In article <5034eo$s2i@itnews.sc.intel.com>, Sze_Boon_Ho@ccm.ipn.intel.com says...

>I am interested to know (for example) given two types of lenses
> 1) 28-70 F2.8 and
> 2) 28-105 f3.5,4.5 ,
>if I set both lens to the same conditions , for instance, to focal length of
>50mm at f6.7. Should I get the same depth of field ?
>In my case (I have both the above lenses), I seem to get much shorted depth >of field from Tokina lens 28-70 f2.8 when compared to the other 28-105 lens. >Is this possible ? If this is possible, can we tell generally that lens which >haslonger depth of field is a better lens that the other (or the opposite ) ?

Since the second lens is variable-aperture, you would need to make the apertures identical before proceeding to compare DOF (setting the 28-105
at 50mm should give you roughly f4 wide-open). Set the (constant-aperture) zoom at 50mm and f4. Compare (and match) angles of view in the finder at
the same focus distance (most zooms change FL with focus, and the marked FL's may not be correct). Compare DOF in the finder with the DOF preview activated, or on film (I suspect that this is what you didn't do - the
lenses must be stopped down to the selected aperture before comparing
them for DOF). You may still not get the identical DOF with both lenses,
even with conditions equalized (though it should be very close - too close
to see a difference in the viewfinder), since a sharper lens will appear
to have slightly less DOF than a softer one (DOF is a relative-sharpness
condition, and if there is a very sharp reference image at the correct
focus, the relatively less sharp image within the DOF will show as less
sharp, decreasing the apparent DOF range).
Hope This Helps