That line’s not straight

Photos that don’t look quite right

 

That line’s not straight•Photos that don’t look quite right [Moonbeams Shop:That line’s not straight]

Have a look at the phtoto on the right. Does something look a little odd? Yes - the horizon isn’t flat. Read on to find out why.

Have you ever taken a photo, looked at it afterwards, and realised there was something odd about it, though you perhaps can’t immedaitely see what? Not just a waste-bin or some other object that you don’t remember being in shot, but something just slightly not-right?

The way cameras work can cause minor distortions in the photos we take. One common one is Perspective Distortion. If you are researching for a science paper on perspective distortion, stop reading now and go instead to the Wikipedia page ‘Perspective distortion (photography)’ for a detailed technical explanation. If you just want a lay-person’s explanation, and some tips for preventing it, see below.

Perspective Distortion

Go to: Why is the horizon bent?What can I do about it?

Why is the horizon bent?

What’s wrong with the picture above is that the horizon isn’t straight. This photo appeared on Facebook™{1} and there was some debate going on in the ‘comments’ about why that would be. Someone had pointed out that, if you go up high enough, the horizon does appear curved. The earth is, after all, a ball, not a disc{2}. But this photo was taken from the cliffs on St Helena - that’s the RMS St Helena in the bay - and they are only about 600m high. That’s nowhere near high enough for the curvature to be as pronounced as it is in this photo. The answer lies in the camera lens.

Every lens has a parameter called it’s ‘focal length’. Look on the front of your camera and, near or on the lens, you will probably see a number that looks like a measurement in milimetres, e.g. 35mm. Exactly what the focal length is needn’t bother us here {3}. What matters is that it affects the width of the image you can capture with your camera. And it also can cause your photos to be distorted.

Early simple cameras tended to have a lens with a focal length of 50mm. Nowadays it is usually 35mm or lower. The smaller the focal length, the wider the image taken, and the change was made so that camera users could easily get more into their pictures. With a wider-angle lens you don’t have to step back to get Auntie Ethel into the picture, or move further away to get the whole Diana’s Peak range in. But, as always, there’s a price.

At f=50mm the image is pretty much undistorted, but as you decrease the focal length the level of distortion increases and straight lines become more and more curved. Take this to extremes and, at focal lengths below 16mm you get an effect known as a ‘fish-eye’ view. Have a look on the Wikipedia for some examples of quite extreme fish-eye view photographs. Which are great for special effects but not so good for taking snapshots of your family & friends. Camera makers aim for something in the middle, giving a wide angle of view with an ‘acceptable’ amount of distortion.

What can I do about it?

Professional and serious hobby photographers use cameras that take interchangeable lenses, with different focal lengths. They have the option to choose a lens that will minimise (or maximise, if that’s the effect they want) the perspective distortion. But the ordinary point-and-shoot cameras that most people use don’t have interchangeable lenses. If your camera has an optical zoom (i.e. the lens actually moves in and out as you change the zoom{4}) you could try using longer focal lengths. Just zoom in, though you may then have to step backwards to get everything in. But there is also a simpler trick that will usually work.

The prominence of the distortion tends to increase towards the edges of the photograph. The horizon in the example above is in the top quarter of the image, and so is quite distorted. Had the photo been composed with the horizon running through the middle of the photograph, the distortion wouldn’t have shown up. Of course the picture would have also had a lot more sky in it, but that could have been cropped out afterwards. Modern digital cameras take quite high resolution images so a degree of cropping is possible while still keeping good image quality. So if it had really mattered that the horizon appear straight, that would probably have worked without having to change positions.

We discuss composition elsewhere in these Photography Tips pages and now you have another tip to add to your set.

 


We hope you find these tips useful. If you have any feedback or suggestions please contact us.

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Footnotes:

{1} Thank you to Fay Howe for letting us use her photo

{2} Though fans of the Discworld series by Terry Pratchett may wish it was otherwise

{3} The answer’s on the Wikipedia of course

{4} As opposed to electronic zoom, which manipulates the image that is captured but doesn’t change the focal length


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