Why it’s almost impossible to have true Par-Focal zooms on the cheaper 35mm camcorders.

XDCAM-sliding-system-v2 Why it's almost impossible to have true Par-Focal zooms on the cheaper 35mm camcorders.

So, first of all what is Par-Focal? Well it’s the ability of a zoom lens to maintain the same focus point throughout the zoom range. In other words as you zoom in and out there is no focus shift. Most DSLR zoom lenses are not Par-Focal, they don’t need to be, as when taking stills photos you would zoom to get your desired framing, then set the focus. Some are closer to par-focal than others, some are pretty good, others are way off.

If you have ever used a full size shoulder mount broadcast camera with a removable zoom lens you’ll know that the lenses used have an adjuster on the rear of the lens to set the back-focus. This adjustment is there to take into account tiny variations in the lens mount, the optical filters behind the lens, wear and tear and temperature extremes. If you don’t adjust this correctly the zoom lens will not be Par-focal. DSLR lenses and most PL Mount zoom lenses don’t have this adjustment. Without a means to adjust the back focus it is the luck of the draw as to how par-focal your lens will be. On the Sony PMW-F3 you can adjust the back focus by adjusting the sensor to lens mount distance with an adjuster screw. However the way the ND filters work, with one stacked above the other will shift the optimum back focus distance depending on which ND filter you use. Most higher end 35mm video and film cameras have mounts that can be fitted with shims to adjust the flange back. In addition the better quality PL mount lenses are designed to be fitted with shims so that all the lenses in a set are corrected for the 52.00mm flange back distance for PL mount.

So, seeing as all the cheaper 35mm and APS-C video cameras have no facility to adjust or shim the distance between the lens mount and the sensor you are relying on the precision of the the camera manufacturer. In addition you are also relying on the precision of the lens manufacturer and the manufacturer of any mount adapters you might be using. Overall, even if the lens you have is truly par-focal the chances of everything being absolutely perfect are small, so the chances of your focus tracking perfectly accurately is also small. As a result, in most cases when using zoom lenses on a lower cost large sensor camcorder your focus will not track perfectly throughout the zoom range.

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