This is a little test I performed in answer to a thread on http://dpreview.com in the micro-four-thirds group. In that thread, someone had used a prime and a kit zoom on a Panasonic GH3 (the premiere mirrorless camera for video) and found that the noise dramatically increased with the kit zoom, despite having not changed any of the settings.
Of course, in the end this is not really possible since the same aperture would return basically the same results. However, there will always be some difference in the micro-contrast and sharpness of each lens, which will definitely make an impression in the final results.
I ran the test in brutally bad light … a pair of really old halogen lights (more than 2 decades old) pointed at the ceiling more than ten feet away on camera left and 8 LED bulbs coming through a bathroom door about 8 feet away at camera right. The video exposure at 1/25s and f/5.6 required 5000 ISO, which is pretty high. Still, you will see that the Panasonic GH2 does a beautiful job at that ISO, possibly because I am running the Cake 2.3 patch, which allows the camera to go – in this case – up to 64Mbps, which is almost as high as the best possible setting on the GH3 and far higher than any other consumer camera will do natively.
I used f/5.6 so that all the lenses could shoot the same aperture, but do note that 5.6 is wide open for one of the kit zooms, and I think it shows in a slight loss of contrast. I graded the entire video track in Sony Movie Studio Platinum Suite 12, raising the brightness and contrast to emphasize the noise. I find the result excellent … strong saturation and very tolerable noise that would vanish if I owned Neat Video or its equivalent.
So … the test. Tripod mount, GH2 set to 1080/24p and the highest quality setting with the Cake 2.3 patch. At 5000 ISO the average bit rate is 64Mbps and at 400 ISO the rate drops to 46Mbps because of the lack of noise and details (much more background blur) to encode. The four lenses in order are: Panasonic 14-140 G Vario Mega-OIS 4.0-5.8 (the mark 1 version), the Panasonic 14-42 X Vario Power OIS 3.5-5.6, the Olympus 45 1.8 and the Pentax M (I say K in the movie but I think it is the M) 50 1.4, which is of course manually focused. I set AF continuous, and none of the lenses pumped focus, and none of them showed any jitter despite me leaving OIS engaged.
My opinion of the results is exactly as I describe above. You see no difference in noise, but you do see a difference in magnification (unavoidable since the lenses are not quite identical in focal length) and in contrast and sharpness. The primes look better, but only because you see them all one after the other. Otherwise you would not notice anything amiss in my opinion.
What I think is really illustrative, though, is the drop from f/5.6 to f/1.4 … an astounding 4 stops. The ISO drops also from 5000 to 400 … and the results are just so pretty :-)
I hope you enjoy it …