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My Canon 1300D DSLR stores the focal length used in the EXIF data of each photo, but there's no way to see this information while I'm composing the shot? Is there a technical reason why the focal length can't be determined before taking the photo or is it just assumed that I wouldn't need that information because lenses are marked with (imprecise) focal length markings? I assume that the lens reports this information to the body somehow.

EDIT: I'm referring specifically to zoom lenses, not primes.

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  • are you asking about focus distance (distance to the subject in focus)? That is different than focal length, which is an inherent property of a lens. – scottbb♦ Feb 1 '17 at 16:33
  • @scottbb No, I'm talking about the focal length. The focal distance isn't recorded in the EXIF data (at least on my camera) and is most appropriately determined by looking through the viewfinder. – Micheal Johnson Feb 1 '17 at 21:43
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My guess is that this information is not shown since it does not affect your photo in any way that is not already apparent from the image in the viewfinder.

For instance, aperture affects DOF (not visible in viewfinder unless DOF preview is used), shutter speed affects blur due to camera and/or subject movement (not visible in viewfinder), and ISO affects noise (not visible in viewfinder). In addition, these values affect exposure, which is also not something the viewfinder itself can show. On the other hand, focal length only affects framing, which is already visible in the viewfinder, so it is not as useful to redundantly display this information in numeric format.

So I think it may simply come down to using the limited resource that is viewfinder area in the best way possible. However, I agree that sometimes it would be useful to be able to see information like focal length or focusing distance in the viewfinder as well.

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    Though I think it's so satisfying to see the varying focal length numbers when you zoom, why would a photographer need that? – user152435 Feb 1 '17 at 18:04
  • I sometimes wish I could see the exact focal length so that I could use, say, exactly 32mm or exactly 50mm. While I can compose it "by eye" and read the markings on the lens, it annoys me when I take a photo that's supposed to be at 50mm but it comes out at 47mm (or 48mm, or 49mm, etc.). – Micheal Johnson Feb 1 '17 at 21:45
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    Note that even for prime lenses the focal length marking is only approximate. A 50mm prime lens might actually be 52mm or 48mm (not sure of the standard allowed variation, surely depends on manufacturer). In the case of a zoom lens you also have the uncertainty of the algorithm the camera uses to tell where in the focal range you are to record the EXIF data. I don't know for sure how that works but it makes sense that it isn't 100% accurate either. – David Rouse Feb 2 '17 at 14:04
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    @DavidRouse focus breathing can change the effective FL too, and that won't be reflected in the EXIF. – Mark Ransom Feb 3 '17 at 6:02
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When you look through the viewfinder, you view and the focal-length are linked. While you do not see the actual measure in millimeters, you are seing it as visible angle-of-view. Therefore you do not really need it since you can see its impact. In contrast, you need to know the shutter speed since you are unable to see its effect while framing. Same with aperture unless you press the DOF-Preview button which exists on quite a few, but not all, cameras.

AFAIK, no DSLR shows the focal-length through the viewfinder. On many Pentax DSLRs, if you have the Status Screen enabled, the rear LCD of the camera shows the focal-length rounded to the closest integer. This is the actual focal-length, not the equivalent one, as you would see an many ultra-zooms. Even so, the focal-length often becomes incorrect since it changes with focus-distance but the display shows the focal-length for infinity focus.

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    Regarding the second part of your answer, that's exactly what's recorded in the EXIF data by my camera: the focal length to the nearest integer, as reported by the lens and as corresponds to the markings on the body (ignoring any effects due to focusing). – Micheal Johnson Feb 1 '17 at 21:47
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It is possible for Canon to display the focal length but they choose not to do it.

If you were to install "Magic Lantern" firmware on your camera, you can unlock many interesting features.

This video shows how you can see the Focal Length along with Focus Distance and DOF information: (DOF Near, DOF Far, and Hyperfocal Distance)

Magic Lantern Video

enter image description here

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  • I have considered Magic Lantern before. – Micheal Johnson Feb 2 '17 at 8:24
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You can't see the focal length on the Canon EOS 600D either. So I guess it is normal for DSLR not to display the focal length on the display or the viewfinder. Just looking at the lens which shows the approximate focal length seems to be to only way to see the focal length before taking the shot.

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Focal length is directly connected with the perspective of the scene and the view-angle. (One usually doesn't switch bodies to alter the viewfield so I humbly ignore the effect of the sensor dimensions.) Therefore you have the information already in the viewfinder, but not as a single value. If there was a dedicated display for such value it would occupy room for the other values to be displayed. One would have to decide whether to reduce the size of the displays (reducing readability), reduce the viewfinder (reduce the viewfinder accuracy) or remove some other display.

Values for aperture, EV, shutter, and ISO are displayed as numbers (bar scale) because the visualisation may introduce more issues than benefits.

The aperture is usually wide open and closes to the selected setup only when the shutter is released. Reasoning is simple:

  • Preview is brighter than the actual image alowing more precise focus
  • Wide open lens have shallower depth of focus alowing more precise focus

If you want to see the actual DOF, you can temporarily close the aperture by the button near the mount (depends on camera design).

Effect of shutter and ISO setup is impossible to preview and EV is calculated from measured light intensity and the values.

If you want to see the actual value of the focal length look at the lens from above. The "zoom" ring has several values printed (engraved) on it. Here you can see a Canon 70-200 mm f/2.8L lens where focal length is set to 200 mm and focused on object 1.5 m away from the camera.

If you are asked to shoot at FL = 90 mm, set the ring accordingly; then look in the viewfinder and adjust the scene by walking around to get the desired scene.

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Because it's not important. Photos are (and always have been) labelled with the F/number, shutter speed, and base focal length of the lens in use. The fact that you adjust the lens to focus at a given distance is not really of interest. It's not that hard to infer that value by examining the image and estimating the distance to things in view.

And in any case, as everyone else has pointed out, focal length is a fixed value for a lens. You are in fact asking about focal distance .

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    Unless your lens is a prime, focal length is not fixed. I am not "adjusting the lens to focus at a given distance" - that would be the focal distance, not the focal length. "Focal length" is a commonly-understood (and technically-correct) term for changing the "zoom" on a lens. – Micheal Johnson Feb 2 '17 at 15:56
  • @MichealJohnson I changed the title to clarify that point. – Carl Witthoft Feb 2 '17 at 16:04
  • it can be critically important if you're shooting real estate photos for a client that requires a specific focal length and you're using a wide angle zoom lens – snaps Sep 25 '18 at 10:07
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    Why would they want that? Also: Simply look at your lens's FL indicator and/or see the EXIF data. – flolilo Sep 25 '18 at 10:16
  • One reason you would want to know the focal length is if you are shooting a pano and have a pano head with stops. How much overlay needed is based on your field of view which changes as you zoom and naturally it is different for different sensor sizes. With a prime lens it is easy to setup the pano head; however, if you put on a zoom it is not so easy. You need the focal length to make the calculations. On the wider end it is not to bad but on the longer end it makes a bigger difference. It was mentioned just look at the lens barrel and you can but it is not very accurate. All that being said – Keith Passaur Jan 24 '19 at 15:36

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