Saturday, 17 December 2011

Choosing A Digital Camera

Relative print and image sizesIT WAS SO MUCH EASIER to choose a camera in the days before digital appeared on the scene. The choice of format was already decided for you by the market you were supplying; but, these days, many photographers base their camera purchasing decisions.upon the size of the pixelated image – often in the expectation that more pixels result in bigger and better images. Few, it seems, consider the fact that the sensor size determines the overall image quality – just as format defines that particular attribute in film.

The other mistake, which is easily made, is to believe that the pixelated image achieved from the camera, and which exceeds the size of most computer screens, will be the same size as a print. Unfortunately it does not work like that; because a large, glossy, magazine photograph is viewed by reflected light, and does not transmit it like a computer screen. Under transmitted light, the individual pixels that make-up the image tend to blend together because of the brightness of the screen; but, in a print, there is no transmitted light to create that effect, and the individual pixels are seen more easily. In order to resurrect the image and maximise its contrast and sharpness, pixelated images need to be printed at some 266 dots (pixels) per inch.

The inset image depicts the relative sizes of the digital image and its resulting print.

It is difficult to compare digital images with film; but the photographic industry equates a 35mm colour transparency with around 20MB of data. (If you scan a 35mm transparency at more than 20MB you get more data; but not necessarily any more detail from the image because all you are doing is providing more pixels to rendering the individual grains in the film).

20MB of data equates to a seven-mega-pixel sized image (20x1024x1024 divided by 3 = 6,990,506 – since 3 bytes are required to record each pixel’s colour components in the RGB colour space). The square root of 6,990,506 is 2,644, so we can say that, if a camera produces an image that is equal to, or more than, 2,644 pixels on its short side, we can be fairly confident of it performing like a conventional 35mm camera.  (At 266dpi, with this size of image, we could produce a sharp 10in x 10in print).

Of course, the comparison does not stop there. Larger sensors are more sensitive to light and can distinguish a greater range of colours and light levels than a smaller sensor can. So if you need to record a wide range of tones and colours for that exhibition landscape shot, you will have more chance of winning the competition if your camera’s sensor is larger than those used by your competitors. Larger sensors produce wide tone, crisp, colourful images with lots of shadow and highlight detail; whereas smaller sensors tend to render shadow areas as murky greys or blacks, and produce less saturated colours.

Photoshop can do a lot to improve a small sensor’s colour rendering and contrast; but it cannot lift detail from an image that is not there.

The following illustration depicts the relative relationships between sensor size and printable images – providing a snapshot of how modern technology’s digital image compares to traditional 35mm film.

Sensor and Print sizes


If your budget forces you to choose a small sensor, then you can utilise some of Photoshop’s advanced features to make its images appear more like those of costlier equipment; but, unfortunately, the basic technique requires that your main image remains static for a burst of bracketed shots.

Bracketing provides a means of obtaining a series of images (made at different speeds to ensure that the same depth of field is maintained) from under to over exposure. Photoshop will then permit you to combine that series of images to form what is commonly known as an HDR (High Dynamic Range) image. In this case, of course, we are not utilising the feature to produce a tonal range that could not be captured in a single shot; but to use the over and underexposed portions of the series to reveal more detail in the shadows and highlights, which the small sensor would otherwise fail to record.

Automatic bracketing is not always available on cheaper cameras; but if your objective is for a winning landscape, then the feature will not necessarily be missed.

Of course, at the end of the day, the quality of any camera’s final image depends upon its lens – and I still curse Canon for changing their lens mount when the company went digital, depriving me of the ability to use their range of fast primes I had built up over the years.

From that moment on, I personally decided there was only one manufacturer that could be relied upon in the photographic industry – and that is why I personally use Nikons for all my journalistic work.

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