When I adjust the focus of my camera, in general, does this affect the focal length? I have heard that it does.

  • @mattdm sounds like the opposite, not a duplicate. – JDługosz Oct 2 '15 at 1:38
  • @JDługosz Errr, sure. But that means the concepts and answers are the same. – mattdm Oct 2 '15 at 1:40
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    Not obviously. In fact, it might inspire the second question. And this affects non-zoom lenses too. Zooming moves the emements around and maybe the complex camming isn't perfect in preserving focal position. That doesn't say anything about the size of objects "breathing" in a prime lens when the focal point is changed. – JDługosz Oct 2 '15 at 1:47
  • Iac, the answers there to not address this at all. – JDługosz Oct 2 '15 at 1:50
  • Here's the duplicate question: photo.stackexchange.com/q/16549/15871 – Michael C Oct 2 '15 at 5:16

In general: possibly yes.

You actually adjust focus of your lens, not your camera. It can be an advantage1 to design a lens in a way that it changes its focal length slightly when focusing. This is not very important if you take still images. This is different for video of course and one reason why cinema lenses are so expensive

But even if your lens is not guaranteed to have a constant focal length, it might still have it. Maybe it's just having that issue at certain focal distances and you'll be able to happily modify focus at other distances without seeing any change in focal length. To what extent a feature is partially existent on a product is something you will never see advertised or in a spec list. If a constant focal length is important to you, you should rent the lens and try it out if it is not explicitly labelled to have this feature.

1 advantage as in the lens designers do not have to worry about it. It costs money to make lens designers think about things. If a company decides that a feature of a lens is not important and the lens designers do not have to think about it, it can make the lens cheaper, which can be an advantage (if you don't care about the feature either)

  • 1
    It might help to clarify why keeping focal length constant wouldn't just happen "by default", or why a lens assembly with a given level of complexity whose focal length changes with subject distance may offer better performance than one of the same complexity whose focal length couldn't change (e.g. the required amount of compensation for chromatic aberration may vary with subject distance, and changing the amount of compensation may in turn change the effective focal length). – supercat Oct 1 '15 at 21:30

I think Alan Marcus is saying that focusing on near objects requires moving the lens farther from the focal plane. You can see that for yourself with a simple lens like a hand-held magnifying glass. That is, the "length" required to focus diverging rays is longer than initially parallel rays.

And this actual length affects f/stop.

I was assuming the question referred to field-of-view and apparent magnification.

But from the essential physics, the "focal length" is different for each subject distance. The lens designer needs to add complexity to compensate for the visible effects of this change, such as f-stop labeling.

  • Close focus increases the back focus distance. The f/stop as labeled by the detent on the lens barrel becomes invalid. At a near focus distance of about 1 meter (3 feet) the error is about 1/3 of an f/stop. To avoid under-exposure, cameras of old stopped the near focus at about 750mm (30 inches. – Alan Marcus Oct 2 '15 at 0:59
  • And lenses of new (e.g. my first EF lens in 1987) don't have a aperture ring or markings at all, and the microcontroller can map the desired f-stop number to the physical iris setting taking the focus position into account. – JDługosz Oct 2 '15 at 1:37

The focal length is a measurement taken when the camera is focused on a far distant object. By distant we are talking about a star. Opticians often make an artificial star target which is simply an illuminated pinhole. A far distant subject is said to be at an infinite distance (symbol ∞). Technically the object must be so far away that light rays from that object arrive at the camera lens as parallel rays. An object like a coin viewed from a distance of 3000 diameters meets these requirements. We are taking about a 1 inch disk (25.4mm) viewed from 3000 inches = 250 feet (72 meters). OK that establishes infinity ( ∞).

Now the job of the camera lens is to refract (Latin to bend back or deflect). What happens is: Light rays enter the lens their direction of travel is altered. The new path resembles an ice-cream cone. The image formed is at the apex (pointy end) of this cone of light. We focus our camera by moving the lens forward or backward. The idea is to cause the apex of the cone to just kiss the surface of film or digital imaging chip. If the distance is right a super tiny circle of light plays on the sensitized surface. If the distance is wrong, this circle of light is perceived as a circle and not as a point of light. The size of this circle of light is the key. It must be so tiny that the human eye cannot discern it. This will be 0.5mm viewed from normal reading distance. Because the modern camera produces a tiny image of the outside world, we must enlarge this image to make it useful. We enlarge when we view this image on our viewing devices or make a print. The circle size of 0.5mm holds so the circle size in the camera must be super minuscule so it can withstand the needed enlargement. Typically this value will be a diameter of about 1/1000 of the focal length. Super high end lenses might be 1/1500 the focal. Thus the typical 50mm lens will deliver a circle size of 0.050mm.

Now the lens has limited refractive power. As you focus on nearby objects, the light rays from these objects do not arrive parallel, they arrive diverging. Nevertheless the job of the lens is to converge these rays to an ice-cream cone shape. To accomplish we must move the lens further away from film or digital sensor. In other words, we provide greater distance so the lens can do its job and attain the tiny circle size. By the way, the circle is called the circle of confusion because under the microscope it is seen as a circle with scalloped edges.

Now you know why the lens is racked further from film or sensor when focusing close. This elongated distance is called back-focus distance. We do not call this the extended focus distance the focal length. That is reserved for an infinity set-up( ∞). However mathematically it plays its part just like the focal length distance. The f/numbers are computed by dividing the focal length by the working aperture diameter. The result of this math is the focal ratio or f/number. These values engraved on the lens are only valid for the infinity set-up. At close distances the elongated back focus cases an error. This error is called “bellows factor”. At super close distances like unity (life-size or 1:1), the error is 2 f/stops. The back focus at magnification 1 is 2x the focal length. In other words at "unity" life-size a 50mm will be 100mm from the film or sensor. When doing close-up work the danger is under exposure due to bellows factor. This is mitigated in the micro lens design and in the modern cameras that measures exposure thru the lens.

More gobbledygook from Alan Marcus

  • Why an "ice cream cone" as opposed to cones in general as a shape? Is it because etable cups commonly called "ice cream cones" are not necessarily cone shaped? The flat bottom could be a representation of the focal plane (a "truncated parallel half cone" but the step-back near the open end doesn't fit anywhere. – JDługosz Oct 1 '15 at 20:49
  • OK – If an ice-cream cone doesn’t do it for you – then how about a dunce cap? – Alan Marcus Oct 2 '15 at 0:48
  • @JDługosz Not all ice cream cones have flat bottom and a flare near the top. Some are the shape of a true cone. – Michael C Oct 2 '15 at 5:10
  • I know. I was pondering on the literary choice of an "ice cream cone" as if we are not expected to know what "cone" (as a math shape) means, yet contains the word cone anyway. The semi-sarcastic musing might help him improve his literary skills, by seeing how people read such prose. – JDługosz Oct 2 '15 at 5:27

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