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10-22-2010, 10:11 AM   #1
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What does "Automatic Aperture Setting" mean?

Hello, I am obviously a n00b to Pentax. I am reading over the "Pentax Lens Series Explained" article right now, and just wanted to clarify the terminology. Does it simply mean the aperture can be set on the camera body? Or does it refer to the diaphragm actually stopping down before exposure?

In other words, if a lens does not have "Automatic Aperture Setting," will the viewfinder get darker as I stop down with the aperture ring?

10-22-2010, 10:30 AM - 1 Like   #2
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Automatic Aperture or A setting on a lens means the body controls the aperture. The earlier SMC or K series and the M series are controlled by the aperture ring only so have to use stop down metering on a "crippled" K-mount. The viewfinder does not get darker as the aperture is not stopped down until you press the shutter release.

Screw-mount lenses have to be used in manual mode so they do darken the viewfinder. The camera has no way to press the aperture pin on a M-42 mount.
10-22-2010, 10:36 AM   #3
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Perfect, exactly the kind of answer I was looking for, thanks, Ben!
10-22-2010, 01:53 PM   #4
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A little more on the subject -- We can see a few different species and sub-species of lenses that can be mounted on a modern K-mount (PK) Pentax dSLR.

1) M42 or M39 screwmount lenses. Adapters are needed to mount.
1a) M39 or M42 screwmount lenses with no aperture automation at all. With or without preset rings, the view darkens when stopped-down.

1b) M42 screwmounts with an auto-aperture pin, but no M/A (manual/auto) switch. Unless that pin is glued or jammed down, aperture is always wide-open.

1c) M42 screwmounts with an auto-aperture pin and an M/A switch. Set the switch to M and it works like the type (1a) above.
NOTE: I have heard of an expensive M42-PK adapter that allows camera control of the aperture pin. When I have an extra US$100+ laying around, I may try one.

2) Various sorts of PK-mount lenses. No adapters are needed to mount normally.
2a) Simple PK mounts with no aperture automation. Such may be T-mounts and their ilk; or mount-reversal rings; or fixed-aperture lenses like mirrors; or lenses with Waterhouse stops (pre-cut apertures, not iris diaphragms); or lenses on non-auto PK adapters or extensions. These again work like (1a) above.

2b) K- and M-type lenses, with a mechanical aperture linkage. These do NOT have an A position on the aperture ring. In any automatic exposure mode, these lenses only work wide-open. Shift to M mode and use the Green button to take exposures, and the camera will stop-down the lens to your desired aperture.

2c) A-type lenses (including A [manual focus], F, FA, DA, and variants [autofocus]). These lenses DO have an A position on the aperture ring, and contact pins on the lens base. Set to A position, aperture can be controlled manually or automatically from the camera. If not in A position, these again work like (1a) above, except that they'll report the f-stop to the camera, which info is stored in the EXIF data.
Only lenses with an A position will report the aperture data. To use any lens with a numbered aperture setting, you must set USING APERTURE RING to ENABLED in the camera's CUSTOM menu. The sticky atop this subforum tells all about using manual lenses.

3) Notice how in (2) above I said, "mount normally" ?? Some abnormal ways of mounting lenses include:
3a) Using any lens from any maker on a mount-reversal ring, which has threads matching a lens' filter threads on one side, and a simple PK mount on the other. This forces VERY close work and is usually combined with extensions (tubes and/or bellows) for macro shooting.

3b) Stuffing any sort of optical material into a bellows, from enlarger or projector lenses to Fresnal sheets to eyeglasses to faceted prisms to kaleidoscopes.

3c) Using a lens-cap PC lens or pinhole or LensBaby-clone or any such simple stuff.

3d) Foreign lenses, modded to fit a PK mount. Olympus OM and Nikon non-AF lenses are easy to mod. They only work as (1a) above.
NOTE: Some aperture-automation with contact pins macro tubes exist, but they're rate and costly. Some folks just get A-type TC's and knock out the glass. AFAIK no A-type bellows exist.

For my next trick, ask about Catch-In-Focus aka Trap-Focus. Have fun!


Last edited by RioRico; 10-22-2010 at 02:25 PM. Reason: addendum
10-22-2010, 01:58 PM   #5
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Thanks for those details, RioRico. I have a LOT to learn all over again.
10-22-2010, 02:12 PM   #6
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the term auto aperture and the description of the function have changed over time, with pentax cameras

the first "auto" apertures were on screw mount lenses, adn these allowed focusing with the lens wide open, and then "automatically" stopped the lens down to shoot.

These lenses had to be "stopped down" to meter as well.

These lenses include auto takumars and super takumars.

the next advancement was to have open aperture metering, by adding an aperture ring coupling lever. Most of these lenses were SMC Takumars.

Then pentax switched to bayonet mount. While the screw mount lenses could be used (and still can be) with an M42 to K mount adaptor, all these lenses need to be stopped down manually. and the image darkens when stopped down.


The K mount lenses all had "auto apertures" meaning that the camera automatically stopped down the lens when shooting, but both metering and focusing were done with open apertures.

With the introduction of the A series cameras and lenses, there was an "auto" setting added to the aperture ring (past the smallest aperture setting) When in the A mode, the actual aperture setting was set by the camera. These lenses also fed maximum and minimum aperture values to the camera via coding contacts in the lens mount.

today, the general description of auto aperture implies lenses with an A setting where you set the aperture via the body, and manual aperture describes lenses where you set the aperture with the aperture ring.
10-22-2010, 02:29 PM   #7
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QuoteOriginally posted by Todd Adamson Quote
Thanks for those details, RioRico. I have a LOT to learn all over again.
And a great way to learn is to hit eBay for cheap manual glass of various species, and try them out. With a bit of judicious shopping, an impressive set of primes can be accumulated for a relative pittance. Of course, GAINING the necessary judgment can be expensive. And finding the deals can be (IS!) rather time-consuming. But I have more time than money, so it's no problem, eh?
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