A900 - What ISO should be used for "base ISO"?

[deathvalleydave]

Hello David,

I'm primarily a landscape hobbyist photographer, and I've been scratching my head for some time over what I should be using as my base ISO on the A900 - 160, 200, or 320? I have run some test shots with highlights on both ends of the spectrum, and have difficulty seeing anything other than 160 seems to overexpose compared to 200 and 320 which look a bit underexposed compared to 160.

I think I read that you said 160 was the A900's true base ISO? Please correct me if I'm misquoting. I've also seen that 320 is supposed to have better shadow detail... But, I'm probably more interested in better highlight detail... My main question however, is how much difference is there in total RAW dynamic range between ISO 100, 125, 160, 200, 250, and 320 on the A900? Can I assume that they all have at least 11 EV of DR, with one of them closer to 12 EV, and one closer to 10 EV? It would be nice if a legitimate test site would post how much total RAW DR is available to you on the A900 at these 6 ISO's.

Thanks in advance for any input here,
Dave

[David Kilpatrick]

According to DxO, the base ISO is 160 but if you set ISO 100 you get 160 overexposed, and if you set ISO 200 you get it underexposed (which fits with my experience). If I was very worried about highlights more than anything else, I'd say use 200 or 320.

Agorabasta, Iliah Borg and others reckon that ISO 320 gives the greatest dynamic range. While I can't confirm that with figures, I can definitely say that 320 appears to give the most flexible raw file for development with Adobe Camera Raw or Lightroom. As you will know, sensors don't really have ISO speed at all, and highlight saturation is an absolute thing - shoot at 100 for example and you most certainly do lose a stop of highlight headroom relative to 200. Sony's indication (lines above and below ISO number) is that 100, 125, and 160 are saturated (overexposed and adjusted in gain-curve). 200 is not. Tests of actual sensor speed show that 160 and 200 are actually the same gain, all that the system does is give 1/3rd stop more exposure at 160.

320 is what I use simply because many tests I've done show that the noise-grain level is exactly the same at 320 as 200, so there is not much point in using 200. Sometimes I get lazy or make an error and end up with shots at 200 and they are nearly always not as good as similar shots at 320.

David

[deathvalleydave]

Thanks David,

I really appreciate your reply. I will study this now, and I'm thinking that I'll end up at ISO 320, based on your detailed response... Thanks again, because my tests bear out exactly what you have said - when I looked at 160 vs. 200 and 320, 160 looked slightly overexposed in comparison to 200 and 320. The difficulty for me is seeing how 320 looks better overall to 200, because it looks like there is more noise, but I have both Bibble 5.1.1 and LR3, and I always perform a Noise/Sharpening ratio/combo as one of my post-process steps... At this point, I should be fine with shooting 320 - one additional advantage is a faster shutter speed!

Dave

[redsim74]

David,

Does the A700 ISO have similar characteristics?

[David Kilpatrick]

The A700 does indeed work much the same way. We have been using the A700 without worrying about ISO anyway since ACR 6.2 arrived. I can file pictures to Alamy, who are very picky, at ISO 1600 with no worry at all.

David

[redsim74]

Thanks, David.

I use Aperture 3.1 for my RAW conversions, which seems pretty good on the noise processing. I think the latest Lightroom/ACR edges it though.

[Simmondsphotography]

David,

Does the A700 ISO have similar characteristics?

I can also confirm that both my A700 and A900 produce the best images at a base ISO 320 and thereafter I try and use multiples of 320, ie 640 rather than 800. I rarely use anything higher than 800. I am using Capture One Pro as my raw file converter program. Nice clean grain free images without blocked up shadows or blown highlights....usually!