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Originally Posted by pascal_meheut
This is just a screen problem. AF screens are designed to support low-speed lenses. Leica could go to AF and give us the choice between AF and MF optimized screens. Some people use screen with manual focus help on Nikon and Canons and are ok with them
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"OK" manual focus isn't good enough, and the manual-focus aids in C and N viewscreens are useless at small apertures (typically smaller than f/5.6).
Quick and accurate is better than "OK", and it's not simply a screen problem. The entire viewing system is involved including and most importantly the semi-silvered reflex mirror. An AF camera's mirror allows more light to pass through because the AF system needs it, and the same would be true of a focus-confirmation system. The screen and pentaprism are designed around the reduced light reflective properties of the mirror to deliver a bright screen image at the expense of image contrast, when it's contrast that our eyes are most sensetive to when focussing. When AF fails (as Canon explains, below) the AF camera is left with a viewing system that is sub-optimal for manual focus.
I want to be able to focus quickly, accurately, and simultaneously anywhere and everywhere on the viewscreen. My subjects are very active, even when they appear to be sitting still. They might yawn, turn their head, shift weight from one foot to the other all in fractions of a second, each time shifting the plane of best focus. The eye could be anywhere, at any time so I cannot pick a particular focus point to watch. For example, are there any focus points at this pheasant's eye? Recall that this is exactly as I saw it in the viewfinder, no cropping:
(note: the software is substituting the web page titles for the URL, click on each to see the photo)
photographs of birds by Douglas Herr
In addition, I often watch multiple points or regions for optimum focus, in this case, eye, back, and tail:
photographs of birds by Douglas Herr
and with this bird, near eye, beak, 'ear' tufts, shoulder:
photographs of birds by Douglas Herr
beak, eye, neck:
photographs of birds by Douglas Herr
eye, chest, shoulder, fruit, toes:
photographs of birds by Douglas Herr
eye, beak, neck, wing, particularly as it twists, dips and flicks feathers while preening:
photographs of birds by Douglas Herr
photographs of birds by Douglas Herr
from tip of beak to tip of tail:
photographs of birds by Douglas Herr
beak, eye, neck, back:
photographs of birds by Douglas Herr
eye, nose, paw, and as much of the seed pod as possible:
photographs of mammals by Douglas Herr
eye, nose, beak and shoulder in lower right corner:
photographs of birds by Douglas Herr
eye, beak, neck, back as much of wings and tail as possible:
photographs of birds by Douglas Herr
These areas aren't all in focus by accident; I'm watching all of these points simultaneously and shifting my position or waiting until the bird lines itself up with my focus plane. When everything comes together my shutter finger has to respond immediately; the fraction of a second used shifting the view for optimum composition is enough time for all sorts of stuff to shift out of the focus plane, and to watch this many regions simultaneously I'd rather use a viewscreen that shows me clearly and unambiguously where stuff is in or out of focus, all over the viewscreen all at once. Let me be clear about this: these areas aren't all in focus by accident.
Perhaps my needs are more specialized than the majority of camera users' needs, but there is no AF camera yet made that has focus points covering the entire viewscreen nor is there an AF camera yet made that can have enough simultaneous active focus points nor is there an AF camera yet made that includes the optimizing algorithm in my brain that can determine which of several active focus points are the most critical ones, and an AF camera in manual mode with its compromised viewscreen is not optimum for the kind of photos I want to make.
Canon's white paper at
http://photoworkshop.com/canon/EOS_Digital.pdf explains using autofocus correctly. In particular see pages 11 and 29:
"For optimum focusing performance with close subjects, we recommend
avoiding the FLR (Focus-Lock-Recompose) technique. Instead, use an off center focusing point or focus manually."
and later in the white paper:
"The closer the subject, the more critical accurate focusing becomes.
One potential problem to avoid is camera or subject movement after
focus lock. This can happen more easily than one might think... Even
slight camera movement or subject movement after focus lock can result
in soft images, particularly when shooting at wide apertures with
narrow depth of field."