Originally posted by vonBaloney But it is mainly a phenomenon with internal focusing lenses? (At least to such a large degree as I am seeing on this 18-250.) Are IF lenses a relatively new innovation or have they been around forever?
to my limited knowledge, IF lenses began at around 1976, and with pentax, the SMC 28 F2.0 which had a Fixed Rear Element. At that time, it was noted that there was a slight decrease in the focal length as the lens was focused in from infinity.
The issue is much more apparent in high ration zooms compared to any other type of internal focused lens. Unfortunately, while the focal lengths are measured only at infinity, there is no reporting of the focal length consistancy etc, as a function of focus distance reported.
While it is understood it happens, and has even been covered in photo magazines, it would be nice for the consumer to know the full performance of their lens. In this respect, I am disappointed by the photo press, who seem to concentrate less on some of the physical atributes of a lens relitive to specification.