On what occasion it is advantageous to take pictures in live view mode? I understand that with a 60d with swivel display can be useful to avoid contortions in certain shots but with a fixed display like in the 7d when there has been helpful?
I don't have a 7d, but I do have a different DSLR with a fixed screen (not on a swivel). At first I was not a live view believer, but I have come around. Pretty much whenever my camera is on a tripod (e.g. landscape, cityscape, architecture Exposure am in live view these days. It buys me a few things:
- The magnification option lets me check for very sharp focus.
- In live view (at least on Canons) you get mirror lockup for free (the mirror is up to support live view and shutter fires without moving the mirror), this helps lower vibration.
- Exposure simulation gives me a reasonable approximation of the exposure, helping with setting up the shot.
That said, I pretty much never use live view when doing sports or portraits as either my subject and/or myself are moving around quite a bit more.
LiveView is generally better when using manual-focus lenses (or focusing most lenses manually, as mentioned by others). The reason is that the focusing screen seen through the optical viewfinder in the 7D is tuned for auto-focus shooting, which makes nailing focus manually more difficult.
From KatzEye Optics:
The standard Canon 7D screen is optimized for automatic focusing. As such, it is very bright, but it gives a less than optimum indication of DOF and has less ‘snap’ (indication of when focus is achieved) than is required for demanding manual focus applications.
Canon has a PDF that illustrates a few different types of focusing screens. Unfortunately, the 7D's screen is not interchangeable.
So, LiveView is almost essential when using manual focus lenses, auto lenses switched to manual mode, vintage/alternate lenses mounted with an adapter, or macro extension tubes, etc. You see exactly what the sensor is seeing, and have the benefit of zooming in for super-sharp focus.
All my lenses are manual focus (i got them intending to shoot video, but of course they still work great for stills), so i basically live in LiveView. I've missed plenty of shots while focusing and zooming on the LCD, but going manual is a personal choice of style and control. Auto-focus through the viewfinder isn't perfect, and can miss focus in certain conditions or if the camera isn't set to an appropriate focus mode or zone. If you're shooting sports/weddings/etc where you can't afford to miss a moment, then you'll probably be using auto-focus lenses anyway.
Contrast AF may work at slightly lower light levels than phase detection. Additionally, I don't recall if the 7d has face detection or not, but if it does, that only works in LiveView. In general, I find I don't use LiveView for photos almost ever though. It's mostly there to make it so the camera can act like a point and shoot when a novice user is using it or for use with video.
When the 7D is on a tripod and I am manually focusing, I often use Live View. If I am shooting objects at a fixed distance I will often use it to focus, then go back to the viewfinder to actually frame and take the shots. This is especially the case if I'm shooting in low light where the extra noise from the sensor being energized for extended periods has the most impact on the resulting photo.
If I am taking a series of long exposures, even in low light, I will often use Live View. By leaving the mirror up between shots it reduces both the time between the end of one and the beginning of the next exposure and also avoids the vibration associated with mirror movement during long exposures.