Contrasting sharpness with softness is a useful creative tool in photography, but it’s not always as easy to achieve as might be expected. Andy Cross explains why.

The subject of selective focus may seem like a pretty lame topic to be writing about in a magazine of this calibre. But if you keep reading you might find that I haven’t completely lost it. As an optical engineer I learned more about lenses, light and their behaviour than I would ever need to make an image with them. So some of the attributes I intend looking at – which are rarely written about – could be useful.

http://www.avhub.com.au/images/stories/igsu/IgnitionSuite_Image(8069).jpgSelective focus achieved with a 24mm full frame equidistant projection Mamiya fish-eye lens focused on a subject 8.0 cm from the lens element. Distortion isn’t noticeable on round subjects.

There are really three ways a photographer can place an emphasis on – or direct a viewer’s attention to – some portion of the subject. The first is through luminosity by putting more light on it. Our eyes are more sensitive to the highlights in an image than they are to mid-tones or shadows. The second is to place an emphasis on colour. The girl in the bright red dress surrounded by the bland colour of masonry would be a good example. The third method is to render some parts of the subject out of focus while others are in sharp focus. We tend to pay more attention to the sharp portions of the subject ahead of anything else. Any of these techniques can be mixed and matched. However, there are some lenses that make achieving selective focus more of a problem than others. These lenses are, of course, ultra wide-angle and fish-eye lenses because of their inherent depth-of-field.

http://www.avhub.com.au/images/stories/igsu/IgnitionSuite_Image(8070).jpgThe distortion created by fish-eye lenses is noticed the most in geometric shapes at the edges of the frame especially when the lens is tilted up or down.

How Wide? At this point I should define what is actually meant by ultra-wide and fish-eye. A lens that has an angleof- view greater than that offered by most normal lenses – which is around 49 degrees irrespective of format –could be called a wide-angle. By the time the lenses approach an angle-of-view greater than 115 degrees, they are often advertised as ultra wideangles. When an angle-of-view of 180 degrees or wider is achieved, they are usually called fish-eyes.

In regard to fish-eye lenses, there are really two types with two sub-categories. They can be the full-frame or circular field types, and can have an equidistant or linear projection classification. As the name suggests, full-frame means the image on area occupies the entire frame. The latter renders the image as a circle in the middle of the frame. Equidistant projection simply means the light rays coming in from the edges of the lens travel the same distance to the film or sensor plane as the central light rays do. Consequently, the exposure remains even across the entire frame. Whereas with the linear projection types you often need to use a graduated centre neutral-density (ND) filter of the correct value to even out the exposure.

http://www.avhub.com.au/images/stories/igsu/IgnitionSuite_Image(8071).jpgPseudo selective focus achieved with a wide-angle lens by ensuring the background contains no texture. The background in this image was still water reflecting an evenly lit overcast sky.

The inherently longer depth-of-field of wideangle lenses means that even when set to focus on a subject relatively close to the lens, it may almost be at the infinity focal distance. Close down a couple of stops to shoot at the ‘sweet spot’ for the lens, and the further increase in depth-of-field will mean you have the foreground and background rendered sharp.