Some kit lens deficiencies are easily resolved in post and some are just ugly to the core, or is that the iris. For today we consider the factors that can be sorted post shot.
Here is an uncorrected image taken with an old 28-85 kit lens on a full frame sensor, whilst the lens is not that bad, the edges of the frame show Chromatic Aberration, there is some vignetting, obvious barrel distortion and some loss of corner detail. At this small size it no doubt looks OK but a good size print would be a different matter.
Chromatic Aberration can be generally sorted and visually to my eyes makes one of the biggest differences, it effects colour and clarity and I personally find high levels of CA visually disturbing, I almost need to have a stiff drink when I see a bad example. CA can be aperture dependent and is always focal length dependent.
With most cameras these days CA can be corrected "in camera" if you are shooting in JPEG format, only the worst cases from the worst offenders will be seen in the final file, however Raw files will normally show the CA in all its awful glory.
It is worth noting that there are two types of CA, longitudinal and lateral. Longitudinal CA is usually magenta - green and is normally only obvious at wide apertures using fast lenses. Basically you get differing coloured fringes around the details dependent upon whether you are looking at the area in front or behind the point of focus. This is hard to fix but far less common and actually rare with slow aperture kit lenses, the good news is if your lens is prone to it, you can prevented with stopping the aperture down to something more sensible.
The other CA, lateral is where we get coloured fringes around high contrast details, the strength of which increases as you move away from the center of the image, hence the term lateral. There is no fringing in the middle of the frame. No, its not the purple fringing that many people refer to, it can be red, green, blue, magenta, yellow or of course occasionally purple.
Many people think the pure purple fringing seen in a lot of high contrast photos is CA but in fact its often something else altogether, “sensor blooming” and is normally controllable by avoiding over-exposure. Note however as said, real CA can also be purple too but is most likely in my experience to be magenta or green.
My experience is that some applications are much better at dealing with CA than others, I won’t make any recommendations as apps are always in a state of flux but don’t give up, if your current application can’t “kill the fringe” try something else.
Ideally you will always get the best sharpness from a lens which exhibits no CA, well chances are you will strike out on that one with a kitty, however it is highly likely that one focal length will display very low levels of CA compared to the others. You will need to dig around a bit. For example, my Sony 18-55 changes its CA characteristics significantly between 25- 30mm with 27mm being close to perfect. No other focal length comes close and as you move away from the 27mm setting things get progressively worse but in different colour directions.
Here we can see a section from the right hand upper corner of the above image at the top of the page,
the disturbing CA is obvious here and we can also see it has messed with the corner sharpness.
This is a very old film era kit lens lens and no doubt, was fine for its time,
but of course digital is far less forgiving.
Unless truly woeful, vignetting is easily sorted, but very high levels can leave your edges a bit noisy once it has been eliminated. I often add vignetting in post production so it could be a benefit to you anyway.
The worst cases of VD ( vignetting disease) seem to occur at the wider end of things, with most kit lenses displaying considerable vignetting between 18-24mm. You may not actually see any vignetting if you're only shooting in JPEG mode, almost all cameras now correct this in processing, but again for Raw files you will need to roll your sleeves up.
Generally a little vignetting is actually quite tolerable for most image styles and unless you are shooting landscapes with lots of sky in them you probably won’t be overly bothered by it.
Geometric distortions are generally easy to sort and again unless really bad you won't notice the issue. The only thing to bear in mind is the more you have to correct the distortion the less final image area you end up with, and you may get an accompanying loss of corner/edge clarity once the problem is corrected. These days most cameras correct the worst of the distortion internally when shooting in JPEG format, so again you may never notice it anyway.
I suspect that many kit lenses actually shoot a bit wider than their stated focal length to allow for the needed cropping that occurs via the in-camera editing to get things squared away. The Sony 16-50 is a case in point, I think prior to processing it is probably more like 14 mm at the wide end. That leads me to a killer tip, by default the distortion correction is turned on on almost all cameras when you receive them, if you need a little extra angle of view, temporarily turning it off might just be enough, so long as you can accept the distortion. Even better if you shoot RAW you can disable the default distortion correction in your Raw Convertor application to get that little but extra in the pic.
Another tip to take to the bank on is that if you are shooting at focal lengths that will need distortion correction, just step back a bit and give yourself some extra wiggle room in the editing phase.
Normally the distortion is only really obvious if you are shooting objects with straight lines in them, for example architecture. I must point out that even many fixed wide angle lenses have considerable distortion so buying an alternative lens may not solve the problem altogether. From an optical point of view distortion is very to eradicate via lens design and usually involves adding more elements to the lens, making it both heavier and far more expensive, since it is relatively easy to fix in post, the justification for buying well corrected but expensive alternative lenses is somewhat diminished.
There are two primary types of distortion, Barrel and Pincushion. Almost without exception wide angle lenses tend towards barrel distortion (which bloats the image outwards and telephotos tend towards pincushion distortion (which pinches the image inwards. Normally for zooms there will be a point where there is no distortion and with most kit lenses this will be somewhere in the 26-35mm range. As a tip, the ideal focal length for easier panorama stitching will be the one that has neither type of distortion.
There is one complex type of distortion known as moustache distortion, in this case the middle part of the frame edge barrels outwards whilst the area about half way between the middle and far outer corners pinches in. Normally such a distortion only occurs with focal lengths at the very wide end of the focal length range. This complex distortion characteristic unless dealt with in-camera generally defies the best efforts of Raw converters and editing programs, though there are some specialised panorama applications that can correct it. Again in my experience unless you are doing a lot of architectural work this distortion is not likely to cause you any grief.
And now we have our finished specimen, the CA is gone, the corners sharper, vignetting removed and the distortion but a memory. Of course it is a little cropped due to the distortion corrections, hence my tip to shoot a little wide if possible.
The final optical Aberration of concern is field curvature, I am not even going to start to deal with that in this blog entry, it deserves its own post so we will save that.