The live view has been introduced some years ago, and nowadays it is found in nearly all SLRs. I have begun using it with the 40D, and now I take all my macro photos using live view! The live view has several advantages:
. It is possible to frame the photo without looking into viewfinder
; this is a great help for macro, where you often have to frame the photo from weird angles. Some camera have even tilting screens, that make the framing even easier!
. You can magnify the image by 10x
, or even more in some cameras, for a very precise manual focus. The angle-finder is no longer necessary. In the past I used a lot the Canon Angle-Finder C for macro photography, to frame a subject from weird angles, but I didn't like it a lot: the built quality is average, it makes more difficult to find the subject, it makes the viewfinder even darker, and it has a crappy dioptric adjustment system (you have to change the diopter every time that you switch from 1.25x to 2.5x). The live view is so much better!
. You can stop down the lens to the effective aperture (with the DOF preview button) to check the depth of field, both on the entire image or in a 10x magnified area...it is fantastic to see in real time the effect of different apertures! And remember that while the viewfinder becomes extremely dark if you press the DOF preview button, the LCD always maintain the same brightness...I love it!
. You can evaluate immediately the brightness
of the image: it is possible to simulate the exposure and to view the histogram in real time! You can see immediately the effect of exposure compensation, too.
. In live view, you can activate a super-imposed grid
, that helps to compose the photo and to avoid tilted horizons.
. In some cameras, Live View replaces Mirror Lock Up
(I'm going to explain in-depth this point in the next paragraphs).
The only downside of live view is that is reduces a lot the battery life; for example, with my 1DsIII I can take about 900 photos when I don't use live view, while with live view I can take about 200-250 photos. For short trips it is not a problem, while for longer trips it is necessary to have some spare batteries, or to be able to charge the battery every day.
Mirror Lock Up
Nowadays, the majority of interchangeable lens cameras still use the mirror to provide the image to the view finder; Panasonic has recently announced the first interchangeable lenses cameras without mirror (the Panasonic G1 and Panasonic G HD, that have an electronic viewfinder), but this kind of cameras is still a rarity.
With SLR camera, the mirror is raised just before the exposure; the movement of the mirror creates some vibrations that reduce the sharpness of the image at slow shutter speeds. The Mirror Lock Up (MLU) allows to avoid this problem: when MLU is active, you can raise the mirror some seconds before the exposure. Until some time ago, MLU was essential for every photo taken at slow shutter speed, and it was even more important for macro, where the magnification emphasizes every minimum vibration. Nowadays, with some cameras you no longer need to use MLU, if you shoot in live view: this is a great advantage, but not every camera that has live view offers this possibility.
Mirror lock up is particularly useful with long lenses: in this case, the lens is mounted on the tripod so the camera is "hanging" from the lens, and every vibration is amplified. If you use short lenses and you mount the camera directly on the tripod, the mirror vibrations are slightly reduced by the tripod, but they may still be noticeable: I highly recommend to use either the MLU or live view. Image stabilization does not replace the mirror lock up.
You don't need MLU, instead, if you handhold the camera (but of course when you are handholding you use relatively fast shutter speeds, otherwise the photos will be blurred by your movements), or when you use flash as main light (the duration of the flash is extremely short so it is an equivalent of a very fast shutter speed).
Different types of Live View and MLU
Nearly all cameras have Live View, but there are four main different types of Live View.
A) The most advanced Live View implementations (currently found in the Canon 7D and 5DII) allow to shoot using the electronic first curtain; in other words, when you press the shutter the camera reset electronically the sensor to begin the exposure, then it closes the physical shutter curtain to end the exposure. With this kind of live view, you no longer need MLU.
B) Another implementation of live view is found into older Canon cameras (1DIII, 1DsIII). These cameras don't have the electronic first curtain; when you press the shutter the camera closes the physical curtain to reset the sensor, it opens the curtain to begin the exposure and it closes again the curtain the end the exposure. With this kind of live view, you still need MLU.
C) A third implementation of live view if found into Nikon and Olympus SLR cameras. These cameras don't have the electronic first curtain; when you press the shutter the camera closes the physical curtain and turns down the mirror, then it raises the mirror, it opens the curtain to begin the exposure and it closes again the curtain the end the exposure. With this kind of live view, you still need MLU; moreover, it has a slight shutter delay (from 1/2" to 1").
D) Some cameras don't have mirror at all. Of course, in these cameras you don't need MLU! That said, the only camera of this type currently in production - the Panasonic G1 - has not electronic first curtain, so it may give the same problems of "type B" live view.
Other than these four types of live view, there is another type of live view that use a secondary image sensor placed into viewfinder. This implementation has many downsides (poor accuracy, impossibility to fine tune the focus at pixel level) and it is currently found only in the Sony A350; I think it will soon disappear, replaced by type A or type D live view.
How does these explanations translate in practice? I have done a simple test, that shows clearly the differences. These images are 100% crops.