focal length defined?

Is there a standard way to how focal length is defined for all lenses?

The reason why I ask is that I've heard that some of the new super zooms are only at their stated focal length when focused to infinity and at focusing distances less than infinity, the lens could appear to be much shorter than it really is.

Second, does this mean all lens manufacturers measure it this way, or is the canon 70-200L lens be made differently and look like a 200mm focal length lens at all focusing distances?

Yes, there is a standard way that focal length is defined for all lenses.

The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus".

The focal length of the lens does not change when the focus distance is less than infinity. The focal length is the same, however if your sensor is not at the focal point then your image will be blurry.

No lens will appear to be shorter than it really is. I think you may have heard wrong, or someone didn't really understand what the focal length is.

sorry, should have referred everyone to this

http://www.dcresource.​com/forums/showthread.​php?t=35235

dontblink wrote in post #6024101
Yes, there is a standard way that focal length is defined for all lenses.

The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the subject (at infinity) is "in focus".

The focal length of the lens does not change when the focus distance is less than infinity. The focal length is the same, however if your sensor is not at the focal point then your image will be blurry.

No lens will appear to be shorter than it really is. I think you may have heard wrong, or someone didn't really understand what the focal length is.

At infinity, under normal circumstances and if the manufacturere have not exaggerated the focal length too much, you are right.
Unfortunately, with modern lens designs and focusing at distances closer than infinity, this is not correct anymore. Many zoom lenses tend to shorten their FLs when getting closer than infinity, although not shorter than their stated shortest FL, and the same is true for many IF lenses.

IOW, FLs may get shorter when focusing from infinity to nearby, at least with some lenses. A prime example is the 100 F/2.8 Macro (pun intended ), which goes from 100 mm to about 70 mm at 1:1. Essentially, IF works a little like a zooming mechanism, and it is not always possible to control it in such a way that FL stays the same, especially not with a long focusing range.

The same is true with a long zoom range, and therefore with zooms it is especially the superzooms which suffer from this fate. The more complex the design, the more difficult it becomes to control this. Off hand, I can't remember any specific examples, but then, I don't own any superzooms .

Kind regards, Wim

rix2357 wrote in post #6025217
sorry, should have referred everyone to this

http://www.dcresource.​com/forums/showthread.​php?t=35235

Ah yes, the Sigma 18-200 shows the point I make a little higher up.

Also note tha tit si common use to allow for a 5% to 10 5 discrepancy in reall FL. IOW, a lens with an FL of 180 mm at infinity may well be marked as a 200 mm, especially if it si a zoom lens.

Kind regards, Wim