Flattening and Bulging

What, you may ask, is this post all about? It is to do with perspective and how it is altered at different focal lengths. This is a brief and very untechnical overview of this.

What is Focal Length?

Whether it is a zoom lens or a prime lens, it will be set at a specific focal length at a single time. Most interchangeable lens’ have this marked on them, most compacts won’t, although it is recorded in the EXIF info on the images. The focal length is the distance between where the light rays converge to form a clear image and the sensor and it is measured in millimetres. In easier to understand terms, it is the amount of zoom on the lens. The longer the focal length, the more zoomed in or magnified the image appears.

Lenses are usually given specific names depending on their focal range. <35mm is a wide angle, 35mm – 70mm is a standard lens, and >70mm is a telephoto (https://expertphotography.com). Although there are more specific terms these are a good general terms.

Now it is easy to think that when taking images at different focal lengths that the perspective of the image remains constant and true but this is certainly not the case. As the focal length changes, the image becomes more or less distorted from reality. Wider angle lens or those with low focal lengths bend the perspective more, whereas longer focal lengths flatten the perspective more.

This leads to two common types of distortion that can occur.

Barrel Distortion

This is the most common type of distortion that occurs. The straight lines appear as curves as if the image was wrapped around a barrel. The diagram below shows this. The image on the left shows how it should look in reality, the image on the right shows the effect of barrel distortion with he image bulging out.

Barrel distortion occurs using wide angle lenses, so the shorter the focal length the more pronounced the distortion is.

Sometimes image distortions such as these are unwelcome but it can often be used to create great effects. Here are some examples

Landscapes

Landscape images often benefit greatly by being shot at shorter focal lengths with wide angle lens. The barrel distortion gives this amazing sweeping effect which really makes landscape pop.

Here are a few examples of landscape images shot with a wide angle lens at about 10mm. Notice the amazing sweeping effect in the skies.

In comparison, here are some landscape shots taken with longer, standard focal length lenses. Notice how much flatter they appear. This may or may not be a desirable effect.

Selfies

Barrel distortion is used heavily in selfies although most people don’t understand it. When you set your mobile / cell phone to take a selfie it has to use a very short focal length to fit your face in at arms length, thus barrel distortion occurs. When people take selfies they position the camera so that the bulging barrel distortion affects their eyes most. This makes there eyes appear bigger which is often seen as a sign of beauty. It also shrinks the periphery of the image making their chin, mouth and neck appear smaller than in reality which is also seen as desirable. Moreover, by tilting your head upwards when you take the shot it masks unwelcome things such as double chins.

This is all well and good, and it makes people feel great about themselves, which is why they plaster them all over social media, but it is not how you look in reality. It is a significant visual distortion from reality just like one of those funny mirrors you get at the fair.

I will discuss the phenomena of selfies in later post but suffice to say they seem to benefit from barrel distortion. But just to drive the point home here is a badly taken selfie of myself which hopefully clearly shows the barrel distortion.

IMGP0815.jpg

Where it is not good?

Usually for portrait photography (except selfies) this distortion is not seen as desirable especially if it for a professional looking headshot or suchlike as you want an accurate depiction of reality. Other such portrait shots such as formal wedding shots also tend to require less distorted images which can be obtained by using longer focal lengths.

I could go on and list other uses for this type of distortion but ultimately it is just an artistic effect, so use it, or avoid it, where you think it would be best.

Pincushion Distortion

This is basically the opposite of barrel distortion. Instead of the image bulging out, it appears pinched into the center as shown below. It is more commonly associated with telephoto lens.

barrel.jpg

I had a look through my photos and couldn’t find any examples of pincushion distortion, probably because I mostly shoot with wide angle lenses. However, again like barrel distortion it is just an artistic effect to be used when desired although I doubt you will encounter it much.

Correcting Distortion

Although distortions can be used to great artistic effect, they often an unwanted occurrence. Here are a couple of examples of barrel distortion which may be better corrected.

Correcting distortion can sometimes be very simple, or sometimes impossibly difficult.

Lens Correction Presets

Photoshop and Lightroom have specific lens profiles with them. By selecting these they should, in theory, correct some of the distortion problems with the images. The lens profiles for the lens used in the above photos was not available so I cannot show the effects of this.

Manual Lens Correction Tools

If the above doesn’t work you can trying and manually correct it with lens correction tools. Again this can be hit or miss. Here is how the two photos look using this option. They have slightly corrected the distortion but not fully.

Manual Distortion Tools

Packages such as Photoshop and GIMP have various distortion tools such as perspective transform and warp tools. These can be used to manually correct the distortion but they can be very time consuming and tricky to use. Due to the time and skill required to do it this way, I really wouldn’t recommend it unless you really have no other option.

Flattening or Bulging

So there is a very brief overview of how different focal lengths affect images. It is worth noting that different lenses distort images differently. e.g. not every lens set at 10mm will give the same barrel distortion.

As a general rule of thumb, the wider the angle of lens and the shorter the focal length, the more bulging barrel distortion will occur. The longer the focal length, the flatter and less distorted the image will appear, although you may get pincushion distortion at longer focal lengths.

These distortion effects can be applied positively as an artistic effect and often are. However, they are often very unwelcome and can ruin a photo. Although distortion correction be done, it is often fiendishly difficult and time consuming, so it is probably better trying to avoid it altogether if you do not want it.

However, don’t be put off using wide and angle or telephoto lenses because of this as these distortions can provide stunning images.

Thanks for Reading,

Neil

 

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