There've been a few posts in the last week or two centering on how our FD lenses & bodies "communicate" with each other, so perhaps a chat on the mechanical aspects of lens & mount is timeous.
Firstly, I'm attaching a couple of pics - one of the back of an FD lens, which is labeled, and a couple of an F1N front. The eagle-eyed among you will immediately notice that the Body pics are reversed! This is so that they can be viewed & compared side by side - imagine you're looking at the lens mount from behind the camera.
First, let's look at the back of an FD lens:
Moving around the lens you'll see an "Aperture Signal Lever". In the pic the Aperture Signal Lever is at the top of it's travel which is where it'll be with the lens set to it's widest. As the aperture ring is moved around to smaller apertures, the Lever will move down.
This Aperture Signal Lever actually sits on top of the matching lever in the body:
at the "10 O'clock" position. As the lens aperture ring is turned to smaller apertures, then the lever in the camera body is pushed downwards.
Now. Whether the lens has a maximum aperture of f1.2 or f5.6, the aperture signal lever is at the same position for full aperture and moves down (or back up) a set distance per aperture change. In effect it's signaling that the lens will transmit 1/2, 1/4, 1/8th, 1/16th, ... of the light it's presently transmitting once it's been stopped down for exposure.
If we look at the TTL meter in the camera, what it does is simply measure the brightness of the light coming through the lens and takes information about film speed and shutter speed. With a particular film & shutter speed set, the meter may for example calculate that the film needs 1/2 of the light it's getting at the moment - hence the lens should be closed by 1 stop, 1/4 of the light = 2 stops, 1/8th of the light = 3 stops, and so on... Making the appropriate indication in the meter readout.
Similarly, with a particular aperture set at 1 stop, 2 stops 3 stops, etc... below the lens maximum, the meter will be told that the film is only going to get 1/2, 1/4, 1/8th, etc of the light it's getting at full aperture and can then indicate a shutter speed.
So... As far as exposure calculation goes for the meter readout, it doesn't really matter what the actual lens maximum aperture is, it only needs to know how far it'll be stopped (or closed) down at exposure.
Still with me? I hope so.
Now onto the function of the "Full Aperture Signal Pin". Given the above, it starts to look kind of redundant, but obviously it isn't. It's a fixed pin which is bigger for "faster" lenses and smaller for "slower" lenses. It pushes in the matching pin on the body to a greater or lesser extent and thus translates the position of the aperture ring from "full aperture", or "down 1-stop", "down 2-stops", etc... Into an actual f-Number value on the viewfinder readout. In effect, it tells the meter readout where to start from in terms of "f-Number" for us to understand more easily.
Continuing around anti-clockwise, at the 6 O'clock" position is the "Stop-Down-Lever" The lever in the body drives the lever in the lens across and this stops the aperture down from fully open to the value set on the aperture signal lever. When you press the shutter button, this is (almost always) the first thing that moves, and the last thing that returns at the end of the exposure (not that you'll notice given how fast it all happens).
Still with me? We're just coming to the interesting bit; the "A" setting, so here's the lens & body pics again to stop Y'All having to scroll up & down:
... And we're round to about the "4-O'clock" position on the pics, where you'll see the "AE Switch Pin". As you rotate the lens aperture ring (and the aperture signal lever goes down - remember), you move past the lens minimum aperture and push the Aperture signal lever right down AND push out the AE Switch Pin. This pushes in the matching switch on the body and this kind of changes the rules. With the AE Switch pin "on" and hence the Aperture Signal Lever right down, the lever in the body will now drive the aperture signal lever UP to a selected point as decided by the meter (and then the lens stops down).
But what about Variable-Aperture-Zooms? (I hear you ask!) For example, a 35-70mm/f3.5-4.5? The Full-Aperture-Signal-Pin is fixed! The meter readout always says f3.5 even at Maximum Focal Length!
If you think about it... That doesn't matter. The meter will still just see the actual amount of light and stop the lens down by a set amount. The only thing that's lost is an accurate measurement of the aperture in the meter readout. You'll get an actual aperture a set amount less than the meter indication depending on the focal length selected. Simple?... Simple!
Similarly, Teleconverters & Auto Macro Extenders transmit the Lens Maximum Aperture and Aperture Signal direct without any attempt to "convert" these values in any way, so for example, f2.8 won't actually be f2.8, it'll be what's set on the lens. Still doesn't matter.
Finally, a quick word on "Stopped-Down-Metering". Most often used with Mirror Lenses & accessories without the appropriate signaling pins.
By activating the Stop-Down-Lever, you're manually carrying out the "first" step of the exposure cycle, and telling the TTL-Meter that "This is how much light the film will get" Thus, the meter indicates a shutter speed only, based purely on how much light it's getting (and film speed).
Hope this helps,
PS: Purely for the sake of completeness... The "Reserved-Pin" signaled the focal length of the lens; Longer for Telephotos & shorter for Wide-Angles.
(Credits to Christian Rollinger & the mir Canon New F1 sites for the pics. The article itself is mine and I allow re-publication without restriction subject to appropriate acknowledgement.)
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