4 Things You Should Know About Focal Length and Composition

4 Things You Should Know About Focal Length and Composition

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Lenses are the eye of your camera. The focal length of a lens (and your point of view) determine how much of the subject your camera sees.

You may already be familiar with the basics, and understand the difference between, say, wide-angle and telephoto lenses, but let’s dive into the the topic a little deeper to see what’s really going on.

focal length and composition

There are four fundamental things to know and understand about the focal length and composition.

1. Focal length is not as important as field-of-view

There are two factors that determine the field-of-view of a lens:

  1. The focal length.
  2. The digital sensor or film size

Field-of-view (sometimes called angle-of-view) is far more important than focal length, because it tells you how much of the scene the lens sees. However, as field-of-view changes according to sensor size, manufacturers tell us the focal length instead. Focal length is a fixed measurement that doesn’t change (it is literally the distance from the middle of the lens to the focal plane, which is the sensor).

Here are some practical examples.

Example #1 – 50mm prime lens

A 50mm prime lens has a field-of-view of 47 degrees on a full-frame camera. This field-of-view approximates what we see with our own eyes. But what happens when you put the 50mm lens on an APS-C camera (crop factor of 1.6x)? The crop factor of the smaller sensor means that the lens now has a field-of-view of around 30 degrees, making it a short telephoto lens.

This change in field-of-view means that you have to move further away from your subject in order to fit it in the frame, which also changes the perspective (giving the compressed effect that characterizes short telephoto lenses).

If you want the equivalent of a 50mm lens on an APS-C camera you need to use a focal length of around 31mm, as it has the same field-of-view (47 degrees).

A prime lens with that focal length doesn’t exist (you could choose between a 28mm or a 35mm depending on whether you wanted a slightly wider or a tighter field-of-view), but you can set that focal length if you have a zoom.

focal length and composition

50mm lens, full-frame camera. The lens has a field-of-view of 47 degrees.

focal length and composition

50mm lens, APS-C camera. The same lens has a field of view of 30 degrees with this camera.

Example #2 – 21mm lens

The same applies to wide-angle lenses. A 21mm prime lens has a field-of-view of around 92 degrees. That’s a nice wide field-of-view ideal for landscape photography, or creating images with dramatic perspective.

But put it on an APS-C camera the field of view narrows to around 65 degrees. It’s still a wide-angle, but the effect is much more moderate. It now has nearly the same field-of-view as a 35mm lens does on a full-frame camera

To get the same field-of-view as the 21mm lens (on a full frame) you would use a 14mm lens (on an APS-C camera).

focal length and composition

This photo was taken with a 14mm lens on an APS-C camera. It has the same field-of-view as a 21mm lens does on a full-frame camera.

Example #3 – 16mm lenses

It’s even possible to have two lenses with the same focal length, but different fields-of-view (on the same camera).

A 16mm wide-angle lens has a field-of-view of 107 degrees – but a 16mm fisheye has a field-of-view of 180 degrees.

They have the same focal length but each one is designed for a different purpose. The 16mm wide-angle is designed to keep straight lines straight. The fisheye doesn’t try to do that, and as a result has a much wider field-of-view.

This table shows the field-of-view of common focal lengths with full-frame, APS-C and micro four-thirds cameras.

focal length and composition

The next points explore the relationship between field-of-view and composition.

2. Wide-angle lenses are lenses of inclusion

You can think of any lens with a field-of-view wider than around 63 degrees as being a wide-angle. That’s 35mm or shorter on a full-frame camera, 20mm on APS-C, and around 18mm on micro four-thirds.

Wide-angle lenses have two characteristics that affect composition:

  1. The wide field-of-view means that you have to move in close to your subject to fill the frame. But, at the same time wide-angle lenses also include quite a bit of the background. The shorter the focal length, the closer you need to get, and the more background is included.
  2. Wide-angle lenses also appear to have more depth-of-field at any given aperture setting than longer focal lengths (they actually don’t, it has to do with lens to subject distance which also changes with focal length).

These two factors combine to make wide-angle lenses, ones of inclusion. You can always fit more into the frame with a wide-angle lens, no matter how close you get to your subject. The background is also more likely to appear more in focus, than it is with longer focal lengths. Getting in close, creates the dramatic perspective that some photographers love. It emphasizes line, and creates a sense of depth, that images taken with longer focal lengths can lack.

The slightest change in your point of view makes a dramatic difference to the composition of the photo. The shorter the focal length, the more this applies. As wide-angle lenses include so much background it can be difficult to simplify the composition and remove all distractions. There’s no way around it, it’s just a characteristic you have to embrace.

focal length and composition

This photo, taken with an 18mm lens (APS-C), includes the buildings, the city wall, the reflection in the water, the city trees disappearing into the distance, and keeps everything in sharp focus.

3. Telephoto lenses are lenses of exclusion

A telephoto lens is one that has a field-of-view of around 30 degrees or less. That’s around 85mm or longer on a full-frame camera, 50mm on an APS-C camera, and 40mm on micro four-thirds.

Telephoto lenses are ones of exclusion. They have a narrow field-of-view. Fill the frame with your subject, and you won’t get much background in at all. It is also easy to throw the background out of focus by using a wide aperture, and making sure there is sufficient distance between your subject and the background.

focal length and composition

This photo, taken with a 50-150mm lens set to 72 mm (APS-C), shows the woman’s hands and the textiles she is selling. There is not much in the background at all.

4. Normal lenses occupy the middle ground

Normal lenses, those with a field-of-view somewhere around 55 degrees, occupy the middle ground between wide-angle and telephoto. They don’t create images with the dramatic perspective that you can obtain with a wide-angle, nor do they exclude the background to the same extent as telephotos.

If you have a normal prime lens you can open the aperture up to defocus the background, sometimes quite dramatically if you get close enough to the subject. But, you can also often stop down enough to get everything within the frame in focus.

focal length and composition

I took this photo with a 35mm lens, a normal lens on an APS-C camera. It lacks the dramatic perspective, and wide field-of-view of the photos taken with wide-angle lenses. But it includes more of the background and shows less compression than the photos taken with telephoto lenses.

Your turn

Can you think of anything else that photographers ought to know about focal length, field-of-view, and composition? If so, please let us know in the comments. I’d like to hear your thoughts.

Note: this is the second in a series of articles on dPS this week talking about composition. See: Using Framing for More Effective Compositions and look for more over the next few days.


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Andrew S. Gibson is a writer and photographer living in New Zealand. He is the author of over twenty photography ebooks – please join his monthly newsletter to receive complimentary copies of The Creative Image and Use Lightroom Better.

  • CopyKatnj

    How are DX lenses on an APS-C sensor camera calculated? Will a DX lenses on an APS-C sensor camera change field of view? Example: Will a 50mm DX prime lens have a field-of-view of 47 degrees on an APS-C sensor camera?

  • The field-of-view always changes when you use a DX lens on an APS-C camera. With your example the 50mm lens will have a field-of-view of around 30 degrees. The field-of-view is always narrower on an APS-C camera than a full-frame one.

  • Doug Sundseth

    “Wide-angle lenses also appear to have more depth-of-field at any given aperture setting than longer focal lengths (they actually don’t, it has to do with lens to subject distance which also changes with focal length).”

    Nope. For any given lens to subject distance, a lens with a shorter focal length will have a greater DoF. (Note that this is largely sensor-size invariant, though the photo-site size actually matters because of the standard definition of “sharp” for DoF purposes.) As a result, the camera on your phone, which typically has a focal length in the 2.0 – 2.5mm range and a fixed aperture near f/2.0, has a nearly infinite DoF at all but very tight macro ranges, and has much deeper DoF at macro ranges. (Camera phones turn out to be very good at small-object macro photography.)

    See http://www.dofmaster.com/dofjs.html (for example) for detailed DoF calculations.

  • Hi Doug, thanks for your feedback. We settled on the wording you quoted after a discussion with the editor. We had to bear in mind the fact that depth-of-field is more or less the same at any given aperture on lenses of different focal lengths when the subject is the same size in the frame.

    For that to happen you need to move closer with a wide-angle lens or further away with a telephoto, hence the mention of lens to subject distance.

    Hope that helps.

  • This article has a good explanation of it: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

  • Marc De Coninck

    Crop-factor x1.6? Only with Canon APS-C camera’s. All other APS-C camera’s have 1.5x crop-factor.

  • Hallgeir_Risenfald

    Nope. There are infact other brands out there than Canon and Nikon 😉

  • Doug Sundseth

    Tru-ish within a narrow range of focal lengths and subject-to-lens distances. So the difference between a 35mm and 50mm lens at typical portrait distances won’t matter much.

    But compare, at f/8 for instance, a 10mm lens at 3 feet (in focus from 1.7′ to infinity*) to a 100mm lens at 30 feet (3.63′ in front and 4.78′ behind the subject*). Same FoV at the subject (so the subject is the same size in the frame), but fairly wildly different DoF. And on a crop-sensor camera, the former focal length could work pretty well for an environmental portrait while the latter works well for a beauty portrait.

    Using your stated simplification is more wrong that right.

    * Note that these examples are both using a 7D, since it’s the default camera in Dofmaster. Other cameras will give similar results.

  • Suhithar Baus

    Yes. Nikon APS-C cameras have 1.5X crop-factor. Canon cropped sensors are a bit smaller than Nikon cropped sensors.

  • Larry Dickerson

    Is there any appreciable difference between shooting a subject with a 50mm macro lens and shooting it with my 16-300mm zoom lens set at 50mm?

  • My thinking here is that any article that talks about focal length should include a mention on how far infinity is. I may be analog here, however when I was talking photos full time and worrying about stuff like this, there was always an infinity end of the particular lens setting. Unfortunately, I don’t have any pics that I’ve taken like this scanned, or I would show an example.

  • Bob Bevan Smith

    There will be a difference in the quality of the result, depending on the subject. The macro lens is designed specifically for taking photos of objects at close range, say within a few centimetres, where you should expect superb results. It will also probably have better distortion correction etc. The zoom lens is a general purpose tool, which gives good results on many different subjects, but is not good at close range, and maybe not quite as pin sharp as the 50mm lens. It will probably not have such a close minimum focus distance as the Macro, meaning you may not be able to focus on objects as close as you can with the macro lens. Also, the 50mm lens will probably have a wider maximum aperture. But for landscapes and the like, you probably won’t notice any difference in quality. The zoom lens is a much more versatile tool for general use. So choose your lens to suit your subject – for example, I wouldn’t use a macro lens at a sports event. The best advice I can give is to experiment with a variety of scenarios, and see which works best for you.

  • Feenix

    Why is it that one can’t actually focus all the way to infinity on most (or all) lenses? Even in the bad old days of film cameras that was an issue. You always have to back off a hair to get distant objects sharp

  • Bob Bevan Smith

    One other behaviour of lenses with different focal lengths is the amount of compression of background distance. For example, a full body portrait taken with a wide angle lens will appear to be a long way from its background, whilst in one taken with a long focal length, showing exactly the same size portrait, the background will appear to be much closer.
    This also affects the shape of a face – a portrait with a short focal length tends to emphasise the nose. Thus a useful portrait lens tends to be of slightly longer focal length than a ‘standard’ length – eg. 90mm rather than 50mm on a 24x36mm sensor, or 40mm rather than 25mm on Micro 4/3rds.

  • Bob Bevan Smith

    Strictly speaking, you can’t say how far infinity is. By definition, infinity does not have a rational length. But all lenses are built with a degree of imprecision because of the limits of accuracy of the machine tools, wear and looseness, so it makes sense to allow a safety margin in the design, so that the occasional inaccuracy doesn’t prevent a lens from achieving a perceived infinity setting. This is usually only significant for astrophotography. Mostly, if you switch to manual focus, you will achieve your desired focus point.

  • See Bob’s answer below – he explains it very well.

  • Good point. One of our other writers recently wrote an article about this.

    http://digital-photography-school.com/how-to-use-focal-length-and-background-compression-to-enhance-your-photos/

  • What Bob said it all true but if you meant is there a difference in the angle of view of either – no they are the same.

  • But Doug the FoV is not the same if the subject is the same size. Field of view is how many degrees from side to side does the lens see. That is not the same.

    In your example you’ve not only changed lens focal length but distance to subject – the latter of which is what alters the DOF not aperture or focal length.

  • Richard

    Doug is right. Depth of field has a precise definition based on the circle of confusion. Ceterus paribus, a wide angle lens will produce smaller circles of confusion and therefore greater DoF.

  • Understanding Composition

    I’m not exactly sure what this article has to do with composition. It seems like an article on different lenses.

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