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2 thoughts made me ask this question:

  • Different lens looks different on the different-sized sensor and has a different 35mm equivalent

  • Many normal and wide-angle lenses' optical center is not exactly the same distance to the image sensor as its focal length described, and are actually zooming in (de-telephoto? I am not sure if there's a term for that)

Then does that focal length only indicate its 35mm equivalent? If there's a lens of an unknown focal length, what determines the focal length of this lens?

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  • \$\begingroup\$ Does this answer your question: What is crop factor and how does it relate to focal length? \$\endgroup\$
    – scottbb
    May 4, 2022 at 0:06
  • \$\begingroup\$ @scottbb I doubt so. I am aware of the crop factor, my question is more like if there's something intrinsic to a lens, say a 30mm, that makes this lens forever 30mm regardless of where and what it is being mounted on. Or does that 30mm only mean it's 30mm on a full-frame camera \$\endgroup\$ May 4, 2022 at 0:23
  • \$\begingroup\$ See What is the reference point that the focal length of a lens is calculated from? \$\endgroup\$
    – osullic
    May 4, 2022 at 11:44
  • \$\begingroup\$ @ Amarth Gûl The focal length engraved on the lens defines the image size of the images it projects on the sensor. It can only be changed by the addition of a supplemental lens. The angle of view generated is a function of focal length and sensor size. Mount a lens on different size sensors alters the angle of view. It has become industry practice try an equate how a lens will perform to its full frame counterpart. \$\endgroup\$ May 4, 2022 at 15:19
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    \$\begingroup\$ Re, "actually zooming in (de-telephoto?...)" How about, "retrofocal?" en.wikipedia.org/wiki/Ang%C3%A9nieux_retrofocus \$\endgroup\$ May 4, 2022 at 21:45

4 Answers 4

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The focal length is a physical attribute of the lens, regardless of sensor size.
The focal length coupled with the sensor dimensions gives you the angle of view, also called field of view, or FOV.

Angle of View = 2 * tan-1(d/(2 * f))
where d is the one of the dimensions of the sensor (X,Y, or diagonal) and f is the focal length of the lens (units must be the same, i.e. millimeters).

You will see equivalent focal length used on crop sensor cameras as referenced to a full frame sensor. This equivalency gives a sense of reference for those who are used to shooting with full-frame cameras.

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    \$\begingroup\$ To directly answer the title question, would add: Interchangeable lens systems usually (always?) report the actual focal length. So 35mm APS-C lens, 35mm MFT lens, and 35mm full-frame lens would all produce the same level of magnification on the same camera. (Not necessarily same FOV because the sizes of the imaging circles are different.) Lenses that are permanently coupled to a sensor often (always?) report the 35mm equivalent focal length. These include phones, compact, and bridge cameras. \$\endgroup\$
    – xiota
    May 4, 2022 at 11:00
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    \$\begingroup\$ Medium and Large Format: "What am I, chopped liver?" \$\endgroup\$
    – Davidw
    May 4, 2022 at 13:02
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Because the 35mm film camera has been around for about 100 years and because it has been popular for 2/3 of that time, many photographers are highly familiar with how it performs. Thus, it has become the benchmark.

Comparisons need to be made as an aid to help photographers choose lenses for diverse formats that deliver the same angle of view as the venerable 35mm. Suppose you are familiar with the action of a 50mm lens mounted on a 35mm camera. Suppose you switch and use an APS-C camera. What lens on an APS-C matches the 50mm on a 35mm?

Crop factor to the rescue. We find out the diagonal measure of both frame sizes and divide to find the crop factor. The 35mm frame diagonal is 43mm. The APS-C frame diagonal is 28mm. We divide and find the crop factor is 1.5. This tells me that the 35mm fame is 1.5 X larger or conversely 1/1.5 = 0.66 or 66% of the 35mm frame.

OK a 50mm lens equates to 50 ÷ 1.5 = about 35mm.

In other words, a 50mm on a 35mm camera and a 35mm on an APS-C deliver about the same angle of view.

As to lens placement: Every lens has two cardinal points. We measure distance to film or sensor from the rear nodal. We measure subject distance from the front nodal. Their locations are not generally published. Lens makers can shift these points around quite a bit. A telephoto lens can be very long and thus unwieldy, Often, the rear nodal is shifted forward, this shortens the barrel length. Conversely a wide-angle lens has a short focal length, and this can be a problem as the lens placement is too close to the camera body to accommodate mirror swing and the like. The wide-angle frequently has its rear nodal shifted to allow the lens placement to be more forward.

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  • \$\begingroup\$ Similarly, M43 (used by Olympus, Panasonic and others) is a 2x crop factor, so a 25mm lens is equivalent to a 50mm lens. \$\endgroup\$ May 4, 2022 at 9:57
  • \$\begingroup\$ 35mm equivilents is a pretty wonky frame of reference when dealing with medium format stuff thats not 3:2 aspect ratio as only the horizontal field of view is equivilent. In 6x6cm for example the 35mm equivilent will appear much tigher as the image fills more of the frame vertically. A lot of medium format lenses doesn't even list 35mm equivilents. \$\endgroup\$
    – max
    May 4, 2022 at 11:50
  • \$\begingroup\$ Over the years, photographic technology allows smaller formats without serious loss of image quality. I switched from 4x5 inch to 2 ¼ x 3 ¼ to 35mm without even thinking about a crop factor. I think the crop factor methodology is useless unless if you don’t have high familiarly using the previous format. \$\endgroup\$ May 4, 2022 at 13:56
  • \$\begingroup\$ The crop factor also reveals the magnification factor. Generally used to compare a smaller format result to the results obtained on the venerable 35mm camera. A crop factor of 1.5 means this smaller format must be enlarger 1.5X more to yield a displayed image of the same size. Now we must consider, can the smaller format tolerate this increased magnification requirement? Mostly camera size shrinks as technology advances. Lilley the answer is yes. \$\endgroup\$ May 4, 2022 at 15:33
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The short answer is: "No"

For Example the Zeiss Distagon 40mm for Hasselblad medium format (6x6) is a wide-angle lens (it would correspond to 20mm when compared to "classic" lenses), and the planar 80mm is mostly "normal" angle.

For the second question: I think if you have a "light spot" (the sun may do) in "infinity" distance (i.e. very far away), the focal length is the distance from the optical center of the lens (that would be another question) to the focussed image the lens creates (move a paper forth and back until the image is focused, then measure the distance).

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  • \$\begingroup\$ Re “the focal length is the distance from the optical center of the lens [...]”: Technically, it's the distance from the image-side principal point to the focused image at infinity. The principal point is not quite an “optical center” and it may well be, in a retrofocus design, outside the lens itself. \$\endgroup\$ May 5, 2022 at 7:55
  • \$\begingroup\$ It's not just retrofocus designs. The actual definition of a telephoto lens is one which has a front nodal point in front of the front of the first lens element. Any lens which has nodal/principle points outside the actual glass of the lens are necessarily compound lenses with multiple lens elements. \$\endgroup\$
    – Michael C
    May 5, 2022 at 8:13
  • \$\begingroup\$ The Sun has an angular size of 1/2° as measured from the surface of the Earth. That's nowhere near a "point source" of light which, in theory, is a dimensionless single point and has an angular size of zero. \$\endgroup\$
    – Michael C
    May 5, 2022 at 8:15
  • \$\begingroup\$ @MichaelC Feel free to suggest an alternative. Despite of that I never said the image of the sun will be a dot; it has to be focused. \$\endgroup\$
    – U. Windl
    May 5, 2022 at 13:24
  • \$\begingroup\$ @U. Windl To my ears "light spot" sounds too much like what google translate might come up with if it translated "light point source' into another language and then back to English, or merely what google might translate the other language's accepted equivalent for point source into English. \$\endgroup\$
    – Michael C
    May 9, 2022 at 14:54
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In general: Interchangeable lenses are marked/designated/marketed with their actual focal length.

Thus, a 50mm APS-C lens and a 50mm FF lens will both give the same angle of view on an APS-C camera. This angle of view will be equivalent to a 75-80mm lens on a FF camera.

In general: Lenses permanently attached to a camera are usually marketed with the FF focal length that would give the equivalent angle of view. But the lens is usually marked with the actual focal length and sometimes the FF equivalent angle of view expressed as a focal length is included in parenthesis following the actual focal length.

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