"...the 40D's sensor is about 1/6 to 1/3 of a stop less sensitive than the
30D's. This means that images shot with identical shooting parameters
(shutter speed, aperture & ISO) are not exposed exactly the same. I should
note here that the 40D's metering is fine - this change does not cause
underexposed images."
How is this so? Surely ISO, is well ISO? If I was using a handheld
incident reading and shot in manual mode, does that mean that the image
would be under exposed?
>Looking at the following link, it says the following:
>http://www.the-digital-picture.com/R...ra-Review.aspx
>
>"...the 40D's sensor is about 1/6 to 1/3 of a stop less sensitive than the
>30D's. This means that images shot with identical shooting parameters
>(shutter speed, aperture & ISO) are not exposed exactly the same. I should
>note here that the 40D's metering is fine - this change does not cause
>underexposed images."
>
>How is this so? Surely ISO, is well ISO?
There are standard definitions for film ISO, nevertheless films do
vary, and the ISO you choose for a particular film might not be the
one stated by the manufacturer. For example, ISO 160 portrait films
such as Kodak Portra 160 and Fujicolor Pro 160 are usually exposed at
around ISO 100.
There is no accepted standard definition of ISO for digital images, so
you should not be surprised that there are differences between
apparently similar ISO ratings on different cameras.
>If I was using a handheld
>incident reading and shot in manual mode, does that mean that the image
>would be under exposed?
That depends what you mean by "underexposed". There isn't an
established standard for any of this, so what appears unexposed to you
may appear perfectly exposed to someone else. There are no absolutes
in digital photography.
> "...the 40D's sensor is about 1/6 to 1/3 of a stop less sensitive than the
> 30D's. This means that images shot with identical shooting parameters
> (shutter speed, aperture & ISO) are not exposed exactly the same. I should
> note here that the 40D's metering is fine - this change does not cause
> underexposed images."
> How is this so?
Canon used fairly pestimistic settings, to the tune of ca
1/3rd of a stop --- so their "ISO 1600" was more like ISO 2000.
They changed that for the 40D and now their ISO 1600 setting is
more like, say, Nikons ISO 1600 setting.
> Surely ISO, is well ISO?
Nope.
> If I was using a handheld incident reading and shot in manual mode,
> does that mean that the image would be under exposed?
With the 40D it would be "perfectly" exposed, assuming
- your meter is correct
- you metered correctly
- the t-stop[1] of the lens you use does not come into play
With the 30D, 20D or 5D your shot would be ~1/3rd of a stop
overexposed --- which is so close to "correct" that it doesn't
count for print film (which can take +/-2 stops off) or slide film
(which you can shoot with cameras offering only 1/2 stop steps)
and digital.
-Wolfgang
[1] t-stop stands for transmission stop. As I understand it,
cinematographic camera work is *very* sensitive to even slight
brightness differences, and thus t-stops matter to them.
t-stops are related to f-stops only as f/stops are always
equal or faster than t-stops. Think of using a lens at f/8,
and then think of using the same lens at f/8 with a 3 stop
neutral density filter --- same f/stops, different t-stops!
If you meter TTL (through the lens), t-stops can be completely
ignored, as the light has already passed through the lens
and has suffered any t-stop effects.
Apart from filters (e.g. ND, polarizer, stronger conversion
filters etc.) and intermediate rings you will see t-stop
creep up (in photography) with catadioptic lenses, as these
mirror lenses block part of the light by design with their
secondary mirror.
40D ISO Rating 
Linear Mode
