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Common Image Problems: Distortion
Before I started to write this page, I thought I would look up the word distortion in a dictionary.
"the act of distorting"
Well a lot of good that one is!
"the modification of unconcious impulses into forms acceptable by concious or dreaming perception"
That one - I don't even know what it means!
"the censorship of unacceptable unconscious impulses so that they are unrecognizable to the ego in the manifest content of a dream"
I think that means the same as the one before - who knows! Who do you think I am, Sigmund Freud?
"A change in the shape of an image resulting from imperfections in an optical system, such as a lens"
At last - something that makes a bit of sense. Sometimes a distortion may be an intentional part of the image, but in most cases, any form of distortion is undesirable. This section looks at the different types of distortion and what you can do to minimize them.
Barrel Distortion
This type of distortion is very common with wide-angle lenses. As the name suggests, the image is distorted into a barrel shape - something that is very noticable is you have horizontal or vertical lines in your image.
On ultra-wide angle lenses, some barrel distortion is inevitable - in fact on a type of lens called a fisheye lens (180 degree field-of-view) it is the most obvious feature of the image.
As the effect of barrel distortion increases towards the edge of the frame, the best quality wide-angle lenses will use aspherical lens elements to provide a gradually increasing correction towards the edges of the image.
barrel distortion - the effect has been considerably exagerated
in this picture to better show the effect. If your lens is this bad,
it's time to buy a new one!
Pincushion Distortion
This is the opposite of barrel distortion - horizontal and vertical lines near the edge of the image tend to bend inwards towards the middle of the picture. This tends to show up at the telephoto end of a long zoom lens - usually being more noticable on a lens that tries to cover too wide a range of focal lengths (i.e. 18-200mm will probably show the effect worse than a 70-200mm)
pincushion distortion - the effect has been considerably
exagerated in this picture to better show the effect.
Astigmatism
An effect where the focus of an image is lost as you approach the edge of the frame. It is rarely seen now on modern lenses although it used to be common on very cheap cameras with a fixed focus lens.
Chromatic Aberration
Light is made up of lots of different wavelengths - and these different wavelengths (which are seen as different colours) are refracted (bent) by glass lenses by different amounts. This causes the different colours of light to come to focus as slightly different position.
In the final picture this usually manifests itself as coloured fringes - usually most noticable near high contrast edges - a dark object against a bright sky. Many better modern lenses use low dispersion glass and have the surfaces of the glass elements treated with a special coating to try and minimise the effect, although it can never be totally eliminated.
The glass used in lower end compact cameras is particularly susceptible to this form of distortion.
picture by Lisa Eastwood. Taken on a 3MP Fuji Compact camera. Image shown at 300% size - the purple colour fringes are very noticable in the high contrast areas of the image.
Vignetting
This is simply a reduction in the brightness of the image near the corners.
Digital cameras have traditionally suffered from this more than film cameras due to the way the sensors are less responsive to light hitting them at an angle. However this effect has been almost eliminated by the use of improved sensor technology and micro lens arrays over the top of the sensor.
The most common cause of vignetting now is using the incorrect lens hood on a camera, so that the hood itself is casting a shadow.
Another possibility is the use of direct flash light - especially with wide angle lenses. Many flash guns have a built in wide angle diffuser, but if you forget to use this, the effect can be quite noticable.
To top image has no obvious vignetting, however the bottom one is what happens when you use the wrong lens hood. Although it is not completely spoilt the picture, had the image been one with a large amount of sky, the overall effect might have been a lot worse.
It's worth noting that no lens is 100% free from distortions, to some degree some or all of the obove mentioned distortions will be present on even expensive lenses. If you look for the totally perfect lens you'll be looking for some time. However, this is an area where cheaper lenses tend to show their weaknesses compared to their expensive counterparts.
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