Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

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On my Canon 500D, I can set aperture and shutter speed in 1/3 stops, however with ISO I appear to be limited to whole stops (100, 200, 400, 800 etc...)

Why is this?

It would appear the camera is capable of partial ISO stops, as if I leave the ISO on Auto, I sometimes see values such as 150, or 320 for the ISO.

Is the reason simply that this is an entry level camera, or is there some other reason I can't set ISO to whatever I want?

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It's a actually a good thing. Intermediate ISO are usually more noisy because they use software amplification. That is one reason why review and benchmark sites, only show full-stop ISOs. –  Itai Jul 19 '11 at 12:37
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3 Answers

up vote 15 down vote accepted

Analogue amplification (pre-digitization) is the best way to implement a variable ISO sensitivity. When you do so you are only amplifying the photon noise. When you amplify the digital data (i.e. by multiplying all the values by a fixed amount) you amplify the photon noise, the read noise and the quantization noise, leading to more noise overall!

Most Canon DSLRs only have analogue amplification circuits for the whole stops (100, 200, 400, 800 etc.), when you select one of the intermediate fractional values (520, 640 etc.) uses the closest analogue amplification stop and then uses digital multiplication to give the correct overall sensitivity. E.g. ISO640 is really ISO800 multiplied by 0.8, ISO500 is really ISO400 multiplied by 1.25 etc.

This is bad as you either end up using a higher analogue ISO and lose highlight headroom, or a lower analogue ISO and get more noise than necessary. For this reason I would advise against using the fractional stops on Cameras that support it.

For an example of how bad digital amplification can be compared to analogue, see this image:

The top image was shot at f/2.8 1/30s and at ISO1600, i.e. 5 stops of analogue amplification. The bottom image was shot at an identical f/2.8 1/30s but ISO100, and then 5 stops of digital amplification were applied in Photoshop. The result is that digital amplification gives a lot more noise.

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Is this only for Canon models or for Nikon too? Is it true for all Canon models having partial ISO capability i.e 60D, 7D? –  fahad.hasan Jul 19 '11 at 11:41
    
Thanks for the answer! I guess I should be avoiding Auto ISO then? –  Tom Jefferys Jul 19 '11 at 12:02
    
@ShutterBug I think it's true of all Canons except the 1D line. As far as I know Nikon implement partial stops in hardware on their cameras. It's not hugely difficult, just as example of Canon being lazy and assuming users wont notice (hint: some do!) –  Matt Grum Jul 19 '11 at 12:21
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@Matt - although your point is understood, note that in order to get a standard exposure,m at the end of the day you will have to adjust the brightness of your image in software (kinda what you did in the example). Thus, I don't see how you can run away from that, and if your camera's firmware is good enough to do that on capture time, then why not just use it. After all, 1/3 of a stop is barely noticeable and your example (making the point, though) is exaggerated. –  ysap Jul 19 '11 at 14:44
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@ysap there are usually other ways to get a standard exposure (change Av/Tv) but let's say you can't, having the camera manipulate the raw data offers no advantage to the raw shooter (assuming they tweak exposure in post anyway) and conveys some disadvantages, i.e. noise/greater likelihood of clipped highlights. These disadvantages may not be significant, but having the camera's firmware irreversibly alter the raw data in a way easily replicated in post seems to go against the philosophy of shooting raw. –  Matt Grum Jul 19 '11 at 15:09
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If I look at the specs of the 500D, there doesn't seem anything like 1/3 EV ISO increments (while the likes of the 7D do offer that).

And this discussion seems to confirm that the 500D only offers full-stop ISO increments when not in automatic mode.

Your first suspicion looks like the good one: it's not high-end enough...

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Even entry level Nikons have sub-stop ISO adjustments. Just saying... :) –  ElendilTheTall Jul 19 '11 at 8:13
    
@ElindilTheTall it's actually worse than that, not even the higher end Canons offer partial stops in hardware, it's faked in software - with sub-optimal results. –  Matt Grum Jul 19 '11 at 10:58
    
@Matt - do you know that Nikon has hardware circuitry for achieving that? –  ysap Jul 19 '11 at 14:45
    
@ysap I don't know for sure, you'd have to be a Nikon engineer for that, but I know they don't display the spikey noise vs. ISO graph that's a dead giveaway that something is up with the fractional stops –  Matt Grum Jul 19 '11 at 15:15
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Its a great mystery why a company like Canon do not include user controllable partial ISO stops when the camera itself is capable of delivering it (in Auto ISO mode). Its not like there's any need of sophisticated hardware or extra electronics.

There is one third party firmware called MagicLantern which will remove this barrier and allow you to control partial ISO stops. But this firmware doesn't come with any warranty and might damage your camera permanently (though I haven't seen anyone with a broken camera).

However, if you really need partial ISO increments, I guess its better to buy an updated model which supports this feature, its a good excuse to upgrade your body.

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Ah but there is a need of sophisticated hardware / extra electronics to implement partial ISO stops properly. Canon cameras with fractional ISOs or hacked firmware just fake the intermediate stops by fiddling the numbers in the raw data, which is bad (see my answer). With auto ISO in M mode you actually need partial ISO stops to get the right output levels in a JPEG. But if shooting Tv/Av without auto-ISO you can always adjust shutter and aperture in third stops to get the right exposure when using a whole stop ISO. –  Matt Grum Jul 19 '11 at 10:55
    
Partial stops have to be part of the auto-ISO function in order to get the correct exposure when using auto-iso with fixed shutter and aperture speeds. However they are implemented in software not hardware so the ISO150 you get in auto mode is "fake", I think the partial stops are unavailable when auto-iso is not on in order to simplify operation of the camera. –  Matt Grum Jul 19 '11 at 12:17
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