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12-19-2008, 11:17 PM   #16
Just1MoreDave's Avatar

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Photos of the M50/f2

In case someone is interested, here are some photos I took a while ago. I had two Pentax-M 50mm f2.0 lenses, one with jammed focus. I tried to keep one mostly assembled as a guide for the broken one. It worked, and I got them fixed and back together.

In the first photo, the left lens just has the mount removed. The right lens has a lot of front disassembly already, the mount, aperture ring and silver ring that rests between the lenses removed. This silver ring has a notch that fits a key on the aperture ring, and a short round bar that should face towards the front of the lens. The short bar fits into the aperture control mechanism and keeps the aperture from opening wider than the aperture ring setting. This ring just lifts off. It doesn't really work or fit upside down. By the way, the aperture ring usually is holding in a small, spring loaded ball bearing that makes the lens click at all the stops. The bearing will try to shoot off into outer space and you'll never find it, unless you are prepared. Open the lens in a plastic bag, box or other controlled area, find the bearing and keep it safe.

The left lens now has the silver ring removed. The right lens has the aperture control mechanism removed and placed between the lenses. This has the lever that goes into the camera, a second lever that goes further into the lens to move the blades, and a return spring. Three screws hold it in. The lens on the right has a copper tab removed. These tabs connect the focusing helix to the lens. If you remove these and unscrew the helix, you might spend an hour getting the lens back together correctly.

Here is some front disassembly, just one lens this time. The "name ring" comes off first, and you'll see three screws. They hold a short barrel that has the filter threads. Remove that and on this lens you'll see more screws. The brass ring holds down the front lens group, which is upside down and to the right. You can't see the three screws still on the lens which attach the focus ring to the lens body. These can be removed but you'll need to adjust the lens for infinity focus upon reassembly.

Other Pentax lenses are similar, The front lens group is frequently mounted in a threaded carrier that screws in. Only the cheaper lenses use the brass ring. Older lenses have common screws instead of phillips, and brass instead of aluminum where the focus ring attaches.

12-20-2008, 01:05 AM   #17
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QuoteOriginally posted by idig4phish Quote
I understand the 1/2 light per stop idea... but what I'm guessing is that 1/2 of f/8 is so small that I don't notice a change to f/16. I hope that makes sense. 1/2 of f/2 is a lot, so I can see it move to f/4.
The difference between f8 and f16 is not 1/2 the is 1/4 the light. Half the light of f8 is f11 and half the light of that is f16.

Half the light of f2 is not is f2.8. Half the light of f2.8 is f4.

Imagine you have a square aperture and it is easier to visualize. Imagine a square 2cm by 2cm. Total size of the aperture is 4 square cm. Double that to a 4cm by 4cm square and you'd have a 16 square cm aperture. Same thing with a circular opening....double the diameter (f2 to f4) and you quadruple the area....two stops....four times as much light. To double the area, you need to open up the hole by a factor of the square root of two. f2 x 1.4 = f2.8.....double the area and therefore double the light. f2.8 x 1.4 = f4.

This is why if you look carefully at the aperture numbers you can see two sets of doubled numbers meshed together like the teeth of a zipper.

2.0 ------+
12-20-2008, 07:09 AM   #18
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yes yes yes... i was in a hurry. I certainly meant f/8 to f/11, but at the time I couldn't see it move at all, so it really didn't matter. Thanks for the mathematics review though... understanding the physics of the aperture is a better way to remember that f/8 halves to f/11... better than rote memorization.

By the way, the pictures of the f/2 look very unlike the f/1.4 I took apart. Certainly helpful for someone I'm sure... but I'm certainly suprised by how unfamiliar it looks. Taking the mount off revealed a "brick wall" basically... there wasn't anywhere else to go. Just a solid black encasement, such that the barrel was made out of the same mold as the back part under the mount.

I'll give you the quick run-down of what was wrong with mine... the aperture mechanism is contained in one peice with the blades sandwiched between two rings, one of which has the pin that interacts with the pin you see on the back of the lens. The whole mechanism is held steady by 3 screws which don't actually screw into the mechanism... they screw in from the side and have sharp ends that pinch the ring in place. These screws had become lose, so that everytime the aperture was used (i.e. every time you moved the aperture ring) it shifted the mechanism a little. This reset the "open" setting to something less than open because the mechanism shifted and the blades slid out a little. So my lens started open at, say, f/2... so by the time I had the setting to f/8, it had closed all the way and turning the aperture ring anymore didn't do anything.

The way you get there is similar to the method described above for the f/2 lens. Take the name ring off (a shot glass turned upside down on rubber gripper used to open jars works great!), then start taking screws out. There's 3 for the first ring around the front element, then 3 more to get it free. Then you have to unscrew the actual lens, then you can see the aperture mechanism between it and the back lens. I also had the K-mount off from the back, but I'm not sure that would have been necessary. I did give me a better look at putting the aperture mechanism back in place to line up with the control pin though.

Sorry I didn't take any pictures, but I was trying to hurry... I was sorta on a timeline.

#1 tip, do not attempt without proper screwdrivers. Not just small electronics screwdrivers, but jewelers tools or something similar. "Small" screwdrivers and a steady hand can get you into the lens OK, but the aperture mechanism is locked in place with countersunk screws that truly require a very small flat-head to access without making a real mess.

Thanks all....

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