The 70mm and 100mm mark I'm referring too are just small marks with lines leading to the distance scale on the lens, so you'll know where you are between 70-200, but it's a rough guide since they are very close together.
[a href=\"index.php?act=findpost&pid=188158\"][{POST_SNAPBACK}][/a]
I believe the marks (in red) indicate the focus offset for infra-red film and are not relevant for normal focussing.
The problem with these zooms and the distance scales is that they are incredibly course (as already mentioned), are logarithmic and only relate to one focal length. Not forgetting the old curved focus field which just compounds the problem further.
e.g. set your 24-70 to 24mm and the focus to between 3m and infinity. What distance have you set the lens to? Do the same but at 70mm. Same problem - it's any ones guess what the actual focus distance is.
Because of this a DOF chart is pretty useless at getting consistent results.
What you can do though is do some tests and work out where on the scale will give you acceptable results at each focal length and aperture (and across the frame from centre to edge).
e.g on my 24-70 at f11 and 1Ds2 the following settings give me minimum focus distance while just maintaining acceptable sharpness at infinity:
24mm 10 feet
28mm half way between 5 + 10 feet
35mm 10 feet
50mm third way between 10 feet + infinity
Sounds screwy but it gives consistent results with near far compositions and gives a base reference point to avoid messing the focus up and missing shots.
