40D ISO Rating

["Followup-To:" header set to rec.photo.digital.slr-systems.]
Ali <me RemoveThis @privacy.net> wrote:
> Looking at the following link, it says the following:
> http://www.the-digital-picture.com/Reviews/Canon-EOS-40D-Digital-SLR-C...ra-Revi

> "...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.
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