But despite this, SLRs soldiers on, serious photographers still want to own them, camera manufacturers still reserve their best technology for SLRs which they bill as their 'flagship' models*. Those who insist that they are obsolete are focusing entirely on a few factors which are important to them and how they use cameras as an accessory to take to a party or traveling; while ignoring how people who take photographs either purely for their own enjoyment, for art or for a living use cameras. Often the people saying this are well-regarded and even professional technology commentators (and apparently, well known cityscape photographer stuckincustoms); however they often view cameras purely as another gadget, purely as something used to make a record of a memorable moment to share on Facebook. The difference between the two views of photography is much like the difference between dancing the Macarena at your friend's wedding and professional dancing (be that ballet or street-dance). One is merely an accessory that improves your enjoyment of something else, the other is an art-form. Personally I think people often miss the distinction between different uses of photography and assume that anyone who wants to take a photograph is engaged in pretty much the same activity and therefore has the same priorities.

One - The Viewfinder

I'll start with the reason why SLRs are called SLRs in the first place. Single Lens Reflex cameras have a mirror which directs light from the lens up into the viewfinder and then pops up to allow the same light through onto the sensor when you take an exposure. This means that when you look though the viewfinder you are seeing exactly what the sensor sees. This mechanism is what makes the distinctive shape (and part of the bulk) of an SLR camera, with the hump on the top. In the days of film, the alternative was compact cameras with viewfinders which were slightly offset from the lens, and if you had a zoom lens then as you zoomed in or out, the viewfinder stayed the same so you had no idea what the photograph would actually look like.

This was the main reason why SLRs always had interchangeable lenses and compact cameras didn't. Many people think that an SLR is a camera with interchangeable lenses, but in fact this was a symptom of the fact that you were able to compose the image by looking directly through the lens making it practical to have more than one lens option (there was a thing called a twin-lens-reflex back in the day, look it up if you're interested, but just so you know I'm not ignoring its existence).

Now we have digital cameras with live-view, so that we look at an LCD screen on the back of the camera with an image direct from the sensor. This also means that compact cameras can now have interchangeable lenses, which makes for so-called 'compact system cameras'. This is one of the biggest reasons people give that SLRs are obsolete, their whole reason for being, and the reason they are so bulky is no longer required.

This is missing the point. The human eye is far more advanced a piece of optical apparatus than any LCD screen. Looking through the viewfinder enables you to see a much brighter and larger image, spot much finer detail, and as anyone who has tried to use a compact camera in bright sunlight will know, not worry about the reflection on the screen obscuring the image. Also when you look through a viewfinder everything around you disappears, all the distractions are gone and you are more focused on the photograph you are making.

An LCD is fine if your only objective in taking the photo is that both your girlfriend and the Eiffel Tower are in the frame, but not if you are concerned with composing the various elements of the image into a certain design, be it simple rule of thirds or something more complicated. So this goes back to my earlier point about something being fine for casual use as an accessory but not so much for serious use.

Two - Lag

Still on the theme of the mirror-reflex mechanism, is the issue of lag. When you take a photograph with an SLR, two things have to happen first; the mirror has to flip up, and the shutter has to open. In practice these two things happen almost instantaneously and the photograph is taken a tiny fraction of a second after you press the shutter button.

When you take a photograph using live-view (and if you have an SLR with live-view as an option then this counts for that as well) more stuff has to happen. The shutter is already open and the sensor is already recording, so first the shutter has to close, then the sensor has to discharge as it will be carrying an residual electrical charge from the image it was recording in live-view, only then can the shutter open again. This whole process takes much longer; and people who try to record fast-moving events using compacts or camera-phones will be familiar with the lag between pressing the shutter button, and an image actually being taken. If you were serious about something such as sports or wildlife photography, then you really would need to invest in an SLR.

Three - Stability

A final point about how using a viewfinder (with a reflex mechanism) is better than using a LCD screen with live-view is a little less well known, but very important. To improve stability when hand-holding a camera, and therefore reduce motion blur caused by movement of the camera, photographers like to hold their camera in a certain way. Both hands on the camera, close to the body with elbows tucked in. They will press the viewfinder right up into their face to use their head as a stabiliser and bring the centre of gravity of the photographer/camera combo as close to their body's own centre of gravity as possible.

Live-view can be useful on an SLR, it enables the photographer to place the camera in places that their bodies can't reach such as up high or through a narrow gap. It can also allow the photographer to take a photo from a viewpoint on the ground without having to lay down (useful in bad weather); but photographers who try this know how much of a hit they take on stability, which they have to compensate for by using a faster shutter speed and in turn a higher ISO rating, or a wider lens aperture (assuming that the reason they can't use the normal stance also makes a tripod impractical, when using a tripod live-view can be useful ironically for stability as it reduces your need to touch the camera).

If live-view is the only option you have then this forces you to always adopt an awkward and inherently unstable stance with the camera held a foot or so in-front of you. Granted the fact that these cameras are much lighter makes it easier to do this, but it there is still the weight of your own arms to consider, the extra space you require if in a crowd and just the general discomfort that comes from having to do this all the time.

Four - It's the ergonomy, stupid

One of the main areas where people who criticise SLRs and those who stick with them seem to be missing the point, is that beneath it all is an assumption that in technology, smaller is always better. Again this stems from looking at cameras purely as gadgets and accessories. If it is small enough to fit in your top pocket, more comfortable to carry around and doesn't require its own bag then this is good. There are no advantages at all to being bigger.

Remember part of the reason SLRs are larger is due to their reflex mechanism, but also because due to this mechanism they are more suitable for serious use, so the size and the fact a camera is an SLR goes hand in hand.

One of the things that photographers always judge a new SLR on is how comfortable it is to hold and how easily usable the controls are. The more comfortable you are with a camera in your hands, the faster you will be able to respond to a situation which forces you to change your settings, and more camera control will become second-nature. This is where SLRs really excel, an it isn't unusual to hear a new SLR model commended for having a good heft.

Not to say that smaller and lighter cameras aren't easier to carry, but how you plan to use the camera affects whether you are prepared to accept small and fiddly controls and an LCD screen that forces you to use only your finger-tips to hold the camera and go through a menu system in order to change each setting in return for something that will go nicely in your pocket; or whether you are prepared to accept a camera and lenses that requires a special bag which will give your abs a work-out in return for being able to grasp the camera in two hands with separate buttons and dials for commonly used controls.

Five - Noise

One other area where smaller isn't always better concerns the technology itself, specifically the size of the sensor and the corresponding signal-to-noise ratio. A smaller camera has to have a smaller sensor; and a smaller sensor (assuming the number of megapixels remains the same) means smaller pixels (photo-sites). These are the little 'buckets' that collect light. The smaller they are the greater the possibility that errant electrical pulses coming from the camera's own circuitry as well as from the atmosphere will overwhelm the electrical charge built up by the photons that have landed in said bucket. The darker it is the less photons there are to begin with and the more this electrical charges will have an effect.

These random pulses show up in the images as unattractive and obscuring noise. The upshot is that the smallest sensors such as those found in camera-phones create an unacceptable level of noise even when the light levels are moderate, and are completely unusable in conditions where an SLR with a larger sensor still performs admirably. Even a decent compact camera suffers more than an SLR would when conditions get challenging.

Six - Big Glass

Finally, a larger camera means larger lenses, and larger lenses transmit more light. The amount of light a lens transmits is a ratio of the focal length of the lens to the diameter of the lens aperture (well specifically the exit pupil but I don't want to get too technical) given as an 'f number' (f = focal length / pupil diameter). This can be varied by adjusting the aperture but each lens will have a maximum 'f number' when the aperture is wide open. The larger the lens, given that the focal length is the same, the smaller the f number, a smaller f number means more light. Because the sensor in a smaller camera is smaller, the focal length can be shorter to achieve the same field of view, so it is still possible for tiny lenses to have small f numbers, but this comes at a cost in terms of sharpness, not to mention the fact that smaller lenses mean any slight aberration in the surface of the glass is magnified. If you want a fast lens that is also of excellent quality then inevitably size matters.

As I have said throughout I think a lot of this is to do with people not distinguishing enough between the activity of someone who wants to share something funny their cat did and someone who wants to create art. Also a lot of newcomers to photography don't realise at first how steep the learning curve is to start producing works of real wonder. At first, a compact camera will fulfill all their needs and they will go through a phase of wondering what people who carry big bulky SLRs are bothering for; it takes a while before they start to realise that they are not yet anywhere near the sort of level that gets recognition and that the equipment you use has its own part to play in getting there. So there you go, my six points. Feel free to add more of your own, and I'd be keen to know if anyone out there disagrees completely with everything I've just said.

*yes I am sad that my own manufacturer Olympus have bowed out of the SLR market, but this is everything to do with the competition from the big names and the fact no-one else bought into the 4/3 format and nothing to do with SLRs being defunct.