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05-08-2012, 11:04 AM   #1
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FA* 85 aperture issues

I have noticed several odd things about the aperture of my FA* 85: First of all the aperture doesn't form a symmetrical nonagon, which is noticeable in the OOF highlights when stopped down to f/8 or smaller. Could it affect IQ in other ways?
Furthermore I noticed, that nothing happens when I set the lens to 'A', pick aperture 1.6 or 1.8 and press the DOF preview button. The aperture doesn't close a bit. At f/2.0 the aperture starts closing. Also there is a difference in the diameter of the aperture depending if I set the aperture ring to a specific aperture or set the ring to 'A' and pick the same f-stop in the camera ('A' setting results in noticeably larger diameters).
Is this a reason to be worried or is this behavior within the limits?

05-08-2012, 11:16 AM   #2
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The asymmetry is normal, I believe. However, at F1.8, the lens should stop down very slightly. Does it stop down when you actually take the shot?

Adam
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05-08-2012, 11:58 AM   #3
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I set the camera to Bulb and 1.8, looked into the lens and pressed the shutter. There's nothing moving...
05-08-2012, 01:33 PM   #4
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Ooooo, That doesn't sound good, With the lens not mounted does the aperature close down when you stop down the ring??? And if you set it to 22, and flick the lever on the mount of the lens does the lever feel nice and free or does it have a "stickyness" to it?

05-08-2012, 02:04 PM   #5
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QuoteOriginally posted by cmohr Quote
Ooooo, That doesn't sound good, With the lens not mounted does the aperature close down when you stop down the ring??? And if you set it to 22, and flick the lever on the mount of the lens does the lever feel nice and free or does it have a "stickyness" to it?
The aperture works very smooth and is not sticky at all. Beside the asymmetry the lens works fine between f/2.8 and f/22 with the 'A' setting. When I operate the aperture with the ring, f/1.4 to f/22 work fine. It's just that the camera doesn't seem to close the aperture far enough between f/1.6 and f/2 with the lens set to'A'.
05-08-2012, 03:03 PM   #6
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Hmm, seems odd, sounds like there may be something slightly amiss between the actuator in the camera that stops down the lens and the lever on the lens itself that holds the blades open. Because if it all seems to open correctly off-body and doesn't feel sticky or slow I wonder If that lever on the back of the lens had even a slight bend in it, that could cause this kind of problem because it would not be positioning correctly with the lever in the camera body.

Its just a thought thats all, just me thinking out loud.
05-08-2012, 03:51 PM   #7
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Thanks for your thoughts, cmohr. I've no clue what's going on. I checked the behavior of my other lenses and they seem to be ok. But they wider, slower lenses, so a difference is hard to tell. I'll check the FA* 85 with another camera (next time I have access to one) to rule out that something with my K7 is wrong.
05-08-2012, 04:24 PM   #8
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QuoteOriginally posted by cmohr Quote
Hmm, seems odd, sounds like there may be something slightly amiss between the actuator in the camera that stops down the lens and the lever on the lens itself that holds the blades open. Because if it all seems to open correctly off-body and doesn't feel sticky or slow I wonder If that lever on the back of the lens had even a slight bend in it, that could cause this kind of problem because it would not be positioning correctly with the lever in the camera body.
I would think that this is the problem. The aperture actuation lever movement is designed to be proportional to the area of the aperture in "A" lenses forward, and if the lever is either bent or worn, it's possible that it is not engaging accurately.This also could be true of the actuating lever on the body, BTW. 1/3 to 1 stop is actually a pretty small difference in the movement of the lever.

The bottom line is how much this will practically limit your ability to use the lens effectively. If you would regularly be shooting this with Auto aperture control at f stops over f1.4 and wider than f2, then it would be worthwhile to have it calibrated by a pro lens tech. If you would normally shoot this lens stopped down past f2, then I wouldn't bother. I realize that it's normal to expect a costly legendary lens to perform superbly in every respect, but if there is no actual practical downside, then why let something that really doesn't matter cloud you perception of the lens' quality? I find that confidence in my gear effects the way I shoot and the results I get.

Working around this does not seem to be much of a problem. At these apertures, framing and metering stopped down 1/3 to 1 stop is not really much of a working handicap, and critical focusing might actually be more accurate since it would be done with the lens physically stopped down.

If this actually practically effects your shooting, then I'd make the investment to have the aperture lever calibrated to the body or work around it. If not, then I wouldn't let this relatively minor fault mar my impressions of a superb lens.

Scott

05-08-2012, 07:15 PM   #9
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Your lens is not defective if that's what worries you. The asymmetrical aperture shape is due to loosely made aperture blades which can be compensated somewhat at the adjustment stage. It can be close to perfect if you got lucky. AFAIK, this started to become obvious with the FA series and some samples can be way off. It will impact the bokeh but not sharpness.

The imprecise stop down of A aperture can be due to a few things:

1) The aperture stop downs are calibrated separately for M (turning the actual aperture ring) and A aperture. If the A stop down is not inline with M aperture, the rear aperture lever can be re-calibrated w/o disturbing the M stop down.

2) Due to the loose tolerance nature of K aperture linkage, the actual stop down is a combination of both the lens and the body, and either or both can be off slightly but still considered within specification. What this means is that your FA*85 can be re-calibrated to match your K-7 perfectly, but can be off again with the next camera you will own.

3) The aperture stop down of the K bodies was originally a piece of thin metal. Pentax have switched that to a piece of thick plastic since the MZ series. Since some of the lens aperture levers aren't perpendicular, the contact surface has changed when stopping down. This will impact the accuracy of the actual stop down too.
05-09-2012, 04:18 AM   #10
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Thanks for your replies! I'm really relieved that my lens isn't defective and it's probably an adjustment issue. I haven't noticed this problem in my images (except the irregular shaped OOF highlights sometimes) and metering seems to be ok. So there is no immediate need to fix this.
05-19-2012, 08:49 PM   #11
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hi there,
i had the same issue with my FA 100/2.8
it is very easy fix if you are brave enough to open the lens.
Unscrew the five bolts on the bayonet, then you will have access to the aperture control. Displace the black O-bracket which is right new to the glass just a little to have access to the two screws. There are two screws on the aperture control, unscrew a little then adjust a little to the left. Screw them again while holding the aperture control. Align the O-ring, place the bayonet carefully, try not to lose any pins(just a tip, place some kind of tape on them so they stay in place. That's it. It is an easy fix, not worth the price you will be quoted to be fixed. And if you are not too scared you may do it in less than 5 mins.
If you want i will guide you through the process with some pictures.

Mine was more noticeable. At 2.8 no problem, but it closes down at F4.0 when i enter a value of F8.0 on the body. when i want to stop at 5.6 i had to enter F13. -1.0EV error. not nice at all.
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