Silly question #42

[nsouto]

Hi folks. Have had this one in my mind for a while, thought I might ask here and someone might enlighten me.


OK, here is my "I wonder" problem. Nikon manual focus slrs have a meter that usually "indexes" to the max aperture of the lens. Ie, if any given lens has a max aperture of 2.8 while another has a max aperture of 4, the camera meter will know about this while taking a measurement.

My 6X6 Arax meter also has a little manual index adjust. So does my RB67 prism meter.



All that is very nice, but: meters supposedly measure light.

In terms of absolute amount of light coming through a f2.8-max lens set at f4, it is exactly the same as a f4-max lens set at f4.

So why do we need an index adjust? Is it because the meter is working on an automatic lens at the max aperture and as such needs to know what that might be?

Or am I miles away from the reason?


After all, my old faithful - the Lunasix F I carry with me everywhere - doesn't give a hoot what lens I'm using: it just tells me what to set it at. And I believe so does every other external meter.

 

[Steve Smith]

This may not answer your question but the way I think the Nikon system works is this:

Metering is always performed at minimum f No. (aperture wide open). The meter does not 'know' if this is f2.8, f4, etc. and it couldn't care less.
What it does need to know is how many stops the aperture will stop down when the shutter is fired. Again it does not need to know the actual aperture number, just the ratio between fully open and selected f No.

Perhaps the Arax and the RB67 prism work differently and do tell the meter the actual aperture selected. Therefore the meter needs to know what the fully open aperture is in order to do the compensation.



Steve.

 

[nsouto]

That makes perfect sense to me!
Thanks a lot.

 

[John Koehrer]

This may not answer your question but the way I think the Nikon system works is this:

Metering is always performed at minimum f No. (aperture wide open). The meter does not 'know' if this is f2.8, f4, etc. and it couldn't care less.
What it does need to know is how many stops the aperture will stop down when the shutter is fired. Again it does not need to know the actual aperture number, just the ratio between fully open and selected f No.

But.....
When you index the lens it changes the start point on an internal resistor band.
It's not just counting steps from the maximum aperture, the voltage changes with the rotation of the resistor band against a fixed contact. The change in voltage moves a coiled hairspring to which the meter needle is connected.