Letus extreme grain affected by focal length?

[Stephen Roe]

hey, I'm having a bit of an issue with a Letus Extreme adapter when using a longer lens. I've shot 24, 28 and 50mm FD lenses on it and everything has be absolutely fine; no grain whatsoever below f5.6. Strangely though, when I mount a 135mm lens there is quite pronounced grain in what I can only describe as streaking patches running diagonally from top left to bottom right of the frame.

The max aperture of the 135mm lens is f3.5 so its already running into grainy territory, but f3.5 still shouldn't really be an issue. I'm trying to determine whether the motor for the ground glass isn't vibrating quite the way it is supposed to and causing the grain (though wouldn't this be an issue at all focal lengths?), or whether this lens is causing the exaggerated grain due perhaps to peculiar magnification; specific to this one lens. Problem is: I don't have access to any other FD mount lenses longer that 135mm to do any tests.

I know that tele lenses aren't usually an issue with 35mm adapters as I've shot 300mm lenses on a redrock with no problems, albeit with its spinning GG.

Anybody got any ideas?

Cheers
Stephen

 

[kodjo]

3 posibilities:

a) Something in your camera is set to automatic. (Check the camera)

b) The lens may be say 3.5 but is really a 5.6 or above. (Check the lens)

c) One of the columns in your ground glass just broke. (Check whether you get this with another lens).