Originally posted by Lowell Goudge the aperture is actually continuously varaible, and not changing in steps. the steps reported only correspond to steps in the 1/2 f stop indexing used by pentax in thier mount. I would be willing to bet, that the exposure actually changes within the constant F stop range and is really only accurate at the middle focal length within the range. In this way the 1/2 stop steps become +/- 1/4 stop deviations from correct (i.e. perfect) exposure.
You're correct about aperture being constantly variable. I did a test on this awhile ago. I set the zoom ring as far as it would go and still report 190mm and f4.5. Then I nudged it a tiny amount so that it read 210mm and F5.6. The shutter speed and ISO didn't change, so clearly the aperture didn't step from 4.5 to 5.6.
My response above about the camera readings being stepped was to explain why people often say "the 55-300 is f4.5 below 210mm" or "the 55-300 is f4.5 @ 200mm". It's because that's what the camera shows.
Quote: Also note that on many zooms (but more dominant on super zooms) focal length changes with focus distance. Focal length is only accurate at infinity.
I also tested this. The 55-300 is close to the same FOV as my 100mm prime at about 100mm, and is slightly wider than my FA 50mm at minimum zoom (55mm). Note that even two primes with the same FL show FOV variances, so I don't think that FL is off with this particular zoom. I suspect that the lenses which show large variances between infinity and close focus, or show a noticeably different FL compared with FOV are internal focus designs. The 18-250 is IF and shows wild swings in FOV when going from infinity to close focus at any given FL. The 55-300 does not.