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02-27-2010, 06:16 PM   #1
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Need help with aperture

I recently purchased a K-x and picked up an SMC Pentax-F 50mm f/1.7 to go along with it. Unfortunately, the lens is giving me issues and I'm not sure if there's a problem with the lens or user error.

The camera seems to disregard the A setting on the aperture ring and always shoots wide open. I just took a series of shots from f/1.7 to f/22 (iso and shutter speed held constant) and the images came out identical.

I did notice that the blades seem to stick when opened beyond f/4, and the lever on the bayonet needs a little tap to close the diaphragm. I'm really hoping there's a setting I'm completely missing, but fear the problem may be with the lens.

Thank your for your patience and assistance.

02-27-2010, 07:25 PM   #2
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It sounds like there might be oil on the blades which sticks them together and makes them move too slow. The camera holds the lens wide open to make it easier to see through the viewfinder. When you press the shutter button, the camera lets the lens's aperture move to whatever setting you've chosen, then opens the shutter and takes a photo. The aperture blades have to move pretty quick to keep up with this cycle. If they don't, you'll get the effect you describe.

[I don't know how Pentax managed to make a million manual focus 50/1.7s with a good amount of grease in them and never have oily blades, then make an AF version with very little grease and start to have oily blades. I see maybe one F50/1.7 every few weeks on eBay. I don't get it.]

You might be able to see something on the blades similar to this:

The other possible problem is a weak spring in the lens. The lens probably needs service, which you might attempt yourself or have done. I'm assuming your camera works OK with other lenses?
02-27-2010, 09:14 PM   #3
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Great info. Thanks! The camera works just fine with the two kit lenses. This lens was a random $40 pickup with an old film body. I see some shiny areas on the blades that look just like your pic.

I guess I'll do a bit of researching on self service and give the local shop a call.

Thanks again!

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