# What determines exposure? [duplicate]

I’m trying to figure out what determines exposure is it Aperture and shutter speed? Or a different combo.

The reason I’m asking this is because iso just brightens the photo or magnifies the exposure that is there already. In that cause why do we even need iso?

Also if the exposure is already chosen regardless of the iso why are shots darker and brighter when using iso?

Curious to know this balance of the exposure triangle.

• Comments are not for extended discussion; this conversation has been moved to chat. Sep 16, 2021 at 3:47

## 1 Answer

As a really bad analogy, consider taking a photo to be the equivalent to filling a bucket with a hose. If the bucket has very little water in it, then that is the equivalent of a dark picture. If the bucket is overflowing with water, then the photo is bright white. If it is half full then the picture looks about right.

Now consider the hose, it has 3 variables:

1. The diameter of the hose (call this aperture (1))
2. The speed the water is coming out of the hose (call this ISO)
3. The time that the hose is turned on (call this shutter speed)

So for a given bucket size you can independently adjust any of the 3 parameters in order to fill the bucket. But in order to fill the bucket to just the half way mark, you need to balance all 3 of them together.

EG if you halve the diameter(2) of the hose, you could compensate by either increasing the speed of the water coming out of it, or you could double the length of time that the hose is turned on.

The exposure of a photo works in a similar way to filling the bucket (and in fact the sensors of digital cameras are know as buckets, and you fill them with electrons).

So for any given image the exposure is a function of Aperture, ISO and Shutter speed. And that you can get an equivalent exposure by manipulating any 2 parameters at the same time. EG decrease the aperture (f/x number becomes larger) and increase the time the shutter is open in order to let the equivalent amount of light pass through in the time that the shutter is open.

For a much better explanation of this, take a look at Exposure Value which goes into the math a lot more.

(1) Aperture indication on cameras is standardized a "f/x" so that the smaller "x" number means that more light is allowed to pass though

(2) Technically halve the area of the hose, but I'm playing fast an loose here

• No, ISO is the scale used to measure how much water is in the bucket, or even the size of the bucket. (Higher ISO settings "fill up" (reach the maximum digital value at ADC) with fewer photons falling on the sensor). The speed of the water coming out of the hose is the brightness of the scene (i.e. how much light the scene has coming from it). Aug 29, 2021 at 21:11
• @MichaelC Yep .. that's a better analogy Aug 29, 2021 at 21:12
• @MichaelC Actually wouldn't you call ISO the volume of the bucket, so the amount of water becomes the EV? Aug 29, 2021 at 21:15
• Please see my edit to the comment above made before your last comment. Aug 29, 2021 at 21:36
• In this analogy, I'd say ISO was the amplification factor of the ADC used to measure the "volume" of the water. E.g. measuring the water using a scale or load cell under the bucket. With a high gain on the ADC of the load cell, you can quickly estimate the amount of water but only require a small amount of water collected (short collection time) and extrapolate, but with the drawback of having high amplification noise and potential estimate error. With a low gain, you can get high precision, but you have to collect a lot of water to have meaningful signal.
– scottbb
Aug 30, 2021 at 1:27