ISO Truly Applies to Film, But What About Digital?

[grafiks]

Sorry if this has been talked about before.

Most people familiar with photography know what ISO means when talking about film. One thing I have never been able to understand is how ISO can relate to a sensor inside a digital camera. How can the sensor become more sensitive by changing the ISO? I can only guess that it is a software function, which to me is pretty lame. It makes me think that ISO is just a "feel good" setting on digital cameras.

I personally do not use high ISO on digital. I simply slow the shutter speed instead. Of course, I have experimented with the setting and saw the extra grain with the high ISO settings.

Is there anyone that can explain why an ISO setting is included with a digital camera and does it really work? And if so, how?

Thanks!

 

[Alex_B]

well, the signal from each sensor pixel is amplified by the same amount. This amplification factor is arbitrary and hence is something which can be chosen by the photographer. In order to give you the same feeling the amplification scale was calibrated to match film ISO.

To get the right exposure, you choose the right aperture, exposure time .... and ISO. So this ISO is just as real as film ISO is real, it just stems from different physics / chemistry in the two cases. Just see film as a chemical and disposable version of the sensor/amplifier combination.

A smaller amplitude signal has to be amplified more, which means also the noise is amplified more (you have an overall worse signal to noise ratio with weaker signals). Hence higher ISO - more noise. Just like higher ISO for fim means more grain mostly.

 

[grafiks]

well, the signal from each sensor pixel is amplified by the same amount. This amplification factor is arbitrary and hence is something which can be chosen by the photographer. In order to give you the same feeling the amplification scale was calibrated to match film ISO.

To get the right exposure, you choose the right aperture, exposure time .... and ISO. So this ISO is just as real as film ISO is real, it just stems from different physics / chemistry in the two cases. Just see film as a chemical and disposable version of the sensor/amplifier combination.

A smaller amplitude signal has to be amplified more, which means also the noise is amplified more (you have an overall worse signal to noise ratio with weaker signals). Hence higher ISO - more noise. Just like higher ISO for fim means more grain mostly.

That's kind of how I thought it would have to work on a digital camera. So it would be software driven, telling the sensor to become more sensitive (in other words, to amplify more or less depending on the current ISO setting). It still leads me to believe that a longer shutter would yield a superior image by capturing a broader range of light with less graininess.

 

[zioneffect564]

It still leads me to believe that a longer shutter would yield a superior image by capturing a broader range of light with less graininess.

Yes i keep my camera at 100 iso for everything but there are times when you cant use low iso for example you're in a place with low light and you dont have a tripod. For me i would bump up the iso because you can fix noise but you cant fix camera shake.

 

[grafiks]

Yes i keep my camera at 100 iso for everything but there are times when you cant use low iso for example you're in a place with low light and you dont have a tripod. For me i would bump up the iso because you can fix noise but you cant fix camera shake.

True. That would be the only good reason to use it, that I can think of.

 

[Digital Matt]

It still leads me to believe that a longer shutter would yield a superior image by capturing a broader range of light with less graininess.

The same is true of film. Lower ISO films produce images with much tighter grain.

 

[darich]

True. That would be the only good reason to use it, that I can think of.

But increased ISO also lets you freeze a moving object that your longer exposure can't so that's 2 reasons why adjustable ISO is useful.

ISO is included for exactly the same reasons that you get various speeds of film.

 

[grafiks]

It does make sense to have it for the purpose of freezing fast objects in low light.

 

[Digital Matt]

Not just for freezing the subject, but also the motion of the photographer.

 

[Alex_B]

well, lets just agree on the fact that it has the same effect exposure wise as on film .. and if you take the analogy grain / noise, it even holds true for the image appearance (of course grain looks slightly different from noise, noise can be slightly less pleasing)

 

[grafiks]

Thanks for all the input. It makes much better sense to me now (regarding how the ISO setting works and is used with digital).

 

[Don Simon]

I think often it's not helpful to draw analogies between different technologies or systems but here it makes sense; sensitivity is just as integral a factor in exposure with digital as it is with film. As for using low ISOs where possible, yes ideally one would do that in order to keep noise to an absolute minimum. But it's not always practical to do that. I think most would agree that it's better to have a noisy sharp image than a clean but blurred one. Software can reduce noise quite well, and anyway I think most dSLRs now can shoot at least up to ISO 400 without noise being all that noticeable.