On Wed, Apr 18, 2001 at 10:04:30AM +0300, Severi Salminen wrote:
> The magnification of Canon's EF 2.8/100mm macro at 31cm (the closest focus
> distance) is 0.48, or 1:2.1
>
> The magnification of the 28-90mm USM "Kit" at 38 cm is 0.31 or 1:3.2.
>
> - these numbers are from Canon brochure and calculated by f/Calc for PalmOS.
> So what is the point I'm not seeing? Canon macro is supposed to give
> 1:1 (I don't know the english word - ratio?). How can that be
> possible if it can focus no closer than 31 cm?
The formulae you are using are inaccurate, or rather make some
simplified assumptions about lens construction that don't hold
for macro lenses (or indeed most modern lenses, although with
most non-macro lenses they're probably close enough).
The 100/2.8 USM macro does indeed give 1:1 ratio at 31cm as measured
from the film plane. The usual formulas for magnification assume a
lens construction where the lens' front "principal point" is at the
middle of the lens, which is only accurate for very simple lenses, and
way off for some like most macro lenses (and for example fisheyes).
Note also that the lens focal length changes depending on the distance.
Take a look at http://www.photo.net/photo/optics/lensFAQ
for more details about the optical formulae.
--
Tapani Tarvainen
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
