though i am not the biggest fan of a focus tab on my lenses (why not focus tab AND knurled focus ring?) i don't really understand why leica only does it for shorter focal lengths. is it due to the longer throw of longer lenses?

-- stefan randlkofer (geesbert@yahoo.com), February 24, 2002

Answers

I'm sure that's part of the answer. But for another thing, with lenses as small as some wide-angles are, it would be difficult to provide a large enough focusing ring. I think the tab helps keep the lens compact.

-- Bob Fleischman (RFXMAIL@prodigy.net), February 24, 2002.

Bob and Stefan have bits of the puzzle. Here's the rest.

1) Tabs make for fast-but-sloppy focusing. Rings make for slow-but- accurate focusing.

2) Wide-angles will work with fast-but-sloppy. Longer lenses need slow- but-accurate.

3) Leica has never made up its mind which works better for 50s - they've made them with and without tabs for 35 years.

4) In additon to compactness, accuracy, and 'focus throw' - tabs also require low-mass (read small, wide-angle) lenses. Imagine trying to focus a Noct or a 75 'lux with just the side of one finger - people already complain they're too stiff using all five fingers!

Since this is likely to turn into another tabs=good/tabs=bad slug fest anyway, I may as well save myself a second response:

....I love tab focusing for 50mm and below - but I understand why I can't have it with the 90s - they're tough enough to get focussed right as it is.

-- Andy Piper (apidens@denver.infi.net), February 24, 2002.

my experience is the same as Andy's above. The tabs let me focus faster, but not as accurately as with the barrel. My shots with the current 50mm lens wide open in the 3 to 10 feet range are noticeably and consistantly sharper than similar ones I took with the tabbed lens. I don't have any trouble focusing The 40mm Rokkor I have with a tab, however, and the lens is so short, it would be difficult to find the focus ring if it had one. People do put zip ties on the barrels of the 90's as a non permanent tab, and someone who posts here once stuck up a photo of their "tabbed" 90mm.

-- Andrew Schank (aschank@flash.net), February 25, 2002.

i think leica designs the focusing tab/no tab and rotational resistance based on focal length; at least i find it true for the range of lenses i have (21-135). the 21 SA w/tab has the least resistance, and it gets progressively harder going to the 35 then 50, etc. i think it's because once focused, if one "bumps" the focus barrel, the change in focus/OOF is larger the longer the lens is:

for a 21, when focused at 10 feet and bumped slightly will defocus +/- only inches, where a 90 it may be +/- feet. so, making the focus resistance higher the focus becomes more "locked".

-- chris chen (chrischen@msn.com), February 25, 2002.