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I'd like to be certain about the focal length written on the cropped lenses, e.g. I have a Sony NEX-5N with the original OSS 18-55mm kit lens. Is it actually an 18 to 55mm focal length with no "recalculation" from the manufacturer?

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    \$\begingroup\$ Some people put far too much importance on “crop factor”. Have you ever used a “Full Frame” camera? If not, then why do you want to know the focal length relative to a Full Frame camera? \$\endgroup\$
    – Mike Sowsun
    Jan 11, 2022 at 3:19
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    \$\begingroup\$ @MikeSowsun I agree that people get way too hung up about comparing different sensor sizes. But I guess the standard is useful mainly due the reason most of the material already written describes focal lengths in terms of FF. Consider all the articles recommending 35mm or 50mm primes. If a beginner were to buy a M43 camera and blindly purchase a 50mm lens, they might get a surprise! \$\endgroup\$
    – Matt Dunn
    Jan 11, 2022 at 11:28
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    \$\begingroup\$ I'm sure you know that focal length is an absolute property of the optics. Attaching to a small-frame camera changes the field of view but not any other properties that are a function of focal length. \$\endgroup\$
    – Toby Speight
    Jan 11, 2022 at 13:51
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    \$\begingroup\$ @TobySpeight you have to be careful with blanket statements like that. DOF for example depends on both the FL and the crop factor. The lens will of course project the exact same on any size sensor, but the blur will be magnified more when you view the cropped image. \$\endgroup\$
    – Mark Ransom
    Jan 17, 2022 at 0:58

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Typically, if a lens is removable it is normally labeled with its true/actual (rounded) focal length, without any consideration for what it might be attached to (I don't know of any exceptions). That is true for the 18-55 kit lens you have as well; it is not adjusted for crop factor.

Conversely, if the lens is not removable the focal length is generally stated with the 35mm crop factor conversion applied.

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    \$\begingroup\$ Some compact cameras with fixed lenses do apply the “crop” factor when labelling the lens. In my experience, most don’t. My Canon compact cameras always said the actual focal length. (Canon S120 5.2-26mm) \$\endgroup\$
    – Mike Sowsun
    Jan 11, 2022 at 3:23
  • \$\begingroup\$ Hasselblad/Zeiss used to be very "generous" in their rounding. The "Zeiss Distagon CFi 4/50" datasheet says it has a focal length of 51.9 mm. Maybe all manufacturers do similar, and only Hasselblad/Zeiss were open about it? It's only their datasheets where I've encountered this. \$\endgroup\$
    – osullic
    Jan 11, 2022 at 17:49
  • \$\begingroup\$ They all round to the nearest "standard" FL. 24 - 28 - 35 - 40 - 45 - 50 - 55 - 65 - 70 - 85 - 90 - 100 - 105 - 135 - 180 - 200, etc. Many 70-200mm zooms are actually about 77-185mm or 74-190mm zooms. \$\endgroup\$
    – Michael C
    Jan 11, 2022 at 19:18
  • \$\begingroup\$ I don't think that's quite right @Michael to say they all round. I see primes on B&H with listed focal lengths of 25, 30, 31, 43, 56, 58, 77... \$\endgroup\$
    – osullic
    Jan 11, 2022 at 20:17
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    \$\begingroup\$ @osullic; F/1.8 is on the more common 1/3 stop exposure scale. \$\endgroup\$
    – Steven Kersting
    Jan 12, 2022 at 20:02
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All this confusion comes from the old "bad habit" to express field-of-view values not by themselves, but by the focal length that gives this field of view on a "standard" 24*36 film area.

Now that sensors come in all sizes (FF, cropped, M43, whatever), that habit fails miserably.

The focal length given with any lens means exactly that, its focal length, and doesn't care about the size of the sensor behind it, be it FF or cropped. And of course, with a given lens, if you have a smaller sensor, that sensor catches a smaller part of the image, thus giving a smaller field of view.

On the other hand, if a pre-digital book on image composition talks about a focal length, it most probably means the resulting field of view (assuming the FF 24*36 format), and has to be translated to the shorter focal length (divided by the crop factor) that gives the same field of view with a smaller sensor.

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The Sony 18-55mm OSS f/3.5-5.6 NEX E-mount has focal lengths of 18-55mm. On a APSC 1.5x crop sensor the field of view is approximately the same as 28-85 mm on full frame 35mm.

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