Graphic Logo for some broswer "Please modify in header.php: line 35"

Photographers brought up on film are familiar with the fact that faster, more sensitive films bring with their speed an increase in grain. In digital photography, the equivalent of grain is noise – but ‘noise’ is much more complicated than ‘grain’.

noise – problems and benefits

Caused by many factors, including heat, the nature of the electrical signal and activity in the sensor itself, the outcome of noise is always the same – irregular, random specks or sparkles that don’t relate to specific details in the photograph. In fact, because noise tends to mask detail it is generally regarded as a flaw that should be avoided or reduced. In the same way that many film-using photographers tried hard to avoid grain in their photographs – by working with the slowest practicable films and developing their images with special formulations – so digital photographers continually endeavour to avoid noise.

It is the electronic nature of noise that determines its rapid rise with increased sensitivity. Setting higher sensitivities means that we are amplifying the image signal – making it ‘louder’. However, in the process, we also amplify the errors (the noise) already present in the signal. While the overall level of noise is falling thanks to clever algorithms, it is still broadly true that noise rises with ISO setting, with pixel count and with the use of smaller sensors.

However, noise is not necessarily a bad thing; it can give texture to what is sometimes too smooth and clinical an image plane. It’s worth experimenting with different ISO settings to learn about the characteristics of your camera’s noise and how to exploit them. If you want to add noise artificially, you will get the best results from specialist software which simulates film grain.

detail and resolution

Resolution is a measure of how much detail has been recorded to a certain level of contrast. A high-resolution setting, however, doesn’t necessarily guarantee a high-resolution image – that is, a high-quality one full of useful detail – it means only that the image contains a lot of pixels. If the camera lens is of poor quality, recording an image with more pixels won’t make it any sharper. The practice of calling the largest file setting “high resolution” is therefore slightly misleading.  In fact, it’s vital to keep in mind the difference between measurements, such as resolution, and assessments, such as quality.

When you choose a resolution setting, you’re making a decision involving more than simply the size of file to save. A high-resolution file needs far more data processing than a low-resolution one, so it takes longer to process each image, slowing the speed of operation – and of course you’ll be able to fit fewer large files onto your memory card. If you have a camera that can record 10 megapixels or more, it’s useful to try the second-largest resolution setting and compare the results with the largest. You may well find that for your needs the smaller file is perfectly suitable.

compression and quality

One way to balance the high storage needs of large files with a desire for good quality is to compress the files, thereby reducing the memory space they occupy. The JPEG protocol is able to reduce files to as little as one-tenth of their original size without visible loss in quality, and even more if visual loss is allowed. However, JPEG is a ‘lossy’ scheme, which means that data lost as a result of JPEG compression is forfeited forever. Virtually all digital cameras can save files in JPEG. Some allow you to set the compression or quality level: the best strategy is to set the highest quality (the lowest compression) if possible.

depth of field

It’s hard to get excited about depth of field, taken in its usual definition. In actual fact, however, it’s about a lot more than merely the span of space within an image in which an object will appear acceptably sharp. Depth of field is really about the plasticity of the image – it’s feel, it’s handling of space.

sharpness and space

In normal vision, barely 5% of what we see is sharp – only the part of the visual field that we can see is in detail. However, a scene appears sharp overall because our eyes continuously refocus as they flit from detail to detail, scanning the scene to build a fully focused picture.

In a typical photograph before the digital era, hardly any of the image was sharp. The premise of the influential f/64 school of photography (created in 1932) was the worship of extensive depth of field – most easily achieved when photographing distant subjects, which made landscapes the group’s favoured subject matter.

This changes in the digital era. With the use of small sensor chips calling for lenses with short focal lengths, depth of field suddenly became very extensive. Everything looked sharp, and at first this was welcomed, but negative reactions swiftly followed: all pictures looked the same and it was difficult to find a way to make the main subject stand out.

observer’s eye

The main controls we have over depth of field are aperture, focal length, and smaller magnification (where the camera is further away from the subject). To decrease depth of field it is necessary to increase lens aperture, use a longer focal lens or move closer to the subject.

However, these controls are insignificant compares to the observer’s opinion about sharpness and how critically the image is being viewed. The perception of depth of field will also be influenced by the size of the print at which the image is viewed. The smaller it is, the greater the apparent depth of field.

quality of blur

Given that so much of a typical image is unsharp, what may matter more is the quality of the unsharpness or blur. It was only in the late twentieth century, and the use of inexpensive lenses with odd-shaped aperture diaphragms in point-and-shoot cameras, that photographers learnt about unattractive sharpness – where the blur was uneven, lacked smooth transitions, and was sometimes fringed. The quality of unsharpness is called “bokeh”, and good bokeh is now an image quality prized in almost equal measure as sharpness.

The net result is that the plasticity of your image – how it feels and the way it conveys the picture space to the viewer – depends heavily on your control of the depth of field. That in turn depends on you using larger sensors if you want to create reduced depth of field and high quality lenses for smooth blur.

shutter settings

The shutter setting determines much more than the exposure time. From past practice, the priority was to use short shutter times in order to ensure that the images were sharp. Once that was satisfied, the appropriate aperture to achieve correct exposure was set. Now, the arrival of cameras and lenses with image stabilization means the guidelines are being rewritten.

exposure times – sharpness and blur

The shutter’s primary function is to set the exposure time. While it’s open, the photographic exposure is created by a summing up of all the light that falls on the sensor. The resulting record is not just that of the light falling, but also it’s spatial relationship with the sensor. This means that the sensor records all changes in position of the subject or of the camera itself, however minute they may be: in short, the sensor records both light intensity and movement.

With a short exposure time, there’s less chance for movement to travel a significant distance on the sensor. This means the blur from any movement cannot be easily seen – the image looks sharp. If we hold the shutter open for longer, movement both in the camera and the subject can draw a blur that’s large enough to be seen.

short exposures & longer exposures

While using the shortest exposure time seems the obvious solution in order to obtain the sharpest picture, it can, in fact, have a negative effect on your image. A brief exposure may require a large aperture, or you may need to use a high ISO setting – sometimes both.

These compensations may undermine your attempts to capture a sharp image: the large aperture reduces depth of field, calling for more precise focusing, and may also reduce image quality. A high ISO setting may increase noise. This reduces the sharpness of detail, not to mention lowering the quality of colours.

Consequently, you may find that if you unshackle yourself from the tyranny of the brief exposure time and its adherence to sharpness, your pictures improve. Allow yourself a longer exposure to work with blur or allow slight movement into your image. Any cameras featuring image stabilization give us a marvellous new way to approach movement. It helps us hold the camera steadily during long exposures, yet we can still capture motion blur.