ISO

[glass eye]

Hi everyone;

How do you avoid noise when using high ISO?

Ive read that if the photo is exposed properly you will get minimal noise.

Can someone explain this in a logical way.

Cheers John

[jethro]

John from ive experienced and tried, low light, high iso, more noise.
Is a flash involved, this will make a world of difference. Conditions and usage of available equipment play a very important role. There are so many variables.
Jethro

[DaveB]

Consider a pixel sensor: a "photosite". Say it's able to capture a value between 0 and 4095 (12 bits worth).
Also consider there's a certain amount of electrical noise to deal with: say +/- 10 values.

Now for areas of dark pixels, the noise is relatively "loud". For example a value that should be 100 could be anywhere from 90-110: a 10% error.
The brighter the pixel is, the less the noise is an issue: for a value that should be 4000 it could be 3990-4010 (a 2.5% error).
It's not that simple, as the linear values from the sensor are mapped into a gamma curve (our visual system is not linear, more logarithmic). But you should get the basic idea.

So noise is most-visible in the shadows. When you increase the ISO you're electrically amplifying the analog signal from the photosite before it gets digitised, and this also amplifies the electrical noise.
By exposing your images so that the values are brighter ("to the right" in a histogram) you can minimise the noise, but obviously you run the risk of overexposing some of the image and losing detail that way.
Nothing's ever simple!

[glass eye]

Thanks Jethro;

I guess sometimes you have no choice but to deal with noise either through a programme [photoshop] or lower the ISO and opt for a longer exposure.

One question if the subject is quiet a distance away [out of flash range] would this lower the noise level?

The point you made has got me thinking now.

Cheers John

[glass eye]

Cheers Dave;

So if i shoot in RAW and the change the image in PP by overexposing the photo and then adjust the brightness [darker] this will rectify the problem?

Hope im on the right track

[DaveB]

So if i shoot in RAW and the change the image in PP by overexposing the photo and then adjust the brightness [darker] this will rectify the problem?

There are two ways of reading that question, so I'll try to word it differently:

Once the sensor has recorded the image into the RAW file then it's recorded everything including the noise. If anything you should expose brighter in-camera (trying to avoid blowing channel highlights!) then reduce the exposure in the RAW converter.

Michael Reichmann's Expose Right article is a useful read.

[glass eye]

Thanks for that i really appreciate it

[Greg B]

And it is interesting that higher speed film (the speed of which was once called ASA, and 100 ASA = 100 ISO) tended to show more grain than lower speed film.

Grain was to film what noise is to a digital image (More or less). I honestly don't know whether there is any correlation between the two aside from the effect - it seems too much of a coincidence to believe that the two are unrelated. It is just difficult to see how the causes of noise - all about electronics etc as detailed above by Dave - are emulated in film where grain is the result of actual grains of silver halide being larger and standing out (visually) from the smaller grains which make up the image.

There is probably a simple explanation (he said, hopefully)

[DaveB]

It is just difficult to see how the causes of noise - all about electronics etc as detailed above by Dave - are emulated in film where grain is the result of actual grains of silver halide being larger and standing out (visually) from the smaller grains which make up the image.

There is probably a simple explanation (he said, hopefully)

You've described the cause of film grain: "clumpiness" in the grains of silver.
In digital images the noise typically takes on two forms: "luminance" and "chroma". Luminance noise looks similar to film grain, but chroma noise is generally regarded as visually "nasty". It can be seen in an area of grey where individual pixels take on coloured tints.

In a typical digital camera the colours are determined by using a "Bayer" filter, where individual pixels are covered by a red, green, or blue filter (the Sony F828 also uses an "emerald" filter, and many older cameras used cyan/yellow/magenta/green). The camera (or RAW converter) reconstructs the colour at each pixel by considering the R, G, or B value at that pixel and combining this with the values of the surrounding pixels.
Now consider what happens when some of those colour values are higher and some are lower than they "should be" because of electrical noise. This is where chroma noise comes from. It looks dramatically different to film grain or luminance noise.

The correlation of increased film grain and increased digital noise at higher ISOs is coincidence, but at least it intuitively makes sense!

[Matt. K]

So....you can minimise noise by slightly over exposing the image. Take care not to blow the highlights. Then you can use a program like NOISEWARE which is available free on the internet to remove some of the noise. But don't overdo it. Bottom line is...don't let digital noise worry you too much. A good image is still a good image even if it has noise. A poor image will always be a poor image....even if it has no noise whatsoever.

[stubbsy]

But more importantly, Matt, if I have no noise I won't be able to smell anything

[Oneputt]

Stubbsy - I am starting to worry about you

[glass eye]

I Think Matt has hit the nail on the head with that comment so thats encouraging i guess