What is that small circular window for?

[Guest tummydoc]

Advertisement (gone after registration)

But the EV the meter reads is going to be dependent on the aperture setting isn't it?

 

With a handheld meter there is no aperture, so the EV is directly related to the amount of light falling on its sensor, but with an aperture the amount of light can be varied by changing the aperture, or am I missing something?

 

Aha. Coffee, I need coffee

[marknorton]

In theory, you don't need to know the maximum aperture value of the lens because comparing the direct and TTL light levels gives you the working aperture as I described earlier. However, if you get wacky results, it helps to know the maximum and minimum apertures for the lens mounted so that you can limit check to avoid embarassment, "the calculation says f0.5 but I know this is only an f2.8 lens...". It would be interesting to know whether estimated f-numbers out of range are regarded as unreliable and ignored.

 

The blue dot sensor is at the back of a "light pipe" which will give it a relatively narrow angle of view but, as Sandy says, if the lens and blue dot sensor are not looking at the same thing (never mind any difference in spectral response), any result is an informed guess at best.

[2Bébèrt]

 

1.4 Brightness sensor1

 

1 Leica M lenses with viewfinder attachments cover the brightness

sensor. Information about the use of these and other lenses can

be found in the sections “The displays/In the viewfinder”, p. 78,

Illustrations inside front and rear covers and “Leica M lenses”, p. 86.

 

2.1 LEDS (Light Emitting Diodes)

(with automatic brightness control, adapts to

the ambient brightness1) for:

 

 

1 The automatic control is not available for Leica M lenses with

viewfinder attachments, since they cover the brightness sensor

1.4 which supplies the information required for their operation.

In such cases the displays always maintain a constant brightness.

[2Bébèrt]

pict of sensor

Welcome, dear visitor! As registered member you'd see an image here…

Simply register for free here – We are always happy to welcome new members!

[Hookeye]

The problem is that the angle of view of the external cell is not the same as the lens based internal cell. Which is why the aperture measure is only approximate. It would be especially bad for long lenses.

 

Well, in theory that could also be compensated for. Because the 6 digit code tells the camera what lens is in use - the angle of view is known. If the sensor were equipped with several concentric zones which could be measured independently, then the angle-of-view issue could be accommodated.

 

I still believe that with the help of some clever engineers the extra cell could indeed give a fairly reliable indication of the apartrure. But then I've just come back from town where I've had a couple of glasses of wine. Just now, everything seems possible. No, it is not lack of coffee which is the problem here..

[marknorton]

I can assure you the blue dot sensor has no such sophistication. Looks to me like it's a photo-diode costing, say, $0.50 in quantity.

[Hookeye]

Advertisement (gone after registration)

I can assure you the blue dot sensor has no such sophistication. Looks to me like it's a photo-diode costing, say, $0.50 in quantity.

 

That, to me, is the mystery of this circular window. It seems the engineers at Leica started off with some great visions for what this sensor could be doing, but then it all collapsed because of a cheap hardware solution (photodiode) and half-hearted attempts at writing firmware for it.