Basically, the aperture in my 28mm is no longer opening. It was fine one minute and the next it just slowly got darker as the aperture closed like i have never seen before. I dont know if it is because i had my settings so high as i was shooting with strobes, or what. But this is my one of my favorite lenses and i can't stand to just let it go as it is still a nice piece of glass, the aperture is just not budging on manual or auto. There doesnt appear to be any obstruction between the connective plating on the lens or the camera.
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If you're using a Nikon camera, you want to lock the aperture ring at its smallest setting (largest f/number). You can control the aperture from the camera body, the same way as on a lens without an aperture ring.
This has nothing to do with the focal length. You can shoot at any focal length from 28mm to 200mm.
Set the aperture ring to its minimum setting (largest f/number) and leave it there. You control the aperture from the camera body, exactly the same way as on a lens without an aperture ring. In the A mode, simply turn the command dial. In the M mode, hold down the aperture (exposure compensation) button and turn the command dial.
I have a Nikon D60, and it has numerous settings, including aperture and shutter priority, and others. It sounds like you have have it set in aperture mode, set for f/16. First try taking a picture in dimmer light, where it's not so bright. That should cause the shutter to open longer. The camera will adjust the shutter speed to take the best picture at f/16 if it's set for that.To adjust it to something other than f/16, on my camera, you use the thumb wheel on the back of the camera. Try changing that if it has a thumb wheel.
Next try it in Auto mode by rotating the dial on the top of the camera to the green setting and see if you get a batter picture.
If all else fails, consult the camera manual, if you have one.
Okay did you happen to take the lens off without shutting the camera off? If so that's not a good idea. First thing I'd advise is to clean the lens contacts using a new red pencil eraser, lightly polish the gold contacts and clean the surface off with a lens cloth not a tissue. Next I'm not totally sure of the Pentax camera you have but does the camera control both the shutter speed and aperture? If so then the lens needs to be placed in the I'm going to say stopped down position before locking it to the camera, now that might be wrong so if it doesn't work then shut the camera off remove the lens and open the aperture to its wides point reinstall the lens and turn the camera on and try that. Sorry it's been a year since I used a Pentax and I'm going on memory. Another thing some older lenses although they fit the mount and appear to work may not actually work (depending on how old the 28-200 is) Some older technology in third party lenses can't keep up with the "new" cameras. In short the new camera may be able to look back in technology but the lens may not have the "chip" to look forward and the two are not compatible. I'm not saying this is the case here because you said it worked I'm thinking dirty contacts or the aperture ring was not set correctly.
If this is the Sigma 28-105mm f/2.8-4.0 as in the title, then the maximum aperture will vary with the focal length. It is probably only 2.8 when the focal length is zoomed to 28mm. It will be 4 at 105mm, and at focal lengths in between it will be in between.
You say the aperture doesn't work- is that on or off the camera? becasue of the unique design of the Canon "breach-lock" lens mount FD lens aperatures were locked open until they were mounted to a camera-
Hi I am SAGHA camera repairer from mumbai,I think your sigma 18-125 dc lens is having electronically operated aperture unit in which one Film PCB runs from mount contacts to aperture unit.Due to multifolds of that film inside the barrel the film gets cracked at one fold location So either you have to get that film changed or the whole aperture unit replaced from an expert.give me feedback for this tip Vote me if you found it useful,ALL THE BEST.
As you have found out, it is not a lubrication issue. It would seem that your lens has a problem with the linkage arm inside the mount. The arm that extends out of the mount could be bent or misaligned. It would be fixable, but the parts are somewhat delicate. Best to let a good repair person have a look.