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01-29-2011, 02:59 PM   #1
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My Poor smc Pentax-M 50mm f1.7 (Aperture Blade Problem...)

I just recently noticed that the aperture blades on my smc Pentax-M are slightly skewed. Instead of the perfect hexagonal shape, they're making a lazy-hexagon from about f8 to f22 (see attached pictures). Does anyone know the implications this might have? Should I try to fix it or just try to pick up another copy of the lens?

At f22:
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At f8:
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01-29-2011, 05:36 PM - 1 Like   #2
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Practical advice:
I doubt it would have any noticeable affect on photos. If the blades still are snappy, it's not worth worrying about. I guess if it was a new lens, I'd consider exchange or warranty repair just so that I don't feel dumb if something serious happens 1 day out of warranty.

What I'd do anyway
I have one of these that I couldn't sell for my price because of a small scratch, so I kept it. I can take it apart in about a minute. So I would just do that to see what's up. The aperture is always inconveniently located in the middle of everything so it's a lot of disassembly. But this particular lens makes it easier than most.

Use a friction tool to unscrew the "name" ring via the filter threads. It can be anything from a real tool fo a rubber crutch tip, as long as it gets a good grip on the ring and doesn't touch the front element. Dents in the filter ring or dirt in the threads make this difficult; fix those first. When the ring is off, you'll see this (almost):



You can remove the filter ring by removing the three screws marked with red arrows above. It may not be 100% necessary for this operation, but it shouldn't be tough either. That makes your lens look like this:



You now need a lens spanner or improvised tool, like needlenose pliers with tips ground down to fit the square slots. The ones you want are marked with green arrows in the above photo. They allow you to unscrew the front group of lenses in one unit, and place that aside. Don't do anything with the screws marked with red arrows in that photo. The screws you want to remove are marked here with red arrows:



Remove the screws. Now you can lift out a center tube that contains the aperture and rear lens group. It looks like this:



The arrow points to a rod that moves the blades. The rest of the lens has a little fork that the rod goes into. You'll need to do that when reassembling:



You can set aside the rest of the lens. I would also unscrew the rear lens group from the tube so it's out of your way. At this stage you can grip it by hand and not need tools, or use the outer spanner slots.

Now you're on your own. I haven't taken mine apart any further than this. I think the construction is similar to this Takumar 55mm f2:



Both lenses have screws around the outside of the tube. They hold some part of the aperture mechanism together. On the 55, it's a plate that keeps the blades from falling out. You'll probably have to see for yourself.

Though that's what I'd do, and it doesn't seem like a big deal to me, it may not be for everyone.
01-29-2011, 05:43 PM   #3
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I forgot reassembly tips. Obviously the rod goes in the fork. At this point, you can rotate the tube left or right, and the aperture blades may also move. You have to set them so that the aperture ring moves the blades correctly. Move the ring to f22 and rotate the tube until the blades fully close and just barely stop moving. Then install one screw and tighten it to keep the tube from rotating. You should be able to turn the aperture ring to f1.7 and the blades will fully retract.They should start appearing when you move the ring off f1.7. When the tube is aloigned, install the other screws and tighten.
01-29-2011, 05:58 PM   #4
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I guess it depends on how it looks toward wide open. If it looks ok, then might not worth the trouble fixing it. Many Pentax lenses have imperfect aperture stop down, just the way they are I am afraid. And imho, yours doesn't look bad really.

01-29-2011, 06:19 PM   #5
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QuoteOriginally posted by wlachan Quote
Many Pentax lenses have imperfect aperture stop down
Many non-Pentax lenses too.


Steve
01-29-2011, 09:19 PM   #6
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I think I heard someone on here say that it could possibly affect your bokeh but I cannot confirm nor deny this.
01-29-2011, 10:01 PM   #7
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The only times this will have a noticeable effect is shooting at night. Out of focus light sources will have a slightly irregular hexagonal shape at f8. At f8, it'll be hard to find any out of focus areas actually. I wouldn't bother fixing this, it's not going to affect the sharpness or usage.
01-30-2011, 11:00 AM   #8
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QuoteOriginally posted by KxBlaze Quote
I think I heard someone on here say that it could possibly affect your bokeh but I cannot confirm nor deny this.
QuoteOriginally posted by hangu Quote
The only times this will have a noticeable effect is shooting at night. Out of focus light sources will have a slightly irregular hexagonal shape at f8. At f8, it'll be hard to find any out of focus areas actually. I wouldn't bother fixing this, it's not going to affect the sharpness or usage.
These are both possibly true. But it's almost a theoretical defect. Is anyone going to shoot at f11+ and then examine the bokeh? Or choosing f11+ for night shots, when going to f8 helps out a lot of other things than the shape of OOF highlights.

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