How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

Started 2 months ago | Discussions thread
FingerPainter Veteran Member • Posts: 8,365
Re: How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

Video-vs-photo wrote:

Let me do it like you

The sincerest form of flattery?

You get style points. Not many points for relevancy.

FingerPainter wrote:

Video-vs-photo wrote:

So randomness could cause missing signal which is not exactly noise :). But distort original image.

No. Noise is variation in pixel values. How visible noise is depends on the Signal to Noise Ratio (SNR) - the higher the SNR, the less noisy an image looks.

Most of the noise in a typical digital photograph is variation in pixel values due to the variation that was present in the light itself even before the light was captured by the camera. When light is created, the timing of the release of individual photons, and the colour of each photon are both random. The camera adds additional variation. In most parts of a typical photo, the amount of noise added by the camera is small relative to the amount of noise in the captured light.

Noise are different things not just SNR of the sensor!

I didn't say that noise is just SNR of the sensor.

About shot noise:

I question the relevance of the context of your sources.


"Originally" when? Before the production of commercially available DSCs, I think. Therefore not of much relevance to our discussion

it was interpreted as arising from the random occurrence of photon absorption events in a photodetector, i.e. not as noise in the light field itself. Intensity noise at the shot noise level is obtained when the probability for an absorption event per unit time is constant and not correlated with former events. However, the existence of amplitude-squeezed light, which exhibits intensity noise below the shot noise level (sub-Poissonian intensity noise), proves that shot noise must be interpreted as a property of the light field itself, rather than as an issue of photodetection only – although a photodetector may be blamed for shot noise if it requires optical attenuation, which raises the shot noise level of the relative intensity (→ relative intensity noise)."

This just backs up what I was saying about shot noise.

So it was but now is different and tomorrow could be different again. Usual for science.

The fact that the cause of the phenomenon was misunderstood decades ago in no way invalidates anything I have said, because what I have said is based on the current understanding, not the previous incorrect one..

To be able to get enough light you need to have enough light so the sensor will be saturated to produce useful picture.

No. Saturation is the maximum capacity of the sensor. If you reach saturation on all pixels you will not have a usable picture. you will have a white picture.

I am non native English speaker and English is known for many different meanings of the words. So in this example I do not talk about FULL saturation of the sensor when get just white dot. But I am talking about lowest level that could be sensed/measured from the sensor.

That's pretty much the opposite of saturation. It is also a lower level than is useful. You need to collect enough photons that their capture is noticed above the noise floor.

Because you can not sense or measure infinite small portions of electrons or photons. So sorry for my bad English!

I wish I had as much facility as you do with a second language.

And if sensor is not saturated enough you will see pattern like grain.

All photos that have not reached saturation on all pixels will have noise because of the (variation) noise naturally occurring in light.

Yes but noise is negligible if we have enough light. All kind of noises will stay below useful information.

Hmm. Not really.

When you have a lot of captured light you have more noise than when you have little captured light. However, the SNR will be higher so the image will look less noisy. Here, your terminology problems are held in common with many English-speakers.

But then we apply amplification to get more use of small portion of the light that we captured and this produce much more signal which does not exist in the image - noise.

No. When you apply amplification (which is only one of several ways of implementing an ISO increase), you don't get more light, and you don't degrade the SNR. Either the SNR remains the same or it is increased. Which of these two occurs depends on whether the camera adds any noise after the gain stage. Most cameras add some noise after the gain stage to so on most cameras that use amplification to implement an ISO increase, increasing the ISO reduces the noisiness (increases the SNR). On many modern digital cameras, the amount of noise added after the gain stage is small, so the improvement in SNR is not readily noticeable.

You don't get more light! This is why of you need low noise pictures you need to have enough light!!!

Oh good. We agree.

Fast lens also does not add more light, but capture big portion from already available light. But It add more light to the sensor though but not to the object.


When you amplify does not matter analogue or digital you amplify everything including the noise ( all kinds of it).

Incorrect, and also a terminology problem. The term "amplification" carries with it the implication of an analog process, not a numeric one. The incorrect part is the claim that analog amplification amplifies everything. In fact, any noise added by the system after the gain stage is not amplified, and that is why increasing the ISO for a given exposure will usually increase SNR.

And because you bring the minimum level of the signal UP but you could not bring the maximum level of signal level/saturation UP

I don't know what you mean by the bolded bit.

your SNR go down

How does that follow? If the signal is constrained, so must be the noise.

and your dynamic range go down.

If you clip, but amplification doesn't guarantee clipping.

Or where before you have dark tones now you have noise.

It must be a large number of stops of amplification to get dark tones to clip. Now you have just run into the same problem as with exposing to saturation.

So no way to cheat with missing light, we just could add more light. Or we can get bigger glass to catch more light.

The effect of adding more light is to add both more signal and more noise. Since the noise in light is the square root of the signal, as you increase the light, the SNR goes up, and the image, despite having more noise, looks less noisy.

So above you said increase the ISO SNR goes UP

Yes, for a given exposure.

now we have increase the light and SNR goes UP?!?

Yes, and usually by a lot more then from a corresponding increase in amplification

So whatever we do SNR goes UP ..........

No, if we decrease exposure, SNR goes down. And if we decrease ISO setting for a given exposure, SNR goes down.

If, at the same time we raise ISO setting N stops and lower exposure by the same number of stops, SNR goes down because the increase to SNR from amplification is much less than the decrease from lower exposure.

Actually this more noise added with more light does not play now because we have enough photons to see clear what we are shooting.

When is "now"?

Amplification hasn't given us any more photons. If you mean after increasing exposure (by using a wider aperture, slower shutter or adding more light to the scene), then we have more noise but its effect is less visible because the SNR is higher. The shot noise only increased by the square root of the signal.

We could make our equipment better so at the end we will have small amount of added noise.

Yes. There are at least two ways to make the equipment better. On is to increase the portion of photons falling on the sensor that is actually captured (increase the quantum efficiency). The other is to reduce the amount of noise added by the camera.


Always nice to find points of agreement.

In this case shooting with cap on and shooting of gray uniform object will provide you of all possible information.

No. Shooting with the cap on results in no signal and no noise in light. It will give you information about camera-added noise, which is usually a small subset of the noise in a usable image.

NO! Actually it is not small subset

Really? OK what do you think typical levels of camera added noise and shot noise are for a mid-tone area of a well-exposed image. Express your answer as a ratio of camera-added noise to shot noise.

and it grows UP with adding more ISO.

No, the ratio of camera-added noise to shot noise decreases as amplification is increased because the portion of camera-added noise added after th fgains stage is not increased, but all the shot noise in increased.

Actually it will give you better understanding of your equipment and so called native ISO.


So this whole scientific blah, blah need to tell you that you need to add more light if you do not like level of the noise in the end images.

The improvement in SNR you get by adding a stop of light is larger than the improvement you get by adding a stop of gain. But only if there is no post-gain added noise will there be no improvement in SNR from adding gain.

You can lower the noise little bit by using bigger glass

or by slowing the shutter, or by bouncing more light off the subject

but this will change DoF


and also will not fix bad colors produced from poor light.


it will improve colour depth

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