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Why is max aperture info needed / used?
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PostPosted: Sat Nov 02, 2019 8:18 am    Post subject: Why is max aperture info needed / used? Reply with quote

I've been fiddling with some Tamron adaptall-2 adapters with several lens mounts and film bodies.

My initial question was: how Pentax K and Minolta bodies get the max aperture information that is needed for correct exposure calculation using full aperture meterig.

Then I realized: They don't and they don't need it. The body just reads the light value wide open and stop by stop correct the time value.

But: why do Contax and Konica bodies use the max aperture value. Both meter exposure wrong if they don't get the max aperture value?

Question Confused


PostPosted: Sun Nov 03, 2019 3:34 pm    Post subject: Re: Why is max aperture info needed / used? Reply with quote

kansalliskala wrote:
I've been fiddling with some Tamron adaptall-2 adapters with several lens mounts and film bodies.

My initial question was: how Pentax K and Minolta bodies get the max aperture information that is needed for correct exposure calculation using full aperture meterig.

Then I realized: They don't and they don't need it. The body just reads the light value wide open and stop by stop correct the time value.

But: why do Contax and Konica bodies use the max aperture value. Both meter exposure wrong if they don't get the max aperture value?

Question Confused


I don't know about Contax or Konica, but some Fujica bayonet slr's are the same, the AX-5 can take advantage of the feature by being able to use M42-screw lenses in open-aperture shutter-priority and program modes, using the appropriate (XD) adaptor ... possibly the only body that allowed that functionality with those older lenses?

Maybe the body "knows" how far to push the aperture linkage in for any given stop, provided it's been "told" where it's starting from?

Either way, I'm happy it did, 'cos it meant I hung onto the screw-mount lenses I'd accrued "back in the day" and am continuing to enjoy them now I've "gone digital" Smile


PostPosted: Mon Nov 04, 2019 8:26 am    Post subject: Re: Why is max aperture info needed / used? Reply with quote

kypfer wrote:

Maybe the body "knows" how far to push the aperture linkage in for any given stop, provided it's been "told" where it's starting from?


That is actually how Nikon pre-ai bodies work: you tell them the maximum aperture by winding the ring back and forth. Smile


PostPosted: Mon Nov 11, 2019 4:50 pm    Post subject: Reply with quote

Each camera brand has its own way of processing this information. With Canon FD cameras, for example, they don't read the actual light value unless they're in "stopped down" mode, something that was a holdover from the FL days, and which the FD cameras had to insure backward compatibility. Instead, a Canon FD lens has a stud on the back of the lens that indexes with the body and tells the body the maximum aperture value of the lens being mounted. As a Nikon user also, I've gotten used to rocking the aperture ring back and forth whenever I mount a lens.


PostPosted: Mon Nov 11, 2019 6:07 pm    Post subject: Reply with quote

Actually, the Minolta SLR does need to know the currently selected aperture on the lens in order to provide TTL metering in a simple way, because they are mechanical calculators that only work with a specific arithmetic. If there is no connection to the aperture ring, then the camera will meter wherever the aperture blades are which means you are using stop-down metering and therefore your viewfinder goes dark if you are not wide-open.
As for the other brands, I don't know what they do. But for Minolta, the XD does not even need to know the minimum aperture value of the lens in order to provide shutter-speed priority, because the final-check metering will compensate the result regardless, and you can set an intermediate apertures as a limit to avoid diffractive aperture values reducing the sharpness which means the lens' MD tab doesn't connect with the camera's MD tab. But, it is only because of the CPU and the MC tab and ISO value being provided that this is possible.