Originally posted by dh4412 so, if i understand, a macro such as the 100 mm's have a better image quality for close up or macro, and are designed for that, than at longer distance , farther away subjects ? and macro lenses in general are to be manually focused for macro shots? and depth of field is such at 2.8 that most people will use f 4 or something around that, for macro?
Well, yes, but... Some macro lenses do well at distance, but many are best at close range. Yes definitely, at macro ranges we need to focus manually -- AF just isn't precise enough. And yes but, while we need to stop-down (and maybe do some focus-stacking) to get macro shots with any decent DOF, some pervs DO shoot wide-open macros, just to exploit the very thin DOF artistically.
What makes a macro lens? 1) Design to focus very close, and 2) edge-to-edge flatfield sharpness. Enlarger lenses (ELs) have these qualities too, which is why I like using ELs on bellows and/or tubes. Some macro and enlarger lenses just don't do well at distance. Some, like my Macro-Takumar 50/4, are built so that focusing beyond 2m is difficult. The lens reviews will tell you which commercial macro lenses are weak afar.
Edge-to-edge flatfield sharpness is important if we're shooting flat stuff, maybe a bit less critical for centered 3D subjects. But if we want maximum sharpness throughout the frame, we need to 1) focus manually and carefully, and 2) stop-down for thick DOF. "Focus carefully" may include using focus rails: we slowly move the subject to the precise necessary spot. And for much stopped-down work, we need flash.
And flash is why we may need AF macro lenses. Pentax P-TTL flash only works with A-type and AF lenses. A-type manual-focus macro lenses aren't real common; AF macro lenses abound. Yes, non-P-TTL flash *can* be used with lenses lacking auto-aperture contacts, but it's trickier, with lots of chimping and adjusting. Some here are masters at that. And they put much time and effort into their brilliant results.
Hope this helps!