Up close and personal

When it comes to portraits, blur can be just as important as sharpness

Naturally, key attractions of a prime lens are that you can expect great sharpness and low levels of distortion. That’s the case with the vast majority of portrait primes, but blur is equally important.

A main facet of ‘fast’ short to medium telephoto lenses is that the focal length and wide aperture enable a tight depth of field (DOF), so you can throw backgrounds out of focus and make the main subject stand out.

Some portrait lenses go wider than others in terms of aperture, most budgetfriendly options having an f/1.8 rating. For head-and-shoulders and even halflength portraits, the depth of field is tight enough to blur somebody’s ears when you’re focused on the eyes. Moving up to an f/1.4 lens, the depth of field becomes tighter still and you’ll often find that only one eye is truly sharp in portrait images. There can be issues with fast prime lenses that might be seen as advantages for portraiture. At their widest apertures, sharpness towards the edges and corners is sometimes compromised but, ultimately, you’ll probably want the outer edges of the frame to be defocused anyway. Secondly, vignetting can be noticeable when shooting wide-open but this results in more attention being focused on the main subject.

A bigger problem is that fast primes often give two kinds of colour fringing. Firstly, there’s lateral chromatic aberration, which can be an issue with any type of lens. It shows up mostly as coloured lines around high-contrast edges towards the corners of the frame. For example, it can be visible around dark branches against a bright sky.

The second type is axial or longitudinal chromatic aberration, also known as bokeh fringing. This is a problem more closely associated with fast prime lenses. It’s a failure of the lens to focus different wavelengths of light at the same point, and can be noticeable as coloured fringes around the edges of objects in a scene that are slightly in front of or behind the point of focus. As such, these fringes can show up anywhere within the image frame, but generally only occur when using very wide apertures. Some fast lenses are worse than others for axial fringing, but this is typically much improved by stopping down a little.