Bill Pierce
Well-known
If you use a small, non reflex camera with Live View focusing, how do you focus?
Phase detect autofocus, normally associated with big DSLR’s, is FAST. (But, because, it is checking the focus on, not the sensor that records the image, but a separate sensor, it can be inaccurate. The problem is solved when cameras bodies using phase detect allow you to set up autofocus corrections for each individual lens.)
Because focus in Live View in the little cameras is read off the imaging sensor, it can be quite accurate. The problem is that it’s SLOW. Here’s a quote from Wikipedia.
“In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. Furthermore, as the autofocus system does not calculate whether the subject is in front focus or back focus, focus tracking is not feasible. As it does not use a separate sensor, however, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate.”
In other words, phase is fast and live is accurate. (Fuji, Ricoh, Nikon, Canon and probably some others I don’t know of have worked out systems in more recent cameras where, more or less, phase rough focuses and live fine focuses.) If you use a little camera with only Live View focusing how do you deal with the problem of slow focus? As an old rangefinder photographer used to manual focus, my answer is simple. I prefocus. It doesn’t matter if its manual, phase or live view. Focusing, along with setting exposure, are something I do before I start shooting. Obviously, in a few situations, this can be a disaster. But in most situations it means less time between seeing the moment and recording the moment.
Again, when you use a camera with Live View focusing, how do you deal with the problem of slow focus (unless you do landscapes or still lifes)?
Phase detect autofocus, normally associated with big DSLR’s, is FAST. (But, because, it is checking the focus on, not the sensor that records the image, but a separate sensor, it can be inaccurate. The problem is solved when cameras bodies using phase detect allow you to set up autofocus corrections for each individual lens.)
Because focus in Live View in the little cameras is read off the imaging sensor, it can be quite accurate. The problem is that it’s SLOW. Here’s a quote from Wikipedia.
“In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. Furthermore, as the autofocus system does not calculate whether the subject is in front focus or back focus, focus tracking is not feasible. As it does not use a separate sensor, however, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate.”
In other words, phase is fast and live is accurate. (Fuji, Ricoh, Nikon, Canon and probably some others I don’t know of have worked out systems in more recent cameras where, more or less, phase rough focuses and live fine focuses.) If you use a little camera with only Live View focusing how do you deal with the problem of slow focus? As an old rangefinder photographer used to manual focus, my answer is simple. I prefocus. It doesn’t matter if its manual, phase or live view. Focusing, along with setting exposure, are something I do before I start shooting. Obviously, in a few situations, this can be a disaster. But in most situations it means less time between seeing the moment and recording the moment.
Again, when you use a camera with Live View focusing, how do you deal with the problem of slow focus (unless you do landscapes or still lifes)?
Bill Pierce
Well-known
Here’s an interesting article I came across today that deals with the advantages of manual focusing, prefocusing, whatever you want to call separating the process of focusing and triggering the shutter. It is written from the point of view of a Fuji X Pro user but the principles apply to many cameras.
http://paulsamolukphotography.com/fuji-x-pro-1-back-button-focus/
http://paulsamolukphotography.com/fuji-x-pro-1-back-button-focus/
rjschell
Established
This raises a question for me. How does f/stop affect the accuracy of focus?
Bill Pierce
Well-known
Phase detect sensors in a sense are much like the old beam splitter pseudo rangefinders found in SLR’s, one with a base within the lens’s diameter. Thus they may black out with slow lenses or not take advantage of fast lenses. Some phase detection systems incorporate a central sensor, that while it blacks out with slower lenses, takes advantage of faster lenses. Phase detect does not take advantage of lenses faster than the f/stop set for that central sensor, usually 2.8.
Contrast detection, like its name implies, searches for highest contrast in the image within the target area. This coincides with the image being in focus. In some cameras contrast detection is done at the taking aperture; so, a fast lens doesn’t provide more accurate focus. Some cameras work with the lens wide open.
Godfrey
somewhat colored
My 'live view' system cameras at present are a Ricoh GXR with A12 Camera Unit and an Olympus E-PL1. The GXR does not have any AF, it take's Leica M-mount lenses. The Olympus does, but I use it mostly with adapted manual lenses anyway.
Focusing is as instantaneous as my skill in seeing the sharpness on the screen or in the EVF. The Oly has image magnification to help. The GXR has image magnification as well as focus peaking. Most of the time, I need neither ... it depends on the lens.
Both are also quite good at ISO 800, which lets me stop down and focus by zone with shorter focal length lenses, at which point focusing is faster than instantaneous ... you don't have to focus at all, other than once at the beginning.
Auto focus only appeals to me when I have time for it. Most of the time I don't.
G
Focusing is as instantaneous as my skill in seeing the sharpness on the screen or in the EVF. The Oly has image magnification to help. The GXR has image magnification as well as focus peaking. Most of the time, I need neither ... it depends on the lens.
Both are also quite good at ISO 800, which lets me stop down and focus by zone with shorter focal length lenses, at which point focusing is faster than instantaneous ... you don't have to focus at all, other than once at the beginning.
Auto focus only appeals to me when I have time for it. Most of the time I don't.
G