What is ISO in digital terms?

[CowGummy]

Hi all,

I've been stumped by this a couple of times recently...
There's not many people in my circle of friends that shoot photography on a dslr and recently I've had a few of my friends ask me to explain manual shooting to them so that they can get a better grasp of their camera and any future investments in a dslr system.

Myself, I've been shooting for over a decade and consider myself fortunate enough to have been taught photography in the pre-digital era - I learned to control a camera by using a Pentax K1000 with a 50mm prime attached - nothing else. So I have no problem explaining how a shutter works and the effect of the aperture on the exposure, but when it comes to iso I can inly relate it to film: ie sensitivity in terms of film speed. So faster film speeds (I used to like shooting on ilford b&w at iso3200...) and the correlation of increased film grain as a result.

But then recently I got asked how that translates to digital, and it was one of those weird things where I had never even given it consideration, but I the honest answer is I don't know. So, what does iso relate to in terms of digital imaging? Is it the speed of the sensor, write-speed to CF card, etc...??

Many thanks in advance!

S.

 

[awinphoto]

ISO is from the International Standards Organization (or also known as International Organization of Standards). In photography terms, it's an industry wide standardized measurement of sensitivity. Short answer, it relates with film... long answer it's a little more dicey... In the film days, film was pretty spot on regarding speed however it was a good idea to buy a "brick" of film or a lot of film from the same batch and company. You would test the first roll and then use those tests to relate to the rest of the brick. Also you tested the lenses with leaf shutters because sometimes the springs would weaken and 1/50 maybe 1/30...

Leap ahead a decade or two and you're in the digital age with electronic shutters and sensors amplifying the signal match the ISO requirements... I read a while ago and cant find it on google, but there was an article with a title like "when ISO 100 isn't ISO 100" or something like that... Dx0 takes the cameras and reads the measurements of each camera's ISO's and points out EXACTLY where that ISO really is... sometimes the ISO's are right on (especially on the lower ISO) but towards the high end they may be a full stop off, which will affect your exposures... If you find it or anyone else knows what article I'm talking about please post it because it was rather interesting...

So yeah... for the most part, ISO's SHOULD relate to film, however there are minor nuances you may want to look into for your specific camera. In regards to exposure, ISO should be the same.

 

[opiuman]

I think what he meant is what does ISO actually change in the digital format.

As far as I know it has to do with the sensor, specifically increasing the gain on the sensor so measured data gets boosted to higher levels. This is why when you increase ISO on digital format you get noise because the gain is high relative to actual data on dark parts of the scene so it essentially turns almost nothing into something where as on the brighter parts there isn't as much noise because data measured there is big in relation to the gain applied (gain relative to actual measured is low/lower), basically SNR.

 

[awinphoto]

ISO changes the signal amplification and sensitivity of the pixels on the sensor changing exposure speeds... Same as film...

 

[CowGummy]

Thanks awinphoto & opiuman!

That most certainly helped me out - and awinphot: That's an interesting notion and I have considered this myself. The idea that iso on a digital camera can surely only be as close a match to film as the processor is capable of amplifying the sensitivity to an approximate match in terms of film speed. Interesting to find out that the higher the iso value on a dslr the more like it is to be somewhat 'off' - i didn't realise it could be as dramatic as a full stop!
Thanks for the help guys.

 

[Edwin Herdman]

Ironically, one of these big features of digital cameras - changing ISO - is actually still an analog process.

The DxOMark article is great reading, you should find it.

The essence of it is that the silicon sensor indeed is much more sensitive than film - but it has a "base sensitivity." There is a range (that looks to be getting even bigger these days) where the sensor is most sensitive - ISO increases don't change the physical sensitivity of the sensor, which is fixed, but increase the gain applied in the process of reading off the chip (by the analog to digital converter, aka ADC, which may be integrated into the "CPU Processor" aka DIGIC but doesn't have to be and is a distinct step from the usual "image processing" steps).

Daniel Browning on Photography-On-The-Net has come up with some amusing terms for various kinds of on-camera ISO indicators. There's 'smart' ISO settings which include the ISO setting which most closely matches the default sensitivity of the sensor (ISO 100 or 200 is usually the "cleanest" looking ISO) and ISOs which are clean doubles of it - I thought that analog gain could be applied essentially as much or as little as wished but apparently it doesn't really work that way (although it is wrong to think that the "real" sensitivity setting always is halved or doubled - the actual curve is slightly different as DxOMark says).

Then there are "faked" ISO settings, which are not even step increments - i.e. a "ISO 640" step (between 400 and 800) could be ISO 400 pushed (not likely) or could be ISO 800 "held back" (more likely) which may keep good highlight detail but add noise to the shadows. Nevertheless, some shooters swear by the "faked" ISO settings for shooting video, at least on some cameras, so it helps to experiment and find out what works best for you.

For some clues to get you started, I find this site to be a good starting point. It takes the DxOMark data and mines it for recompilation into a different kind of presentation. According to this data, my humble T1i (i.e. 50D sensor) does pretty well up to ISO 800. In practice, I like to limit it to at most ISO 400 - ideally to ISO 100 or 200 (there's a big jump in noise from 200 to 400). The read noise plot demonstrates this.

One thing you can learn from the Sensorgen.info charts is that at a certain point the effect of using in-camera ISO may be worse than manually or "artificially" brightening the image later (since more light = brighter photograph). You also can see that dynamic range and saturation are impacted by higher ISO selections.

To tie it all together with a neat list of some observations:

- The sensor has a set sensitivity
- ISO settings introduce tradeoffs to attempt to capture more data at higher ISOs
- The ISO scale is defined scientifically, but real camera settings are probably based at least partly on the appearance and marketing factors to make the very edge of what's reasonable with ISO increments appear to be more of a gain than it may really be
- When you attempt to map the actual sensitivity of the camera to the real ISO scale, the plot will probably be skewed one way or another, and it is in fact totally arbitrary because the manufacturers can explicitly define one camera ISO setting to be equivalent to something way off the mark - and sometimes it has happened, apparently. Probably a bigger issue with older cameras, although I wonder if newer sensor tech can cause "cheating" the other way (i.e. underreporting an on-camera ISO setting's sensitivity to be more conservative, maybe?)
- Your camera might be "missing" low ISO modes because it makes no sense to throw away data hitting the sensor - just darken the image afterwards (since ISO is essentially just brightening the image).

 

[gmrza]

Thanks awinphoto & opiuman!

That most certainly helped me out - and awinphot: That's an interesting notion and I have considered this myself. The idea that iso on a digital camera can surely only be as close a match to film as the processor is capable of amplifying the sensitivity to an approximate match in terms of film speed. Interesting to find out that the higher the iso value on a dslr the more like it is to be somewhat 'off' - i didn't realise it could be as dramatic as a full stop!
Thanks for the help guys.

In most cases - at least when you are using the camera's metering system - it doesn't really matter that the stated ISO setting is "off" from the true ISO sensitivity. The camera's metering system will compensate for that. For the folks using external light meters, ISO accuracy, or at least calibration between camera and light meter, is more of an issue.

 

[Mt Spokane Photography]

Its a complex issue,

DXO has tried to intrepret the standard and setup a standard test that they use to test all cameras.

Here is their excellent discussion of the meaning.

http://www.dxomark.com/index.php/About/In-depth-measurements/Measurements/ISO-sensitivity

Here are their testing protocols.

http://www.dxomark.com/index.php/About/In-depth-measurements/DxOMark-testing-protocols/ISO-sensitivity