What is ISO for in a Digital Camera?

Can you please share me what is the perpose of the ISO in the digital camera...

Hi Kranthi,

Welcome to the CiC forums from me.

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Originally Posted by kranthi54321

Can you please share me what is the purpose of the ISO in the digital camera...

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In a nutshell; it allows the photographer to trade quality of a recorded image against the ability to take the image (correctly exposed, etc.)

OK, what does that mean in real life?
If take a picture outside on a sunny day, you would normally use a low ISO setting (e.g. 100 or 200) to give maximum quality.
However, if you want to take the same shot on a dark, cloudy day, you could use a higher ISO setting (e.g. 500 or above).
Why would you do that? Well, so you could use a suitable aperture to provide sufficient Depth of Field for the subject and also use a high enough shutter speed to freeze any subject movement.
The main downside of using a high ISO setting all the time is that it introduces "noise", or visual multi-coloured 'snow', to an image, hence using a lower ISO number on brighter days or other times when there is plenty of light.

If you use a low ISO setting on a dark day and take an under exposed image, you can improve it in Post Processing (PP) e.g. with CS4, however, doing it this way (alone), especially with jpg images, results in worse noise than if you had used a higher ISO setting in camera.

Cheers,

ISO in film reflected the speed at which a chemical reaction to light took place. So, a ISO 100 film took longer to expose but because the crystals on the coating very small it gave a fine detailed look, whereas ISO400 or higher would have larger grains and gave that grainy textured look. The advantage was that ISo400+ needed less exposure time so you could use a smaller aperture, say f/8 in dark conditions and still get a reasonable shutter speed.

With digital, ISO doesn't mean quite the same thing. Ideally you should always shoot at ISO100, but in practise you can't because you wouldn't get a decent shutter speed. Digital cameras therefore 'boost' the digital signal, and depending on what ISO setting you have selected it will boost the digital signals an appropriate amount. The problem is that boosting any electrical signal can cause distortion, which in a camera comes out as 'noise'. Grain (in film) can look creative. Digital noise does not look creative and can affect colour rendition. Best bet is to use one of the latest cameras such as Canon 7D or 5DMk2 which have processors that are better able to deal with noise at high ISO. Always try to shoot at ISO100/200 whenever you can, but don't settle for too slow a shutter speed as you will just end up ruining your shot. better to have a slightly noisy well focused shot.

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Originally Posted by Dave Humphries

In a nutshell; it allows the photographer to trade quality of a recorded image against the ability to take the image (correctly exposed, etc.)

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Quite right of course, although it's probably worth mentioning that camera design has improved so much with recent generations that with many ISO settings (200, 400, 800 even) it's getting pretty close to the proverbial "free lunch" (ie "no real downside").

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Originally Posted by Colin Southern

Quite right of course, although it's probably worth mentioning that camera design has improved so much with recent generations that with many ISO settings (200, 400, 800 even) it's getting pretty close to the proverbial "free lunch" (ie "no real downside").

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Yes, I was being a little conservative, certainly if you shoot RAW and PP with something like Neat Image (after ACR NR) and only web display or don't need to print big, then with a modern camera can do 800-1000 no problem, and more.

One of my better ISO 6400 (Nikon D5000) images:
http://www.pbase.com/dhumphries/imag...6/original.jpg
EXIF 1/45s at f5.3, 112mm.

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Originally Posted by Colin Southern

Quite right of course, although it's probably worth mentioning that camera design has improved so much with recent generations that with many ISO settings (200, 400, 800 even) it's getting pretty close to the proverbial "free lunch" (ie "no real downside").

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Yes but there still is with mine; however I would point out I would rather have it repaired than replaced with another intermediate. I can think of a use for all those pixels with cropping but noise isn't one of them.

The 7d though, wow; I would love that but if somebody said they would buy me your ideal camera it would be a hasselblad NO No sorry I meant 5d with L type lens.

Realistically the 7D is all I will ever need and noise wise fantastic. Does the Nikon do 100 iso and I'm assuming iso is the same as asa or din 21?

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Originally Posted by arith

Does the Nikon do 100 iso and I'm assuming iso is the same as asa or din 21?

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Nope; 200 is the base ISO, there is a "Lo1" which gets it there, but I'm not sure how they achieve it. I have never used it.

Yes, I believe all those standards equate to the same thing.

Cheers,

Increasing the ISO increases the gain applied to the circuits in your camera. As the gain is increased, the signal level increases. All electronic circuits have noise inherent to them. Unfortunately, as the signal level is increased, the inherent noise present in the circuit increases too.

The Nikon P90 will let you go down to ISO 64.

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Originally Posted by Dave Humphries

Nope; 200 is the base ISO, there is a "Lo1" which gets it there, but I'm not sure how they achieve it. I have never used it.

Yes, I believe all those standards equate to the same thing.

Cheers,

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Originally Posted by Dave Humphries

Nope; 200 is the base ISO, there is a "Lo1" which gets it there, but I'm not sure how they achieve it.

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I've heard that Canon & Nikon both do the "psudo" ISO modes by simply left-shifting or right-shifting the data, as opposed to a change in amplifier gain (same goes for intermediate ISO steps) - so it's handy from a shutterspeed perspective, but at the expense of increased noise.

The noise problem is being progressively improved with most new models, though the D90 is still slightly better than the D300s which is a later release.

Thanks for all your information provided for ISO that was really helpful information for me, as a fresher for this photography.

Thank you all,

Have a great day!

Every one has been correct. However, more simply stated: ISO is a measure of sensitivity of your sensor to light. The higher the ISO number, the more sensitive the sensor and the lower the ISO number, the less sensitive.

The more sensitive the sensor, the higher shutter speeds (which stop action and to some degree camera shake) can be used AND/OR the more sensitive the sensor, the smaller aperture (larger f/number) can be used allowing you greater depth of field.

There is no free lunch in photography however. As you increase the ISO, you normally degrade the image somewhat mostly due to noise. The extent to which an image is degraded as you raise the ISO depends on several parameters; the most important of which is the camera model.

Present generation DSLR cameras are capable of achieving satisfactory imagery at quite high ISO and the quality of that imagery achieved at a high ISO can be further improved by using a noise reduction software. There are many noise reduction programs on the market including Noise Ninja which is a favorite of many photographers. I use Topaz Adjust which has many other great facets along with its noise reduction capability.

As a photographer with well over fifty years professional experience, I marvel at the ISO capability of today's DSLR cameras. They can achieve what to me is mind boggling imagery at ISO levels which we never dreamed would be possible when I first started in photography. However, there is still a great difference in capability between various brands and models of DSLR cameras.