topic Re: Interpreting specified focal lengths in EF & RF Lenses

Interpreting specified focal lengths

ehz — Wed, 18 Sep 2013 18:15:54 GMT

Are the focal lengths specified for the EFS lenses true focal lengths, or have they been adjusted to be 35mm equivalent? For example, if an EF lens with a specified focal length range of 18mm-55mm is mounted on an EOS 70D which has a 22.5 x 15mm sensor, does the angle range from somewhat wide to somewhat telephoto, or does the angle range from very wide to barely over standard.

I had assumed that the focal lengths listed are true, but then I noticed Canon describing a lens labeled 50mm as "standard" which would be the case for 35mm film, but not for a 22mm sensor.

(Sorry for asking such a basic question. I'm new to DSLR.)

- Ehz

Re: Interpreting specified focal lengths

Skirball — Wed, 18 Sep 2013 19:53:33 GMT

The focal lengths on the lens are true focal lengths. They're the same regardless of EF or EF-S. Putting them on a crop sensor will narrow the FOV. So the 18-55 on a 70D will behave something like a 24-70 would on a 35mm camera (technically it'd be like 28 - 88).

Re: Interpreting specified focal lengths

RexGig — Wed, 18 Sep 2013 20:02:57 GMT

EF-S lenses are designated by their true focal ength, not the 35mm-equivalent. Keep in mind that on a cropped-frame camera, the image is cropped, which changes the apparent angle-of-view, but the focal length remains the same. For example, a wide-angle lens with focal length that makes a close-range subject's nose appear too large, or distorts vertical lines if not held level, will do so to the same degree on both a 35mm sensor, and a cropped-frame sensor.

Re: Interpreting specified focal lengths

RexGig — Wed, 18 Sep 2013 20:14:13 GMT

Another example is my 100mm 2.8L Macro. Whether I use is on a 5D, a 1D Mark II N, or a 7D, it behaves the same, reaching 1:1 at the same distance from a subject. Focal length is focal length, regardless of the crop factor. It is the display device, or the print, that performs the "apparent" magnification.

Re: Interpreting specified focal lengths

ehz — Wed, 18 Sep 2013 20:37:24 GMT

Great!

Skirball and RexRig, thank you both for posting.

- Ehz

Re: Interpreting specified focal lengths

TCampbell — Wed, 18 Sep 2013 20:47:04 GMT

As Skirball says -- they're "true", but everything in photography is rounded off.

It's not unusual for careful testing to determine that a 100mm lens is really (and I'm making this up as an example) 97mm, etc. f-stops are also generally rounded to the nearest whole or 1/3rd stop increment. They'll always be so close that the difference wont affect the exposure.

A focal length is considered to be "normal" or "standard" when it's angle of view approximates the angle of view of the human eye (and that assumes you're looking directly ahead and not moving your eye around). The angle of view is in the range of 40-50 degrees.

When a lens provides the same (or very close) angle of view, so that the camera sees approximately what the eye can see, they considered it to be a "normal" angle (because it seems normal enough to us.)

On a full-frame (sensor is the same size as 35mm film negative frame) camera, that turns out to be about 50mm. On an APS-C crop frame it turns out to be around 31mm (and there is no 31mm lens -- so anything in the range of 28mm to 35mm is close enough.)