I think I understand what the problem is here. It's something very
easy to illustrate and test with most any lens. It's the meter
sensitivity of the E-1 itself.
According to the specs (depending on which ones you look at, because
there is some variance between published specs), the detection range
is ESP and Center-weighted averaging is EV 3 to 20 (with 50/2 lens,
which is a significant hint), and EV 3 to 17 in Spot. Some of the
specifications say 1-20 or 2-20 or 3-30 or 2-17 or 3-17, but with MY
camera, I'm pretty confident that the metering range is 3-17 for
accurate metering, with 1-3 being depending upon quality of light or
color of light.
So, let's say you have a normal E-series lens mounted on the camera.
When you are metering the scene, the lens is wide-open and the camera
extrapolates the exposure based on known stops between the wide-open
aperture and the set aperture. This way, by metering wide open, the
exposure determination is almost always within the acceptable range of
EV 3 to 17. The DMC-L1 specs say 1-20 with the viewfinder, but I am
very very confident that those numbers are fantasy land.
But what if you place a legacy lens on the E-1? If the lens is wide
open, the camera will likely meter the scene like it did with the
E-system lens. (with a caveat which I'll address in a moment).
However, if you stop down the lens to your desired aperture and try
metering, what happens is that you may no longer be within the
acceptable range of the exposure sensor. As you get to the edge of the
range, the meter becomes less accurate.
Illustrate this by running a series of auto exposures with a
"normalish" lens. Adjust the apertures in full stops across the entire
range. You'll see that wide-open, a lens like the 50/1.4 will not be
accurate and stopped down at F11 or F16 it won't be accurate, either.
As you get to the edges of the range, you'll get over exposures.
So, with this 300/2.8 and 2X converter, you're already knocking two
stops off by using the teleconverter. Combined with another factor
(which I'm still getting to), and the camera is highly unlikely of
getting an accurate exposure because you're leaning into the edges of
the EV range of the exposure sensor.
Now for the other factor... Lens design. in the specifications for the
E-1, it clearly states the 50/2 lens. This is a nifty hint that not
all is going to be perfect in exposure land. The exposure detector in
the E-1 is located in the prism hump. (When using spot metering,
depending on the camera, it may actually use the center-point AF
detector). As it is measuring the light in the viewfinder prism kinda
sorta off the focus screen, you are subject to odd optical traits
through this optical chain. The focus screen is NOT a frosted glass
type screen that we grew up with. Instead, it's actually a clear
screen with a very fine microprism mesh. The angle of light from the
50/2 follows a specific light path that the viewfinder and metering
system is optimized to. The "telecentricity" of the E-system lenses
also provided for a uniform transmission through this optical chain
too, so the metering is pretty accurate for all E-system lenses. But,
if you change lenses to a design which passes light through the screen
at a different angle (by not being telecentric, or being outside the
acceptable aperture range of the microprism design), the meter is
unable to accurately measure the light. We see this with our own eyes
as we stop down the lens and we don't actually see any exposure change
in the viewfinder. Stick a 50/1.4 on the E-1 and you'll have to click
down to F4 before the viewfinder brightness changes in any significant
manner.
So, the factors involved are:
1. Brightness level of the scene being metered.
2. Telecentricity of the lens designed.
3. Dimming factor of the lens as it is stopped down.
4. Dimming factor of the teleconverter.
I use the Tokina AT-X 100-300/4 and OM Zuiko 300/4.5 on my DMC-L1 all
the time for moon/sun shots. The metering with the viewfinder is
always going to be off to one extent or the other. As to live view,
the rule of thumb is that if you can see the subject on the LCD, it
will meter correctly.
--
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
--
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