Thanks for those links. I had started to read the first when you (?)
posted it a week or so back but got interrupted and never finished it.
I think I understand now but am still having trouble with his comments
about large pixels and sensor saturation. He says:
----------------------------------------------------------------
1. Sensor ISO is based on the amount of light it takes to saturate the
sensor to pure white. This is what DxOMark measures and reports.
Sometimes that yields counterintuitive values. Double the collection
efficiency of your sensor so it only takes half as much light to
saturate the sensor, and you've doubled the ISO. That makes sense.
Suppose, though, you just make larger pixels that can hold more
photoelectrons. What happens then? Well, it takes more light to saturate
the sensor, so the sensor ISO drops even though the sensor still has the
same sensitivity to light.
---------------------------------------------------------------
The first paragraph makes perfect sense. But in the second he says
large pixels hold more photons so it takes more light to saturate the
sensor. That doesn't make sense to me. It certainly takes more photons
but the pixel size is an area measure. The pixel will collect more
photons in a given unit of time just by virtue of its size. It's not
evident to me that sensor ISO should drop. More photons were needed but
more should have been received during the same exposure time. There's
something here that's not stated or I just don't understand. Equated to
film it sounds to me synonymous with saying that the ISO of 2-1/4 film
should be less than that of 35mm film because the collection area is
larger. Eh?
Chuck Norcutt
On 12/2/2012 4:04 PM, Moose wrote:
> On 12/2/2012 4:05 AM, Chuck Norcutt wrote:
>> Thanks, Mike, that's really interesting. The EM-5 equals or exceeds the
>> 5D (and 7D) in every category except that the 5D holds a one stop
>> advantage in low light noise. Another big surprise though it that the
>> entire EM-5 ISO range seems to be overstated by about one stop. ISO 200
>> is really 100, 400 is 200, etc. I'm wondering how this relates to the
>> camera's own metering measurement vs that of an external meter. Could
>> make a big difference in, say, using an external meter for studio flash
>> work. Not a problem as long as you know it exists.
>
> That's why DxOMark gives you a plot of camera ISO vs. sensor ISO. It doesn't
> reveal that a camera manufacturer is
> "fudging" or "lying." It tells you how the two different kinds of ISO, which
> are legitimately determined in very
> different ways, compare. That's all. It's not an exposé. It's just technical
> information. So far as real photography
> practices go, we care a lot more about camera ISO.
> - Ctein
> <http://theonlinephotographer.typepad.com/the_online_photographer/2012/09/why-iso-isnt-iso.html>
>
> Much more detail, including how it relates specifically to the E-M5.
> <http://theonlinephotographer.typepad.com/the_online_photographer/2012/10/raw-is-not-raw.html>
>
> In my tiny pond, I'd say that in actual practice, shooting the things I
> shoot, the E-M5 ISOs are about the same as 5D
> and 60D and the compact cameras I have. No tests, just moving from one camera
> to the other and seeing the image files
> that come out.
>
> If It Moves ... Moose
>
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|