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http://thephotographerwithin.com/2015/05/back-button-focus/
What are the advantages of BBF?
Faster shooting with static subjects: you can lock focus with the AF-ON button once then let go of the button for subjects that don’t move. Great for landscapes, macro, even sleepy newborns. No need to refocus every time you press the shutter.
Easier to focus and recompose: you can lock focus using your center focal point with the AF-ON button then let go of the button and recompose your image very easily without having to re-do your focus each and every time you press the shutter. Useful for Canon cameras with limited focal points or if your initial composition isn’t optimal.
Better focus for moving subjects: you can shoot continuously with the AF-ON button depressed and your focus will track your moving subject.
Override and fine-tune autofocus: you can ensure tack sharp images when using Live View and zooming your focal point in to 100%, then locking in that focus by using the AF-ON button. You can even adjust your focus manually by turning your focus ring on the lens without turning the switch to MF on the front of the camera (if required).
And finally, and this is the HUGEST advantage I’ve found, you never, ever need to change your auto focus mode again. Setting it to AF-C/AI Servo AF and using BBF allows you to access all of the focus modes with this one button. BBF eliminates the need for switching focus modes between AF-S/One Shot, AF-C/AI Servo, and AF-Auto. BBF makes AF-C/AI Servo AF usable for both moving and stationary subjects. Wow, right?!
The average interocular of humans is considered to be about 65mm (2.5 inches.) When this same distance is used as the interaxial distance between two shooting cameras then the resulting stereoscopic effect is typically known as “Ortho-stereo.” Many stereographers choose 2.5” as a stereo-base for this reason.
If the interaxial distance used to shoot is smaller than 2.5 inches then you are shooting “Hypo-stereo.” This technique is common for theatrically released films to accommodate the effects of the big screen. It is also used for macro stereoscopic photography.
Hyper-stereo refers to interaxial distances greater than 2.5 inches. As I mentioned earlier the greater the interaxial separation, the greater the depth effect. An elephant can perceive much more depth than a human, and a human can perceive more depth than a mouse.
However, using this same analogy, the mouse can get close and peer inside the petals of a flower with very good depth perception, and the human will just go “cross-eyed.” Therefore decreasing the interaxial separation between two cameras to 1” or less will allow you to shoot amazing macro stereo-photos and separating the cameras to several feet apart will allow great depth on mountain ranges, city skylines and other vistas.
The trouble with using hyper-stereo is that scenes with gigantic objects in real-life may appear as small models. This phenomenon is known as dwarfism and we perceive it this way because the exaggerated separation between the taking lenses allows us to see around big objects much more that we do in the real world. Our brain interprets this as meaning the object must be small.
The opposite happens with hypo-stereo, where normal sized objects appear gigantic. (Gigantism.)
http://dashwood3d.com/blog/beginners-guide-to-shooting-stereoscopic-3d/index.html
http://3d-con.com/2014/files/NSA2014-MACRO1.pdf
http://nzphoto.tripod.com/stereo/macrostereo/macro3dwindows.htm