exposure, yes, but you also said changing the ISO changes the amount of light. How? How does changing the ISO change the amount of light that reaches the sensor or film? Because that's what surajck's question is about. He thinks ISO does not affect the amount of light on the sensor, but instead changes exposure through amplification. And he's right. (Except it's not the light that's amplified, it's the electronic signal). Or in the case of film, film of different ISO has different sensitivity to light.

    @RoelSchroeven changing the ISO changes the way the camera meters for the F-stop and shutter speed, so it does change the exposure but not directly.

    @MarkRansom Let's not make things even more confusing; let's keep aperture and shutter speed equal and only vary the ISO. The question is what happens then. There is no doubt that that affects the exposure, but what Stan claims is that it does that via affecting the amount of light captured and that is just not true.

    @RoelSchroeven in any discussion of exposure the topic of ISO is bound to come up. It's important to know what it affects and what it does not. It *does* affect the exposure, but not in the way people might naturally expect as you point out.

    My question was about what happens if all the other parameters of the camera are held constant, and two shots are taken at ISO 100 and ISO 200. In both these scenarios, the 'amount of light' hitting the sensor is the same. Thanks @RoelSchroeven for clearing that up.

    @Stan changing the exposure by one stop, yes that could be the words you guys use on the field. But it is not done by letting more light in in my specific case. It just gives the same result as changing the exposure by one stop.

    @surajck Okay, I think I understand your line of questioning. When you change ISO/ASA setting, the sensitivity to E by the sensor is altered (increased or decreased) not the stimulus affecting the sensor per se. Does this make sense to you? The same E will affect a more sensitive sensor more than it does a less sensitive sensor.

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