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

Started 2 months ago | Discussions thread
FingerPainter Veteran Member • Posts: 8,350
Re: How Do I Measure Noise Using Photoshop? And What Is A Significant Change?

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.

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.

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.

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.

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.

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.

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