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01-08-2010, 07:36 AM   #1
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Aperture Priority Shooting

Good morning all, I get the fact, that to use an older lens on aperture priority, on say a kx or k7 I got to set it up for ring ok, and green button to meter before shot. Why doesn't this work like on the me super? You know hold shutter half way then camera sets speed. Also if you use a fa or dfa lens will those work the way I'm used to on the ME, you know set aperture, when in aperture priority, and camers set speed for me without having fiddle around with green button. Thanks any clarification would be great. Dan

01-08-2010, 08:07 AM   #2
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for K mount lenses with no A setting, pentax DSLRs all behave the same way.

in Av mode they expose wide open only, because there is no coupling that reads the aperture setting of the lens.

in M mode, the green button stops down the lens and the camera sets the shutter speed to match the light coming through when the lens is stopped down

with A lenses, set the lens in A and the camera can set the aperture.

this is because the KAF or KAF2 mounts on the DSLRs are all missing the paerture ring linkage.

Some refer to this as the J mount because the lenses with this missing are FA-J lenses or later.
01-08-2010, 10:21 AM   #3
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What does it mean that the camera 'Stops down' the lens?
01-08-2010, 11:05 AM   #4
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QuoteOriginally posted by Winnie Quote
What does it mean that the camera 'Stops down' the lens?
Wow...huge question. The answer requires a bit of a history lesson.

In the beginning...
All lenses had a simple aperture diaphragm where the size of the opening was always the same as the f-stop setting on the lens. This was very reliable, inexpensive, and worked well with the cameras of the day (rangefinder, view, zone focus, and twin-lens reflex). This type is called a manual aperture or manual diaphragm.

Then came SLR...
Doing focus and composition through the taking lens created a usability issue. It was difficult to focus with the lens at the set aperture (i.e. "stopped down") due to a dark viewfinder and excess DOF. Solutions came in three flavors
  • Pre-set aperture/diaphragm
  • Semi-automatic aperture/diaphragm
  • Automatic aperture/diaphragm
Pre-set
With these lenses, there is a lever or ring that allows the aperture to either be fully open for focusing or stopped down for actual exposure. While the approach is old, some lenses still made today use this technology (e.g. the Russian Jupiter-9 85mm).

Semi-automatic
These lenses are wide open to focus and "automatically" stop down to the set aperture at the time of exposure. They must be manually reset with a small lever to reopen the diaphragm blades.

Automatic
These lenses are wide open for focusing and stop down "automatically" to the set apertures just before the shutter opens. They then "automatically" reopen immediately after the exposure. Almost all modern lenses fall into this category. Back in the day, this was a major feature in a lens and was often included in the lens name. That is why many older lenses have the word "Auto" in the name. This designation causes some confusion today since the label refers to the automatic diaphragm and does not mean that the lens was designed to support automatic exposure or automatic focus.

While the automatic diaphragm is a feature of the lens, that feature is supported by and actuated by the body. That is why we say that the body "stops down" the lens.

Steve

01-08-2010, 11:45 AM   #5
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The short answer, though i the "stops down" means if you've set an aperture like f/8 - anything other than the maximum aperture (smallest f-number) - the camera tells the lens to actually close its aperture blades to that aperture. Normally with a K-mount lens the aperture ring doens't take effect immediately - the aperture blades don't actually close to the elected aperture until you press the shutter.

As for *why* Pentax DSLR's require this when older film bodies didn't - it's because the older film bodies were able to "read" the aperture ring and know where you had set it, thus enabling them to select a shutter speed speed. At some point after Pentax started building auto-aperture lenses and cameras that could control them (lenses with an "A" position on the aperture ring, and controls on camera for setting aperture), they stopped building the capability to read the aperture ring into their cameras.
01-09-2010, 06:13 AM   #6
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Ergo, for the purpose of this thread:
I have an older lens.
I have my camera's menu set up to allow me to manually set the aperture.
I'm in the M mode.
I have it set on f5.6 and I line up my shot and focus.
I half press the shutter, and press the green button.
At this moment, even though I already have the lens set to f5.6, the sound I hear is the camera 'resetting' (ie stopping down) and then springing the lens's blades back to f5.6, in order to calculate my shutter speed?

Is that about right? Because in practice it's really the easiest thing. Pressing the green button is becoming second nature. And M mode seems to work much better than Av mode for me.

Last edited by Winnie; 01-09-2010 at 08:48 AM.
01-09-2010, 06:52 AM   #7
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QuoteOriginally posted by Winnie Quote
Ergo, for the purpose of this thread:
I have an older lens.
I have my camera's menu set up to allow me to manually set the aperture.
I'm in the M mode.
I have it set on f5.6 and I line up my shot and focus.
I half press the shutter, and press the green button.
At this moment, even though I already have the lens set to f5.6, the sound I hear is the camera 'resetting' (ie stopping down) and then springing the lens's blades back to f5.6, in order to calculate my shutter speed?

Is that about right?
Almost.

Until you press the green button you are looking through the lens with the aperture wide open.
Pressing the green button reduces the view to the aperture setting you have already selected; in this case f5.6. It has 'stopped down'.
At that instant the K10D chooses an appropriate shutter speed for the chosen (stopped down) aperture setting.
Once the shutter speed is set, the camera re-opens the aperture again to it's widest possible setting, as it was before you pressed the green button.
01-09-2010, 08:49 AM   #8
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Ok, totally get it now. Thanks.

01-09-2010, 01:40 PM   #9
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Nice thread. Also nice review Steve. I have figured most of this by reading countless threads. good to see it summarized.

One question, though. I have old M42 mounts and use them in Av mode. Of course, closing the aperture ring down immediately leads to a darker VF. I have never owned an M series lens. I was wondering is that how the M series behave. I had assumed that it would, but then from some of the comments here made me think otherwise. Is it that one could set a desired aperture and still focus wide open (with a brighter VF) and only until pressing the green button and the shutter would the camera actually stop aperture down to the set aperture?
01-09-2010, 02:21 PM   #10
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Nope the M series lens do not act like the M42 type lens. For the M lens shooting in Av mode means you will always be wide open. Only in manual mode will you be able to stop down the lens. The reason why the M42 type gives you a darker viewfinder is because it uses a pin to move the aperture blades which the K mount system does not use. The K mount lens control the aperture blades via a flange.
01-09-2010, 06:21 PM   #11
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QuoteOriginally posted by Winnie Quote
Ok, totally get it now. Thanks.
Try turning the camera around so you're looking down the lens while you press the button. You'll easily be able to see exactly what is going on.
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