Funny, what was that in above about Mamiya three focus points??? Another Google search by Fred??? I had AFDIII and DSLR in past. DSLR has many focus points, sure... care to tell me which one focus on that eye???
Not really interested in responding to Anders's provocation, but I'll respond to the questions with all those question marks;) in the interest of other readers
First of all I do not need to do a google search to know that the DF only has 3 focus points. I owned both the Phase One DF645 and the Phase One AF645.
Both have the same focus point layout. No indication is given as to which focus point locks focus, but you can manually choose the one you want to use.
Regarding 35mm DSLR cameras when you have 64 or 104 focus points you have a much higher chance of a focus point being on the eyes or face.
Like here where i was shooting for 5x4 aspect ratio.
There are times where the eye will not fall behind a focus point, but in that case there will be one close by. Thanks to it being close by there will be less focus error
when recomposing. Also if the photographer is composing with the face quite far away from the center of the frame there is the option of using live view focus.
With live view focus the focusing point can be moved to any point on the screen and quite quickly with the joy stick. Some cameras even have face detection or feature detection.
With the D800 feature detection AF works like this.
First in non live view mode:
You set the AF mode to 3D focus. In this setup the focusing system uses a single focus point
as a target. Start by pointing the chosen focus point closest to where you will be placing the feature to focus on. Then if you change the composition
the system will choose the cameras focusing point that is closest to the remembered feature. The area where focus adjustments are made is limited to the
total area of the focus points. When you go beyond that area the focus stops at where the last available focus point left off.
Second in live view mode:
In live view mode there are two tracking options. Face detect and feature tracking. Both work in a similar way, but face detection is
based on automatic face detection (camera "knows" what faces look like) and feature detection requires a snapshot (needs to be told what to look for).
With face detection it looks for for a face in the frame and targets it for focus and will follow the face around the frame doing it's best to keep it in focus.
It is actually really quite fast, but not as fast as phase detection.
With feature detection you switch to target focus. You get a square to start with and press the center button of the multi function control on either the
body or vertical grip. This tells the camera what to look for and the camera will follow this feature around the frame and even if the photographer or subject move forwards or backwards. The feature can change size and still be followed accurately.
Here is a video that shows the live view focus tracking.http://youtu.be/JzIxNOBPaaM
Here are a few quick tests to show how well the different focus options on the d800 work in situations where with a single
focus point you would have to recompose.
The first one is regular viewfinder focusing, but instead of using the center most focusing point I used the one closest
to the top right corner and then recomposed putting the subject right up in the right hand corner. Focus is on the ledge
under the numbers. 100% crop
The following shot is using live view focusing using using a manual set focusing point with the joystick/multifuction button
with the subject in the top right hand corner. 100% crop
The following was shot using live view target mode. First the target is set using the multi function button.
Then I recomposed and the live view focusing tracks the subject. I moved the subject around the frame
and had the tracking follow. I then chose the composition with the subject in the top right corner of the frame.
Stopped for a fraction of a second and shot the frame. 100% crop
The following photo was taken with the face recognition live view method. In doing this test I moved around so as to somewhat simulate a model
making changes to a pose. I even did some figure of 8s with the camera to challenge the live view focusing. I also move forward an backwards.
The final composition was again with the subject in the top right of the frame. What is very nice about the facial recognition focusing is that
the size of the focusing box scales continuously with size of the face hence producing accurate focus. 50% crop to keep some text in the shot.
One other really neat feature of face recognition with the D800 is face review after shooting.
Regardless of what focusing method was used when reviewing a photo you can quickly look at a closeup
of each face. This is really handy if you are shooting groups on location.
It is also really useful if you are shooting two models in one shot with a tilt shift lens wide open and one model close
and one model furthur away. Like in this shot:
Shot on film, just using it to describe the setup.
Top is without tilt bottom is with tilt.
Here is how after the fact face recognition works when reviewing images.http://youtu.be/yNajUFMpISs
This is also useful when reviewing a fashion accessory shot. I did a shot where I wanted an accessory to be in focus as well as the face, but shoot wide
open for shallow depth of field and an 85mm tilt shift to focus face and purse. I would review the shot zooming in on the purse and then with no hunting around just one click of the front dial. It would find and zoom in on the face. This also works on HDMI output. Very handy for quick review on portable HDMI monitors.
In conclusion. While there are some gradual improvements in AF and focus review with medium format cameras many issues remain to be addressed.
This is particularly relevant if one keeps in mind that one of the repeated marketing points of Medium Format is shallow depth of field.
Hasselblad with true focus has at leased addressed the recompose issue with shorter focal lengths.