Re: [Leica] Leica M5(metering)

["Mark Hammons" <astair@ix.netcom.com>]

>Mark,

>That is what I was trying to say - I musn't have explained it very well.
>
>As you say, if you quote EV you need to quote three things: film speed, EV
>and aperture. It seems to me to be more direct, useful and simple to quote
>film speed and shutter speed only.
>
>The sensitivity of the M6 would then be quoted as equivalent to 2 sec with
>100 ASA film (I made an error in my original post). That simple statement
>gives directly useful information - you can quickly calculate the longest
>shutter speed that the meter will work at for any film speed. So, for
>example, you know that the meter will work to 1/2 sec with 400 ASA film,
>regardless of the aperture. It has nothing to do with aperture.
>
>The CLE's sensitivity was quoted by shutter speed and film speed. I guess
>that the only reason for using an EV/film speed/aperture combination is to
>allow comparison with hand-held meters.
>
>Any comments?
>
>Regards,
>Malcolm
>


I'm a little confused here so I'll just give my understanding of EV values.

Exposure value (EV) is a measurement of the total light.  The EV is a function
of light AND film speed.  It is calibrated such that EV 0 would indicate an
exposure setting of aperture set at F1 with a shutter speed of 1 second.  Thus
EV 10 would be consistent with a F4 aperture at 1/60th of a second and so forth.

Given that EV is a function of the light intensity AND film speed then it
by itself is independant of any "lens" -- that is why handheld lightmeters
can give the EV for a given amount of light when the meter is set to the
film speed of interest.

But thru-the-lens light meters in cameras must also take into account the
speed of the lens on the camera to guage the minimum and maximum
EV values they can measure.  If you are in EV 1 light and you have
a camera with built in meter that is just barely reading the light with
an F1.4 lens on the camera, will the camera be able to meter the
light if you put an F4 lens on the camera?  The answer is no.  The
EV 1 light will be cut down by another 3 f stops and will be below the
minimum sensitivity of the camera's meter.

Thus, when quoting the sensitivity of a TTL light meter in a camera
specs are usually given as:

EV 1 @ 100ASA and F1.4

By the way, this would be equivalent to

EV 0 @ 100ASA and F1.0.

Hope this clarifies the point I was trying to make.


Mark Hammons