How does a fully manual lens stop down at the moment of exposure?
Asked 7/9/2016
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On a DSLR, the viewfinder stays bright because the lens is normally held wide open until the shot is taken. With autofocus/electronic lenses, the camera tells the lens what aperture to use and then stops it down just before exposure.
How does this work with a fully manual lens that has no electronic contacts, such as a Samyang 8mm f/3.5 on a Nikon D5000? The camera reports “no lens attached,” yet the lens still appears bright in the viewfinder and then uses the selected aperture for the shot. What mechanical connection makes that happen?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Nikon has a very noticable lever just inside the lens mount on the left side (viewing the body without the lens from the front).
Photo source: Nikon D5000 DSLR: Announced and Previewed
On the lens there should be a matching lever. You can move it and see the diaphragm close.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
10y ago
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Generated from our catalog & community — verify before relying on it.
On Nikon F-mount bodies, this is handled mechanically, not electronically.
Inside the camera mount there is an aperture lever. A compatible manual lens has a matching lever connected to its diaphragm. The camera holds the lens aperture wide open for viewing, which keeps the viewfinder bright. When you press the shutter, the body moves that lever, the lens stops down to the aperture you selected on the lens ring, the exposure is made, and then the lens reopens.
So the lens doesn’t “know” electronically when to adjust aperture—the camera body physically actuates it.
If you remove the lens and move the lens’s aperture lever by hand, you can usually see the diaphragm blades close. Likewise, you can spot the body’s lever inside the Nikon mount. That mechanical linkage is what makes stop-down-at-exposure work even when there are no electronic contacts.
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