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robminchin

Learning the E-620: Auto ISO

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Unlike on my H2, where there was not a wide range of ISO levels without intrusive noise, the Auto ISO on the E-620 can, potentially, be useful (particularly for those, like me, who are just starting out and haven't quite got the hang of where the ISO button is yet). However, like all tools, it requires understanding in order to be used properly.

The Auto ISO setting applies, by default, to most modes. It can be activated for M mode, but cannot be set to anything other than ISO 200 for the 'portrait' and 'night portrait' scene modes (even if the default is set to something different). If it is not activated for M mode, switching to that mode forces ISO 200 (although, unlike the portrait modes, this is reported as ISO 200 rather than ISO-A 200).

I've read in a number of places on the web that Auto ISO cannot select ISOs lower than 200. This is untrue. There are two settings that can be specified: the default and the maximum. These come set to 200 and 800, which seem reasonable levels, but both can be set in the range 200 - 3200, the maximum having to be at least as high as the default (obviously). Auto ISO starts at the default and increases up to the maximum to prevent under-exposure or a too-long exposure time (of which more later). However, it can also decrease from the default to ISO 100 in order to cope with over-exposure. This can most easily be seen in S and M modes (indeed, I've only be able to make it happen in these modes), although presumably it could be forced in P and A modes with a bright-enough light source.

For M mode, the way Auto ISO works is fairly simple - it adjusts the ISO to maintain a balanced exposure as much as possible. This means that if you are setting an over-exposure in M mode with Auto ISO enabled you will always end up at ISO 100 and if you are setting an under-exposure you will always end up at the Auto ISO Maximum. It is not possible to add an exposure compensation to this, so you will always almost always end up at the metered exposure.

For S mode, operation is also fairly simple. The camera will adjust the aperture as far as possible (this, of course, depends on the lens) and will then adjust the Auto ISO. Once it runs out of Auto ISO it will start complaining (flashing the exposure numbers).

For A mode, P mode and the various other automatic modes where the camera is setting the exposure time, the situation is slightly more complex, and the control rather idiosyncratic. Obviously, the exposure time cannot be allowed to climb to 60s before the Auto ISO starts escalating. Instead the camera starts raising the ISO once the exposure time reaches the reciprocal of the 35-mm equivalent focal length (the old 1/f rule of thumb for shake-free shooting), as long as this falls within a couple of limits. By default, these limits are 1/200s and 1/60s - if the lens is longer than 100mm (200mm EFL) then the exposure time is allowed to drop to 1/200s before the ISO climbs, while if the lens is shorter than 30mm (60mm EFL) the ISO climbs at 1/60s anyway. No account is taken of IS in deciding this.

These are, as I said, the defaults. They can be changed - and here we hit an Olympus idiosyncrasy. I looked at these numbers and thought "these look familiar". I then went an played with some settings - the long limit is the flash X-sync time; the short limit is the minimum flash exposure. By changing these values (under 'custom flash' in the gears menu), you can control the behaviour of Auto ISO. Just in case you missed that, I'll repeat it. The behaviour of Auto ISO is controlled by the settings in the 'custom flash' menu. I'm sure this made sense to somebody in the Shinjuku Monolith!

For P mode, A mode and the art filter modes, this is saved; for Auto mode and the scene modes (those which don't have their own Auto ISO settings), the values reset each time. I have therefore set the minimum flash exposure to 1/30s, which means that when shooting at 14mm I don't end up with climbing ISOs until I hit an exposure time of 1/30s. At the long end, the X-sync cannot be made higher than 1/180 so Auto ISO cannot be made to kick in until 1/200s. For 300mm (my longest lens), this is 1.6 stops below the 1/f rule, which should be within the limits of the IS.

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Updated 27th October 2009 at 09:39 PM by robminchin

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Comments

  1. AndyElliott's Avatar
    Rob

    It's not surprising that the x-sync affects the auto-iso. The behaviour on the 620 appears to be similar to the other E cameras. A couple of interesting things... The minimum auto value appears to be 200 on the 12MP cameras, but you can configure it to be 100 on the E-3 - this appears to be to do with the base sensitivity of the sensor (about 125 on the E-3 and about 200-and-a-bit on the 12MP beasts).

    The max x-sync is a function of how fast the shutter curtains traverse the focal plane - faster on the E-30 and E-3, so the fastest speed that the sensor is exposed in one go is 1/250s on the E-30/E-3 and 1/180s on the other cameras.

    Andy