a difference in the image brightness that shows on the histograms or when viewed in Lightroom
or is there just a difference in the shutter speeds to give the same visual rendering?
#1 could indicate a problem in one of the lenses or in the electronic communication between camera and lens. Cleaning the contacts on the lenses and in the lens mount of the camera body might fix this.
#2 could indicate a difference in the light transmission of the lenses. I would expect to the zoom to reduce the light a bit compared to the prime. I would not expect the difference in transmission to cause nearly the one stop difference you are reporting.
As others have asked, an unprocessed image of the same subject from both lenses would help the diagnosis. A JPG straight from the camera unprocessed other than downsizing for posting would be best.
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I will try and get the images up once I work out how to do it.
I tried this test again with exactly the same results, needing 0.9 of exposure adjustment to correct. I tried the 70-200 f2.8 at 70mm against the 24-120 set to 70mm to compare results and both zooms matched up, meaning the prime is actually overexposing, although to my eyes it appears correct with the zooms being darker.
The histograms are all identical in shape with the prime placing it slightly to the right.
I was told that the colour inside the lens casing may be different, being perhaps lighter in color and/or more reflective of the light getting through. I have no idea if this is true or not but it could explain it.
Other than this, all lenses are sharp with accurate colours so at a loss to know why its happening.
Since it seems to be the prime that is over-exposing (rather than the reverse), it could just be that the diaphragm in the 50mm is not stopping down to the pre-set aperture, perhaps because of a sticky linkage in the lens. If that is the case, you should not see overexposure when using the 50mm wide-open (because then no stopping-down is involved).
You could check this out by setting up a test, taking the same shot with the 50mm in Aperture Priority (and with Auto ISO turned off) - first at f/1.8 and then at (say) f/5 - three stops down. The camera should choose shutter speeds three stops apart, and the two shots should have the same exposure. If the f/3.5 shot is significantly brighter, there is the problem
Hi Buzz, I suggest making a series of test shots using full MANUAL mode with each lens while the body is on a good, stable, tripod and while the scene illumination is expected to remain constant. Shoot a series with each lens, varying shutter speed in step with aperture for at least four shots at 1 or 2 stop intervals. Obviously, ALL shots should yield "identical" exposures.
Then, post eight shots (as four pairs, resized but otherwise unprocessed and with their EXIF intact) - and let us take a look...