Understanding Lenses: Part I, and is the third in a series of lessons about camera lenses. Links to the others are at the bottom of the article.
No lens is perfect. No matter how good the quality, there will be lens aberrations of one kind or another. It’s just that on a good quality lens there are less of them.
One of the benefits of digital photography is that it is easy to correct some types of lens aberration using software. Before digital cameras came along there was little you could do except accept what your lens gave you, and splash out for more expensive gear if you weren’t happy with its performance.
Now, as long as you shoot Raw, you can use software such as Lightroom and the latest versions of Photoshop CS to eliminate or minimise the most obvious lens aberrations*. While this doesn’t mean that your lenses will suddenly take on the qualities of their most expensive counterparts, it does help you get the very best out of the equipment you already have.
*If you shoot Jpeg, some cameras have a facility for correcting vignetting and chromatic aberrations in-camera. You can also get rid of the worse effects of chromatic aberration, vignetting and distortion with Photoshop, but it takes longer. Using Raw makes it much easier.
There are three types of lens aberration that you can correct using software. The photo above, taken with a discontinued Canon kit lens, has them all:
This is a characteristic of wide-angle lenses and is where the lens bends straight lines outwards. You are most likely to see barrel distortion at the widest end of inexpensive zooms and super-zoom lenses. You can see it in the above photo and in the opening photo (a really extreme example).
Pincushion distortion is the opposite of barrel distortion and is a characteristic of telephoto lenses. Straight lines are bent inwards. The effect is usually slight and you would normally only notice it if you are photographing something with straight lines, like a building.
Chromatic aberrations and coloured fringing
Chromatic aberrations are the coloured fringes along the highlight edges within your image. They are caused by the inability of your lens to focus all wavelengths of light at the same point. You may also see purple fringing caused by charge leakage from your camera’s sensor.
The crop above shows chromatic aberrations along the edges of the trees.
Vignetting is where the edges of the frame are darker than the centre. No lens is exempt from vignetting. It is strongest at the widest aperture of the lens and gradually disappears as you stop down.
There are times when vignetting is a benefit, and you won’t want to correct it. A good example is a portrait – if the edges of the frame are darker it helps direct the viewer’s eye towards the sitter.
But there are times when vignetting is detrimental, and it’s useful to know how to correct for it.
Broadly speaking there are two types of vignetting. One is caused by the optical characteristics of your lens. This is the sort you can correct in post-processing.
The other is caused by stacking filters or using the wrong lens hood. You can’t do anything about this type of vignetting in post-processing except crop.
Correcting lens aberrations
I use Lightroom for most of my Raw processing so that is the software I will demonstrate in this article. The same functions are available in most Raw processors, including the latest versions of Adobe Camera Raw in Photoshop CS and the free software that comes with your camera. Check the documentation or the help files to see that the capabilities of your software are.
I’m a Canon user and I’m familiar with Canon’s Digital Photo Professional. While it’s not as quick and convenient to use as Lightroom, and doesn’t support discontinued Canon lenses or lenses from other manufacturers, it still does an excellent job. You can learn more about DPP in my ebook Understanding DPP.
The first thing to do is go to the Lens Corrections panel, click on Profile and tick the Enable Profile Corrections box. Then go to Make and select the brand of the lens you used to take the photo from the drop down menu. Lightroom should detect the lens automatically from there, if not you can select it yourself. If there is more than one profile listed, stick with the Adobe profile. Depending on how pronounced the lens aberrations in your photo are, you should see a difference right away:
There are three sliders at the bottom of Lens Corrections panel: Distortion, Chromatic Aberration and Vignetting. The default setting for each is 100, and you may adjust the sliders to vary the strength of the corrections.
For instance, with this particular photo, I felt that Lightroom had overcompensated for the barrel distortion, so I dragged the Distortion slider left until I felt it was right. A grid appears when you do this to help you see whether the lines in the image are straight. I also had to increase the Chromatic Aberration slider to 200 to eliminate the chromatic aberrations. The comparisons below show what a difference it makes:
You may also choose to reduce the vignetting correction, as mentioned previously, for creative reasons.
Now what happens if your lens isn’t included in Lightroom’s profiles? This may happen, for example, if you have a new lens like Canon’s EF 40mm f2.8 pancake lens, a recent purchase of mine. In that case you have to make corrections manually.
Click on Manual in the Lens Correction panel and you’ll see a different set of sliders (above):
These sliders are for correcting barrel and pincushion distortion. You may also rotate the image to straighten the horizon. If you use the Distortion slider, tick the Constrain Crop box to tell Lightroom to trim the image accordingly.
This is for making the edges of the image lighter or darker. The Midpoint slider determines what area of the image is altered by the Amount slider: move it right to lighten or darken the very edges of the photo, or left to include the central areas.
Removing chromatic aberration is a little more involved. Set Defringe to All Edges then use the Red/Cyan and Blue/Yellow sliders to try and eliminate chromatic aberrations. You’ll need to zoom into the image to check the results. You should scan the entire image, because sometimes the slider settings required to eliminate colour fringing in one part of the image introduces it in another.
Sometimes it is a matter of compromise, and you have to settle for minimising chromatic aberrations rather than getting rid of them completely. You may also get better results by setting Defringe to Highlight edges, depending on the content of your photo.
Focal length and perspective
My previous article sparked some debate in the comments regarding the effect of subject distance on perspective. Here are three photos taken from the same position with a 17-40mm lens (on a full-frame camera) set at different focal lengths. You can see that using the longest focal length has the effect of the bringing the island in the distance closer, and the widest focal length pushes it away. This is the effect that the focal length of the lens has on perspective.
These are the previous articles in the series. My next article will be about flare; exploring the causes, how to prevent flare and also how to use it creatively.
Understanding Lenses: Part I
If you liked this article then take a look at my latest eBook, Understanding Lenses: Part I – A guide to Canon wide-angle and kit lenses.