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

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Video-vs-photo New Member • Posts: 13
Re: How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

Let me do it like you

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!
About shot noise:
"Originally, 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)."
So it was but now is different and tomorrow could be different again. Usual for science.

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

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.

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!!!
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). And because you bring the minimum level of the signal UP but you could not bring the maximum level of signal level/saturation UP your SNR go down and your dynamic range go down.
Or where before you have dark tones now you have noise.

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 now we have increase the light and SNR goes UP?!? So whatever we do SNR goes UP ..........

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

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.


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 and it grows UP with adding more ISO. 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.
You can lower the noise little bit by using bigger glass but this will change DoF and also will not fix bad colors produced from poor light.

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