Incident-light meter - Wide angle and Telephoto

Hi
When using an incident-light meter, holding it at or near the subject and
aiming the meter's light-sensitive cell back toward the camera.
I notice a difference when shooting at 10 ft and when shooting at 20 ft.
Say when shooting at 10 ft with a wide angle lens I need to give it f8 but
when taking the same
shoot from 20 ft with a telephoto lens the shot is under exposed.

Is this a known problem or am I doing something wrong?

Thanks
J

 

You need to check you lenses and see if they are performing properly.
Perhaps one of them has a sticky diaphragm.
Incident meters measure light falling on a subject which is obviously
independent of the camera and lens.
Do a test, set you lens to a fixed stop (say f8) and shoot a uniform
subject like a blank wall. Make sure your autoexposure is off.
Fill the frame with the subject.
Now do the same thing for your other lens. Same f stop, but move the
camera to get the same image captured.
You can repeat for several other f stops (like f16) if you wish.
If there is any difference between the exposures then you have a
mechanical problem.