Re: [Leica] ... 28-70 zoom...

[Andre Jean Quintal <megamax@abacom.com>]

At 09:39 -0400 28/08/99, claire wrote:
. . .
>I focus on a subject 3 metres away at 28mm focal length, then I zoom out to
>70mm without re-focussing.
>The focus looks off !

	--> you must remember that the depth of field
	of longer length focals (say at 70mm) is much shallower
	for the same aperture than for a short focal (here, 28mm).

	--> notice also that the (probably) main subject
	you are locking on will be far more detailed in your
	viewfinder at a long focal, any out of focus zone
	then becomes far more obvious.

	--> what seems "pretty good" focus at 28mm
	may actually be in an hyperfocal distance
	that also shrinks as you zoom in, thus bringing your
	subject out of the accurate focus zone as you
	recompose and zoom-in.
	Since you don't have hyperfocal focus to help
	when you shoot a subject at 1 metre, the same phenomenon
	does not then occur, as you are forced to procure
	accurate focus for optimum results, both at
	28 and 70mm that close to the subject, as it then
	sits well "in front of" the hyperfocal interval,
	regardless to focal length.

	--> makes a good point for those clamoring for
	FOCUS CONFIRMATION (or prediction)
	on future Leica-R bodies, as a useful benefit.

	--> I tend to check my focus zone in many situations
	at top focal before I get on with shooting so-so photographs,
	knowing that if the subject is in focus at 70mm,
	it will be at 28mm. It's a matter of habit.

	In hope this helps,

	Andre Jean Quintal