What is focal length?

Paul Oosterlaak wrote on Friday 9 February 2018 at 11:19  | reply

"The focal length has to do with the lens of your camera, and if all is well, it is also written on it. Technically speaking, this is the distance in millimeters between your camera sensor and the center of your lens. "

No, really not. It is the distance between the optical center of the lens and the point at which parallel light rays are bundled into 1 point. The optical center depends on many factors, including focusing distance and lens design.

"The focal length of your lens determines how much can be seen in your photos and how large the subject appears in the frame."

The focal length of your lens together with the size of your sensor.

"This is also called wide-angle. There are even shorter focal lengths, for example 12mm, which is then called super-wide-book."

Obviously, the boundaries between super-telephoto, telephoto, standard, wide-angle, super-wide-angle and ultra-wide-angle are vague. But it is more convenient to talk in terms of angles of view or possibly, as is usually done, in terms of full-frame equivalent focal lengths. So wider than 35mm is wide-angle, wider than 24mm is super-wide-angle and wider than 16mm is ultra-wide-angle, for example (these are not absolute values).

"Wide-angle lenses can cause distortion in your image, especially if you use them to photograph your subject from a short distance."

Every lens causes distortion, because this depends on your perspective. The distortion is only more clearly visible in certain situations.

"So they are not suitable for portraits."

With this, you are doing many portrait photographers a disservice. I know photographers who shoot portraits with fish-eye lenses. Extreme perspective distortion and deformation are deliberately used then. You would be better off saying that if you want less distortion in your portrait, you are better off working with longer focal lengths.

Paul Oosterlaak wrote on Friday 9 February 2018 at 12:11  | reply

Portraits: so many people, so many preferences, so many lenses...
A tight head-and-shoulders portrait is indeed often shot at 100+ mm, but a full-body portrait is more often shot at 50mm or wider. If I have the space, I shoot almost everything at 135mm, but sometimes I don't have the space and/or you want more context with the portrait and I use 50mm (or even wider). So it's not that clear-cut, so add more nuance when you talk about this.
The gist of this article is of course that it shouldn't be too long, but adding a little nuance here and there would also make it more complete and more accurate.

100 wrote on Saturday 10 February 2018 at 12:10  | reply

"The focal length has to do with the lens of your camera, it should also be written on it if all is well. Technically speaking, this is the distance in millimetres between your camera sensor and the centre of your lens. "

‘No, really not. It is the distance between the optical centre of the lens and the point at which parallel light rays are bundled into 1 point.’

Actually, neither statement is correct for the lenses people use on their cameras, because those are not “thin lenses” but “thick lenses” or lens systems, and different definitions apply to those which are interesting to optical engineers, but will not interest the average photographer.

I think the three example photos are much more important than the optical theory because you can clearly see the effects of the different lenses there. The most striking difference is the difference in compression. The “longer” the focal length, the closer the foreground and background come together.
With the focus on the violinist’s head, you can see that the violin becomes proportionally smaller and the fence in the background becomes increasingly larger. An additional effect is the greater background blur, which increases the separation between subject and background.

Portraits:
You can shoot a portrait with virtually any lens, but it is not for nothing that 85mm lenses are considered portrait lenses. Little distortion and relatively simple optics mean that “fast” (f/1.8) and reasonably affordable (a few hundred euros) go together, making them interesting for people who take portraits and want something more than an 18-55 kit lens.

100 wrote on Sunday 11 February 2018 at 00:20  | reply

@Michelle
Certainly, a great moon example is also the video from a few years ago by Mark Gee.
https://vimeo.com/58385453
Shot with a focal length of 1,000mm (EF 500L + 2x extender) from 2.1 kilometers away from the people.

martinphili wrote on Sunday 11 February 2018 at 10:13  | reply

The distortion in portraits (large nose and so on) has mainly to do with the distance between photographer and subject. That is perspective. In that light, I therefore completely agree with the statement that every lens is suitable for portraits.

100 wrote on Sunday 11 February 2018 at 12:57  | reply

@martinphili
“Distortion in portraits (a big nose and so on) has mainly to do with the distance between the photographer and the subject. That is perspective.”

Yes and no.
You can only get a “big nose” if the lens has negative compression (stretching), and that applies to lenses with a focal length shorter than a standard lens. If we divide lenses up simply, you only get stretching with wide-angle lenses (<35mm) and compression only with telephoto lenses (>70mm). Between 35 and 70mm (the boundaries are arbitrary, but it is about the order of magnitude) you get an image comparable to what we see with our eyes.

So if you photograph with a standard lens (let's say 50mm), you will see that photographing from far away or up close has little or no effect on the proportions of a face. The best example is most macro lenses, with which you photograph at very short distances and yet have little trouble with stretching.

Paul Oosterlaak wrote on Sunday 11 February 2018 at 15:52  | reply

@100: focal length for a thick lens or lens system is indeed not as straightforward as with a thin (ideal) lens (without spherical aberration), but assuming an optical center (wherever that may be) is accurate enough.

As for that 50mm. That is simply an arbitrary value that was once easy and cheap to produce. It has nothing to do with distortion. And as martinphili already says, you can just as well get distortion with it. Many a head-and-shoulders portrait taken with a 50mm shows visible distortion.
As I said earlier, you always have distortion. If you take a photo of railway tracks with an 800mm lens, you don't see parallel lines either.

Paul Oosterlaak wrote on Sunday 11 February 2018 at 18:20  | reply

Compression depends on distances and not on focal length: https://flic.kr/p/BctdW1
As far as macro lenses are concerned: a macro lens actually effectively renders the image of a longer focal length, which is why the maximum aperture also decreases (you can also see this neatly on Nikon cameras). What you mean is focus breathing

Michelle thanks for your explanation. Great responses everyone! But reading the article again, it was mainly intended for the beginner (hobby) photographer! In other words, the responses are at a completely different level! And was that really the intention. Perhaps these could be better included in a topic for advanced photographers!! I'm afraid that a lot of beginner hobby photographers won't understand a thing after reading all the responses and will give up!

Michelle Peeters wrote on Wednesday 14 February 2018 at 21:49  | reply

@Rob and Liesbeth: the article has already been supplemented with a number of tips from comments!