DSLR 'Live view' mode is a recent new specification originated by Olympus that is now being incorporated in one form or another into many of the latest DSLR's to arrive. What it is and how it works is causing some confusion amongst many digital camera users, and in reality this is not surprising because although the idea is to allow DSLR's to produce a 'live image' of the scene captured by the cameras lens on the rear LCD screen, just like digicams can, there are different ways this is being achieved, and with different consequences as a result. 

Anyway, this page has been written to lay out the basic details for those who are interested and/or confused.

Digicam v DSLR

One of the main differences that has existed between SLR's and many other type of cameras is the way the scene to be captured by the camera is viewed by the user. One of the main advantages of using an SLR has always been that the view seen through the viewfinder is that through the lens (TTL). This is achieved by using a mirror and a glass prism - called a penta-prism - inside the camera body. This is the origin of the designation Single Lens Reflex. This is different to that of many other types where the view seen is through a separate viewfinder and is not always the view that the camera will capture because it is not accurate due to parallax error problems. We have dealt with these differences on our page  Digital Camera Basics  but the aspects relating to this are worth repeating here because of the development and introduction of 'live view' in DSLR's.

Below is a very basic diagram of a digicam. As you can see, when you look through the optical viewfinder most cameras of this type have, you do not get the same view as the camera's lens sends to the sensor. In general use this is okay, but it can cause some trouble when taking a shot close up, due to parallax errors. You can't frame the shot correctly and you might not get quite the picture that you expect, the angle of view and area captured being different.

This has always been a major drawback with film compact cameras but with a present day digicam there is another way of viewing the shot you wish to take, and this by using the LCD screen on the back. Most digicams switch on the LCD screen automatically when their macro mode is chosen, as this is the only way of seeing what it is that the camera will take a shot of in this mode because of the parallax error problems. But the rear screen can be used at any time for viewing the scene, whatever mode it is in. This is known as pre-viewing, as opposed to re-viewing, which can be done once a shot has been taken and shows the captured image file.

Pre-viewing shots on the LCD is useful because in most cases the screen will show the shot as it will be after it is taken, with white balance and any other effects you have chosen applied to it. Not only that, but what you see is what the lens sees, because the image on the LCD comes from the camera's sensor, so in effect you get the TTL viewing [Through The Lens] that has previously been the sole preserve of SLR's. This is as a result of the basic design of digicams, and is something that hasn't been achievable when using a Digital SLR. 

It's made possible because, unlike film cameras and Digital SLR's, a digicams shutter is not closed except when taking a shot, but remains open all the time, and only shuts down to the correct aperture to take a picture. This has a disadvantage as well however, in that it takes longer to take a shot when you press the shutter button because of the extra sequences the digicam has to go through before the shutter can work, turning the sensor 'off' and then 'on' again for the shot. This is known as 'Shutter lag' and is the time taken between pressing the shutter button and the shutter 'firing', that is opening and closing, thus taking the shot at the correct exposure value.

Here is a basic diagram of a DSLR. You will see that unlike a digicam, when you look through the viewfinder you see through the lens (TTL). This is achieved as we have said earlier by using a mirror mechanism and a prism. (Some recent DSLR's now used a cheaper to make penta-mirror system using mirrors - hence the name - instead of a glass penta-prism. The main difference is the (lower) viewfinder quality that results)

A Digital SLR works in exactly the same way as a film SLR, in that when you press the shutter button to take a shot, the mirror flips upwards out of the way, the shutter opens and closes, and then the mirror falls back down into place. Because the mirror's normal position is down - so you can see through the viewfinder - it means the path to the sensor is blocked. So unlike a digicam, the shutter stays shut as in a normal film camera, and no pre-viewing of the shot using the LCD is possible. The LCD screen on a Digital SLR is used purely for re-viewing shots already taken. 

The advantage of this is when you look through the viewfinder the resolution you see is what your eye is capable of seeing, and thus it is easy to see how the shot looks in terms of whether it is focused correctly, is sharp, and includes all that you want to capture in the shot. An additional benefit of this is that no power is used, as its all done optically, whilst another is that the shutter lag is minimal, and is generally un-noticed in comparison to a digicam. 

DSLR 'Live View' mode

As DSLR's have become cheaper and sales having risen as they have been bought in preference to digicams, efforts have been made to incorporate into DSLR's some of the beneficial advantages digicams have over them. One of these is the ability to preview the scene to be taken on the rear LCD. Although the main advantage of a DSLR is the TTL optical view through the viewfinder, there is no doubt that there are certain circumstances where being able to see and compose the scene to be taken on the rear LCD is an advantage. 'Live view' mode has arisen as a means of providing that choice with a DSLR. There are actually two different general methods that have been introduced by DSLR camera makers to provide a rear screen live view mode. 

The original Olympus E330 'live view' DSLR

The first DSLR camera with a live view mode was introduced by Olympus, and actually had both the live view modes now being used. It was looked upon as a novel system at the time, but not without considerable drawbacks in one way or another, either low resolution or slow and cumbersome to use. The camera was novel in other ways as well as it didn't have either a penta-prism or penta-mirror viewfinder but a penta-porro one. This uses mirrors like a penta-mirror but works sideways rather than vertically, so the cameras viewfinder is to one side of the lens instead of above it, and the mirrors are located separately instead of together. This gave the camera a unique box like design amongst DSLR's that wasn't widely appreciated and hasn't been repeated.

Secondary sensor use

The E330's penta-porro design allowed Olympus to fit a 5mp CCD sensor into the viewfinder system capturing the view that would otherwise be seen by the user looking into the viewfinder. This has some advantages, the AF system is not affected and nor is the main camera sensor. This is important because the longer a sensor is 'on' the hotter it gets, and the higher noise levels can then become. But the downside with this is that a low resolution image generally results because the image view isn't coming off the main sensor. 

The external digital viewfinders made by Zigview that slide over the viewfinder work in roughly the same way, using a sensor to capture the viewfinder image view, and Sony have now produced a similar system in their new A300/A350 DSLR's, using a modified penta-mirror design, which they have called 'quick AF live view', to reinforce the fact that normal lens AF capacity and accuracy as well as shot capture capabilities remain the same and are not affected by the use of live view mode. However viewfinder size has been reduced to accommodate the sensor along with the image magnification produced. What is gained with the one hand is thus lost with the other. And considerable power is consumed, as with digicams, when both the sensor and rear screen are continuously powered on.

Main Sensor use

The other live view mode Olympus incorporated into the E330 which most live view enabled DSLR's are using actually works in exactly the same way as that in a digicam, the main sensor staying 'on' and relaying the image to the screen. To enable this to occur the mirror has to be raised out of the way and the shutter opened. It is in fact similar to taking a shot in 'bulb' mode. There are some disadvantages that arise in doing this. A lot of power is consumed, not only in powering the rear screen and sensor, but in keeping the mirror raised and the shutter open. Another problem is that normal DSLR autofocus cannot be used. In most DSLR's autofocus relies on a small amount of light hitting the reflex mirror being diverted to a sensor in the camera that measures it and adjusts the lens to suit - see  Autofocus .  With the mirror raised out of the way this cannot occur and only manual focusing can be undertaken. Some of the latest 'live view' enabled DSLR's have an option to autofocus when in live view mode and this involves the mirror dropping momentarily to enable autofocus and then raising again once this has occurred. Others are using the main sensor to enable digicam type autofocus, contrast detection AF, but how successful and accurate this is will only become clear with long term use of these cameras. It is whatever, considerably slower to operate.

Conclusions

Although live view mode in a DSLR has the same basic disadvantages that apply when viewing a digicams rear LCD screen, relatively low LCD resolution, difficulty with viewing in very bright and contrasty lighting conditions etc, there are advantages in difficult conditions when looking through the cameras' viewfinder is awkward or nigh on impossible.

Quite a few new DSLR's are also being provided with articulated rear LCD screens of one kind or another, and similar to those that have featured on digicams for some years. This is another distinct advantage in certain circumstances.

An advantage is also gained when the camera is connected to a computer. In a studio environment for example the camera can be mounted on a tripod or similar, hooked up to a computer, and with the right software, (supplied by the camera maker with the camera), can be controlled by it, the live view image being seen on the computer screen, while shots can being taken and downloaded straight to it and viewed on-screen, bypassing the memory card altogether. And of course the latest DSLR's with their generally larger and higher resolution screens ( 2.5" - 3") give a better rear LCD screen view than early digital cameras do

The big question anyone has to ask themselves is really how often they might end up using live view with a DSLR, and whether it is worth either the extra cost, or the downsides that accompany it, in comparison to one that doesn't.

Just make sure if you decide to buy and use one of these live view enabled DSLR's that you have plenty of spare batteries to hand. Or use a mains adapter where possible. And be aware that like digicams, using live view mode with contrast detect AF is a lot slower that taking a shot another way.