Choosing a Lens

Wednesday, December 15 2010

Today I had a question from a budding film maker about lens quality and what things to look for when buying one. This person was asking specifically about wide angle adapters that attach to the front of an existing lens. Initially, you can check the physical quality of the lens to make sure there’s no damage to the lens body or any scratches on the glass elements, but there’s really no way to test for picture quality unless the lens is mounted to a working camera. Here are some things to look for when considering any lens purchase, wide angle or otherwise.

1) Lens vignetting at the wide end. Make sure the actual lens barrel is not visible at the frame edges when pulled back full wide.

2) Barrel distortion at the frame edges. A lot of wide angle lenses will show curvatures at the edges of the frame on things like vertical lines (edges of buildings, telephone poles etc.) and the end user has to decide how much curvature is acceptable for the type of project they’re doing. Generally, the more expensive the lens the less barrel distortion there will be.

3} Chromatic aberration. This is when the RGB values, the primary colours red, green and blue, don’t all arrive at the recording sensor or imager all at the same time. The indication is a rainbow-like effect on the edges of objects. Most lenses have a certain amount of chromatic aberration but the user has to again decide if the CA is bad enough to be unacceptable for the type of shooting you’re doing (like a motion pic ture). After this it gets a bit more complicated. Video cameras like mine have CA (chromatic aberration) compensation built into the camera’s software. You tell the camera what type of lens is on the front and it will compensate for that lens’ known CA values and return a picture with little or no CA. That part of the camera’s software is again part of a larger data base called a “look up table” or LUT, that among other things, keeps a record of all the lens types that can be attached to the front of that particular camera. Some wide angle lenses have no barrel distortion and little or no CA at all but those lenses are very pricey.

Years ago I was looking at a true WA lens (not a WA adapter) from Canon for use on cameras like the Sony F900. The lens was in the $40k plus range and I wondered what made it so expensive until I attached it to a camera body. What really impressed me was that the field of view was so enormous but with no barrel distortion, no vignetting and no perceptible chromatic aberration.

It’s like any thing else: you get what you pay for.

Written by:

Don Greening is the owner and both senior editor and videographer for Reeltime Videoworks. His arts background as a classical musician and composer gives him an uncompromising eye and ear for detail that is very important in video production. Don also hones his technical skills by updating them through courses at Emily Carr Institute of Art + Design.