Thanks useful information: 
A point I want to be clear on:
don't be discouraged by my remarks about the limitations of focusing screens.
if you have any interest in manual focusing in any way, eg. you like to have more/better control over your lens .. or that you want to delve in old, unique, lenses for the fun of it.
In that situation, the focusing screen will help!
I just don't think it will for macro work.
Note that some cameras work differently to others when using liveview mode. Check to see how your particular models operates:Originally Posted by Physioz
1/ some cameras working in Lv mode is that the lens diaphragm operates in real time.
That is, as you set the aperture value, during Lv mode, the aperture may actually be set to that value as you make the change.
2/ some cameras work differently in that they maintain the same method of diaphragm operation as they do in normal vf operation.
The advantage in this is that you get a brighter image. BUT! .. the disadvantage is that you don't see the DOF nor as much contrast as you do in real time aperture operation.
The ideal camera would allow switching between each method of operation.
That is, one mode for wide open Lv mode and auto stopping down when the image is to be exposed, and real time mode so that you can see in real time the DOF, contrast, focal plane .. etc.
Note that it's not uncommon for a lens to have what's known as focus shift.
That is, in normal operation the aperture is held wide open.
When you expose for the image, the aperture is stopped down only for that moment.
Lenses with focus shift result in the problem that at wide open aperture setting, the focus is along one plane, but when stopped down the plane of focus moves(and usually backwards .. that I've ever read of and seen).
The DOF usually masks the issue for normal imagery, but at macro level, it may not mask the issue enough.
I doubt that the Canon 100/2.8 is affected by focus shift, but you may want to do some research on it.
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