Mastering Your Kit Lens - Part 2

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 CA, 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

Chromatic Aberration can be generally sorted and visually to 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 so only the worst cases will usually be seen in the final file, however Raw files will 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 you have it it can be prevented with stopping the aperture down.

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 purple fringing that many people refer to, it can be red, green, blue, magenta, yellow or of course 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.

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.

Vignetting

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 bothered by it.

Distortion

Geometric distortions are generally easy to sort and again unless really OTT 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. Theses days most cameras correct the worst of this internally when shooting in JPEG format, so again you may never notice it anyway.

I suspect that some 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.

The tip to take to the bank is, if shooting RAW files that will need distortion correction step back a bit and give yourself some wiggle room.

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 lens have considerable distortion so buying an alternative lens may not solve the problem altogether.  From an optical point of view this is a hard thing to eradicate via lens design and usually involves adding more elements to the lens, making it both heavier and far more expensive, and since it is relatively easy to fix in post it removes some of the justification for buying well corrected but expensive alternative lenses.

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 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 moderate to 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.

Soft Corners

Poor corner resolution can be improved somewhat by selective sharpening using layers but will likely be your hardest issue to really fix, however in the near future using radial deconvolution sharpening this may well be able to be resolved OK.....love the sound of that "radial deconvolution correction....most discombobulating.

Currently Photoshop offers a work around for fixing this issue, but an auto option would be just peachy

Some kit lenses never get sharp in the far corners regardless of the aperture and this is especially the case at the wider focal length. Realistically the lack of far corner sharpness is rarely a big deal, if you think about it how many shots do you take that really need full corner to corner sharpness, I imagine not that many.  

The main type of photography that benefits for corner to corner perfection is landscape work, but even then it matters little for most shots and even small degrees of cropping remove the offending corners.  In fact if you regularly shoot in 16:9 format you will likely never see a rubbish corner unless the lens is utterly hopeless.

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 series, it deserves its own set of blog posts so we will save that for a time when I get bored.

Next Time we will examine the Mechanics of the Kit Lens.