Discussion in 'Photography Beginners' Forum' started by malbec, Oct 17, 2009.
does your focal length change when your lens focuses by extending inward or outward?
Depends on how complicated you want to get. The simple answer as fokker made short and sweet: "no." If you're using a prime focus lens and there is no internal focusing or internal elements of any kind moving (only the entire lens moves in and out), then definitely no.
With today's more complicated internal focusing lenses, zoom lenses, macro lenses, internally correcting lenses it is possible to have some slight shift in "true" focal length during focusing. But the average user would never know. Just like the average user doesn't realize that his zoom is incorrectly marked for the focal lengths. They are rough estimations and they're even different between 2 identical lenses.
I'm curious why you ask.
I notice on my 50mm 1.8 when I move the focus ring, It does zoom in slightly / out
However I cannot decide wither this is caused by the subjects being pulled into focus and pushed out of focus creating the illusion of zoom or what...
Actually, the answer is often Yes; as a cell-focusing lens focuses closer, it often loses focal length, so many,many lenses like 50mm 1.8 normals from Nikon and Canon,as well as many other non-internal-focusing lenses LOSE focal length as they are focused closer.
On lenses like macro lenses, like the Tamron 90mm AF-SP for example, by the time the lens is focused at its closest focusing distance at 1:1, the actual focal length is around 73mm if my memory serves me correctly.
I'm not sure of the build and focus mechanism of the 50mm f/1.8, but my guess is that the entire lens moves in and out in sync to achieve focus. I have many old manual focus lenses from the old days of film and they all focus this way. Like I said above many newer lenses optimized for different things have internal elements moving with respect to one another which changes the optical "structure" of the lens. This can change true focal length.
As long as all elements move in unison, it's impossible to change the focal length. It's physics. The focal length is what it is. If you see any change in scale it's probably because your subject is so close that you focusing in and out is changing the distance to subject which would of course have a "zoom" effect. The experiment would be more meaningful if your subject was at infinity. And even then it's difficult because the more out of focus something is, the larger it appears, and therefore looks zoomed.
If focusing is accomplished solely by extension then the answer is "no".
While the focal length doesn't change, the angle of view will change; it will get narrower as you focus closer.
Focal length is only one factor in calculating angle of view. The primary factors are the image size (film or sensor) and the distance the lens is from the image (measured from the lens' exit pupil to the sensor). This distance from the sensor/film is equal to the focal length plus any extension from the infinity position that is the result of focusing the lens on a closer object.
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