The Truth About ISO

The effect of increased ISO – in camera “push processing” automatically lift the exposure value to where the camera thinks it is supposed to be.

Back in the days of ‘wet photography’, we had rolls and sheets of film that carried various ISO/ASA/DIN numbers.

ISO stands for International Standards Organisation

ASA stands for American Standards Association

DIN – well, that’s ‘Deutsches Institut für Normung’ or German Institute for Standardisation

ISO and ASA were basically identical values, and DIN = (log10)ISO x10 +1, so ASA/ISO 100 equated to DIN 21….nope, I’m not going to say anything!

These numbers were the film ‘speed’ values.  Film speed was critical to exposure metering as it specified the film sensitivity to light.  Metering a scene properly at the correct ISO/ASA/DIN gave us an overall exposure value that ensured the film got the correct ‘dose’ of light from the shutter speed and aperture combination.

Low ISO/ASA/DIN values meant the film was LESS sensitive to light (SLOW FILM) and high values meant MORE sensitivity to light (FAST FILM).

Ilford Pan F was a very slow mono negative film at ASA 50, while Ilford HP5 was a fast 400 ASA mono negative film.

The other characteristic of film speed was ‘grain’.  Correctly exposed, Pan F was extremely fine grained, whereas correctly exposed HP5 was ‘visibly grainy’ on an 8×10 print.

Another Ilford mono negative film I used a lot was FP4.  The stated ASA for this film was 125ASA/ISO, but I always rated it (set the meter ASA speed dial) to 100ASA on my 35mm Canon A1 and F1 (yup, you read that right!) because they both slightly over-metered most scenes.

If we needed to shoot at 1/1000th and f8 but 100ASA only gave us 1/250th at f8 we would switch to 400ASA film – two stops greater sensitivity to light means we can take a shutter speed two stops shorter for the same aperture and thus get our required 1/1000th sec.

But, what if we were already set up with 400ASA film, but the meter (set at 400ASA) was only giving us 1/250th?

Prior to the release of films like Delta 1600/3200 we would put a fresh roll of 400ASA film in the camera and set the meter to a whopping 1600ASA! We would deliberately UNDER EXPOSE Ilford HP5 or Kodak Tri-X by 2 stops to give us our required 1/1000th at f8.

The two stops underexposed film would then be ‘push processed’, which basically meant it was given a longer time in the developer.  This ‘push processing’ always gave us a grainy image, because of the manner in which photographic chemistry worked.

And just to confuse you even more, very occasionally a situation might arise where we would over expose film and ‘pull process’ it – but that’s another story.

We are not here for a history lesson, but the point you need to understand is this – we had a camera body into which we inserted various sensitivities of film, and that sometimes those sensitivities were chemically manipulated in processing.

That Was Then, This Is Now!

ISO/ASA/DIN was SENSITIVITY of FILM.

It is NOT SENSITIVITY of your DSLR SENSOR….!!! Understand that once and for all!

The sensitivity of your sensor IS FIXED.

It is set in Silicon when the sensor is manufactured.  Just like the sensitivity of Kodak Tri-X Pan was ‘fixed’ at 400ASA/ISO when it was made at the factory.

How is the sensitivity of a digital sensor fixed?  By the SIZE of the individual PHOTOSITES on the sensor.

Larger photosites will gather more photons from a given exposure than small ones – it’s that simple.

The greater the number of photons captured means that the output signal from a larger photosite is GREATER than the output signal from a smaller photosite for the same exposure value (EV being a combination shutter speed and aperture/f number).

All sensors have a base level of noise – we can refer to this as the sensor ‘noise floor’.

This noise floor is an amalgamation of the noise floors of each photosite on the sensor.

But the noise floor of each photosite on the sensor is masked/obscured by the photosite signal output; therefore the greater the signal, the larger the signal to noise (S/N) ratio is said to be.

In general, larger photosites yield a higher S/N ratio than smaller ones given the same exposure.

This is why the Nikon D3 had such success being full frame but just over 12 megapixels, and it’s the reason that some of us don’t get overly excited about seeing more megapixels being crammed into our 36mm x 24mm sensors.

Anyway, the total output from a photosite contains both signal and noise floor, and the signal component can be thought of as ‘gain’ over the noise floor – natural gain.

As manufacturers put more megapixels on our sensors this natural gain DECREASES because the photosites get SMALLER – they have to in order to fit more of them into the finite sensor area.

Natural gain CAN be brought back in certain sensor designs by manipulating the design of the micro lenses that sit on top of the individual photosites. Re-design of these micro lenses to ‘suck in’ more tangential photons – rather like putting a funnel in a bottle to make filling it easier and more efficient.

There is a brilliantly simple illustration of how a sensor fits into the general scheme of things, courtesy of digital camera world:

The main item of note in this image is perhaps not quite so obvious, but it’s the boundary between the analogue and digital parts of the system.

We have 3 component arrays forward of this boundary:

1. Mosaic Filter including Micro Lenses & Moire filter if fitted.
2. Sensor Array of Photosites – these suck in photons and release proportional electrons/charge.
3. Analogue Electronics – this holds the charge record of the photosite output.

Everything forward of the Analogue/Digital Converter – ADC – is just that, analogue! And the variety of attributes that a manufacturer puts on the sensor forward of this boundary can be thought of mostly as modifying/enhancing natural gain.

So What About My ISO Control Settings Andy?

All sensors have a BASE ISO. In other words they have an ISO sensitivity/speed rating just like film!  And as I said before THIS IS A FIXED VALUE.

The base ISO of a sensor photosite array can be defined as that ISO setting that yields the best dynamic range across the whole array, and it is the ISO setting that carries NO internal amplification.

Your chosen ISO setting has absolutely ZERO effect on what happens forward of the Analogue/Digital boundary – NONE.

So, all those idiots who tell you that ISO effects/governs exposure are WRONG – it has nothing to do with it for the simple reason that ISO effecting sensor sensitivity is a total misconception….end of!

Now I’ll bet that’s going to set off a whole raft of negative comments and arguments – and they will all be wrong, because they don’t know what they’re talking about!

The ‘digital side’ of the boundary is where all the ‘voodoo’ happens, and it’s where your ISO settings come into play.

At the end of an exposure the Analogue Digital Converter, or ADC, comes along and makes a ‘count’ of the contents of the ‘analogue electronics’ mosaic (as Digital Camera World like to call it – nice and unambiguous!).

Remember, it’s counting/measuring TOTAL OUTPUT from each photosite – and that comprises both signal and noise floor outputs.

If the exposure has been carried out at ‘base ISO’ then we have the maximum S/N ratio, as in column 1.

However, if we increase our ISO setting above ‘base’ then the total sensor array output looks like column 2.  We have in effect UNDER EXPOSED the shot, resulting in a reduced signal.  But we have the same value for the noise floor, so we have a lower S/N ratio.

In principal, the ADC cannot discriminate between noise floor and signal outputs, and so all it sees in one output value for each photosite.

At base ISO this isn’t a problem, but once we begin to shoot at ISO settings above base, under exposing in other words, the cameras internal image processors apply gain to boost the output values handed to it by the ADC.

Yes, this boosts the signal output, but it also amplifies the noise floor component of the signal at the same time – hence that perennial problem we all like to call ‘high ISO noise’.

So your ISO control behaves in exactly the same way as the ‘gain switch’ on a CB or long wave radio, or indeed the db gain on a microphone – ISO is just applied gain.

Things You Should Know

My first digital camera had a CCD (charge coupled device) sensor, it was made by Fuji and it cost a bloody fortune.

Cameras today for the most part use CMOS (complimentary metal oxide semi-conductor) sensors.

• CCD sensors create high-quality, low-noise images.
• CMOS sensors, traditionally, are more susceptible to noise.
• Because each photosite on a CMOS sensor has a series of transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons striking the sensory photosite array hit the transistors instead of the photosites.  This is where the newer micro lens designs come in handy.
• A CMOS sensor consumes less power. CCD sensors can consume up to 100 times more power than an equivalent CMOS sensor.
• CMOS chips can be produced easily, making them cheaper to manufacture than CCD sensors.

Basic CMOS tech has changed very little over the years – by that I’m referring to the actual ‘sensing’ bit of the sensor.  Yes, the individual photosites are now manufactured with more precision and consistency, but the basic methodology is pretty much ‘same as it ever was’.

But what HAS changed are the bits they stick in front of it – most notably micro-lens design; and the stuff that goes behind it, the ADC and image processors (IPs).

The ADC used to be 12 bit, now they are 14 bit on most digital cameras, and even 16 bit on some.  Increasing the bit depth accuracy in the ADC means it can detect smaller variations in output signal values between adjacent photosites.

As long as the ‘bits’ that come after the ADC can handle these extended values then the result can extend the cameras dynamic range.

But the ADC and IPs are firmware based in their operation, and so when you turn your ISO above base you are relying on a set of algorithms to handle the business of compensating for your under exposure.

All this takes place AFTER the shutter has closed – so again, ISO settings have less than nothing to do with the exposure of the image; said exposure has been made and finished with before any ISO applied gain occurs.

For a camera to be revolutionary in terms of high ISO image quality it must deliver a lower noise floor than its predecessor whilst maintaining or bettering its predecessors low ISO performance in terms of noise and dynamic range.

This where Nikon have screwed their own pooch with the D5. At ISOs below 3200 it has poorer IQ and narrower dynamic range than either the D4 or 4S.  Perhaps some of this problem could be due to the sensor photosite pitch (diameter) of 6.45 microns compared to the D4/4S of 7.30 microns – but I think it’s mostly due to poor ADC and S/N firmware; which of course can be corrected in the future.

Can I Get More Photons Onto My Sensor Andy?

You can get more photons onto your sensor by changing to a lens that lets in more light.

You might now by thinking that I mean switching glass based on a lower f-number or f-stop.

If so you’re half right.  I’m actually talking about t-stops.

The f-number of a lens is basically an expression of the relationship between maximum aperture diameter and focal length, and is an indication of the amount of light the lens lets in.

T-stops are slightly different. They are a direct indicator of how much light is transmitted by the lens – in other words how much light is actually being allowed to leave the rear element.

We could have two lenses of identical focal length and f-number, but one contains 17 lens elements and the other only 13. Assuming the glass and any coatings are of equal quality then the lens with fewer elements will have a higher transmission value and therefore lower T-number.

As an example, the Canon 85mm f1.2 actually has a t-number of 1.4, and so it’s letting in pretty much HALF a stop less light than you might think it is.

In Conclusion

I’ve deliberately not embellished this post with lots of images taken at high ISO – I’ve posted and published enough of those in the past.

I’ve given you this information so that you can digest it and hopefully understand more about how your camera works and what’s going on.  Only by understanding how something works can you deploy or use it to your best advantage.

I regularly take, market and sell images taken at ISO speeds that a lot of folk wouldn’t go anywhere near – even when they are using the same camera as me.

The sole reason I opt for high ISO settings is to obtain very fast shutter speeds with big glass in order to freeze action, especially of subjects close to the camera.  You can freeze very little action with a 500mm lens using speeds in the hundredths of a second.

Picture buyers love frozen high speed action and they don’t mind some noise if the shot is a bit special. Noise doesn’t look anywhere near as severe in a print as it does on your monitor either, so high ISO values are nothing to shy away from – especially if to do so would be at the expense of the ‘shot of a lifetime’.

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