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07-25-2013, 06:43 AM   #1
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Possible to force K-5 to stop down to shoot when using non-A lenses?

Hi all,
I just got my first non-A lens, a Tokina 50-250. While I like the lens so far, I feel like the camera could easily use this lens MUCH more effectively if it just behaved a bit differently, and I want to know if this is possible.

The K-5 will do stop-down metering in M mode when a non-A lens is mounted, or it can shoot in Av / TAv, but it won't stop down the lens to shoot. This would be a LOT simpler if the camera would simply stop down the lens to shoot in other shooting modes... I could just turn on TAv, set the lens for the aperture of my choice, and dial in exposure compensation equal to however many stops down from wide open I set it for. But the camera refuses to stop down to shoot in any mode but manual.

What I want to know is whether there's a fix for this. Can I, for example, through going into debug mode, set the camera to actuate the aperture control motor in Av / TAv mode?

07-25-2013, 06:57 AM   #2
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When shooting A lenses and newer, the camera body has a little lever that moves the aperture arm in the lens. It does this knowing that a certain distance moved will equal a certain aperture on the lens. This linkage is designed as part of a system.

The older M and K lenses do not follow the same aperture arm-distance-lever linkage. Thus, the camera doesn't actually know what aperture the M or K lenses are. For example, if you set your A lens at F4 in-camera, the lever moves the aperture arm a certain amount, knowing that this equals F4 in the lens. If the camera tried to do the same in the M lens, it would end up at some unknown aperture size. The reason why this won't work with the camera is because the camera meters the scene, reads the aperture, and selects the shutter speed (or displays the EV) accordingly. With an unknown aperture, the camera can't give an accurate meter reading.

So no - there is no "fix" to it as far as I know.

I think M42 lenses can be made to shoot in Av mode with auto-aperture, but that requires a bit of work I think.
07-25-2013, 07:05 AM   #3
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That's not what I meant. I'm not talking about the lens-to-body aperture communication linkage, which communicates the lens's set aperture to the camera, or trying to make the camera control the aperture on a pre-A lens.

What I'm talking about is that when a pre-A lens is mounted on the K-5, it will shoot wide open ONLY, in all modes other than manual. It'll stop down the lens in M mode, but not any other mode. THAT'S what I'd like to be able to fix.
07-25-2013, 07:09 AM   #4
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So, if I am reading this right - do you want to take a M lens, set it to F4 manually, shoot in Av and have the camera stop down the lens to meter in Av mode?

07-25-2013, 07:19 AM   #5
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Your concern is about the cripple... if you look at the back of your lens, there is the aperture lever, and then a shorter lever connected to the aperture ring (it moves when you rotate the ring). In older bodies, this engaged with a lever on the inside of the bayonet mount to indicate the setting of the aperture. In our newer digital bodies that input is missing from the flange. Specifically feature #4 in all the diagrams...

some good diagrams here from D. Bonazza



and more



The Mount is crippled - there is a thread requesting Pentax uncrippled the mount here: https://www.pentaxforums.com/forums/pentax-dslr-discussion/200075-pentax-rico...-petition.html

Last edited by mattt; 07-25-2013 at 07:21 AM. Reason: added text
07-25-2013, 07:31 AM   #6
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Again, that's not what I mean. Yes, it sucks that the crippled KAF3 mount doesn't allow the camera to read the lens aperture, but that would require a hardware fix. I'm proposing a software fix.

Here is what I want:
I mount an f/4 K lens.
I set my lens to F/8 (2 stops down from wide open; aperture remains wide open for focusing as per normal operation)
My camera body, set to aperture priority mode, meters based on incoming light (i.e. f/4). IF I took a picture now, and the lens stopped down to f/8, my image would be 2 stops under-exposed.
SO, I dial in 2 stops of positive exposure compensation. THUS, although the camera body is SEEING f/4, it will EXPOSE for f/8, even though it has no way of knowing what aperture it is stopping the lens down to.

This should be achievable by a software fix.
07-25-2013, 07:48 AM   #7
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How camera knows you mount a f/4 lense? Like for the SR, enter F manually?
Use M, use the mettering, and set it 2 stops up in order to compensate when the aperture closes. Well, is not as easy as Av mode.
07-25-2013, 08:06 AM   #8
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From reading another thread, the way the aperture coupling works is this:

In M mode, the aperture linkage disengages, and the lens is free to stop down to whatever your aperture ring is set at. In Av mode, the aperture linkage engages and expects to move the aperture arm. It proceeds to meter wide open, and does the calculation for stopping down based on wide open.

So to do what you want, you need the camera to disengage the linkage when mounting a M or K lens (so the lens can stop down to your aperture ring setting). Then you can use exposure compensation to modify based on the wide-open metering. I don't really see any downsides to doing it this way, and it might be feasible.

The only issue that might occur is when people who don't understand the system mount M or K lenses and use Av mode, then they would get underexposed shots if they set their aperture ring to anything but wide-open.

07-25-2013, 09:11 AM   #9
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QuoteOriginally posted by Sicariphus Quote
Again, that's not what I mean. Yes, it sucks that the crippled KAF3 mount doesn't allow the camera to read the lens aperture, but that would require a hardware fix. I'm proposing a software fix.

Here is what I want:
I mount an f/4 K lens.
I set my lens to F/8 (2 stops down from wide open; aperture remains wide open for focusing as per normal operation)
My camera body, set to aperture priority mode, meters based on incoming light (i.e. f/4). IF I took a picture now, and the lens stopped down to f/8, my image would be 2 stops under-exposed.
SO, I dial in 2 stops of positive exposure compensation. THUS, although the camera body is SEEING f/4, it will EXPOSE for f/8, even though it has no way of knowing what aperture it is stopping the lens down to.

This should be achievable by a software fix.
I fail to understand how this would work. With an "A" lens in Tv mode you set the shutter speed, the camera meters wide open and sets the aperture. Adding compensation changes the aperture. With a non-"A" lens the camera has no way to know what aperture is set or accurately setting the aperture. So even if the lens stops down at shutter release exposure compensation can only be achieved by either changing the ISO or the exposure time. If the shutter speed is changed then it is not Tv.

Perhaps the only real solution to use K and M lenses would be to un-cripple the mount and add back the aperture setting coupler function. As much as many of us would like the mount to be fully compatible with existing K-mount lenses there isn't much incentive for Ricoh to do so unless they want to start manufacturing non-"A" lenses again.
07-25-2013, 09:47 AM   #10
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I don't want the camera to compensate by auto-adjusting the aperture, I want the camera to compensate by auto-adjusting the ISO in TAv mode.

What I'm picturing is pretty much the same as TAv mode, except with the aperture set not by the E-dial but (indirectly) by exposure compensation, and the shutter set by E-dial, with the ISO varied by the camera body to match the current shutter speed & aperture (+ EC added to compensate for a taking aperture different than the metering aperture).
07-25-2013, 10:12 AM   #11
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I don't have a non-A lens to check otherwise I'd do this myself. If you shoot in M mode with floating ISO, press the green button for metering, and then press the +/- button for EC, what happens when you move the front dial? With auto lenses, this adjusts the ISO up or down as displayed by EC value.
07-25-2013, 06:02 PM   #12
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@Sicariphus It is possible - sort of. You have to do two things - #1 plug the 'A' contact on your body to make your body think you have mounted an A-series lens and #2 insulate the lens mount so your body knows the available aperture range.

See THIS POST and then scroll down a couple posts to see the picture.

What you then do is set both the lens aperture ring and your body to the same f-stop value prior to taking your picture.

The remaining issue is that an A-series lens aperture lever is calibrated, non A-series aperture levers are not. So, other than wide open, exposure may be slightly, but consistently off. You can run test shots and use the histogram to determine exactly how far off and use exposure compensation to compensate. In most cases the exposure error is close enough that you can just handle it in post processing in all but the most critical shots.

In ANOTHER POST I compared using a KA 50mm f/1.7 and a KM 50mm f/1.2 using P-TTL flash* while using the above technique to make my body think I had a KA 50mm f/1.2. Download and check out the .pdf with all the histogram screen shots.

*as you know, you normally must have an A-series lens to perform any type of P-TTL flash.
07-25-2013, 06:58 PM   #13
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Hi Jim,
that's fantastic! I love this. I do like this Tokina, but because of this issue I ordered an SZ-X 60-300 to replace it... Who knows, if the SZ-X isn't as nice optically then I may wind up modding the 50-250 rather than sending it back. It DOES have a rather nice macro.

One thing I'm confused about: If you ARE relying on the camera to attempt to set the lens aperture (albeit somewhat inaccurately), what's the use in setting the aperture on the lens to the desired aperture? Why not keep it at f/22, as with and actual 'A' setting?
07-25-2013, 07:15 PM   #14
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QuoteOriginally posted by Sicariphus Quote
Hi Jim,
that's fantastic! I love this. I do like this Tokina, but because of this issue I ordered an SZ-X 60-300 to replace it... Who knows, if the SZ-X isn't as nice optically then I may wind up modding the 50-250 rather than sending it back. It DOES have a rather nice macro.

One thing I'm confused about: If you ARE relying on the camera to attempt to set the lens aperture (albeit somewhat inaccurately), what's the use in setting the aperture on the lens to the desired aperture? Why not keep it at f/22, as with and actual 'A' setting?
You can give it a try, but I suspect the inaccuracy will rise. By setting the aperture on the lens, the aperture cannot get any smaller than that setting. Any inaccuracy will be in the form of over exposure.

Keep in mind that the amount of inaccuracy will vary from lens to lens. Cameras like the old K-1000 just let the lens close down and halt itself at the aperture set on the ring. It didn't matter that the distance an aperture lever traveled to reach mid-range differed between lenses. With the advent of the A-series lens, the body is supposed to halt the lens' aperture lever at a specific point - and that requires consistency between lens models (which is also why the body and lens use contacts to determine the aperture range).
07-25-2013, 07:49 PM   #15
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Will it harm the body's aperture control motor to be brought up short by the lens's aperture ring?
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