Mamiya 645 Pro incorrect metering with AE Prism Finder N

[nick_brown11]

<p>I've realised that my Mamiya 645 Pro is not metering correctly with the AE Prism Finder N but goes progressively out of sync at higher apertures.<br>

I'm comparing the metering with a Gossen Digipro F light meter<br>

Changed the camera battery<br>

Cleaned and inspected all contacts between AE Prism Finder.<br>

Tried different backs at different ASA/ISO values (I have 3 backs)<br>

Checked and cleaned contacts between the backs and camera<br>

Camera lens correctly attached with aperture arm engaged<br>

For example, I meter a white object on spot metering with a constant light at ISO 400<br>

f2.8 60th second - Matches Light Meter<br>

f4 15th second - Light Meter says 30th second<br>

f5.6 1/2 second - Light Meter says 15th second<br>

f8 - LT flashing - Light Meter says 8th second<br>

I noticed that the 645Pro has 1 more connection contact than the AE Prism N.</p>

<p>Any ideas of what could be the problem please?</p>

[c_watson1]

<p>Have you looked through the online manual? It's '80s circuitry that may not be ageing very gracefully. Got a handheld meter? Use it.</p>

[Dustin McAmera]

<p>The lens aperture selector is at 'A' not 'M', right? That is, you're not metering with the aperture stopped down?</p>

[ondebanks]

<blockquote>

 

<p>The lens aperture selector is at 'A' not 'M', right? That is, you're not metering with the aperture stopped down?</p>

 

</blockquote>

 

<p>That's what I suspect also.</p>

<p>If the lens is on "M", then every time you think you are making a 1 stop change, you are actually making a 2 stop change:<br>

- a 1 stop reduction in the amount of light directly reaching the meter (the lens on "M" is physically closed down a stop, there and then - but the meter still "thinks" it is wide open and simply meters whatever it receives);<br>

- and another 1 stop that the meter adds, because the aperture pin tells it that the lens is going to be stopped down at the moment of exposure (the meter cannot realise that this stopping down has already happened, thanks to "M"!)<br>

<br />Thus, setting an f/2.8 lens to f/8 is not a 3 stop change in the meter, it's a 6 stop change.</p>

 

[nick_brown11]

<p>Thanks for the responses - you are right. I was metering with the lens set to M. I assumed that the meter would operate the same way as a modern DSLR.<br>

Much appreciated, saved me sending the camera off for an unnecessary repair.</p>