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10-31-2009, 05:23 PM   #1
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Sears 135mm: Av stop-down does not match aperture ring

I just received a Sears 135mm f/2.8 Macro lens, and the first thing I did when I unboxed it was to remove the Ricoh pin. This went smoothly, but now I'm wondering if I've messed something up.

The problem is that the lens doesn't seem to allow either of my cameras to meter properly in when set to "A". A picture taken in, say, Av mode with the camera setting aperture for f/2.8 will be darker than a picture of the same scene taken at f/4 or f/8, which will be over-exposed by a about 1 stop in comparison. It's as if the camera isn't physically stopping down the lens enough with the lens set to "A", and letting too much light in for the calculated shutter speed.

To check this I did a comparison of the position of the aperture blades using the aperture ring and witht he camera stopping them down with the lens set to A. I set the camera for "optical preview" and looked through the front of the lens at the aperture blades as I made the camera do a DOF preview. I did this with the aperture ring set to f/4, and noted the position of the blades. I then did this with the ring set to "A" and the camera in M-mode set to f/4. In that case, the blades were noticeably more open then with the ring set manually. I'd say that instead of moving the whole stop, they're moving about a third the required amount. The same thing occurs for f/5.6 and f/8: the blades don't quite close the same amount as they do when using the aperture ring. And with the lens set to "A" and the camera set to f/3.2, the blades don't appear at all...the same as wide open f/2.8.

But, overall, the "offset" is the just about constant across all stopped-down apertures. If an image is properly exposed at f/2.8, it will be 2/3 to 1 stop overexposed at f/3.2 to f/22. So it's as if the camera is moving the aperture blased the right proportional amount between intermediate f-stops, but the first step is "behind" and the blades always held too far open.

Just to make sure that wasn't proper behavior (I didn't think it was), I repeated this procedure on my FA 50mm f/1.4. In that case, the amount of stop-down provided by the camera matched the amount set by the aperture ring at all settings. f/2 set by the lens looked just like f/2 set by the camera, etc.

I suspected that maybe the Sears lens wasn't reporting its min and max apertures correctly, meaning something was wrong with the KA-mount contacts. If that were the case, the camera would change the way it set the aperture for in-between stops. But when I checked the available range of apertures on the e-dial, I noticed that I can scroll through the proper range of F/2.8 to F/22. So the camera knows what it should be doing with the lens, but something mechanically isn't working right.

So, any idea what could cause a lens to act this way? Did I not link something up properly when I reassembled the lens? I notice that the little aperture lever on the lens doesn't move in conjunction with the aperture ring (reducing its range of motion for larger apertures) the way my FA50 does. But upon examining the mechanism in the sears lens, I don't see any way for that to happen.


Last edited by aerodave; 10-31-2009 at 05:33 PM.
11-01-2009, 08:32 AM   #2
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When I got my 135, the aperture was kinda iffy. It would sometimes stick wide open, and other times the mechanical linkage in the back of the lens jammed up completely. Turns out there was a bent arm inside, I only had to take out the 4 small screws to fix it. Now it works like a champ! They are very simple lenses, but then again mine's an old PK mount manual lens so it's probably different than yours.
11-01-2009, 02:24 PM   #3
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I have a Cosmicar 28MM "A" lens that behaves the same. Kind of behaves just like using the Green Button on my K10D, the higher the aperture number, the more overexposed it is. The lower the number, the more it underexposes. At F4, it seems to be fine though. So, I have to use EV compensation with it. Luckily, I got it cheap, so it isn't such a huge deal.
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