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Real vs Actual Focal Length — Breathing

A lens can change its focal length as it is focused. This “breathing” is of concern especially for video, because it alters desired relationships, and changes the expected working distance as well as depth of field. It’s also a practical matter for still photography, because it affects working distance.

Relative focal length at close range

I compared the 70-200mm f/2.8L IS II to the 200mm f/2L IS by first focusing the 200/2L as close as it would go, then swapping lenses. This yields a relative focal length.

Assuming the 200/2L as 200.0mm, then the 70-200/2.8L IS II is 191mm, or 5% shorter focal length. Note that at distant focus, the difference is 0.5%, so that indicates a shortening of focal length at close range.

The 5% reduction is far better than the Nikon 70-200mm f/2.8G VR II.

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70-200mm f/2.8L IS II focused at closest focusing distance

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