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I think Pentax have missed a trick by keeping the lowest ISO rating on their two new cameras at 100. It's not so much from a quality point of view as there's virtually no noise at 100 ISO with the K1D and the new models should be even better. However, a lower setting would have turned the K20D into a great street shooting machine - and here's why. I suspect not too many people bother to use the K10D's TAV setting but it has a lot of potential. It works by allowing the photographer to select an f-stop and shutter speed and then varies the ISO level automatically to keep the exposure correct at those settings regardless of changing lighting conditions. In other words, if you want to shoot at 250th at f5.6 all day long for some reason, it is supposed to allow you to do so.
At first I thought this was just a gimmick. I mean, who really needs to stick to just one f-stop and one shutter speed? And then it occurred to me how great this feature would be for street shooting. When you're out on the street and you're focusing by selecting an aperture and setting the lens to a given distance to ensure that everything from, say, 5 - 15 ft is acceptably sharp, you're really not in a position to change f-stops. On aperture priority, you can choose the appropriate f-stop and let the camera take care of the shutter speed. But what if the sun goes behind a cloud and you turn to take a shot of someone in a doorway? You could find that the shutter speed has dropped too far. For pictures of people moving just feet from me, I like to keep the shutter speed at 125th or above.
That's where TAV should come in, keeping the aperture and shutter speed where you want them by varying the ISO. As the old saying goes, in theory, practice and theory should be the same but in practice they're not. And in practice, TAV doesn't work as well as it should. And the reason is that there are not enough truly usable ISO speeds on the K10D. If I'm after good quality results, I won't use anything faster than ISO 400. That gives me just three EVs to play with - 100, 200 and 400.
Let's see how this would work on the street. It's a cloudy bright day (this is Scotland we're taking about, after all, and that's about as good as it gets much of the time). I have a 28mm lens on the K10D as that's the closest I can get to my 35mm film format favourite of 40mm. With the lens focused at 8 feet, I need an aperture of f11 to get acceptable sharpness from just under five feet to just over 26 feet (no need to work that out: just check out these depth of field tables). I pick my 125th minimum shutter speed and work out that on a cloudy bright day, I'd need to set the ISO at 200 for a proper exposure. I dial the K10D to TAV mode and now I'm all set.
But what if the suns pokes out from behind the clouds (it does happen from time-to-time, even in Scotland)? Bright sun would mean a two stop difference in exposure forcing the camera to drop down to ISO 50. But I only have ISO 100 so the image will be one stop over-exposed. Or perhaps a particularly black cloud passes in front of the sun at exactly the same time as Elvis emerges from a dark doorway riding Shergar. That's a difference of three or possibly even four stops from an open scene in cloudy bright conditions and would require an ISO of 1600 or 3200.
OK, I'd get the shot and it would make the front pages of all the papers but it wouldn't make the best A3 print I've ever seen. From what I've read on the 'net so far, the K20D and K200D will have usable 800 ISO and the dearer of the two passable 1600 ISO. For the K20D, that's a range of 5 EVs - better, but still not enough to make TaV the perfect street shooting mode. I wonder if there are any other DSLRs out there with a TAV mode, ISO 50 and a fully usable ISO 800?
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