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Profile of the Digital SLR

Last Update: 11 September 2009

The digital SLR is closely based on the film SLR but offers significant advantages. The basic operation has not changed as a mirror is still used to direct light to a pentaprism into a viewfinder and when the shutter is activated the mirror moves to expose light onto the imaging sensor rather instead of on film. Autofocus technology is essentially the same but requires improved calibration. In common with all digital cameras a LCD monitor is on the back of the camera for image playback and menu display and exposure status display.

Just like the film SLR, looking though the viewfinder of the digital SLR means that you are seeing directly through the attached lens. However, there is a difference. The majority of the D-SLRs are using an imaging sensor that is smaller than 35mm (135 format) film. They are based on the APS-C format which is smaller (22.5x15mm) which means that the image captured by the lens and seen in the viewfinder is effectively cropped. As a result, it can me more difficult to manually focus a lens (more about this later).

All the usual exposure controls are present, aperture priority, shutter priority, manual and program mode. Not only has the photographer got to consider these standard controls but there is now other features that demand attention. Contrast, sharpness, colour saturation and white balance. The first three items once set do not usually need to be changed very often. In many ways they help to define the recorded image characteristics.

There are three main formats used for saving images; JPEG, TIFF and RAW. All these formats have their advantages and disadvantages.

The JPEG (.jpg) is a compression format that produces image files of relatively small sizes. The greater the compression the smaller the file size but the lower the image quality.

The TIFF format produces much larger files than the JPEG but (even when compressed). It's main advantage is that there is no image degradation which makes it handy for making large prints. It's disadvantage is that as the resolution of the image sensor increases (increased megapixels) the more unwieldy it becomes to handle tiffs. As a result, the number of cameras with this format is in decline.

The RAW format is not a strict format as it varies with the camera manufacturer. Here are a few names;

Canon: CR2
Nikon: NEF
Pentax: PEF
Sony (Alpha): ARW
Olympus: RAW

These formats basically record the original data produced by the imaging sensor. The fact that the original data is recorded means that the very best image can be reproduced that the camera is capable of. Unfortunately, this format requires the use of an application that can convert the data back into a viewable image. Once converted, RAW will usually produce a much more detailed and dynamic image than that derived from a JPEG produced in camera.

Aperature Priority, Shutter Priority, Full Manual and Programme provide the necessary controls to allow the photographer to capture subjects under various conditions. Understanding the relationship between aperture and shutter speed settings is the key to controlling the light that enters the camera and forms the final image.

The are three other parameters worth keeping in mind, All digital SLRs provide adjustment for image sharpness, contrast, and colour saturation. Unlike the main exposure controls, these controls do not usually require regular adjustment. They help to tailor the characteristics of the final image but their importance is lessened if images are stored in the RAW format.

Capturing the lightest and the darkest areas of a scene under high contrast lighting conditions remains a serious challenge for any digital camera. If the highlights are protected then some shadow detail may be lost. Alternatively, if details in the shadows is maintained there is the risk that highlights may be blown out. Image processing is getting better at handling this but there is still some way to go.

An alternative way to improve the dynamic range is to reprocess the image in camera. Manufacturers in recent times have been employing sophisticated image processing algorithms that analyse image data and make adjustments in order that shadow information is maintained. The highlight information would be protected by the exposure being adjusted slightly for underexposure. The areas readjusted to maintain detail is achieved without ruining the overall balance of the image. Sometimes this effect can appear rather subtle but the determination is down to how the algorithm assesses the scene.

This feature is best employed when the set ISO is relatively low. As adjusted image areas are the result of luminance and colour boosting, there is the potential for those areas to show increased image noise.

This effects of this image processing is only available on the JPEG format. The RAW file information remains raw.

Auto focusing systems in D-SLRs work by detecting contrast differences in order to determine correct focus. This is similar to the way the human eye works. The accuracy and reliability of auto focus systems varies from brand to brand but all should work well enough for most purposes.

The electrical motor which drives the lens comes in two main types; in the camera body or in the lens. Having the lens in the camera body means that motor couples an attached lens by way of a drive shaft mechanism. This works fine for a range of lenses but there can be issues of AF speed and accuracy with lenses that have heavy optical elements. Also, the focusing can be noisy, though the noise level is usually dependant on the lens. Having the motor in the lens means that the motor is specifically designed for the lens in question to achieve the best performance. Ultra sonic motors allow for quiet, fast and highly responsive auto focusing. These are used almost as standard for Canon lenses and Nikon have a steadily increasing range of lenses with this technology. Both Pentax and Sony are set to follow Nikon by having camera bodies with built in AF motors but with the capability of taking advantage lenses with built in AF motors.

Focus accuracy is critical especially as there is a tendency to view images at 100% when checking for sharpness. Poor or out of calibration AF module can lead to front or back focus issues (where the lens will focus ahead of an object or behind an object respectively). The error may be slight but can become more apparent depending on the lens attached. Usually optically fast lenses used with their apertures at their widest can show if there are focusing issues. This usually results in the DSLR having to be sent back for servicing. To combat is, some cameras are now provided with a method to calibrate the DSLR for a range of lenses.  Note that this focusing problem does not occur with contrast based AF.

The standard feature on digital compact cameras has found its way on to the DSLR. Rather than viewing the scene through the viewfinder, with Live View enabled, the scene can be viewed directly on the LCD monitor. This feature becomes useful when the camera cannot be held to the eye to get the shot (e.g. in the case of shooting above a crowd or shooting at an awkward or low level). Its use is enhanced if an articulating LCD monitor is used as the monitor can then be adjusted as required. All is not lost on the standard LCD monitor as modern screens tend to have a wide viewing angle.

Implementation of Live View differs from one camera brand to another with some designed for greatest flexibility in use and others for speed of operation. The phase detection AF cannot be used on most designs and so the camera instead focuses based on contrast read from the imaging sensor. This is the same technology as used in compact camera and does not possess the focusing speed as phase detection.

With the availability of Live View on the DSLR it is not surprising that video recording followed. This feature effectively makes a DSLR a convergence device as not only is video recording implemented but sound recording too. One major advantage of having video on a DSLR is that the sensor is much later than that used in the consumer camcorders. This makes it possible to have more control of the depth of field.

Currently, the cameras offering video recording do not permit autofocus. The standard phase detection AF cannot be used because the main mirror in the camera body has to remain up. Contrast based AF is not implemented either. Initially this may seem to be a serious setback but those use to making their own video movies will tend to set up a shot and manually focus. This feature is still new to the DSLR so there are bound to be enhancements to the technology with each generation.

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