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There are issues about how we talk about the focal length of lenses that often lead to confusion - see this question for example. I recently read, and agree with “35mm Equivalents”: A Plea For Clarity which suggests a way forward:

Basically when talking about lenses for cropped sensor cameras, you give 2 numbers - the actual focal length and the 35mm equivalent focal length. But you can shrink it to, for example (using APS-C), 50mm (80e) - the "e" is short for "equivalent". Which, to me, is nice and succinct.

Obviously there are times where using this would be more confusing than not - say discussing Canon EF lenses without a particular body in mind - but a lot of the time there would be only one crop factor to think about - Canon EF-S lenses, Nikon DX lenses, any 4/3 or u4/3 lens ...

I'm not suggesting we should enforce the use of this shorthand, but if quite a few of us started to use it, it could become a de facto standard on this site at least. We could set up a question to explain the term (whether on photo.SE or meta.photo.SE) and link to the question when we use the shorthand - eg. 50mm (80e).

So do you think this is an idea we should try and encourage? Or is it just going to be too confusing?

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  • \$\begingroup\$ +1 on the blog post. While I'm not persuaded by his arguments on notation, I think he's got it very right in analysis of the problem. \$\endgroup\$
    – Reid
    Aug 10, 2010 at 22:30

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I'm not sure this is amenable to any specific format, but the (80e) definitely appeals when applied in context.

Personally, my guidelines run roughly thus:

  • Answer in context. e.g., if the question is about a small-format camera, do not say something like "on full frame you'd use X, so here you'd...". What you'd use on full frame is extra information, largely irrelevant.

  • Many people already understand what's going on, so you do not need to be exhaustive, merely clear. If you say "I'd use 50mm (or 35mm on APS-C)," and someone uses 4/3 (or whatever), they can ask, or work it out from their own experience.

  • Use the actual focal length of any actual lens being discussed.

  • Relative descriptors are often just as useful: wide, portrait, telephoto, normal, etc.

  • Avoid whenever possible terms like "focal length multiplier" as they're ultimately misleading.

  • If the format is ambiguous, state your assumptions about the format, or wait for clarification. e.g., "I'd use 50mm (or 35mm on APS-C)".

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Ah, but the equivalent varies. For Canon APS-C, a 50mm has the equivalent field of view of 80mm, but for Nikon/Pentax/Sony, it would be 75mm. So, that could pose its own problems. I think, if the question is relevant for focal length and sensor size, then encouraging including the camera model is sufficient, as most experienced types can quickly determine the equivalent that is relevant.

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  • \$\begingroup\$ I think the difference between 75mm and 80mm FOV is probably small enough to be neglected. \$\endgroup\$
    – Reid
    Aug 10, 2010 at 22:27
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    \$\begingroup\$ @Reid until you get to other formats, then it isn't so much. I just used the fact that even APS-C isn't a standard size to illustrate the point and, as the lens focal length increases, the differences become more pronounced. Net effect, stating the camera amounts to the same thing as stating the equivalent for anyone who is in a position to answer a question where the crop factor is relevant. \$\endgroup\$
    – Joanne C
    Aug 11, 2010 at 2:55
  • \$\begingroup\$ Oh, I get it. Yes, I agree. \$\endgroup\$
    – Reid
    Aug 11, 2010 at 3:17
  • \$\begingroup\$ I think this is an excellent point. Even though Canon may have the lions share, it is by no means the most talked about, and the wide variety of crop factors really makes it difficult to come up with any kind of "standard". The mathematics isn't difficult (focalLength * cropFactor), and unless someone notices that the focal length is clickable, the average user may never understand what were talking about. \$\endgroup\$
    – jrista
    Aug 11, 2010 at 4:25
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    \$\begingroup\$ I would suggest simply not using it when the context is ambiguous with regard to what size sensor the lens would be used with. So it would not be a complete solution. But I do sometimes see comments (on, say a post about a 50mm lens for a 4/3 sensor) saying "Is that 50mm, ie 100mm equivalent, or actually 50mm equivalent?". But if the article said "50mm (100e)" then there would be no confusion. The key confusion is that some articles talk in terms of equivalent and some in terms of actual focal length. By having a concise notation to state both, readers don't have to wonder which is meant. \$\endgroup\$
    – Hamish Downer
    Aug 12, 2010 at 12:51
  • \$\begingroup\$ @Hamish yes, that's exactly my usecase for the (100e) as well. I think it's worth a FAQ entry for sure. \$\endgroup\$
    – ex-ms
    Aug 12, 2010 at 20:24
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    \$\begingroup\$ Also, the 1D uses an APS-H sensor, which confuses matters further... \$\endgroup\$
    – Rowland Shaw
    Aug 16, 2010 at 18:19
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Another way to handle the issue is to simply refer to the field of view directly.

For instance I have a Canon XSi and often use a 100mm (12.7° fov) lens with it. The kit lens (18-55mm) has a 96.0°-22.8° fov range.

It does take a bit of thought to work it out, but it's easy enough to calculate, and it's actually a much more useful measurement.

You do need to use a bit of html magic to display the degree symbol (°) correctly though.

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  • \$\begingroup\$ A requirement for questions essentially about FoV to use a calculator is a great alternative. A discussion on what will be the easiest to standardize and enforce will be helpful, as I don't see a clear winner between these two at the moment. A calculator that has smaller formats or even a box to type in would be great. \$\endgroup\$
    – eruditass
    Aug 10, 2010 at 20:07
  • \$\begingroup\$ Focal length has been the established descriptor for lenses for over a century. Picking a new standard is not a good idea. \$\endgroup\$
    – ex-ms
    Aug 10, 2010 at 20:21
  • \$\begingroup\$ Focal length on 35mm has been an established descriptor for field of view. Focal length by itself has been an established descriptor for a physical property of a lens, which is not being discussed here. I am beginning to dislike the idea of having to go to a calculator instead of ball-parking a multiplication like 1.5x or 2x. \$\endgroup\$
    – eruditass
    Aug 10, 2010 at 20:50
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    \$\begingroup\$ I think the basic problem is that no one thinks in degrees; for better or worse, they think in mm even though it's totally wrong (even me). So while technically much superior, no one is going to understand degree quotations, and no one is going to go to the effort of pulling up a calculator. \$\endgroup\$
    – Reid
    Aug 10, 2010 at 22:29
  • \$\begingroup\$ Fair enough, I'm just offering it as the logical answer even though it is most likely never going to work. Worth a try :) \$\endgroup\$
    – chills42
    Aug 11, 2010 at 0:16
  • \$\begingroup\$ @chills42 probably because most people wouldn't really have a clue as to how to visualize the angles. I suspect that's a big part of why it wasn't the 'focus' of lens information even in the heyday of film where 35mm was the size for all SLRs. \$\endgroup\$
    – Joanne C
    Aug 11, 2010 at 2:59
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    \$\begingroup\$ FWIW, you can also just type a ° with your keyboard. No &s required. :) \$\endgroup\$
    – mattdm
    Jan 6, 2011 at 0:53
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Mike Johnston (inventor of the spelling "bokeh") suggests that people should use 35mm-e, but I agree that it's pretty unwieldy.

It's worth noting that "e" has a nice property of not already meaning much else. Oh, sure, e is Euler's number, and it has some relatively obscure technical uses, but if someone sees it and doesn't understand, the worst is that we'll get a question "What does 50e mean?" — which is much better than someone misinterpreting completely.

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