Technology Explained (aka 'The Science Bit'...)

Why Shoot Raw?

The addition of raw mode is one of the most obvious things that distinguishes cameras in the high-end, enthusiast, 'luxury' class of compact cameras from more mainstream consumer-oriented models. There are other features, too, which are traditionally the preserve of these higher-end products (like full manual control, a hotshoe, and fast, wide lenses) but raw mode is perhaps the most important. Some consumer-level compact cameras offer raw capture, but slow operational speed in this mode, and sometimes dubious image quality gains often make it much less useful than you might think (or hope).

The benefits of shooting in raw mode, compared to JPEG, are many and various. Raw files contain more data, so you can make more extreme tonal adjustments to them before you start to see a penalty in image quality. You can adjust the white balance of images shot in raw mode easily, and save as many JPEG copies as you like without fear of degrading the original file.

Nikon P7700 - JPEG (ADL 'Normal') Processed Raw (using ACR 7.3RC)

The JPEG+RAW images above were captured simultaneously on the Nikon Coolpix P7700. The original JPEG was shot with Active D-Lighting turned to 'Normal', by accident and as a consequence it's flat and lacking in contrast. The camera's AWB system has reproduced the (actually quite chilly) scene accurately, but boringly. A few minutes working on the Raw file in Adobe Camera Raw and we've been able to warm the image up, boost the contrast, and also tweak the sharpening to get considerably more detail out of the file. Click on the thumbnails above to see the full-sized original results. 

Canon G15 - JPEG (ISO 12800)  Processed Raw (using ACR 7.3 RC)

The images above were shot on the Canon PowerShot G15, at ISO 12,800. The 'straight' JPEG file actually looks pretty good but careful processing of the Raw file reveals more detail (important if you're aiming to make big prints) and we've chosen to add some warmth back in, to more accurately reflect the color of the street lights in the original scene. The G15's AWB system has done its job by neutralizing this warmth in the JPEG but the final result is a little lifeless. The converted Raw file is not 'better' necessarily, but punchier, more detailed, and more vivid.

Looking on the bright side - why a fast lens makes a difference 

In very broad terms, there are two advantages to large maximum aperture lenses. Firstly, a so called 'fast' lens lets more light into the camera, which is useful in low-light conditions, or when you simply need faster shutter speeds (useful for shooting people, sport, and to avoid camera shake at long focal lengths).

Panasonic FZ150, F5.2, ISO 400 @ 600mm (equiv)  Panasonic FZ200, F2.8 ISO 100 @ 600mm (equiv)

In the images above, we set up two superzoom cameras - the Panasonic Lumix DMC-FZ200 and its predecessor the FZ150 - at the long end of their zooms. Shot 'wide open' at the same shutter speed the new FZ200, with its F2.8 maximum aperture, can capture a well-exposed image at ISO 100. The older FZ150 has a slower lens, which means that it has to capture this scene at ISO 400, at which setting noise and noise-reduction have a negative effect on image quality. 

Canon G12, F4.5, 1/8sec, ISO 1600 @ 140mm Canon G15, F2.8, 1/20sec, ISO 1600 @ 140mm 

These two images show another benefit of having a faster lens in poor light - you can shoot at higher shutter speeds, which helps you avoid blur from subject or camera movement. Here, we shot two low light portraits on the Canon PowerShot G15 and its predecessor the G12, which has a smaller maximum aperture. If you click on the thumbnails above you'll see that the G15 has delivered a sharp image, thanks to its higher shutter speed, but the G12's smaller aperture forces it to shoot at a slower shutter speed, which has introduced blur. The only solution would be to shoot at a higher ISO sensitivity setting, which of course degrades image quality.

The other advantage of a faster lens is that all other things being equal (which you can't always assume - something we will explain in the following paragraph) a larger maximum aperture means less depth of field, which allows you to blur backgrounds more. This is useful for portraiture, or any application where subject-background separation is desirable. 

Everything you ever wanted to know about aperture, sensor size and depth-of-field...

Where things get complicated is that in depth of field terms, reported apertures aren't necessarily equivalent from camera to camera, because sensor sizes vary so much. The different sensor sizes of the cameras in the high-end compact class actually makes it very difficult to directly compare their lenses. How different? Here's an illustration. None of the cameras in this roundup have 1/2.3" sensors, but it's the standard sensor size for almost all compact cameras, superzooms and travel-zoom compacts. 

When talking about aperture in terms of depth of field control, you must take sensor size into account. The graph below shows the equivalent apertures of each camera in this class, with their various-sized sensors, spanning the range of their equivalent focal lengths. Equivalent apertures allow you to understand how cameras of different sensor sizes will compare in terms of depth-of-field at the same equivalent focal length. Equivalent apertures also give an insight into the camera's light gathering capability, which is an indicator of image quality.

In this graph, both X and Y scales are logarithmic, so that a one-stop change of aperture brightness is a consistent height and a doubling of focal length is always represented by the same width along the bottom on the graph. Very simply, the lower the line at any given equivalent focal length, the more you can blur backgrounds. For example at 100mm equivalent, both the Olympus XZ-2 and the Fujifilm can deliver more-blurred backgrounds than the Sony RX100, despite its larger sensor. 

Canon PowerShot S110 @ 120mm, F5.9 Olympus Stylus XZ-2 @ 112mm, F2.5 

The images above show pretty clearly the real-world impact of the varying degrees of depth of field control that are possible from the cameras in this selection. On the left we have the Canon PowerShot S110, which has the joint-smallest sensor of all of the cameras in this group, and the slowest lens. Subject / Background separation is relatively poor, even at the long end of the zoom. The Olympus XZ-2 on the other hand has a much faster lens, which 'wide open' offers much better control over depth of field.