Dec 292011

Let me make one thing clear: I’m an idiot and this is obvious, because it took me a dozen days to figure out how to use this camera. A dozen!

Well, this is about auto-ISO. Different camera manufacturers have different ideas about how it should work, and so far I have not found any camera that does it right. Nikon comes close to how I think it should work, but only close.

What is auto-ISO anyway? When we take an image, there are really only three important parameters that determine the outcome: aperture, shutter speed and ISO. For a proper exposure you can freely choose two of them, but the third must be a function of your choice. These three parameters determine the recorded image.

Note please that white balance is not elementary. It is nothing but a suggestion as to how the image probably should be interpreted, but whatever you set as white balance (or the camera chooses for you) does not influence what the sensor records.

Auto-ISO is a camera setting where the camera decides about proper ISO and you decide either about aperture, shutter speed or both. Nikon is often criticized for the fact that auto-ISO, when active, also works in manual mode, i.e. when you set both aperture and shutter speed, but they are perfectly right. Think of it from a photo journalist’s point of view. You make an image, you need a certain aperture for DOF, and because the subject moves, you also need a certain shutter speed. So now, what shall the camera do? For a proper exposure there is no other way than to calculate a matching ISO.

Of course the camera and you could disagree about what a proper exposure is, but cameras nowadays are quite good at figuring out how to pack as much of the scene’s contrast as possible into the dynamic range of the sensor. If you need the image, it’s not a bad idea to rely on that, and if it really bugs you because the resulting image is too light or too dark for your artistic feeling, for heaven’s sake fix it in post-processing or turn auto-ISO off.

What exactly do I expect of a sophisticated auto-ISO function? Well, let’s look at it in aperture priority mode, and first with ISO set to a fixed value, for instance the sensor’s native value.

I set the aperture to a certain value, supposedly to achieve a certain DOF or to catch as much light as possible, and I expect the camera to adjust shutter speed. If light gets low, there will come a point where I can’t possibly hold the shot, where shutter speed is so low that camera shake will ruin the picture. Without auto-ISO I will have to raise ISO manually, with auto-ISO the camera does it for me automatically.

The same works for shutter priority mode. Say I need 1/250 s to get an image without motion blur. When light gets low, the camera will compensate for it by opening the aperture, but at a certain point the lens is fully open, and at that point only ISO can be raised, either manually or automatically.

The same is true for program shift automatic and for full manual mode. In all cases we want ISO to be raised only if the primary compensation (shutter speed in aperture priority and aperture in shutter priority mode) is exhausted.

Normally I use aperture priority and here we have the question of the proper crossover speed, i.e. the minimum shutter speed that, when reached, causes the camera to raise ISO. On Nikon cameras I can set this minimum shutter speed, and I can also set the maximum ISO that I want to accept. Once maximum ISO is reached (remember, always one must yield), shutter speed again is lowered, this time below the minimum.

This is already very useful. It would even be better if I could set different values for different focal lengths, because in reality long lenses (or zoom positions) need higher shutter speeds than short ones.

Unfortunately on the Olympus E-P2 I can’t set the minimum shutter speed, I can only set minimum ISO and maximum ISO. In my opinion setting minimum ISO is pretty worthless. Of course I set it to base ISO, i.e. best quality. Why would I ever choose something else?

Minimum shutter speed seems to be hard-wired to 1/60 s. I currently have only the 17 mm lens available, but although I can set focal length in the stabilization menu (why must I set it at all, the camera knows about the lens anyway), doing so seems to have no influence on the crossover shutter speed. It’s always 1/60 s.

Well, with a 17 mm lens and an effective focal length of 34 mm, I need no stabilization at 1/60 s, or if I have stabilization, I should be able to go much lower with shutter speed.

The way I was using the camera, with an ISO range between 200 and 1600 and wide open in low light, I was always getting needlessly high ISO values. I didn’t realize it for a dozen days, because ISO 1600 on this camera is pretty good, about as good as ISO 400 on the LX5, but nevertheless, today I began to ask myself why I don’t get a better overall quality on this supposedly better and heavier and more expensive camera with its bigger sensor. Now we know why.

The lesson is, and I’m an idiot that it took me so long to come to the conclusion, that it is stupid to use aperture priority on a camera where I can’t adjust the crossover shutter speed, especially when I shoot in low light and always wide open, to gather as much light as possible.

Shutter priority is the way to go. I choose 1/8 s, a shutter speed that I can repeatably hold at this focal length, and the camera opens up aperture as long as possible. Only then does it raise ISO. As a result, if light is not so low, I get more DOF (which in my current style is welcome in all but very select cases), and the chosen shutter speed becomes the effective crossover speed. It’s so easy if you think about it 🙂

The Song of the Day is “Idiot Wind” from Bob Dylan’s 1975 album “Blood On The Tracks”. Hear it on YouTube.

  3 Responses to “1898 – Idiot Wind”

  1. I think this is why I always use manual ISO on the EP-1 with aperture priority mode. I feel more comfortable with the Panasonic algorithms, although they are almost worse in the opposite way–choosing a too low shutter speed rather than raise ISO. This is fine for still scenes, bad when people are moving about.

  2. Tech talk aside, this is a very fun image. 🙂

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