Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

Mirrorless cameras always use the sensor's image, transmitted electronically, to the rear screen or an electronic viewfinder. Focus is usually done by evaluating contrast, and it looks at the area covered by your chosen focus point for contrast, and tweaks the focus fore and aft until perfect focus is achieved. It is slower, but seriously more accurate.

In similar fashion, using live view on a DSLR does the same thing. A DSLR also has an optical viewfinder, which works with a mirror that has two semitransparent areas at each side that let some of the light through to a secondary set of mirrors that reflect light to an array of focus sensors. It uses a phase shift between left and right sensors to arrive at focus. With all of the complexity of the light path and mechanical tolerances, dead accurate focus is difficult to achieve 100% of the time, though it does come close enough to present sharp, crisp images.

Also, using live view in a DSLR or a mirrorless camera, the actual shooting aperture is used for determining focus. Because of all of the light losses in the optical path of an optical viewfinder DSLR system, most cameras are precluded from using lenses that are slower than F5.6 or F8 for a maximum aperture if you want to use the phase detect AF system. But in live view, or in a mirrorless camera, you can manually stop your lens down to F22, which will be the actual shooting aperture, and still be able to focus just fine.

When using the optical viewfinder at less than maximum aperture, you are focusing wide open, then stopping down to the shooting aperture. Nearly all lenses on the market today exhibit some degree of focus shift when stopping down - some worse than others. If your lens is relatively slow, like F4 - F6.3 for its max aperture, it is less of an issue, and the depth of field at the smaller shooting aperture is usually deep enough to have nicely focused images. But if you are using an F1.2 or F1.4 lens, the likelihood of focus shift affecting the ultimate sharpness is something to consider.

Using live view on a DSLR usually means slower, but more accurate focusing, and you can pretty much put the focus target anywhere in the image. Some mirrorless cameras, like Nikon, Canon, Fuji, Sony and Olympus - have hybrid focusing systems, which use phase detect AND contrast detect hybrid sensors - providing extremely fast focus acquisition AND dead on accuracy at shooting apertures.

Sony in particular seems to have the lead with the best AF hybrid system on the market today - equaling or exceeding the AF performance of the top DSLRs. Olympus is catching up but not quite there yet. Every new generation of cameras bring innovation and improvement - and AF performance on mirrorless cameras has seen substantial improvements.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

I just tried to focus with a D800 and my 70-200 lens in live view - it too will focus when stopped down to F22, even though it's only rated for F8 with a handful of cross-type focus sensors when using PDAF.

Timmers Loc: San Antonio Texas.

Great information regards the focus systems.

There is a non technical side to the issue. It goes some what like this:
When shooting any SLR camera in which you are 'seeing' through the view finder you are momentarily 'blind' at that moment of making the exposure (mirror up). Some find this a problem, some don't. With ranger finder cameras in the old days when you tripped the shutter you still saw the scene and what was occurring during the exposure.

With the Mirrorless camera the camera can be at your eye or as used as viewing with the camera before you but not at eye level you are seeing the moment, but more importantly you are in the action but holding the camera much like a cell phone device or with the camera anchored by tri pod. It may be true that the Mirrorless is 'blind' at the exposure but the manner of the photographer's action of holding the camera creates a more integrated and homogenies contact with the subject.

Appears rather 'not important' on the surface but as one thinks about this idea of a manner of making photographs one can see how this is different than the traditional camera planted at the photographer's face. There is a concept that does with this it is called 'Being Digital'. There is a published book about 2000 that came out by that same name Being Digital. It goes into issues like tis, the psychology of it and how being digital alters our perceptions and how we see and construct our idea of the world.

bleirer

So thinking about it, I guess there are pros and cons to the depth of field options. Having it on always vs. Being able to turn it off with a menu vs.holding down a preview button. I would think any camera would focus faster wide open, for one. I wish full time dof preview were an option on my RP, but I have the movie button programmed to dof preview and that works ok. I have to focus at a given aperture then preview then change the aperture then preview again to see the effects on the out of focus area, but on the other hand I can focus wide open the rest of the time. If I read it right, with Sony you could turn off continuous and assign a button if desired.