GN Topcor M 50mm f/1.4

Lightshow · Joined: 04 Nov 2011 Posts: 3669 Location: Calgary

Your description of the focus tells me your lens has issues because it should be smooth, the change from a threaded helicoid to a cam "helicoid" was so they can change the rate at which the optics move compared to how much the focus ring is moved, ie a variable rate helicoid, normally the bulk of the helicoid movement is used up in close range focusing and a small portion is left for near infinity(10m+).
It's not my preferred method of focusing, because there is more opportunity for things to go wrong, a helicoid only requires to be clean and lubed and it will last forever, cams also require the cleanliness and lube, but also the cam follower must not have issues, some are simple pins and others are bearings with rubber or plastic sleeves over/around the outside as the contact surface with the cam, grit can enter the cam from anywhere along the path whereas with a helicoid it can only enter at the 2 ends.

Blazer0ne · Expire: 2024-12-07