In-camera distortion correction: Can software ever take the place of optical excellence?
Richard Butler looks at the trend towards cameras correcting lens distortion and what it means for photographers.
What's more important, the final image or how it's achieved? Over the past year or so, we've encountered an increasing number of cameras with lenses that capture significantly distorted images which are then corrected using in-camera software. It's a process that has divided the forums, with some arguing passionately that it's cheating and that the lenses aren't really as good as they appear, while others maintain that it's the final results that matter, rather than how they're achieved.
In the film era, distortion correction had to be designed into the imaging lens, and this was (and remains) true for digital SLRs too (the need for the captured image to match what the photographer saw through the viewfinder limits what the camera can do to the shot once taken).
The advent of live view digital cameras (fixed and interchangeable lens) not only allows software correction of distortion (something impossible in an analog camera), but crucially also lets you see the result of whatever distortion corrections are being applied at the point of shooting, so the preview image matches the captured image.
There's no reason to believe this is a particularly new approach for camera makers (it would help explain the sudden trend for including 28mm equivalent wide-angle zooms in compact cameras), but its appearance in enthusiast cameras such as the Panasonic DMC-LX3, which output RAW data, means it has become potentially visible to the end user.
With the arrival of Micro Four Thirds, we now have an interchangeable lens system with many lenses that require software correction of distortion. So what are the manufacturers playing at? Why are they releasing lenses with distortion, what effect does it have, and should dpreview review the distorted or the corrected output? We spoke to a series of leading camera and lens makers, as well as the creators of leading RAW converters to find out just what's going on.
So what do we mean by distortion anyway?
First of all, it's important to understand what we mean by distortion here. Quite simply, we're talking about the lens rendering lines which should be straight as curves. This is is known as rectilinear distortion, and comes in two basic types: barrel distortion, where lines bow outwards away from the center (making a rectangle resemble a barrel), and pincushion distortion, which is the opposite.
Why have manufacturers left this distortion in their lenses?
The simplest answer to this question is that they can. On film, if you captured a distorted image on your negative, it would be very difficult to subsequently correct it. As more powerful in-camera processors have become less expensive, distortion can be corrected and, because many cameras offer only electronic displays, the user can frame and compose their shots based on a pre-corrected preview.
|The preview on the back of the Canon S90 shows the same corrected image as will appear in the final image, with a closer relationship between the framing of the preview and final image than you'll get with most DSLR's, most models of which tend not to have viewfinders offering 100% coverage of the scene.|
Olympus, a founding member of the Micro Four Thirds system, explains the possibilities this creates: ‘this lens distortion correction by software enables us to greatly reduce the length and volume of a lens, and gives manufacturers flexibility of small and light weight designs.'
'By using the software correction, front lens groups with strong optical power are not necessary, and the size of the front optical element can be reduced. As the total length of the lens can shorten, it is easier to design light weight and compact size lenses.'
Canon, whose PowerShot S90 also applies correction to its output, makes a very similar point: ‘In designing any optical system we need to consider multiple aspects including cost, target market, product size, manufacturing complexity / efficiency and sensor. It gives more flexibility to the design of the whole system if software correction is considered.'
Kazuto Yamaki, Chief Operating Officer for lens maker Sigma explains how this can work in more detail: 'compromise in distortion and chromatic aberration is a very effective in making lenses more compact and reducing cost. In general, compromising the distortion at the wide-angle end affects the compactness of the lens and compromising the CA influences cost reduction.' It's not without its drawbacks, he says: 'it's also true that doing this deteriorates the image quality if we correct them with image processing. It may not be very obvious if you look at the images in small format, but it becomes recognizable when enlarged. In the case of users who are very, very insistent on top image quality, they may not be 100% satisfied with such images.' As a result, no Sigma lenses currently require software correction.
The extent to which software correction is used depends on the purpose of the lens, he says: 'If the very best image quality is the desired goal then we might not be able to use it but if compactness and reasonable price is considered a priority over the image quality (for those who want such lenses), we can use that methodology.'
All the manufacturers we spoke to make clear that the basic lens design process isn't fundamentally changed when developing a lens that will include a degree of software correction – they aren't using optically-uncorrected, fish-eye lenses or anything so extreme. However, Olympus sheds a little light on the process: ‘Generally an optical correction of barrel distortion tends to cause a moustache distortion,' which is very difficult to correct for in post-processing. ‘We consider that we can improve the final image quality with a balance of optical distortion corrections and software correction,' the company says: ‘rather than with only with an optical design solution.'
It's a question of balance between the image quality and size/price, explains Sigma's Mr Yamaki: 'we can compromise somewhere between the two - correct the aberrations nicely in the lens design and also correct them non-aggressively in the image processing. It's not the matter of choice between the two options.'
What's being corrected?
From our testing, we've seen different manufacturers take different approaches and to different degrees. Panasonic's Micro Four Thirds lenses contain information about both geometric distortion and chromatic aberration (color fringing), Olympus's only contain geometric distortion information. Panasonic's camera bodies correct whatever they receive information about from the lens, and also pass that information on to their RAW files, while Olympus' only correct and retain distortion data. Samsung tells us its NX series doesn't yet do any correction but has the capability to do so if needed in future lenses.
What's the cost?
Understandably, many people are wary of the idea of what they perceive as 'optically imperfect' lenses and the image manipulation that consequently has to go on. However, while it is easy to make assumptions and to criticize as a point of principle, it's harder to prove any great damage is being done.
Here we've taken the same shot using the Olympus 14-42mm kit zoom on an E-620 DSLR and using the corresponding Micro Four Thirds lens on an Olympus E-PL1. The DSLR version of the lens produces 1.38% distortion at this setting, according to our lens test, while the Micro Four Thirds version natively produces 4.5% distortion (one of the highest levels of distortion we've measured), and corrects it to around 1.8%.
Our copy of the DSLR lens seems to have developed a degree of asymmetry since we last checked it, but is likely to still likely to be representative of kit lens performance in the real world. They are, in any case, the closest we could find to comparable systems, one of which relies just on optical correction, the other of which combines optical and digital correction.
Overall, the best corners of the DSLR lens that makes use of a purely optically-corrected design have an edge over the Micro Four Thirds lens that combines both optical and digital correction. But as you can hopefully see from these crops, it's pretty slight - you have to look at the pictures very closely to notice any difference.