Mamiya 645 AE Prism Finder N

Having taken my first photographs with my 645 Super I was a bit disappointed with the exposure results using this finder.
I have read glowing reports about the light measuring this way but mine were erratic.

So I decided to RTFM again, and have begun to wonder whether I have misunderstood the instructions.

I thought that at its most simple setting AV that when you selected the 'round circle' mode it was in effect an aperture priority mode and that the led display showed you the selected shutter speed.
( I understand the implications of LT and Over display.) As long as you are careful to ensure that no one tone dominates ie very bright or very dark this should deliver.

I also chose SP and metered certain shots using this but these were underexposed. Is the spot area the whole of the centre circle i.e. includes rangefinder, microprism and matte? If so this is a much larger area than I am used to and it might be I have picked up backlighting in this reading. I will have to learn to use AE lock!

Using Auto A-S ought to choose the best method for metering based on the two readings and their ratio one to another. This does not appear to deliver consistent results.

Any comments welcome; either to tell me I have got it hopelessly wrong or to suggest your preferred settings, warnings etc.

Thanks
Graham

The AV means "centre weighted averaging" - it averages the reading over the scene, with extra weight given to the centre.

SP means spot, and it is indeed responsive to a moderately large central area - about what you describe. It is difficult to describe that area exactly, because the description varies with different screens. I find I can see where the sensitivity falls off on the screen if I carefully move the camera side to side and up and down while looking at a small light source.

The Auto A-S measures both AV and SP readings and automatically chooses one or the other, based on an algorithm that tends to favour a spot reading when the two diverge.

One thing I note with my AE Prism finder N is that it is susceptible to battery inconsistencies. Are you using silver oxide batteries?

Are you sure you haven't set some unexpected exposure compensation on the finder?

The system depends on setting the film speed on the back, and having that information communicated through the camera. You might check that the meter responds correctly when you adjust the film speed.

I'd also recommend cleaning the various electrical contacts.

I have a Mamiya Super with the AE Prism finder N and have had pretty consistent results. From what you are saying, you appear to be using the meter correctly. The only thing I can suggest is that you meter a scene and compare the readout from the prism finder and compare it to a hand held meter that you have already calibrated and have achieved successful results. If the meter findings differ, than you can attribute the discrepancy to a low battery or other condition. I have several cameras with built in meters and they can vary considerably. The only way is to test.

NEVER EVER forget that...

1. the A vs. S mode only differentiates the amount of frame area which it reads, that is all.
2. a reflected light meter assumes that the scene averages to 18% tonality (in the case of A mode) or that the object under the spot is itself an 18% tonality area (in the case of S mode).

So IF your scene averages to less/more than 18%, or IF you spotmetered area is brighter/darker than 18%, you still need to crank in Exposure Compensatiion for the amount that it is more/less than 18% tonality, or else your metered exposure will be in ERROR

Thanks everyone - more care needed then!