0

I tried to take several pictures with a DSLR ( Nikon D3200 - 18 to 55mm zoom lens) with manual focus mode. I changed the manual focus randomly and clicked several pictures one after another. I was expecting to see the change in the manual focus somewhere in the metadata of the camera. However, no change was visible. All the pcitures I took had identical metadata with focal length, aperture, shutter speed, ISO, etc.

What parameters are triggered by rotating the focus of the lens? Shouldn't it be the focal length?

  • 1
  • @infoclogged - please keep it civil. An explanation is not required in order to downvote. Insulting someone for downvoting you without an explanation is rude. – AJ Henderson Aug 14 '17 at 17:18
  • @aaaaaa - SE is trying to be a repository of answers for questions. If it isn't already on the site, being findable by google isn't a reason to not ask. "L2Google" isn't a valid response. This seems to be a decent question about the difference between focal plane and focal length, which is certainly a relevant and non-trivial topic. – AJ Henderson Aug 14 '17 at 17:18
  • @AJHenderson i disagree completely with your views about downvoting. OPs should be taught and not kicked. So, I guess, you are spreading a negative view. here it is: meta.stackexchange.com/questions/135/…. and sorry, for the frustration on an anonymous downvoter who downvoted my first question on this site ! – infoclogged Aug 14 '17 at 17:35
  • @infoclogged - please don't get me wrong. I edited my comment a bit to make it more clear. I much prefer that a downvoter comment on why and help improve as well (I'm one of the 895 up votes on that meta post you linked), but it is very intentionally not a requirement. I share your frustration when I see a downvote and have no idea what the user felt the problem was as I feel powerless to fix their concern, but insulting them is certainly not ok. – AJ Henderson Aug 14 '17 at 17:49
4

The focal length expresses the power of the lens. We use this value for lots of stuff, we can calculate the angle of view, is this lens a wide-angle, a telephoto, or does it provide a normal angle of view. The focal length is a measurement taken when the lens is imaging a far distant object. If the lens has a fixed focal length, such a lens is called a “prime”. If the lens is adjustable as to focal length, we call this lens a “zoom”.

Say your camera is fitted with a 30mm lens, that tells us that the distance from about the center of the lens barrel to the imaging sensor at the back of the camera, will measure about 30mm when focused on distant mountains. When you focus on nearby subjects, you must focus to get a tack sharp image. What is happening is: When the lens is imaging nearby subjects, their images fall further downstream from the lens. They will come to a focus at a further distance from the lens. To focus nearby subject we rotate the lens. A screw mechanism moves the lens somewhat forward. The degree of forward motion required is greater if the object is very near. Some lenses allow super close photography. To accomplish the screw mechanism racks the lens many millimeters forward.

All the while, the focal length does not change because this is a measurement taken only when the lens is imaging a far distant object. When working in close, what changes is the “back focus distance”. The EXIF does not give the back focus distance.

  • thanks for the good explanation. When you say, rotate the lens, does lens really physically rotates? I mean, AFAIK a lens is symmetric, so what difference does it make by the rotation? I cant understand the difference between the focal length and the back focus distance. Shouldnt the back focus distance more important for the user, coz it tells the actual distance between the lens and the image sensor plane. – infoclogged Aug 10 '17 at 6:39
  • The key point: The lens acts like a projector; it forms an image in the air behind the lens. We must adjust the distance, lens to imaging chip to obtain a focused image that plays on the surface of this chip. The focus mechanism if manual is moved by you. If automatic, an electric motor does this job. In either case, the lens is moved forward or backwards to obtain a focus. As the lens moves, some mechanisms rotate, the lens some don’t. This is a design thing. Most times the actual back focus distance is unimportant data. This data can become important when making super close-up pictures. – Alan Marcus Aug 10 '17 at 15:35
8

No. Focal length is characteristic of the lens. e.g. a 50mm non zoom lens will always show 50mm in the metadata / EXIF. For a zoom, like an 80-210mm the metadata / EXIF will show 80mm at the wide end, and 210mm at the telephoto end, and everything inbetween.

What you are looking for in EXIF is "subject distance" which may or may not be supported depending on which camera and lens you are using, and may or may not be visible depending on what EXIF reader you are using.

  • So, the zoom might be showing the effective focal length and not the focal length of individual lenses? I am assuming that the zoom has more than one lens, to be able to change the path of the incoming rays. – infoclogged Aug 10 '17 at 6:28
  • @infoclogged Of course, all lenses consist of multiple lens elements. The focal length of individual lens elements is never shown in EXIF data. Only the resulting focal length is shown and used in calculations. – Gerhardh Aug 10 '17 at 7:36
  • @KeiferJ, is the "back focus distance" ( see Alans answer ) and the "subject distance" the same? Can you give me an example of a camera / lens / EXIF reader combination that supports "subject distance".. – infoclogged Aug 10 '17 at 12:54
0

I will answer my question myself. Its just for added explanations to the back focal distance as answered by @Alan.

Quote wiki To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear nodal point and the film, to put the film at the image plane. The focal length (f), the distance from the front nodal point to the object to photograph (s1), and the distance from the rear nodal point to the image plane (s2) are then related by....:

( 1/s1+ 1/s2 = 1/f )

The back focal distance ( distance between the vertex of the last optical surface and the imaging plane. ) for objects at infinity is the same as the focal length. Put s1 as infinity.

The back focal distance, however changes for objects that are not at infinity ( s1 is not infinity ) . By rotating the manual focus, one is adjusting the distance ( s2 ) between the vertex of the last optical surface and the imaging plane.

So, basically, focal length is constant all the time, because focal length is the property of the lens.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.