srt101fan

Bob, if ISO is an adjustment to the sensor output, you can't really say that ISO "governs how the sensor will react to light", can you?

jackm1943 Loc: Omaha, Nebraska

But adjusting the ISO does change the aperture and/or the shutter speed unless you're in manual mode, thereby changing the amount of light hitting the sensor. What am I missing?

letmedance Loc: Walnut, Ca.

You forget that you have to change one of the other settings if you change your ISO. Try setting the camera for a shot at 100 ISO manually then snap a shot, next change only the ISO to 1000 and take the photo. Which photo are you going to keep, the good one or the white screen one?

letmedance Loc: Walnut, Ca.

The amount of light reaching the sensor is determined by aperture and shutter speed, when one changes ISO he must also change the amount of light hitting the sensor via Apt and or SS, if not you will be either under or over exposed. The video may have shown how ISO change is accomplished by gain of the processor but it mistakenly takes ISO out of the Triangle.

rmalarz Loc: Tempe, Arizona

It's a quibble but I'll give you that. The sensor will predictably react to light hitting it. After the sensor dumps its sensel data, that data is manipulated by the amount of amplification dialed in through the ISO setting.
--Bob

rmalarz Loc: Tempe, Arizona

The initial premise of this post is that ISO is not related to exposure. Exposure is the amount of light (aperture setting) and the duration of that light passing through the aperture ( shutter speed). That's it.

Now, photographic exposure will take into account the photosensitive material which that light strikes.
--Bob

rmalarz Loc: Tempe, Arizona

The focus of the post was exposure. Technically, exposure is simply the amount of light (aperture) and the duration of that light passing through the aperture (shutter speed). Exposure does not require a photosensitive material. Photographic exposure takes the photosensitive material into account.
--Bob

Ysarex Loc: St. Louis

You're conflating exposure, which is the amount of light reaching the sensor, with your desired lightness in the final image. There is no "must" requirement when changing ISO such that you "must" change shutter speed or f/stop. ISO is related to exposure because in addition to exposure ISO will lighten the final output image. However it doesn't do that by changing the exposure. Instead it applies processing changes in the camera to achieve that result.

Actually he took a moment at the end of the video to say that although ISO does not change the amount of light reaching the sensor (exposure) he nonetheless thinks the Exposure Triangle is a useful learning aid. I can see that but it does cause confusion as well especially concerning ISO.

plumbbob1

And I always thought it did have to do with sensor sensitivity. HELP!!

rmalarz Loc: Tempe, Arizona

Exposure and photographic exposure are two similar but different processes. As stated throughout this article, exposure is simply the amount of light and the duration of that light. Photographic exposure involves the addition of a photosensitive material.
--Bob

Ysarex Loc: St. Louis

Changing ISO only causes the camera to change shutter speed and/or aperture when the camera is in an auto/semi-auto mode and the shutter speed and/or aperture are under the control of the camera's metering/exposure system. What you're missing is the need to keep cause and effect clear. In your description here you still acknowledge that it's either the shutter speed or aperture that actually change the exposure -- the ISO change is recognized by the camera meter/exposure system and so can change either of the two exposure controls if it has access.

Cause and effect matter. If you look at any modern presentation of the Exposure Triangle it muddies up cause and effect. The presenters feel this need to make the triangle balance and so they present this set of relationships:

Shutter speed controls exposure + rendition of motion.
Lens aperture controls exposure + depth of field.
ISO controls exposure + noise.

A classic example: https://creativeraw.com/wp-content/uploads/2021/02/exposure-triangle-photography-explained-1.jpg

What that does is convince a whole lot of people that ISO causes noise. Again ISO is related to noise but it doesn't cause it -- the noise we see in our modern cameras in low-light photos is a function of exposure. ISO is related to exposure but it doesn't cause it.

letmedance Loc: Walnut, Ca.

ISO just like ASA determines the speed of the medium that light is hitting, it is a definite part of exposure. Knowing the speed of your medium is necessary to capture a photo, film ASA is set by the emulsion but by processing and pushing it can be changed. In a digital camera the ISO or ASA is changed by gain in the camera processor. If we change either film ASA or Sensor ISO a change in one of the other points on that triangle must also change.

PS: how is that slide attachment working?

Ysarex Loc: St. Louis

It does, but there's lots of details to flesh out. What you often hear that's misinformation is that changing ISO changes the sensor's sensitivity. It doesn't. The light sensitivity of a digital sensor is fixed in manufacture and doesn't change. You can't make it more or less light sensitive.

So your sensor comes with just one light sensitivity. With your camera set to it's lowest ISO you have the potential to use the full recording capacity of your sensor -- good thing as you get the max SNR (signal to noise ratio) and best overall IQ.

We raise ISO when we don't have enough light and in order to keep the shutter speed fast enough or aperture small enough. When we raise ISO the sensor doesn't change it's light sensitivity -- we simply expose the sensor less and then use a processing method to lighten the final image to compensate.

Ysarex Loc: St. Louis

There you go with "must" again. No.

Did you catch the part where you said you're wrong in your description of what happens? You just said, "In a digital camera the ISO or ASA is changed by gain in the camera processor." Well if you're right about that then how is the exposure, which would already be over at that point, changed by ISO?

Wikipedia has a good definition of exposure: "In photography, exposure is the amount of light per unit area (the image plane illuminance times the exposure time) reaching a frame of photographic film or the surface of an electronic image sensor, as determined by shutter speed, lens aperture, and scene luminance." my bold https://en.wikipedia.org/wiki/Exposure_(photography)

I bolded the clause in the definition that identifies the factors that determine an exposure.

jackm1943 Loc: Omaha, Nebraska

This whole thing is starting to sound to me like just a question of semantics and definitions, a little like the epic one we had a year or so ago about "perspective".