Gene51 wrote:
How does that work for you when you are at a soccer match, a motocross event, shooting birds, etc.?
Those are all situations where you would be much better served using autofocus anyway (and hopefully a lens with a high performance AF system). I shot sports & wildlife with manual focus cameras and lenses for many years. I was darned good at it, too. But modern AF with the right camera and lens combo is faster and more accurate than I EVER was doing it manually!
While you CAN manually focus with all Canon DSLRs... should you?
It depends upon the lens.
USM or ultrasonic focus drive lenses have "Full Time Manual" focus. You can override the AF manually any time without concern of damaging the lens.
STM or stepper motor focus drive lenses also can be overridden manually without concern, but are "fly by wire" lenses that must be powered up even to manually focus them. (A few USM lenses are also "fly by wire".)
Micro motor focus drive lenses SHOULD NOT be manually overridden without first turning them off at the switch. Canon lenses that use a micro motor are any that aren't marked either USM or STM. A lot of third party lenses are micro motor too, consult the manufacturer regarding proper usage. (Tokina, for example, use a "focus clutch" mechanism that makes it impossible to manual focus their lenses without first taking them out of autofocus mode.)
Some Canon (and other) lenses are strictly manual focus. For example, all Canon TS-E "Tilt Shift" and the Canon MP-E 65mm macro lens are manual focus only. The type of photography done with those lenses, using Live View typically isn't a problem.
And, yes, you CAN change focus screens in many cameras (even some that aren't designed to facilitate it, though it might call for some disassembly).... adding features to help with manual focus such as a split image rangefinder and micro diaprisms. HOWEVER, you need to be aware that these often "mess with" the camera's metering system. At the very least, in most cases Spot Metering will no longer be accurate. Partial metering will likely be heavily skewed, too.... Other metering patterns may be more or less effected, too.
P.S. The various types of manual focus aids work with different types of lenses. For example, "split screen" work well with shorter focal lengths.... 85mm and less. With longer telephotos "micro diaprisms" work better. Overall matte field screens might be the most versatile, but also the hardest to see for critical focusing (I often used a magnifying loupe when working with my 4x5 camera, which only had a matte screen). Many SLRs and medium format cameras had focusing screens that incorporated two or all three types of manual focus assist features. Cameras designed for manual focus also tend to have bigger, brighter viewfinders than the typical DSLR.
Rangefinder cameras use a completely different method of focusing.... Often a dual image that aligns when in focus. Since this is not Through The Lens (TTL) focusing, the focusing system needed to be calibrated with the lens. Plus there are "parallax" issues when focusing close with RF cameras (the image seen framed in the viewfinder doesn't match up with the framing of what's actually being taken).