ISO Expansion Question

I'd like to ask what many of you will no doubt consider to be an elementary question. I'd just like to clarify what I think to be the case with ISO expansion on an EOS 1DMkII.

I have toyed with activating the ability to shoot at the lower expanded ISO (50) in the menu several times at the races so that I could shoot a moving race car in daylight at a relatively slow shutter speed (for wheel and background blur) and a decreased depth of field (sometimes there is signage in the background that detracts from the shot if it is too clear). The main thing that has kept me from doing this is reports of a reduced dynamic range at the expanded ISO settings, but I think I should see what I think myself about this by trying it and seeing what happens. It's only one stop, but sometimes that's enough to make a difference, and since these are images of race cars, not portraits or something, the reduced dynamic range may not be as destructively important for me as it might be for some others. Optionally, of course, I could just use an ND filter on the front of the lens, but for as often as I might want to use this option that route might not be worth the cost of the filter or the hassle of installing/removing it.

Anyway, here's my question. If I enable the expanded ISO feature in the menu, and leave it enabled all of the time, the dynamic range limitation will apply to only frames shot at the expanded ISO setting, correct? I could leave that feature activated for easily selecting and de-selecting ISO 50 without going back to the menu and still shoot at other ISOs (100, 200, 400, whatever - except 3200, of course) with absolutely no difference in the dynamic range results from what I would get if that feature were not activated, right? That is my impression from the little I have read on the subject. I'd just like to confirm that is correct. I would not want to leave the expanded ISO feature activated if it were going to negatively affect the quality of frames shot at "normal" ISO settings.

Not that it matters, I suppose, but I shoot RAW all the time.

Bob,

What you presume, I think, is correct. 50 ISO merely shows as "L" in your ISO options (as 3200 shows as "H"). Ratings in between are unaffected by their presence. I've had mine set from day one.
The ND option could give you a bit more latitude with achieving even slower shutter speeds: ND 4X =25 ISO, 8X = 12.5 ISO. (from a 100 ISO base)

John.

There is NO dynamic range limitation at ISO50...

Bob Krueger said:

The main thing that has kept me from doing this is reports of a reduced dynamic range at the expanded ISO settings,

Click to expand...

ISO50 = ISO75-80, in reality, and to get same (or even better) dyanmic range of ISO100, make sure you dial -0.3 to -0.7 EC, while shooting at ISO50. You will not get true ISO50, but you will maximize sensor's performance, though.

Results are excellent, and you actually get to the top, to the very best you can pull out from the 1D MKII sensor (this is based on my N, which should be the same in RAW as my former non-N).

Enjoy!

Ferenc Harmat said:

ISO50 = ISO75-80, in reality, and to get same (or even better) dyanmic range of ISO100, make sure you dial -0.3 to -0.7 EC, while shooting at ISO50. You will not get true ISO50, but you will maximize sensor's performance, though.

Click to expand...

There really needs to be some distinction of different levels of DR. DR is just an abstract concept, unless you strictly define what you mean by it, as there really is no hard limit on what the lowest usable S:N is. I tend to look at things from the viewpoint of the ability to use very deep shadows. In that regard, ISO 100 is always going to give the best "DR" results with the 1-series cameras, because the S:N will be better 8 stops or more below saturation at ISO 100 than 50, because the blackframe read noise is basically the same in ADUs, and ISO 50's saturation point is a much lower ADU than ISO 100's.

If you were to graph the S:N with both ISO 50 and ISO 100, and didn't adjust with the EC you suggested, the lines would be the same near black, and then eventually the ISO 50 would start having a higher S:N as you got to the brighter areas, but the line would stop before ISO 100 did.

If you graphed the S:N lines using the EC compensation for ISO 50 (X axis resresent stops below saturation), then ISO 50 would be better in the midtones than ISO 100, but not as much better as the uncompensated situation. Going down near black, the ISO 50 starts to get a *higher* S:N than ISO 100, because you have boosted the read noise, which is basically the same at both ISOs, uncompensated. Of course, compensated, the ISO 50 line goes all the way to saturation.

The bottom line is that any S:N curve has two very distinct elements to it; shot noise and blackframe read noise are somewhat independent, and their variations can mean that one ISO could have the highest S:N 5 stops below saturation, and the other could have a higher one 8 stops below saturation. Noise is not monolithic.

Interesting, same view here, over time...

John Sheehy said:

I tend to look at things from the viewpoint of the ability to use very deep shadows.

Click to expand...

Exactly. This is precisely where I notice the ISO75-80 sensitivity allowing me to push shadows up, very, very easily (right from C1) with a minimal noise toll. But here is where the mystery deepens: that is, CLEARLY less noise than ISO100 on the bottom-end, while still having excellent top-end tonality (basically SAME as ISO100!).

In that regard, ISO 100 is always going to give the best "DR" results with the 1-series cameras, because the S:N will be better 8 stops or more below saturation at ISO 100 than 50, because the blackframe read noise is basically the same in ADUs, and ISO 50's saturation point is a much lower ADU than ISO 100's.

Click to expand...

It should behave as you describe, indeed. That is how ISO50 should respond, but... when EV compensating for the "true" sensitivity level (thus avoiding forced "under" or "over" exposing) results turn out somewhat different, indeed.

I have a couple of files that I can send to you (via www.yousendit.com), if you desire, and compare for yourself. They were correctly exposed (dead-on for ISO100 and -2/3EV for ISO"50" (or real ISO75-80). You should see shadow noise, dynamic range, etc. A true revelation

Let me know.

John Kirby said:

Bob,

What you presume, I think, is correct. 50 ISO merely shows as "L" in your ISO options (as 3200 shows as "H"). Ratings in between are unaffected by their presence. I've had mine set from day one.
The ND option could give you a bit more latitude with achieving even slower shutter speeds: ND 4X =25 ISO, 8X = 12.5 ISO. (from a 100 ISO base)

John.

Click to expand...

Thank you, John. That's what I thought. I think I'll give it a try at the next race and see what I get. It's obviously less time consuming, as the cars go by, to simply reset the ISO than to get into the bag, get the ND filter, screw it on, etc.

Bob Krueger said:

It's obviously less time consuming, as the cars go by, to simply reset the ISO than to get into the bag, get the ND filter, screw it on, etc.

Click to expand...

But you do have to remember to decrease the EC, if you are already in the custom of using all the RAW DR. If you're already exposing in such a way that normal-contrast JPEGs don't clip, then this is probably not going to be an issue for you.

John Sheehy said:

But you do have to remember to decrease the EC, if you are already in the custom of using all the RAW DR. If you're already exposing in such a way that normal-contrast JPEGs don't clip, then this is probably not going to be an issue for you.

Click to expand...

Well, yeah, I guess. But even that's only a button push and wheel turn, which should be a lot quicker than the filter option.