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I keep seeing people talking about focal-length of lenses without saying how large the frame is. Is there a more sensible way to talk about how much a lens “zooms”?

(I have seen so many adverts etc that just have the focal-length(s) of a camera so this is a common problem)

Ian
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Focal length is focal length. Field of view is field of view. Unfortunately, sometimes they're conflated and the former means the latter.

If it's a concern in a particular question, I'd suggest asking that the OP clarify with his/her crop factor.

Reid
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  • what is a crop factor? – Ian Jul 28 '10 at 15:54
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    @Ian, see http://photo.stackexchange.com/questions/139/what-is-the-difference-between-focal-length-and-crop-factor for a good answer to that – Reid Jul 28 '10 at 16:26
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    I much prefer effective focal length than having crop factors thrown around. It will give a one common term to use that everyone can relate to, instead of both a focal length and people's varying crop factors.

    I do understand that many might not know how to calculate effective focal length, but that should be in the FAQ.

     – eruditass Jul 28 '10 at 17:46
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    But "effective focal length" is a meaningless term. I shoot APS-C and I know what 18mm will give; it's unfair to insist that I mentally convert that to 24mm whenever I talk about it. It's an 18mm lens and it has a certain field of view on one form factor and a different FOV on another. – Reid Jul 28 '10 at 18:32
  • I'm not insisting you convert it whenever you talk about it, but only when referring to applications that reference to field of view or angle of view, such as "what lens should I use at a basketball court?" It's either the one OP converts the figure, or all the readers multiply two varying numbers in their head. If it provides a common term across all formats, how is it meaningless? Just because it doesn't measure something physical does not make it meaningless.

    For most questions, I don't envision EFL being mentioned at all, just as crop factor does not need to be mentioned.

     – eruditass Jul 28 '10 at 19:34
  • I'm only saying that it will act as a normalizer across all formats(0.64,0.8,1x,1.3x,1.5x,1.6x,2x,4.55x,7.6x,20x...) and get everyone (including superzoom and P&S users) involved and understanding what it all means. As opposed to converting from your crop factor to the OP's crop factor, each person converts to one common term. – eruditass Jul 28 '10 at 19:46
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    @Eruditass: my impression has always been that "effective focal length" loses the "effective," both in writing and in people's heads, and then you end up explaining that the focal length of a lens has as much to do with the sensor as does its mass. I think there are better terms for the situation you describe, e.g., "equivalent/effective field of view". – ex-ms Jul 28 '10 at 23:32
  • Any solution will require re-education to the masses. I feel that asking people to memorize degrees numbers is less likely, but would definitely be ideal.

    I view both of these solutions as better than the crop factor suggestion, which I believe has been a major source of confusion. People have been stuck on crop factors instead of what they do, which is give an equivalent focal length. I believe assigning emphasis on the equivalent focal length, instead of crop factor, would make people think more about understanding it. You only need to know your own, not others.

     – eruditass Jul 29 '10 at 15:26
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    But it's not equivalent focal length. It's field of view. We shouldn't perpetuate this confusion. – Reid Jul 29 '10 at 15:33
  • @Reid, what else would equivalent focal length mean? Clearly, it's not a physical measurement of focal length, or else it would be focal length. That's the definition of equivalent... it is equivalent in some other fashion (field of view) to the focal length of ###mm on 35mm.

    Equivalent field of view, for example, does not make sense. That is it's field of view.

    If you want to re-educate everyone, I support you in making a chart of the focal lengths in all the different formats and their field of views in the FAQ.

    This is like applied vs theoretical physicists having a discussion.

     – eruditass Aug 10 '10 at 14:57
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    The point is that effective focal length does not exist. It's a complete misnomer. Our argument is that saying e.g. effective field of view of 50mm communicates exactly the same thing as effective focal length of 50mm and does not perpetuate confusion. – Reid Aug 10 '10 at 15:36
  • We just need to drop the term and start using focal lengths only, unless we're specifically describing the difference between sensor formats. "Effective focal length" is only used in small-format digital photography, and despite a chart in a manual here and there (like Bronica), has never been particularly popular when comparing formats before now. People are smart enough to know what "45mm" means to them, and in a large number of cases descriptors like "normal" or "wide angle" are perfectly adequate. – ex-ms Aug 10 '10 at 17:30
  • @Reid, so you are arguing semantics/terminology. Please clarify your position of agreeing or disagreeing with my idea of going to a common format over crop factors and degree numbers. – eruditass Aug 10 '10 at 17:33
  • @matt, Yes, in most cases. My point is when a photographer is asking about what lens to use in a basketball court on the sidelines, having never been to one. The answer is different between APS-C and FF. If you read my answer, this is one of the few situations where it would even need to be brought up. He does not know what 45mm means to him at the basketball court or any other environment he has never set foot in. He needs to know the equivalent field of view. – eruditass Aug 10 '10 at 17:36
  • @Eruditass: no, I'm done here. This is a long discussion and I don't understand your position. Feel free to open a new question on meta. – Reid Aug 10 '10 at 17:44
  • @Reid, please ask questions and understand the position before injecting criticisms. – eruditass Aug 10 '10 at 17:59
  • Sorry to seem short. My point is that I've been following this discussion since the beginning, it's been going on long enough, and it doesn't seem to be going anywhere. Thus, I'm tired of participating. There's a new question on meta FWIW. – Reid Aug 10 '10 at 18:24
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    @Eruditass Yes, there are always exceptions. But even to your hypothetical I'd suggest that "wide" is sufficient, as people do tend to reason fairly well from experience, and it is likely he's been in a gymnasium before, or attended a sporting event other than basketball. If he thinks that means 18mm where you might suggest 24mm precisely, that's not a big deal; he may be correct in the sense that he likes the results, or he may learn from the experience. Neither one of those outcomes is a particular problem. – ex-ms Aug 10 '10 at 18:35
  • @Matt, your counter-argument ignores my requirement of the person never having set foot in the area. What about a new sport with a new sized arena across the country, what focal-length (equivalents) are required for the closest and furthest spots of the arena? What prime lenses and zoom lenses should he buy, rent, and bring? What about a particular concert venue and known disatnce? What prime to bring? That is a legitimate question and when deciding to use a new standard for discussion, one needs to address these potential situations. Otherwise, what is the point of creating a new standard? – eruditass Aug 10 '10 at 19:29
  • @Eruditass No, it doesn't. It simply assumes a real person who has typical life experience, from which they can extrapolate; I think any other assumption is pedantry. Even at a known distance, opinion will vary: 85? 105? 135? 80-200? Where will they set it? You make a mountain of a molehill. – ex-ms Aug 10 '10 at 19:42
  • @Matt, you say it doesn't ignore the requirement, then you go on about you are ignoring my requirement. Opinion will not vary if one person wants specific information to get head-shots or upper-body portraits from 3 rows back in a regular music venue. That is a question is perfectly suited for this kind of site.

    If you want to make arguments for a new standard (since there isn't really one now), you better get used to this kind of analysis before blindly accepting one.

     – eruditass Aug 10 '10 at 19:58
  • @Eruditass OK, let me put it this way: your "requirement" is a straw man who does not exist, and your example is contrived to the point of parody. Specific information to get head-shots or upper-body portraits from 3 rows back in a regular music venue? Do you maintain the "requirement" that they have never set foot in any music venue from which they could extrapolate? How does anyone give a specific answer to that question? You've used up my 'benefit of the doubt' here, this is a waste of time. – ex-ms Aug 10 '10 at 20:12
  • @Matt, fair enough. I rescind my requirement of having never set in to having never paid attention, although there are plenty of places I've love to shoot that I've never been and cant check out. I have been to many concerts but never brought a camera before. I am still working on my intuition of distances, field of view, and framing. Before I go, I'd ask those who have taken pictures in venues with real experience what kind of equivalent field of views for distances, as I'd likely bring only one lens, and it would be a prime for low light and discreteness.

    I'll make sure not to ask you.

     – eruditass Aug 10 '10 at 20:23
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To put it simply, the field of view of a camera body, which depends on its sensor size, determines the effective focal length of a lens when used on that body. There are a variety of sensor sizes and body depths, and therefor a variety of fields of view, for cameras these days. If we take just Canon, they have three sensor sizes for their DSLR cameras: Full-Frame 35mm (1x Crop), APS-H 28mm 1.3x Crop, and APS-C 22mm 1.6x Crop.

When it comes to lenses, a single lens may be able to be used on multiple camera bodies. Again, if we take Canon as an example, the bulk of their lenses are the EF mount. A single EF mount lens, say the 24-70mm focal length L-series lens, supports all three of Canon's DSLR sensor sizes (and therefor all three fields of view.) One may buy the 24-70mm lens for their first Rebel series 550D body, and later upgrade to a full-frame 5DMkII body. When buying an expensive lens that should have a very, very long life, the field of view of the camera body should not really be a factor.

The focal length of the lens itself is really the key factor, and as long as you know the appropriate multiplier for your sensor, you can calculate the effective focal length for each body it might be used on, and its usefulness on that body. This little fact was useful for one of my recent lens purchases. I have a Canon Rebel XSi (450D), and I needed something in the 24-70 range. Since I know my crop factor (or focal length multiplier) is 1.6x, it was easy enough to calculate that the 16-35mm L would effectively be a 25-56mm lens, which generally fit the bill. I also know that when I upgrade to a 5DMkII (or III) in the relatively near future, that this lens will behave as a very nice, ultra-wide to wide angle 16-35mm zoom lens ideal for landscape photography.

If lenses were rated in their field of view, it would be rather confusing to make such a simple determination as effective focal length when a lens is used on different bodies with differing sensor sizes. Lenses are lenses, and should be rated in focal length. Camera bodies are camera bodies, and there should be a simple way of determining their focal length multiplier due to the field of view the sensor provides. In most cases, cameras have a known multiplier, and if not, the information can be easily gleaned (Canon has 1x, 1.3x, and 1.6x, Nikon has 1x and 1.52x, etc.)

jrista
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    Totally agree. Focal length and effective focal length are immediately intuitive ways of understanding the uses of particular lenses. – Nick Bedford Sep 20 '10 at 22:19
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From the wikipedia article on focal length:

The focal length of a lens determines the magnification at which it images distant objects. The focal length of a lens is equal to the distance between the image plane and a pinhole that images distant small objects the same size as the lens in question.

So the focal length of a lens is an optical property - it does not change when attached to a different camera. However photographers are used to what the focal length numbers mean when attached to a 35mm film SLR - which is equivalent to a full frame DSLR.

The "crop-factor" multiplied by the focal length gives you the focal length of a lens that will produce the same field of view on a 35mm sensor. (However it will not be exactly the same image.)

I don't have any particular suggestion of what else to use. I don't find it too confusing myself. You could use the field of view angle in degrees. However that would have to specify the sensor aswell and lots of prime lenses are used on cropped sensor bodies, so it wouldn't make that much sense.

Edit: Just found this article which argues for using the suffix "e" for "35mm equivalent". So for APS-C cameras, you might say "This is a 60mm (96e) lens" - which is succinct and saves mental arithmetic. When it's not clear what sensor size it might be used with, it would be best to go for just the focal length.

I rather like this idea.

Hamish Downer
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  • I personally think that focal length and the use of 'effective' focal length is one of the easiest ways of gauge the field of view of a lens. Photographers learn early on what different focal lengths achieve in terms of field of view, but I've personally never thought in terms of degrees, it's usually a feeling of what it gives. I think in terms of "oh a 50mm lens on my camera would make a nice portrait lens" or "18mm will still give me a decent wide angle for group photos". – Nick Bedford Sep 20 '10 at 22:13
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In theory you should probably talk about angle of view -- but almost nobody knows that. Most people are accustomed to thinking in terms of focal length X on a 35mm camera, so that's how things like "effective focal length" are usually expressed.

When you look at things more carefully, however, there are still differences that can't take into account. For example, depth of field depends entirely on aperture and distance from the subject. This is why some people say that a full-frame camera gives less depth of field than a cropped-format camera. Technically, it's not really true -- if I mount (for example) an 85mm lens on a full-frame camera and then on an APS-C camera, and shoot at the same aperture and distance from the subject, both will show exactly the same depth of field -- but given the narrower field of view on a crop-format camera, with the same lens you'll be farther away from the subject to get (roughly) the same field of view, and that extra distance from the subject will increase your depth of field.

Jerry Coffin
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Depends entirely on their question. If they are talking about what lenses to use in certain situations, I believe we should encourage the use of effective focal length, as personally, I don't memorize field of view or angle of view numbers.

When talking about lens reviews and comparisons, unless specifically about its application and a specific type of environment, it is unnecessary to mention effective focal length.

The practice I adhere to is usually mentioning both.

eruditass
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If you have blue paint and yellow paint and you mix the two you get green paint. It is utterly useless to talk about the “green potential” of your blue or the “green potential” of your yellow until you know which blue AND which yellow you actually have.

AWilding
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    This is a good analogy, but it'd probably be stronger if you explicitly spell out the connection — this stuff isn't obvious to everyone. – mattdm Oct 25 '18 at 11:43