Mike is a leading DP in London, England and editor of High Definition Magazine.
For a few years I’ve assumed that it could be worth spending many dollars on high quality HD zooms to improve the performance of wide shots that all seem a little soft on HD. In the past few years I’d noticed lateral chromatic aberration on standard quality HD zoom lenses (colour fringes either side of verticals), mostly on wide shots and assumed a high quality $70k HD lens would reduce the colour fringes.
What has been annoying is that all the frame grabs I had ever seen of my DSC charts in my studio look pretty good, as do everyone else’s frame grabs, but location wide shot frame grabs still suffer the lateral aberration.
First, lens manufactures have a hard time focusing Red Green and Blue light to an equal distance through the optical block. In fact they get a helping hand from the camera manufacturers who created a standard where the red green and blue CCDs are not set at equal distances on the optical block; this has enabled TV lenses to be designed with 40x or 100x zoom ratios and film lenses (where all wave lengths have to be focused on the same plane), become virtually impossible to make past 25x.
However, it is still a devilish task to create a zoom that is perfect at all focal lengths, impossible in fact, but each zoom lens does have what DPs call a sweet spot. This is a focal length where RGB rays are as good as they can get. There is a residual lateral aberration in the form of an inherent problem in the prism – (well, camera manufacturers pass the responsibility to the lens manufacturers and lens manufacturers blame the prism itself!)
The problem becomes increasingly evident below 12mm and is really quite a problem at 5mm or 6mm. Having tested many lenses on many cameras, it is evident on all lenses and cameras.
This sweet spot varies with each lens. A sweet spot on a Canon 40x lens, for instance, is 100mm whereas a 21x may be 14mm. Both focal lengths aren’t too long or too wide so that a test chart can’t be shot in a lab without knocking down a wall.
So most of the charts I shoot, say testing gamma curves, are usually well above the focal length where the aberration is a problem and also are in the sweet spot of the lens! So I have a brilliant collection of frame grabs with the DSC chart neatly filling the frame where there is near zero aberration, and a collection of wide shot frame grabs with problems.
So why hasn’t this aberration been noticed on our test charts? It is only recently we have pixel perfect registration in LCD monitors and, combined with 1920×1080 10 bit recording rather than 1440×1080 pixel 8 bit recordings, we are seeing more fine detail.
The main reason is that we rarely shoot a test chart on a wide lens where it fills the frame.
Since there are no 10 foot wide charts my tech tip for testing the wide end of zoom lenses is to shoot the DSC chart on the left and right edge of frame at 4 meters.
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