Distance scales on lenses that *aren't* parfocal

[mmodica]

Hi all,





So I know my 28-135mm lens definitely isn't parfocal, but that begs the question - how can the distance scale be accurate if it changes its mind when I change my focal length and refocus?





I see there are the infrared marks on the side but I'm pretty sure they're only for when shooting on IR film (and I'm shooting on digital now [:P])





Thanks for the input!

[Dr Croubie]

But the distance scale isn

[mmodica]

It is pretty close yeah, seems to be only for really close subjects that it tends to be more off. But also for some reason the distance scale on the 28-135mm IS is just labeled "Macro" for anything closer than like three feet, although I notice that its also like that on the 15-85mm IS.


Speaking of which, is it possible to view the focusing distance information from somewhere other than the distance scale? I don

[Dr Croubie]

Just had a check of my photos, and I can

[neuroanatomist]

Speaking of which, is it possible to view the focusing distance information from somewhere other than the distance scale? I don't see it stored anywhere in the EXIF data but it must pass it onto the camera for the flash.

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It seems that one specific distance is not recorded with the EXIF data - there are two values, Focus Distance Upper and Focus Distance Lower. I can see those with a Terminal (command line in Windows-speak) tool on my Mac (called ExifTool). It looks like Lightroom does some sort of calculation on those values to estimate a subject distance, or just uses one of the values, and so does the picture info viewer in Windows 7.


Looking at some of my own shots, the actual focus distance falls in between the two of them, a little closer to the Lower value (I save my LensAlign Pro AF microadjustment shots - and for those, I actually set a specific focal plane to test target distance with a tape measure).


It seems that DPP uses these data for lens aberration correction - in the Tune dialog box of the aberration correction section, there's a 'shooting distance information' slider. However, that's useless as the only marking on the slider is&infin; at the right side; there are no actual numbers there.


Furthermore, when you use DPP for RAW conversion, it actually strips theFocus Distance Upper and Focus Distance Lower fields from the EXIF data! They are there in the RAW file, but not in the resulting JPG.



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[scalesusa]

USM lenses have a slip clutch that lets the focus ring override the AF motor. This makes the focus scales totally worthless unless you first set the focus to infinity or minimum each time. Don