Does changing shutter speed and aperture create equalized exposure?

[vintagesnaps]

I don't know about the OP but I skipped on ahead... what was the question again? lol

I'd say, meter the scene. Then you'll know when you adjust ISO/shutter speed/aperture if you're getting the amount of light needed for a proper exposure.

 

[Ysarex]

You seem to be contradicting yourself and confusing me and your student. On the one hand you say that raising ISO doesn't have a direct effect on noise that it's because you reduce light to the sensor. Well, if you raise the ISO, you have to change the aperture and shutter settings to reduce light to the sensor.

No you don't have to change the aperture and shutter settings.

So, ipso facto, raising ISO causes noise.

No, reducing the exposure causes noise. Raising ISO does not. Again ISO correlates with noise but doesn't cause noise. Here's some help understanding the difference between correlation and causation: Correlation vs Causation

So your student is right.

I told my student to raise the ISO without changing the exposure -- go back and read it. Because ISO suppresses read noise raising the ISO without changing the exposure produces less noise in cameras that exhibit read noise.

You also now agree that changing ISO itself causes noise due to the amplification of the signal.

I do not agree and I did not say that. I said, "What ISO is doing in the camera doesn't directly alter exposure and so doesn't cause shot noise. ISO implementation in the camera via amplification of the analog sensor signal suppresses read noise."

Since this is the most used method, noise occurs most often in most cameras when you raise the ISO.

ISO correlates with noise. Most people raise the ISO to compensate for reducing exposure. Reducing exposure is the cause of the noise. Amplification of the sensor signal does not cause noise. In many cameras it will suppress noise. I've already said that.

It seems that the problem is compounded because there is a concurrent reduction in the amount of light on the sensor as well.

 

[West Coast Birder]

It is useful to remember what the f-stop or f number is. The f-stop is the ratio of the focal length of the lens to the diameter of its entrance pupil. So, the f-stop is inversely proportional to the diameter of the lens. Now, you may be wondering why these funny numbers 2.8, 4, 5.6, 8, 11, etc. You can see that each of these numbers is roughly 1.4 times the previous one. Since the area of the aperture is proportional to the square of the diameter, when you go from 2.8 to 4, you basically are reducing the area of the aperture by a factor of (1.4)^2 which is roughly 2. In other words, going from f/2.8 to f/4 cuts the area of the aperture in half which means it lets in half as much light. So, to get the same exposure, you need to leave the shutter open for twice as long. And so on....

 

[AlanKlein]

No you don't have to change the aperture and shutter settings.

No, reducing the exposure causes noise. Raising ISO does not. Again ISO correlates with noise but doesn't cause noise. Here's some help understanding the difference between correlation and causation: Correlation vs Causation

I told my student to raise the ISO without changing the exposure -- go back and read it. Because ISO suppresses read noise raising the ISO without changing the exposure produces less noise in cameras that exhibit read noise.

I do not agree and I did not say that. I said, "What ISO is doing in the camera doesn't directly alter exposure and so doesn't cause shot noise. ISO implementation in the camera via amplification of the analog sensor signal suppresses read noise."

ISO correlates with noise. Most people raise the ISO to compensate for reducing exposure. Reducing exposure is the cause of the noise. Amplification of the sensor signal does not cause noise. In many cameras it will suppress noise. I've already said that.

You're just confusing everyone. The fact is if you raise ISO, you have to reduce exposure which causes noise. That's the ultimate issue.

Regarding raising ISO without lowering exposure, how do you do that without overexposing? I've never heard of such a process.

 

[Ysarex]

You're just confusing everyone. The fact is if you raise ISO, you have to reduce exposure which causes noise.

You got the cause right: reducing exposure is what causes noise.

That's the ultimate issue.

Regarding raising ISO without lowering exposure, how do you do that without overexposing? I've never heard of such a process.

ISO correlates with noise and it also correlates with the brightness of the camera's output image (JPEG). As you raise ISO the output image gets brighter. And both these correlations between ISO and the camera's output image (JPEG) are strong. There's little you can do to break them.

But as I previously noted the correlation between ISO and noise in raw files is much weaker -- same goes for the correlation between ISO and image brightness. You don't absolutely have to change exposure with an ISO change.

Raising ISO without lowering exposure won't over brighten the output image if it's too dark to begin with. This is what I caught my students doing. Misinformed as they were about ISO causing noise, and aware that they couldn't increase exposure because of motion blur they would raise ISO to what they thought was their noise tolerance level and then leave the camera output image too dark -- I noted my student said he planned to fix it in PS. They thought they could game the system by not raising the ISO even higher and so have less noise in the photo they adjusted in PS. They were wrong because they were misinformed and confused about ISO causing noise.

Because the correlation between ISO and noise/image brightness is weak with raw files it's easy to do this: Took two photos for you.
One at ISO 200 (base ISO of my camera) and the other at ISO 3200 -- both the same exposure: 1/15 sec, f/5.6. In both I processed the raw files.

A) ISO 200: ISO-200-noise.jpg
B) ISO 3200: ISO-3200-noise.jpg

Because the exposures are the same the shot noise level should be the same in both photos and it is. The ISO 3200 photo has very slightly less noise because of some read noise suppression -- camera is doing that when the ISO is raised. Here's a 100% peak of the two side by side:

If raising ISO really is the cause of noise why does the ISO 200 photo have more noise than the ISO 3200 photo? If ISO amplification is causing noise then why does the ISO 200 photo (less amplification) have more noise than the ISO 3200 photo? The cause of noise is reduced exposure.

 

[AlanKlein]

You got the cause right: reducing exposure is what causes noise.

ISO correlates with noise and it also correlates with the brightness of the camera's output image (JPEG). As you raise ISO the output image gets brighter. And both these correlations between ISO and the camera's output image (JPEG) are strong. There's little you can do to break them.

But as I previously noted the correlation between ISO and noise in raw files is much weaker -- same goes for the correlation between ISO and image brightness. You don't absolutely have to change exposure with an ISO change.

Raising ISO without lowering exposure won't over brighten the output image if it's too dark to begin with. This is what I caught my students doing. Misinformed as they were about ISO causing noise, and aware that they couldn't increase exposure because of motion blur they would raise ISO to what they thought was their noise tolerance level and then leave the camera output image too dark -- I noted my student said he planned to fix it in PS. They thought they could game the system by not raising the ISO even higher and so have less noise in the photo they adjusted in PS. They were wrong because they were misinformed and confused about ISO causing noise.

Because the correlation between ISO and noise/image brightness is weak with raw files it's easy to do this: Took two photos for you.
One at ISO 200 (base ISO of my camera) and the other at ISO 3200 -- both the same exposure: 1/15 sec, f/5.6. In both I processed the raw files.

A) ISO 200: ISO-200-noise.jpg
B) ISO 3200: ISO-3200-noise.jpg

Because the exposures are the same the shot noise level should be the same in both photos and it is. The ISO 3200 photo has very slightly less noise because of some read noise suppression -- camera is doing that when the ISO is raised. Here's a 100% peak of the two side by side:

View attachment 269282

If raising ISO really is the cause of noise why does the ISO 200 photo have more noise than the ISO 3200 photo? If ISO amplification is causing noise then why does the ISO 200 photo (less amplification) have more noise than the ISO 3200 photo? The cause of noise is reduced exposure.

My understanding is raising ISO without changing aperture or shutter only causes noise depending on how high and which camera. Some cameras are better at allowing raising the ISO much higher before you get noise. Other cameras create noise much quicker.

 

[Ysarex]

My understanding is raising ISO without changing aperture or shutter only causes noise depending on how high and which camera.

Raising ISO doesn't cause noise. Reducing exposure by changing the aperture and/or shutter speed causes noise. I just demonstrated that -- I raised the ISO without changing the exposure and got a little less noise.
Raising ISO correlates with noise because most people raise the ISO to brighten the image when they're forced to reduce exposure -- reducing exposure causes noise.

Some cameras are better at allowing raising the ISO much higher before you get noise. Other cameras create noise much quicker.

 

[smoke665]

Raising ISO doesn't cause noise. Reducing exposure by changing the aperture and/or shutter speed causes noise. I j

While I agree with your premise I believe your delivery might be confusing. Reducing exposure (shutter/aperture) at any ISO creates shot noise. Even within a properly exposed shot you can still experience shot noise in the shadows. Adjusting shutter and aperture to get the best exposure for shot parameters, will result in the lowest SNR. That doesn't mean there won't be times when raising ISO is necessary to increase your shutter speed, or stop down for greater DOF. @AlanKlein DP review has an interesting article that might explain it a little clearer What's that noise? Part one: Shedding some light on the sources of noise

Like all things, understanding the consequences of actions helps you make the right choice.

 

[AlanKlein]

While I agree with your premise I believe your delivery might be confusing. Reducing exposure (shutter/aperture) at any ISO creates shot noise. Even within a properly exposed shot you can still experience shot noise in the shadows. Adjusting shutter and aperture to get the best exposure for shot parameters, will result in the lowest SNR. That doesn't mean there won't be times when raising ISO is necessary to increase your shutter speed, or stop down for greater DOF. @AlanKlein DP review has an interesting article that might explain it a little clearer What's that noise? Part one: Shedding some light on the sources of noise

Like all things, understanding the consequences of actions helps you make the right choice.

The second article states that raising ISO just by itself can increase noise a lot depending on the camera. Am I misreading this?

Sources of camera noise part two: Electronic Noise

Following on our look at the effects of shot noise, our attentions turn to the electronic noise added by your camera. In this second part of the series, we look at read noise and how your sensor's behavior defines what your camera is capable of and consequently, how you should shoot with it...

www.dpreview.com

 

[Ysarex]

While I agree with your premise I believe your delivery might be confusing. Reducing exposure (shutter/aperture) at any ISO creates shot noise. Even within a properly exposed shot you can still experience shot noise in the shadows. Adjusting shutter and aperture to get the best exposure for shot parameters, will result in the lowest SNR. That doesn't mean there won't be times when raising ISO is necessary to increase your shutter speed, or stop down for greater DOF. @AlanKlein DP review has an interesting article that might explain it a little clearer What's that noise? Part one: Shedding some light on the sources of noise

Like all things, understanding the consequences of actions helps you make the right choice.

From back on page three, I said; "There are two common types of noise that concern us in our photos: shot noise and read noise. Shot noise is by far and away dominant and typically what we see when we see a noisy photo. Read noise, from the camera's electronics, is less a problem and getting less all the time as the hardware improves. Shot noise is entirely a function of exposure -- SNR (signal to noise ratio) get's better with more exposure and worse with less exposure. What ISO is doing in the camera doesn't directly alter exposure and so doesn't cause shot noise. ISO implementation in the camera via amplification of the analog sensor signal suppresses read noise. What's that noise? Part one: Shedding some light on the sources of noise"

 

[smoke665]

The second article states that raising ISO just by itself can increase noise a lot depending on the camera. Am I misreading this?

Yes, no, maybe. This is what I think Joe was trying to get across, ISO does not raise/lower exposure (the amount or lack of) light that hits the sensor, but the lack of exposure allows for greater "shot noise". Once you click the shutter any "shot noise" in the scene is baked in. When you raise the ISO you are amplifying the analog signal from the sensor. Any "shot noise" in the data is also amplified. During the process of amplification you encounter "read noise" which increases as you increase amplification. This is created by the camera, not exposure. "Read noise" + "shot noise" = the noise you see in your image. If you increase exposure shutter/aperture you increase the amount of light hitting the sensor which decreases "shot noise". With less "shot noise" in the file an increase in ISO won't result in higher visible noise in your image. "Read noise" is a function of the camera and varying with model.

 

[smoke665]

From back on page three, I said; "There are two common types of noise that concern us in our photos: shot noise and read noise. Shot noise is by far and away dominant and typically what we see when we see a noisy photo. Read noise, from the camera's electronics, is less a problem and getting less all the time as the hardware improves. Shot

Yes I read that, and agree you are right. Just saying I understood what you were saying, but some of it could be confusing especially for someone who doesn't.

 

[Ysarex]

The second article states that raising ISO just by itself can increase noise a lot depending on the camera. Am I misreading this?

Sources of camera noise part two: Electronic Noise

Following on our look at the effects of shot noise, our attentions turn to the electronic noise added by your camera. In this second part of the series, we look at read noise and how your sensor's behavior defines what your camera is capable of and consequently, how you should shoot with it...

www.dpreview.com

The second part of the article addresses read noise which is CAUSED by the electronics in the camera and not ISO and not ISO amplification. The article notes two types of read noise and notes that downstream read noise is suppressed by raising ISO. ISO amplification reveals both shot noise and read noise. It doesn't cause them or increase them it shows them by brightening them up.

But wait. First put this in perspective. The article starts: "In the first part of this article, we discussed the fact that a lot of the noise in a lot of your images doesn't come from your camera at all: it is you being able to see the randomness of the light that you've captured and it is almost solely dependent on how much light you were able to capture." That's describing shot noise which is in the signal and by the way note the author says it is solely dependent on exposure. Shot noise swamps read noise by a lot. This article is dated 2015. By then the author was already saying, "Most modern sensors have pretty low upstream read noise, the big differences between different sensor designs tend to be in how low the designers have managed to get the downstream read noise." It's the upstream read noise that ISO amplification reveals and in 2015 it was already pretty low. The downstream read noise is what ISO amplification suppresses.

Since 2015 advances have occurred in sensor tech such that read noise has been further removed from our modern cameras. It's on the verge of being a non-issue. When you see noise in your photos you're seeing shot noise almost exclusively -- and that's the noise that is caused by reduced exposure.

 

[Ysarex]

Yes, no, maybe. This is what I think Joe was trying to get across, ISO does not raise/lower exposure (the amount or lack of) light that hits the sensor, but the lack of exposure allows for greater "shot noise". Once you click the shutter any "shot noise" in the scene is baked in. When you raise the ISO you are amplifying the analog signal from the sensor. Any "shot noise" in the data is also amplified. During the process of amplification you encounter "read noise" which increases as you increase amplification. This is created by the camera, not exposure. "Read noise" + "shot noise" = the noise you see in your image. If you increase exposure shutter/aperture you increase the amount of light hitting the sensor which decreases "shot noise". With less "shot noise" in the file an increase in ISO won't result in higher visible noise in your image. "Read noise" is a function of the camera and varying with model.

Just as a point of further clarification: Shot noise is dominant and read noise today in 2023 is approaching insignificance so when we talk about shot noise and read noise existing in our photos it's not a 50/50 situation but more like a 95/5 situation -- don't need to worry too much about the 5.

 

[AlanKlein]

The only reason to raise the ISO is because you either want to increase DOF requiring a smaller aperture, or you want to increase the shutter speed to prevent subject movement blur. In both cases, you reduce the amount of light to the sensor when you concurrently increase the ISO. So, the affect, whether called shot noise or something else, is to get a noisier pictures. Everything else is just conversation and confusing to the photographer.