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2012-07-31 12:00:00

Focal Length

Focal Length - Or Why your Zoom lens isn't just to 'Spy on Things' or 'Get More In'.

The photographer Robert Capa said that 'if your pictures aren't good enough, then you're not close enough'. It's excellent advice; you've probably heard it before. But is it understood and practiced, or has it just become another snippet of information to be collected?

Let's delve in and see. Getting closer to your subject does two things. Firstly, it makes them (or it) fill more of your frame, so they'll dominate the picture. Secondly, it gives your viewer an artificial sense of being in the thick of the action.

Focal length essentially determines the angle of view. Imagine a cone with its pointy end closest to the back of your lens. That cone is a projection of your scene. The further that the film/sensor is from it, the smaller the amount of the scene that's shining on it; so the greater the apparent magnification.

If our lens captures a wide field of view, we call it a Wide-Angle lens. The lower the number, the wider the lens. On a full-frame camera, a 14mm lens is an extreme wide-angle; 24mm is a wide-angle, and 35mm is moderately wide. Smaller sensors capture less of the scene, so the lenses seem less wide for the same focal length

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Longer Telephoto lenses move all that glass away from the camera and show a narrower field of view. We'll only capture a tiny part of the projected image. The longer the focal length of these lenses, the more the scene seems magnified. On full-frame, 90mm is a moderate telephoto lens, 200mm is a bit wilder, and 800mm is positively feral.

Capa would have used Prime lenses; lenses that don't zoom. They have a fixed focal length (such as 24mm or 300mm) so they were simpler to design than Zoom lenses, which have to cover a range of focal lengths (like the 17mm-55mm or 70mm-200mm)

 

Computer-aided lens design revolutionised photography. It made zoom lenses cheaper and smaller; and many zooms now have comparable sharpness with the best Prime lenses (e.g.Nikon's 14-24mm).

The kit lens that may have been bundled with your camera would probably have been a zoom lens. Before, it would have been a 50mm prime lens.

You used to have to 'zoom with your feet'; now you can stay where you were the instant you felt the impulse to take a picture and experiment with different framings by changing your focal length.

This lulls new photographers into bad habits; and it's not just the size of your lens, it's how you use it. If you automatically zoom in to get closer, or zoom out to 'get the whole group in', chances are we can improve your pictures.

A lot of cheap zoom lenses have a smaller maximum aperture at the telephoto end (the higher numbers). This forces you to use a higher ISO or a slower shutter speed if you want to use the lens 'wide open' (the lowest f-number/ the largest aperture). This exacerbates the appearance of camera shake.

But just as importantly, Focal length changes perspective as well as framing. Because the field of view is larger with a wide angle lens, more of the background is captured; so it seems smaller.

Conversely, telephoto lenses capturer a narrower field of view so they ’see’ less of the background; so objects look larger. Those photos with the huge moon would have been made with extreme telephoto lenses (or with Photoshop by scoundrels!)

Therefore, if we want to emphasise our background, we can make it larger by stepping back and zooming in. If we want to shrink distractions in the background or include a wider perspective to set the scene, we can opt for a wider lens.

But due to the way we judge depth, identifiable objects will seem closer or further away depending on their size. This means that telephoto lenses flatten perspective, and wide angle lenses extend the illusion of depth; especially if we include foreground detail which will be disproportionately magnified.

Therefore, if we want to make a scene seem expansive, we can use a wide angle lens and include common foreground and background objects. If we want to unify subjects at different distances to the camera, we can choose a longer telephoto lens; and physically move back and forth to adjust our framing.

You may have seen cinematographers playing with this; they'll zoom out (to a wider focal length) while physically stepping forward in order to keep the subject's face the same size in the frame. The background is thereby magnified so that the scene appears to close in on them, inducing feelings of claustrophobia.

We can use this flattening of perspective more pragmatically too. If you've ever photographed someone close-up with an extreme wide-angle lens, no doubt you've enjoyed a hearty laugh at how silly they look.

And yet most people turn on their camera and instantly take portrait photos using the wide end of their zoom lens. Because the subject's nose is closer to the camera, it is magnified in relation to their face. Generally, zoom in to the telephoto end of your lens and step back for portraits.

Professional portrait photographers generally use short telephoto lenses for a more natural perspective. The use of these lenses is at least partly due to the space limitations of the typical studio. 

For outdoor portraits, longer focal lengths compress the perspective more, and also isolate the subject from the background better. Some photographers use extreme telephoto lenses (400mm or 600mm) for portraiture and communicate with their subject and assistants via a walkie-talkie! I love Nikon's 180mm prime for people pictures.

Photographers talk like aggressors, 'shooting', 'taking' or 'capturing'. I'm there going to take the huge liberty of concluding with an analogy that likens photography to (movie-based) assassination; please take it in the ironic sense in which it's meant.

In the film, 'Leon', young Matilda is being taught how to 'take the shot'. She starts by using a long-range sniper rifle filled with paint; let's compare this to the photographer with the long telephoto lens standing back detached from the scene. The isolating perspective offers the viewer a voyeur's connection with the subject; intimate but detached.

Leon goes on to say that the last thing she'll learn is the knife. This could be our equivalent of the wide-angle lens, photographing in close proximity to our subjects. This natural perspective gives the viewer a sense of being in the midst of the action, allowing an almost interactive relationship with the scene.

While my comparison with a killer's trade-craft is tongue-in-cheek, the point remains. Distortion aside, getting closer with a shorter focal length wide-angle lens gives our viewer a real sense of connection with the subject.

So what's the value in all this? Hopefully to make you more aware that the choice of focal length has several effects beyond framing your photographs. Photographers used to primarily :-) use prime lenses, and each had different characteristics. 

Zoom lenses with no hard stops between focal lengths may make you less clear about the effects that physically changing your position has. Start actively 'zooming with your feet' and let your focal length be determined by the specific 'look' you want, not by your framing.

All images © Ben Evans

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Joseph W Hirn III Said:

<p>Interesting article. Informative.</p>


on 2012-08-15 16:33:28

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