Dual card slots

Many digital SLRs aimed at keen amateurs and professionals have two memory card slots, and this is not just to increase the overall storage capacity.

Then can be used for this, of course, but high-capacity memory cards are cheap these days, and storage space isn't the problem it was.

Some cameras use dual slots to offer support for two different card formats. This Nikon D-SLR can take both SD cards and Compact Flash.

Instead, you can use the two card slots to store different types of data which you want to keep separate. Depending on whether the camera supports it, you could do the following:

• Save photos to both cards at the same time so that you always have a backup in case one memory card fails or gets lost

• Use one to store RAW files and the other to store JPEGs (for cameras which offer simultaneous RAW/JPEG shooting)

• Use one card to save photos and the other to save movies

Shutter life

This is one indication of a digital SLR's durability, and it's measured as the number of exposures, or 'cycles' the shutter is designed to be able to withstand during its life. (You don't usually see this figure quoted for other camera types.)

Makers won't usually bother quoting a figure for basic or beginner-orientated cameras, but shutter life is often used as a selling point for more advanced or professional models.

A few years back, 100,000 cycles was regarded as the standard for professional cameras, but this has crept up and Nikon, for example, now quotes 150,000 cycles for its D7000 model, while its high-speed D3s pro SLR has a shutter rated at 300,000 cycles.

This sounds a lot, but it's not a warranty of any kind, just an indication of how long the designers expect the shutter mechanism to last - and it may go on much longer than that.

Amateur photographers are unlikely to wear out their cameras' shutters before the camera itself becomes obsolete and is replaced. If you do carry out any back-of-the-envelope calculations with your camera's shutter life, do remember that for every picture you save, you might take many more which you discard, and they all count towards the total shutter life.

Continuous shooting

This is where the camera keeps on taking pictures in rapid succession while you hold down the shutter button. This can be an important feature for sports, wildlife and any kind of action photography.

All digital cameras allow continuous shooting, but there are big differences in (a) how fast they can shoot, and (b) how many shots they can take before they have to stop and save them to the memory card.

The continuous shooting speed is called the 'frame rate' and is quoted in 'frames per second', or 'fps' for short. Many compact cameras struggle to shoot more than a couple of frames per second, but digital SLRs and hybrid cameras can shoot faster.

How fast do you need?
• 3fps is about as low as you'd want to go. It's adequate for fairly slow-moving subjects, but it's easy to miss the key moments between frames.

• 5-7fps is a lot more useful for sports and action, and you've got a better chance of capturing crucial moments.

• 8-10fps is the sort of speed achieved by professional SLRs designed for sports, action and press photography. At this speed you can be pretty sure of capturing the perfect moment.

• 30fps+ speeds are achieved by a few high-speed compact cameras, notably Casio's high-speed continuous-shutter cameras. Even digital SLRs can't match these speeds, but because these high-speed cameras use smaller sensors and can't sustain these speeds for long, they don't offer either the quality or the capacity needed by most professionals.

How many shots you can take?
At some point the camera will have to pause to process and save all the frames you've shot, though some cameras can shoot JPEGs indefinitely, up to the capacity of the memory card.

How many shots you can take will depend on the camera's processing power, the size of its images (megapixels), the size of its internal 'buffer' (short-term memory), and whether you're shooting JPEG or RAW files. You can save many more JPEGs than RAW files because the files are smaller.

Some compact cameras can shoot at high speeds too, but you may find the capacity is limited. Fujifilm's FinePix F550EXR, for example, can shoot 8 frames a second, but only for 8 frames.

Zoom range

A lens's zoom range is the ratio between its minimum and maximum focal lengths. So a 28-84mm lens, for example, has a zoom range of 3x, because its magnification at 84mm is 3x its magnification at 28mm.

Zoom range is an important selling point, especially for compact cameras, where the average buyer may have difficulty working out the lens's range from its focal lengths alone.

Superzoom cameras like the Canon PowerShot SX30 IS have the largest zoom range of all. This camera has a 30x optical zoom, and the diagram below shows the huge difference in scale between the wideangle end of the zoom range (the tiny figure on the left) and telephoto end (the large figure on the right).

It can be misleading, though, if you start to assume that the bigger the number  the better the lens. One problem is that the zoom range doesn't tell you the lens's widest angle of view, which is very important. Being able to magnify distant subjects is all very well, but in everyday photography a wideangle lens is more useful for 'getting everything in'.

It is important, then, to check the focal lengths anyway. A 5x zoom with a focal range of 24-100mm equivalent is likely to prove a lot more useful in everyday use than a 10x zoom with a focal range of 35-350mm, because a lot of the time the 35mm minimum focal length just won't prove 'wide' enough.

The other problem is that zooms with a very long range don't always perform that well, particularly at their maximum zoom, where pictures can often come out quite soft-looking and low in contrast. This can happen even if the camera uses a very high shutter speed, demonstrating that it's the lens that's the problem, not camera shake or user error.

The performance of superzoom cameras can often be quite disappointing for this reason, and this applies to pretty well all lenses with an extra-long zoom range. They're more versatile than normal lenses, but the quality is usually compromised to some degree.

Maximum aperture

The maximum aperture of a lens represents its light-gathering ability, and it's an important selling point. The smaller the number, the wider (or 'faster') the lens aperture.

This is the scale of aperture values. These are whole 'f-stops', but there are apertures in between ('half stops' or 'one-third stops'):

• f1 (very rare)
• f1.4 (some fixed focal length lenses)
• f2 (some zooms)
• f2.8
• f4 (most standard zooms start at f3.5, which is mid-way between f2.8 and f4
• f5.6 (the maximum aperture of most standard zooms at their maximum focal length*)
• f8
• f11
• f16
• f22

* Zoom lenses rarely offer the same maximum aperture throughout their zoom range, which is why the lens specs will quote something like 'Nikkor 18-55mm f3.5-5.6'. That means the maximum aperture at 18mm is f3.5, but at 55mm it drops to f5.6.

Each aperture value in the list above is one stop (or 1EV) 'faster' than the one below it. F1.4 is two stops 'faster' than f2.8, which means you can use an ISO setting two stops lower or a shutter speed two stops faster.

There are zoom lenses with constant maximum apertures, but these are much bigger, heavier and more expensive.

Fixed focal length lenses like this Nikon 50mm f1.4 have wider maximum apertures because their optical design is simpler and the designers can push the envelope in different directions.

LCD displays

There are two main selling points for LCD displays: the size, which is usually quoted across the diagonal, and the resolution, or the number of 'dots'.

The size makes a big difference, seemingly out of all proportion to the actual figures. A few cameras still have older 2.5-inch screens, but these seem very small compared to the latest 2.8-inch and 3-inch screens.

The resolution is also an important factor. Basic screens start at around 230,000 dots these days, but this does produce a slightly pixellated display, especially with menu text and icons. 460,000 dots is much better, but the best screens available at the time of writing have 921,000 dots, and are found on better quality hybrid cameras and digital SLRs.

Viewing angle is a third factor and addressed by new OLED (Organic Light-Emitting Diode) displays. These maintain their brightness, contrast and colour better even when the camera is tilted away from you. OLED displays offer higher contrast ratios, reduced power consumption and faster response times. Samsung uses a variation on this technology (AMOLED) in its latest cameras.

Art Filters (Olympus)

Art filters apply special effects like grainy black and white film, pop-art colours, soft focus effects and more directly in the camera. They're found on many Olympus models.

Applying effects like this, rather than doing it in Photoshop, won't be to everyone's liking, but the effects are striking, and they do save time spent in front of the computer later. You can also see the effect as you shoot, instead of having to wait until later.

Accessory Port (Olympus)

A multi-purpose connector on the top of some of Olympus's latest hybrid and high-end compact cameras which allows you to plug in stereo microphones, viewfinders and even macro lights.

Micro Four Thirds

A development of the Four Thirds format originally developed by Olympus and then Panasonic, which keeps the same sensor size but changes the lens mount and lenses to allow smaller, hybrid cameras.

The sensor size is 17.3 x 13mm, which is smaller than the APS-C format used for most digital SLRs, but still many times larger than the sensors used in compact cameras. Technically, Micro Four Thirds cameras should not be able to match APS-C cameras at higher ISO settings, but in practice the differences are not always easy to spot.

A combination of good quality lenses and compact, efficient camera designs means that Micro Four Thirds hybrid cameras are seen to offer pretty much the same quality as digital SLRs.

Direct vision viewfinder

This is an optical viewfinder which doesn't show the view through the camera lens but is lined up with it so that it's as close as possible to what the camera will see.

They work very well on cameras with fixed focal length lenses, including Leica's rangefinder cameras and Olympus's E-series Pen cameras when the fixed focal length 'pancake' lens is used in conjunction with the optional viewfinder accessory.

Their advantages are they they provide an extremely clear, bright image which may be superior even to that of a digital SLR's, and because they show what's outside the frame as well as within it, many photographers find they make it easier to keep up with fast-moving subjects and anticipate picture-taking opportunities.

Disadvantages include the lack of any kind of focussing ability (except with rangefinder cameras), a degree of optical distortion and some framing inaccuracy, especially with nearby subjects.

And although they work well on more expensive cameras with fixed focal length lenses, they're less successful on compact cameras. Here, they have to use a zooming mechanism to stay aligned with the camera's own zoom setting, and the result is very small, cramped viewfinders with lots of distortion and poor framing accuracy.

Rangefinder cameras

An older style of camera design which has a focussing system based on mirrors and triangulation. It's surprisingly fast and effective, and is still used on some modern-day Leica cameras.

You view the scene through a separate 'direct vision' viewfinder on top of the camera, but there's a second viewing window a little distance away. As you turn the focussing ring on the lens, it rotates a mirror inside this second window, and when the main viewfinder image and this secondary image line up, the subject is in focus.

It's fast and effective, though it does require precision engineering and top-quality components. It was also used in Epson's R-D1 rangefinder, now discontinued.

Focal factor

The focal length of a lens doesn't really tell you its actual angle of view because this depends on the sensor size too. But you can use a 'focal factor' to work out what the equivalent focal length will be.

For example, most digital SLRs have an APS-C sized sensor measuring around 24 x 16mm. This is smaller than a full-frame sensor, or a 35mm negative size, which is what most people go by. The difference is 1.6x (a full-frame sensor is 1.6x wider).

So if you use a 50mm lens on an APS-C camera, you multiply this focal factor by 1.6x, which gives you an equivalent focal length of 80mm.

In other words, because the sensor is smaller, a 50mm lens fitted to an APS-C camera actually looks like an 80mm lens.

Noise reduction

Image processing to reduce the noise produced by small sensors or high ISOs.

Noise is caused by random variations in the light captured by each photosite (pixel) on the sensor. Some cameras have sensors which apply 'on-chip' noise reduction, where electronic circuitry adjusts the signal values captured by the sensor's photosites. This is probably the best sort.

Otherwise, noise reduction is applied by the camera to the raw image data as it's processed to produce the photo. This reduces noise, but can result in smudging and loss of detail.

The same applies to noise reduction processes applied using software on a computer, though with more processing power available and more time to find the best settings, this may produce better results.

In any event, noise reduction is something you'd rather wasn't necessary at all. Using a small sensor and applying noise reduction is not as good as using a larger sensor which doesn't need it.

Olympus E-PL2

• Launched: January 2011
• Type: Compact style hybrid
• Sensor size: Micro Four Thirds, 17.3 x 13mm
• Resolution: 12.3 megapixels
• Lenses: Micro Four Thirds
• Key features: Accessory Port, Art Filters
• Links: Product page

Summary: Compact style hybrid for beginners and enthusiasts. It's an update to the E-PL1, and comes with an improved 14-42mm kit lens with 'silent' focussing, a higher-resolution LCD and ISO boosted to 6400.

Olympus XZ-1

• Launched: January 2011
• Type: High-end compact
• Sensor size: 1/6.3-inch
• Resolution: 10 megapixels
• Lens: 28-112mm equivalent f1.8-2.5
• Key features: Accessory port, 'sweep-style' panoramas
• Links: Product page

Summary: High-end compact with very similar specifications to existing rivals, including a 10-megapixel 1/1.63-inch sensor and an f1.8 4x zoom lens, plus HD movies.

Fujifilm FinePix F550EXR

• Launched: January 2011
• Type: High-end compact
• Sensor size: 1/2 inch
• Resolution: 16 megapixels
• Lens: 24-360mm equivalent f3.5-5.3
• Key features: GPS, 15x zoom, Full HD video, 8fps continuous shooting, EXR sensor
• Links: Press release | Product page

Summary: The small sensor size puts the F550EXR at the limit of our definition of a 'high-end compact', but it does shoot RAW files and full HD movies, it has a 15x zoom, and Fujifilm's innovative EXR sensor.

HD movies | Standard HD versus Full HD

Today's digital cameras can shoot video as well as stills, making conventional camcorders redundant for many people. And nearly all high-end cameras can now shoot HD video, which means a massive leap forward in picture quality.

Cameras with full HD movies 

There are currently two HD formats in use:

• Standard HD (1280 x 720 pixels)
• Full HD (1920 x 1080 pixels)

Both formats use the 16:9 'widescreen' ratio used by the latest TVs. Full HD offers the best picture quality, but standard HD shot by a good camera is still more than adequate for most casual users and even professional movie makers.

This diagram shows the relative sizes of full HD, standard HD and the 640 x 480 pixel resolution of the movie modes on older compact digital cameras (roughly equivalent to standard TV definition).

Full HD may not always offer the advantage in definition which the numbers suggest, though. This is because some full HD devices use 'interlacing' to achieve this definition. Here, two 'fields' of 540 pixels resolution are interwoven to give the impression of greater resolution.

Interlaced video is indicated by an 'i' suffix. It's common to see HDTV or camcorders described as offering '1080i' resolution, for example.

The alternative is 'progressive' video, where each frame is drawn in its entirety. This gives better results, but is not always possible at the higher resolution. Generally, standard definition devices produce progressive video, which is therefore described as '720p'.

As the hardware and technology improves, there is a move towards progressive video even with full HD, and this would be described as 1080p.

HD cameras shoot video in one of two main formats:

Motion JPEG/AVI/QuickTime

These files can be copied across from the camera's memory card and double-clicked for playback on a computer, uploaded to YouTube or imported into video editing software. It's as simple as that, and much more straightforward than the old days, where the camera had to be connected to the computer so that video could be 'captured' (recorded) in real time. The main disadvantage is file size. You need a big memory card and/or plenty of spare hard disk space on your computer.

AVCHD

This is a file format developed by Panasonic and Sony. It uses a more sophisticated compression system than the Motion JPEG format to produce movie files which take up half the storage space with little or no loss in quality. The disadvantage is that they use a complicated directory/folder structure on the memory card and can't simply be dragged across and double-clicked like Motion JPEG files. You need AVCHD-compatible software to transfer, import and edit these files, though the latest video-editing programs do provide this.

Touch-screen displays | Are they useful?

Touch-screen displays are now common on mobile phones and compact digital cameras. They're most effective where space for external control is limited, though they have the potential for some novel and interesting uses too.

The advantage for the manufacturer is simpler manufacturing and the ability to design (and update) much more sophisticated user interfaces.

They haven't appeared much on higher-end cameras, though, with the exception of recent Panasonic G-series models, where a touch-screen display is used to offer a range of camera controls as well as touch-controlled autofocus and shooting.

Here, you touch the screen to set the focus point and take the picture. It's a very intuitive and novel way of taking pictures, though it does mean that you have to take one hand away from the camera body to use it, and it could for most users prove to be little more than a novelty that you soon get tired of.

Touch-screen control is good for manufacturers, but not necessarily so good for users. Lower-quality displays can be sluggish or vague, without the tactile feedback you get from buttons and dials. And if you're using gloves, a touch-screen display could prove very difficult to use.

Megapixels

'Megapixels' refers to the number of pixels (in millions) captured by the camera's sensor. It's still used as a principal selling point for digital cameras, even though it's no longer anywhere near as relevant as it used to be and, in some cases, more megapixels do more harm than good.

In the early days of digital photography, cameras didn't have many megapixels. This limited the size of the pictures you could print before the blocky pixel pattern became visible. It also meant that cameras couldn't resolve much fine detail.

So higher megapixels meant bigger, smoother prints and finer detail, but only up to a point, because the other limiting factor is the physical size of the sensor.

Compact digital cameras use very small sensors. There's a limit to how much lenses can resolve on tiny sensors, no matter how good they are. And the smaller the sensor, the more you have to blow up the image to get a same-sized print. We'll call this the 'enlargement factor'. With enlargement factors over 30x, you can expect to see the image quality deteriorating, no matter how many megapixels you've got.

The table below should make this clearer. It shows how many megapixels you need for different-sized prints, but it also shows the enlargement factor required for these print sizes with different types of sensor.

As you can see, the small sensors in compact cameras are hitting very high enlargement factors very early on. This is what limits their picture quality in big print sizes, regardless of how many megapixels they have.

In fact, higher megapixel ratings bring serious technical difficulties which do affect the picture quality. If the megapixel count goes up but the sensor stays the same size, it means the individual pixels ('photosites', to be exact) on the sensor have to get smaller. This makes them less sensitive and more prone to random noise. As a result, makers have to use strong noise-reduction processes to make the picture quality acceptable, and today's high-resolution compact cameras often display weak definition and a 'smoothed-over' look to subtle textured details. The pictures you get are no better than those from cameras of five years ago with half the megapixels.

So why do makers keep increasing the megapixels? It's because megapixels sell. Most buyers will be unaware of the technical implications and just see a bigger number as being 'better', and as long as that keeps happening, the makers will keep on increasing megapixels regardless of whether it's a good thing or not.

To sum up, increasing megapixels in compact cameras are achieving nothing except camera sales. In fact, in many ways it's driving picture quality downwards not upwards.

Hybrid cameras and SLRs are different because the sensors are much larger and they haven't quite reached the same level of  'megapixel saturation'. Even here, though, sensor size counts for more than megapixels, and you shouldn't necessarily expect a proportional increase in resolution just because one camera has 18 megapixels, for example, and another has 12.

AF Points | How many do you need?

Nearly all cameras have several AF points and will select the best focus point automatically - usually the one corresponding to the object nearest the camera. Most cameras allow you to choose the focus point yourself if you want to.

Below is a simulation of an 11-point focussing system in a digital SLR. The green rectangles indicate where the focussing points are, and those highlighted in red are the ones the camera is using to focus on this particular subject.

So how many AF points do you need? The number of AF points is used as a selling point for the camera, the assumption being that the more there are, the better the camera's autofocus system.

This is true, up to a point, but having lots of AF points can also be confusing. Sometimes you need to override the camera's selected focus point, and this can take longer when the camera's got 9, 11 or (on some Nikon models) 51 AF points to choose from. Don't forget that however AF points the camera has, it can only focus on one thing.

Cameras with many AF points may offer predictive autofocus, where the camera can follow the subject's movement across a number of AF points and anticipate the correct point of focus based on this movement. This may prove useful for action photography, though many photographers still prefer the simplicity of 'pre-focussing' manually.

Some older, simpler cameras may have as few as three AF points. This is not the disadvantage it might sound in general photography, since they'll be spread in a line across the frame and can cope with off-centre subjects very well. It's much easier to override simple systems like these too.

Indeed, many photographers prefer to use their cameras' centre AF point for focussing, even when the camera has many more.