A distorted view? In-camera distortion correction
2 A distorted view? In-camera distortion correction
What if I don't want to use the manufacturer's RAW converter?
In all the instances we've encountered, the cameras are recording un-corrected data in their RAW files and including information about lens distortion characteristics as additional information (metadata) alongside the underlying capture data in the RAW file. However, Panasonic says that the Micro Four Thirds standard allows for both approaches: ‘it depends on manufacturers' policy whether corrected image or native image would be recorded.' So this may not always be the case in the future.
Canon, Olympus and Panasonic all make clear that they provide their own RAW conversion software with their cameras that automatically corrects images. However, although metadata describing the distortion is included in the RAW file, it is up to individual software makers how much of this information they use when making conversions; and Olympus says it does not disclose its RAW file format to third-party software providers.
In co-operation with camera makers, Adobe has developed both its Camera Raw (ACR) converter and the open DNG format to recognize and incorporate distortion correction parameters. This means you get the same geometric distortion corrections for supported cameras as in the manufacturer's software.
DXO Labs' DXO Optics Pro software, meanwhile, is based around lens profiles developed by the company based on camera testing, rather than data supplied by manufacturers. At the time of writing this article, neither the Panasonic G1 nor Olympus E-PL1 were supported by the software, however.
Meanwhile, Phase One says its Capture One converter provides ‘native lens correction for certain cameras and lenses. The support is dependent on the manufacturer's willingness to provide information on lens errors and correction requirements.' Capture One Pro also provides a lens correction tool allowing the correction of geometric distortion, fall-off and CA based on individual lenses: ‘there is a substantial difference in using a generic model and using a model where the measurements/correction take place with basis in a specific lens.'
Overall, the result is that, whether you're using the manufacturer's software or the most popular third-party conversion software, you'll get the same undistorted image that you composed on the cameras' screens or viewfinders and that you'd get from the camera's jpegs. But some other raw converters won't provide properly corrected output, so if you choose to use one of these, you'll have to fix the distortion yourself.
Isn't it cheating, though?
Every time somebody does anything with a digital camera that couldn't be done in a film darkroom, people have called 'foul' and make accusations of cheating. However, 20 years after the appearance of Photoshop, it's safe to assume that a degree of post-shoot 'retouching' is the norm, rather than a sneaky exception (And remember that plenty of secrets could hide in the darkrooms of skilled practitioners). At which point, there's a chance that one person's cheating might turn out to be progress for the majority.
After all, this isn't the first instance of automated, non-optional image correction that has no darkroom analogue. Most current Nikon DSLRs assess their images for lateral chromatic aberration and correct it in their jpegs. This could just as easily be seen as cheating, since it results in a final image that hides some imperfections that optical design alone hadn't eliminated from lenses. The situation is slightly different in RAW, since no correction data appears to be included with the file but, given that Nikon's own software applies exactly the same corrections and that many third-party converters can also apply similar lens corrections (either automatically or controlled by the user), the distinction is subtle.
The only fundamental difference is that most Nikon lenses pre-date the application of these corrections, so can't have been designed to take them into account. However, only a handful of people within the company will ever know whether the correction is considered when developing new optical formulae. After all, if fractionally under-correcting CA allowed a slightly simpler, lighter, sharper or less distorting designs, then wouldn't such a design be preferable, if the cameras were going to remove that additional CA anyway?
What about reviews on dpreview?
This brings us back to our original question: ‘What's more important, the final image or how it's achieved?' because it's one we have to answer every time we review a camera or lens. It's our job to give readers a good understanding of the product they might be thinking about buying, but does it make sense to criticize it for something that most people will never be need to be aware of?
Lens reviewer Andy Westlake explains the approach the site takes: ‘In our opinion, what matters most is the final image quality that the end user is most likely to see - which means what they'll get from jpegs, or raw files developed using either the software supplied with the camera, or the most popular raw converters such as Adobe Camera Raw.'
This informs the attitude taken in reviews, he explains: 'we'll be basing our conclusions on the results you'll get when corrections have been applied - and therefore measuring any loss of sharpness that results from 'stretching' at the corners. But we also think it's important to show the user exactly what corrections are being used, so we'll also show how much distortion and chromatic aberration is present in an uncorrected image.'
Finally, he says, it's important to keep things in perspective: 'There seems to be a tendency to assess the performance of a lens from its sharpness alone, but we think there's more to it than that, and it's the balance between sharpness and image defects such as distortion and chromatic aberration that really counts when assessing a lens. For example some conventional SLR zoom lenses exhibit very strong barrel distortion at their wide ends which is complex in character and difficult to correct in software, and this can spoil an image no matter what size its printed or displayed. The use of software correction simply gives manufacturers another tool they can use to address such problems and deliver better-looking images with less distortion and colour fringing - and as long as it doesn't have too much of an impact on sharpness, we believe this is surely a good thing'.
Jul 7, 2014
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