Can computer corrections make simple lenses look good?
Modern lenses tend to be large and expensive, with multiple glass elements combining to minimise optical aberrations. But what if we could just use a cheap single-element lens, and remove those aberrations computationally instead? This is the question scientists at the University of British Columbia and University of Siegen are asking, and they've come up with a way of improving images from a simple single element lens that gives pretty impressive results.
|Image scientists are looking at whether a complex lens can be replaced by a simple one, along with lots of computation.|
The method is described in detail in the researchers' paper. It works by understanding the lens's 'point spread function' - the way point light sources are blurred by the optics - and how this changes across the frame. Knowing this, in principle it's possible to analyse an image from a simple lens and reconstruct how it should look, through a computational process known as 'deconvolution'.
|The Point Spread Function diagram for a simple f/4.5 lens of the plano-convex type (i.e. one side curved, the other flat). The centre shows broad discs due to chromatic aberration, while the cross-shapes towards the corners are due to coma and astigmatism. The researchers split the image up into 'tiles', each with its own PSF.|
This isn't a new idea, but the team of researchers claim to have made some key advances in the field, making their method more robust than those previously suggested. For example chromatic aberration means that simple lenses can give detailed information in one colour channel with significant blur in the others, so they've decided to use cross-channel information to reconstruct the finest detail possible.
One serious problem with deconvolution approaches is that they often struggle to reach a single 'best' solution. The group claims to have solved this by optimising each colour channel in turn, rather than trying to deal with them all simultaneously.
This is all very clever, of course, but does it work? The group shows several before and after examples on its website, shot using a simple F4.5 plano-convex lens on a Canon EOS 40D, and the results are quite impressive.
|Original version (click for original)|
|Image de-blurred using deconvolution (click for original)|
So will this be coming to a camera near you anytime soon? In this precise form, probably not - the system still has problems understanding areas of the image which are slightly out-of-focus, and won't work with large aperture lenses. And while the images are certainly improved, they're unlikely to satisfy committed pixel peepers. In fact we'd guess it's most likely to be useful in smartphones, where the mechanical simplicity and robustness of simple lenses should be appealing. However, it certainly offers an interesting glimpse of the way results could be improved when shooting with a 'soft' lens.
Sep 22, 2016
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