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05-14-2011, 06:40 AM   #1
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The green button

I know what the green button does in terms of manual lenses. However, I am curious of how it does it.

I know for manual lenses it allow you to measure and set the shutter speed. However, how does it do it? I can hear and zip sound from the camera. My first thought was that it moved the mirror up to allow the sensor to capture a sample. However, the zip sound doesn't sound like the mirror flipping up. Is the zip sound from the level that activates the aperture blades? If so how does the camera measure the light?

05-14-2011, 09:15 AM - 1 Like   #2
Aegon's Avatar

Join Date: Jan 2008
Location: Portland, OR
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The mirror stays down because the exposure sensors are in the prism.

When you hit the green button, the aperture lever "zips" to stop down the lens to the set aperture selected by the lens aperture ring. You can try it yourself if you want; unmount a lens, select an aperture, gently depress the lever.

Once the aperture is stopped down, the exposure sensor by the prism detects the light levels and selects an exposure time. Next, the lever pulls back and the lens opens up again.

This all happens pretty quickly. It sounds like "zip zip".

Wikipedia has a nice cutaway picture of a DSLR:
The exposure sensor is the one right above the viewfinder. It only works when the mirror is down and the light path goes through the prism.
05-14-2011, 12:26 PM   #3
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More than you wanted to know

Here is a basic K-mount lens, with the aperture lever at the top:

The original design of the K-mount camera has a corresponding actuator to move the aperture lever like a light switch, just on (stopped down to the aperture selected on the lens's ring) and off (aperture fully open for focusing). At the bottom of the lens photo, there's a rectangular tab that moves in a slot, changing position along with the aperture ring setting. The camera had an aperture follower to track this tab, and tell the light meter where the user set the aperture ring. The system allowed metering at any aperture setting and wide open focusing. That's in about 1975.

When camera makers wanted to add automatic modes, they needed a way to tell the camera more information about the lens, and control it more precisely. Pentax made a couple of changes to their mount, adding electrical contacts for lens information, and making the aperture lever move much more precisely. Now the camera could tell what a newer lens was capable of, and control it. Meanwhile, older lenses would still work the old way, and if you were used to using an aperture ring, you could. That's in about 1983.

About 10 years later, Pentax started eliminating the aperture follower from some cameras. Those cameras required a lens with an A position on the ring. Then 10 more years later, they introduced their first DSLR. Without checking my facts, I think the *ist D didn't originally have stop-down metering - it was activated with a firmware upgrade. This solution meant the camera didn't need any lens information. It just stopped down the lens and activated the light meter.

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