Getting Real About Comparing ISO Settings

800 ISO f4 @ 1/50 sec with 18mm with OSS,  NEX 5n,
a very useable combination, why would I need to push the ISO higher?

Heres a newsflash for you, raising the ISO on your cameras always trades off image quality....always...no exceptions.  When you raise the ISO you underexpose the image at the sensor level and maximum image quality is always optimal when the exposure is at its maximum....right before the highlights actually clip.
And why dear friend do we then raise the ISO?

To access faster shutter speeds to either minimise camera or subject movement.

To access a more adequate level of depth of field without going too slow on the exposure. 

Or to shoot hand held under very low light.

With me so far, good, now answer me this, why do I get all uptight when testers start bagging say 1" or M4/3 sensors when they look mushy at 3200 ISO and above? Simple, we are not comparing apples to apples!!!!

Accepted wisdom is that FF cameras are heaps better at high ISOs, well they dammed well better be, because you are much more likely to need “them there high ISOs’. You see folks it’s never just about the shutter speed or the aperture you need, it's about the interaction between the two of them and the mechanics of the camera in question.

Let me run some numbers for you, I love numbers they can be so edifying, and so confusing as well when mis-used, anyhow lets get some edification happening.  Say you are shooting a group of people in a restaurant at night, you know the sort of thing, a bunch of friends sitting at a table, and you want everyone sharp, so no-ones nose gets out of joint or focus.  So there you stand 1.5 m from the nearest person and the furthermost bod is 4.5 meters away. Flash won't help, the inverse square law will see that,  Mr 1.5m is rendered a ghost and Miss 4.5m a woman of the dark shadows.

Nope you need to run with available darkness. Now lets say you have a lovely 24 mp FF DSLR fitted with a 35mm lens which is just wide enough to fit everyone in without producing any Humpty Dumpty egg shaped head distortion or empty foreground syndrome. And lets face it, no one wants a head like Humpty Dumpty!

So here we go, according to the DOF scales on my iPhone, you would need to focus at about 2.2 metres with an aperture of f10 to cover your needs.  But wait what if we instead use APSC, now the lens will be set at 23.5 mm for the same angle of view and using the same focus point we can run with f6.4 for basically the same result. In udder wordz we get to use a whole F stop and a bit wider or an ISO step and a bit lower or perhaps helpfully a shutter speed faster.

Ah but what if we have a lovely little 16mp M4/3 pocket rocket, the focal length will now be 17.5 mm and the required aperture according to the DOF Master, just f3.2, if I move the focus a little further out to 2.4 m  Ah yes that's right f3.2, now that is near enough a whole 3 f stops or 3 ISO steps lower than the full frame...like we can use 1600 iso instead of 12800 ISO! Now seriously do you really think that 12800 iso on a FF camera is cleaner than 1600 on a state of the art M4/3 camera. It might just be with say the new Nikon DF but generally I would say no.

Ah but lets dig a little deeper, lets dig right down to a 1" sensor like the one in the sony RX 100. Our crop factor is now 2.7x, so 35mm equals 13mm.  Guess what aperture you will need, allowing for the slight backwards shift of focus....f 2.  Basically we don't have that option on the Sony devices but we sure could in the future and Nikon has a lens that will comply with f2, but heck, f2....that's 4.3 stops wider than the FF DSLR or 4.3 ISO steps less. In other words about 640 iso instead of 12800 iso. And trust me, that award winning rx100 performs pretty well at that ISO level.  That’s a big deal fellow photographers and it provides a whole bunch of other options when the light packs up an leaves.  You of course have the option to also shoot much faster shutter speeds and go up a bit on the ISO for example.

But there's more, well there usually is...DSLRs have mirrors and going low on the shutter speeds often incurs a sharpness penalty. Trust me, I am a pretty steady shooter but there is no way on earth I can hand hold my Sony A900 with say an 100 mm lens anywhere near as slow as I can go on my Nex 5n with the Electronic first Curtain shutter enabled and a 55-210 OSS lens set at say the equivalent 65mm or so. I calculate I need about an extra 2 shutter speeds higher on the A900 to get the same results clarity wise due to camera movement. That folks gives the NEX 5n about a 3 stop advantage for real world low light work....or I can use lets say 400 iso instead of 3200. Don’t bother arguing with me I actually shoot paid jobs this way and I know from painful experience what works and what is just wishful full frame thinking.

And still more! On most cameras, noise reduction or to put it more accurately detail reduction starts to kick in around 800 to 1600 iso, even with RAW files in many cases. Any camera that lets you stay under say 1000 ISO is going to have some obvious advantages, regardless of format.

And on it goes, for a great proportion of photos the ISO you will need will be closely related to the efficiency of the cameras image stabilisation and it is here that many DSLRs start to drop back because the stabilisation is normally in the lens....or not at all. Non-stabilised glass will ultimately force you to a tripod 2 to 3 stops earlier, or cause you raise the ISO by an equivalent number of steps.

Currently the Olympus OM is probably is the king of the hill where practical shooting under low light is concerned, its 5 axis image stabilisation offers 4 to 5 stops of compensation with pretty much any lens, but maybe a bit less with some. On top of that remember that m4/3 is 3 stops ahead on DOF to start with. So up front the advantage compared to a regular DSLR with non- IS lenses in marginal light could be a massive 7 stops where the subject itself is still! Or lets put this another way, you could use 200 iso instead of about 25000iso. Ah but I hear the DSLR fanboys yelling, "yeah but man, we got da fast glass and the super clean high ISOs, and I respond..so?

Just read back a few lines, I said “practical shooting” not DXO lab test king...practical.....like actually holding the camera in your hands, you know, without a tripod.  The OM gives you access to some equally fast native glass of top quality and it can use all your fast glass via adapters with full IS too.

Hell man you could even use a Metabones speed booster for another f stop boost plus fast glass and IBIS on the Olympus!

Now you can argue all you like, and I know those Big Boy Canon and Nikon users will, but using say a 85 mm 1.4 lens wide open under low light for anything even mildly close to the camera is never going to give you a reliable real world usable DOF for anything other than low res web images, to hell with the current shallow DOF fashion I say, it is just not practical.  

Like an example, lets say we take a shot at 5m, 85mm, f1.4, and what pray tell is your total DOF....oh about 17 cm or half a head! Your focus better be totally spot on 100% of the time otherwise its a crap shoot. Whats the chance of you reliably nailing a singer in a nightclub actually singing and moving around against those DOF odds?

Now.....just hold on a bit now as I have to go into the bedroom and pop my Ultra Flameproof suit on.........dum de dum dumm dum....Ok here I am all back.

I often run low light night photography workshops and one little aspect that became really obvious early on was that 400 ISO on one camera is not necessarily 400 ISO on another. Many Canon DSLRs in particular seem to be, ah how can I put this...ah ....using ISO ratings that are fantasy compared to say Nikon or Sony. I am talking about people shooting on full manual at fixed ISO with identical apertures and shutter speeds and the good old Canons ending up around 2/3 to 1 stops under exposed compared to their peers. No its not all Canons, the 6D for example seems to be fairly honest but frankly theres more than a fair share of porkies been told.

Don't bother arguing with me, I am not going to be convinced, remember this is when taking groups of people under real world conditions with various camera models and brands and looking at the resulting images side by side, it's not isolated, it has happened in every single workshop, I don’t make this stuff up.

Moving beyond the settings and brands, camera style and ergonomics has an enormous role to play in what you can get away with. Currently Sony has the A7r and the RX1 in their catalogue, one has a smooth as silk leaf shutter and no IS, the other a rattle gun shutter ( sorry, I mean a slightly loud and little bit clunky) ....oh forget it....the shutter is crap. Anyhow the RX1 can be shot at very low speeds despite the lack of IS, the A7r is by all accounts a bit hit and miss dependent upon the shutter speed and lens fitted so is far more likely to actually need those marvellously clean high ISOs.

Granted there are some very specific circumstances where a FF DSLRs better high ISO performance will translate into better, or at least lower noise images. Mainly these are situations where DOF really doesn't matter, like astronomy or shooting really distant landscapes under very dim light, or perhaps arty super shallow DOF stuff, of if you shoot using a tripod.

Lets say you shoot a moonlit landscape at F2 using a 50mm lens, so long as the nearest element you want in focus is at least 20 metres away you're good to go....or not.  There is a fly in that ointment, we are assuming you have a lens that is able to actually perform well at that aperture under low level but high contrast light, don't assume that can done at any sane price point for a FF format lens.

On the other hand, your puny little m4/3 camera can probably access a reasonably priced 24mm f1.4 lens that really will deliver at f2 (maybe even f1.4)....its just easier to achieve this with smaller image circles, period.  Realistically with full frame you will at least need to stop down to f2.8 to clear up the residual deficiencies in most 50mm FF lenses, barring of course the new $4000.00 Zeiss Otus or perhaps that new super duper Sigma on the horizon.

So we have arrived at an end, I think those with open minds have perhaps got my drift but just to be sure, what is my take away point?  

ISO ratings and high ISO performance by themselves are quite meaningless, unless of course you're in an argument in the Pub and the next shout is at stake, then tell whatever porkies are required. 


What matters is how the system as a whole works because that is what will determine what you or I can actually get away with.  In other words unless you can tell me about, shutter action smoothness and shutter type, DOF requirements, real ISO rating, lens choice, camera ergonomics, IS or no IS, all your doing is sprouting useless DXO numbers that may or may not translate into improved real world shooting results.

So the next time you hear some knowledgeable camera tester berating an m4/3 or 1 inch sensor cam for not having stellar 6400iso performance, think carefully before you start nodding in agreement at their infinite but flawed wisdom.

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