Use of ISO?

[tdodd]

the d3 and d700 have hi settings after 6400 though

I'm afraid I have no information relating to cameras outside the Canon brand. In theory there is no reason why the higher ISO values may not also be doubled, and doubled again, and doubled again, but I really don't have information for Nikon. It may be the case for Nikon that the higher values are real values, generated by analogue amplification within the sensor circuitry rather than simple digital doubling of values as is the case with Canon's H/H1/H2 settings.

 

[dod]

What I'm getting at is that shooting at 6400 ISO for a higher shutter speed is no different to shooting at 3200 ISO and underexposing by 1 stop in order to get the same shutter speed that you would get by shooting at 6400 ISO. i.e. if you are using an autoexposure mode then shooting at 3200 ISO and -1 EC will be the same as shooting at 6400 ISO and 0 EC. The "real" raw capture will be the same either way. By setting 6400 ISO all that happens is the camera "fakes" the extra stop by doubling the captured values.

Either way, the sensor will gather an identical amount of light and the pixel values at the sensor will be the same. All that shooting at 6400 ISO accomplishes is that the camera then takes those values and doubles them within the camera before writing the raw file. This might force some pixels into clipping territory when actually the sensor pixels were not saturated at all. If you shoot at 3200 ISO the values are left just as they were captured. You then get to do the doubling (or some other degree of adjustment) in your raw processor and you get an extra stop of control over whether pixels get pushed into clipping or not.

Thanks, I'll have to give that a try to see what the real world difference is

 

[AlexW]

I understand why you might do that but, if my understanding is correct, an exposure at 6400 ISO is actually an exposure at 3200 ISO at the sensor and the pixel values from that (under?)exposure are each doubled to "create" new values as though you had shot at 6400 ISO. The reason, I suspect, that L and H options are disabled by default, and that the manual recommends ISOs from 100-3200 is because the L and H ISOs aren't proper ISOs and should be used with caution (or not at all (unless you shoot JPEG)).

If you shoot to JPEG then that may well be worth doing, since nobody wants an underexposed or blurry JPEG. However, if you shoot to raw then there is nothing to be gained by simple numerical doubling of pixel values within the camera. You can do that just as easily in your raw processor. The downside of doing it in the camera before writing out the file is that you might send pixels into clipping that were actually captured perfectly well at the sensor.

Thus, for raw shooters, in the same way that you might sort out WB in post processing you might as well save "faking" your 6400 ISO exposure until then too and then you'll have the option whether to send pixels into clipping or salvage them. In effect, using 6400 ISO loses you 1 stop of dynamic range in the highlights.


......that's if my understanding is correct. (which I think it is).


In a similar way, 50 ISO images are actually captured by the camera sensor at 100 ISO and then the camera halves the resulting pixel values before saving the file. For a save to JPEG this may effectively perform a "highlight recovery" to some degree within the camera. Once again, for raw shooters, there seems to be no point. You actually end up with an overexposure of 1 stop, which might mean you clipped highlights that would not have been clipped if you'd had a "correct" exposure at 100 ISO.

My understanding of these things comes from raw file analysis work by a chap who goes by the name Panopeeper on some forums and GaborSch on others. Here's a thread with a discussion about "fake" ISOs and noise levels - http://photography-on-the.net/forum/showthread.php?t=625337. I tend to believe what Panopeeper says. So far I have not been let down.

That was fascinating. thanks for going into so much detail

 

[petemc]

Opposite to most of the replies, I use ISO50 on occasion to 'slow' the shutter down, an example of this is shooting waterfalls, I can often get a slow enough shutter speed to blur the water at f22 at ISO50, which perhaps I couldn't at ISO100.

Agreed. It may not be a real ISO as on my Nikon its classed as Lo1 but it can help with longer exposures which you can't fix in RAW later.