On the AF-100 forum there was recently a discussion of one of the more subtle shortcomings of the Lumix 14-140mm f4-5.8 zoom - exposure fluctuations while zooming during a video take. These brief flashes happen not only at the widest aperture settings, but at apertures above f5.8 as well. And as it turns out, the kit lens is not the only auto-focus zoom that suffers from this flaw.
Panasonic and Olympus auto-focus zooms do not function like traditional manual zoom lenses which maintain precise focus and aperture settings by mechanical design. Four Thirds lenses are not designed to hold focus and aperture while zooming - they are utterly dependent on active electronic compensation to dynamically correct their inherent shortcomings. Even the slightest touch on the zoom ring will disturb both focus and aperture settings, forcing the camera to revise its internal lens adjustments.
When an auto-focus lens is zoomed during a video take, the camera immediately detects the aperture change and loss of focus-lock and attempts to correct it. However, continuous zooming will repeatedly invalidate the camera's attempts to correct the aperture and regain focus-lock, forcing it to abort and retry its lens adjustment procedures over and over again. If you zoom quickly enough, the camera will simply fail to keep up with your pace, and the lens will visibly drift out of focus until you pause and allow the camera to catch up. This hazard is particularly severe with the Lumix 14-140mm due to its extremely wide zoom range.
To find out how prevalent this problem is among auto-focus zooms, I conducted a series of zoom tests using a Panasonic G2, which supports AFS auto-focus mode on all of the following lenses:
Olympus 35-100mm f2.0
Olympus 14-54mm f2.8-3.5 II
Panasonic 14-50mm f2.8-3.5
Panasonic 14-45mm f3.5-5.6
Panasonic 45-200mm f4-5.6
Olympus 50-200mm f2.8-3.5 non-SWD
Panasonic and Olympus auto-focus zooms do not function like traditional manual zoom lenses which maintain precise focus and aperture settings by mechanical design. Four Thirds lenses are not designed to hold focus and aperture while zooming - they are utterly dependent on active electronic compensation to dynamically correct their inherent shortcomings. Even the slightest touch on the zoom ring will disturb both focus and aperture settings, forcing the camera to revise its internal lens adjustments.
When an auto-focus lens is zoomed during a video take, the camera immediately detects the aperture change and loss of focus-lock and attempts to correct it. However, continuous zooming will repeatedly invalidate the camera's attempts to correct the aperture and regain focus-lock, forcing it to abort and retry its lens adjustment procedures over and over again. If you zoom quickly enough, the camera will simply fail to keep up with your pace, and the lens will visibly drift out of focus until you pause and allow the camera to catch up. This hazard is particularly severe with the Lumix 14-140mm due to its extremely wide zoom range.
To find out how prevalent this problem is among auto-focus zooms, I conducted a series of zoom tests using a Panasonic G2, which supports AFS auto-focus mode on all of the following lenses:
Olympus 35-100mm f2.0
Olympus 14-54mm f2.8-3.5 II
Panasonic 14-50mm f2.8-3.5
Panasonic 14-45mm f3.5-5.6
Panasonic 45-200mm f4-5.6
Olympus 50-200mm f2.8-3.5 non-SWD