The Secrets of Differential Focus

Shallow depth of field can be used effectively to isolate subjects and make your visual statement clear.

It has commercial uses wherever something needs to be lifted out of a literal context, such as food shot for a magazine or menu. You don't need to know exactly what the plate or the tablecloth are like, or even the entire dish; it's enough to home in on a small area which looks just right, and let the rest dissolve into blur.

Anyone laying text or inset photographs over a background image will appreciate shots with very limited zones of sharp focus and large areas of diffused detail. This is not a consideration for the amateur, or has not been in the past, but the availability of electronically printed albums (photo books) with layout software on your own PC means you can now think like a magazine designer when putting together an album. Family, events, holidays, parties and hobbies are all popular subjects for photo-book production.

Silk poppy: you can experiment with odd optics for effect. This soft result with muted colour and contrast comes from mounting a vintage Super Takumer 105mm lens on the Sigma SD10 and using it wide open. 1/500th at f/2.8, aperture priority auto with -0.7 compensation.

Of course, you can use a tool such as Focal Point, from OnOne Software (distributed by Colour Confidence in the UK). This is a powerful Photoshop plugin which lets you define the parts of your image you want in sharp focus, and without using layer masks or other advanced Photoshop editing, create realistic lens-quality blur.

However, nothing quite beats the results from a real lens used wide open. Sigma has one or two outstanding tools for differential focus or deliberate blur. My favourite is the 150mm f/2.8 Macro, becauseit combines long focal length with a very wide aperture and ultra-close focus. In the studio, this lens can earn its keep just by shooting small objects in a stylish sharp-soft mode. The 50-150mm f/2.8 is a close second, and the shorter macro lenses (both being f/2.8) an accessible choice for many existing owners.

Euro note: the Sigma 150mm f/2.8 EX DG Macro is a great lens for smooth bokeh and extremes of differential focus. Here, it was used on a Fuji FinexPix S3 with illumination from the tungsten modelling lamps of studio flash. Tripod, 1/45th at f/2.8.

One thing you need to be aware of is that SLR viewfinders do not show differential focus very accurately. Sigma's are relatively good, using a more traditional ground glass simulation than some ultra-bright screens designed to cope with low magnification, low efficiency mirror prisms. Just be aware that what you see through the finder at f/2 or f/2.8 really looks like the depth of field you get at f/5.6. You can check this by making some exposures and viewing them on the rear screen.

There's a good explanation of this phenomenon as seen on high-end Canon kit at dPhotoexpert.com - http://www.dphotoexpert.com/2007/09/21/live-view-versus-the-cheating-dslr-viewfinder/ - this article has become a standard reference for explaining the issue to those not familiar with it yet. Once you are aware, you will not rely as much on your viewfinder to judge depth of field, and you'll make more use of reviewing the image on-screen when setting up shots.

One effect you will see with Bayer pattern sensors is 'colour bokeh' error. It's not a noticeable problem with the Foveon X3 sensor, and is absent in the SD9 which has no microlenses. The lens - however well colour corrected - may show defocused neutral gradations as slightly magenta in the foreground, slightly green behind the subject. This effect was never visible on film. It's caused by the shift in relative colour corrections out of the plane of focus, and depends on whether the rays are diverging or converging when they strike the sensor. You need to be aware of it and perhaps desaturate unfocused zones which have a visible hue shift.

Lemur: the full aperture defocused image of the Sigma 80-400mm f/4.5-5.6 EX DG OS image stabilised tele zoom is very smooth. Shot at 400mm, 1/250th at f/5.6, with the Fuji FinePix S3 Pro set to ISO 800.

The defocused image quality depends on the lens design, the shape and position of the iris (aperture) being critical. Since other aspects of performance are more important, 'good bokeh' or a pleasant out of focus transition is not something which goes with cost or optical performance. Some lenses have it, others don't; it is harder to find in complex, long range zooms than in simple designs. A sure way to get good bokeh is to work wide open, which means a perfectly circular aperture. At least, that's the theory, but many zooms will still show oddly enhanced circles of confusion. Again, you need to try different lenses to find which work best for differential focus.

If the light is too bright, consider using a neutral density filter (a polariser can be pressed into service, but may have other unwanted effects). Most photographers are familiar with using an ND filter to get long exposures for waterfalls or waves, well stopped down. Its other use is to allow wife-open working in bright conditions.

Very sharp grain - or well rendered digital noise - can be effective with shallow depth of field. So can desaturated pastel colours. If anything, vivid colours are to be avoided unless you use them deliberately. They can dominate defocused areas, as any fashion photographer who has shot a street scene at f/2.8 and accidentally included a lit traffic light will confirm. Lights, street decorations, and reflections all take in their own life when thrown out of focus.

Detail of an Apple Mac screen: with a bellows and an old enlarging lens, the SD10 captures a tiny zone of sharp focus from an angled computer monitor screen. Mounted on a mini tripod, manual exposure by trial and error, 1/13th at f/4.

Above all, experiment. Try removing the lens entirely from your camera and holding it with your hand like an extension tube. You don't need a special flexible lens at £200 for creative angled-optic effects - a couple of Sigma close-up lenses screwed together and held in your hand will work quite well! Actually, a magnifying glass can do the job, or that loupe you once used to examine slides. Try macro bellows, or reverse mounted lenses.

The most effective differential focus comes from medium focal length and wide apertures, rather than long teles at moderate apertures. Lenses to experiment with range from the 30mm f/1.4 to the 70-200mm f/2.8. What if you don't have a wide aperture lens? You can get very effective results using something like the 55-200mm DC zoom wide open, or an 18-200mm the same way. It may only be f/5.6 or f/6.3, but if your subject is a close-up, the isolation and foreground/background blur will be enough.

Noodles and stir-fry: food is often best shown using very restricted focus zones. Sigma SD10, 50mm f/2.8 EX DG macro, ISO 100, 1/6th and f/2.8 hand-held by natural light.

In the professional studio, high power flash used to limit choices. In the 1980s, the standard colour negative stock was Vericolor Type S at ISO 160, and most monobloc or pack flash systems didn't turn down below 1/4 power. Even at portrait distances and with large light shapers, you could end up needing to use f/11 at minimum power. When digital SLRs began to take over, the flash makers had to rethink and offer 1/16th or 1/32nd power - the first DSLRs sometimes had ISO 400 or 320 as their lowest option. Habits had already begun to change, with scanning digital backs and continuous light, persuading pro shooters to explore differential focus more.

The result is seen all over today, from Marks & Spencer 'not just food' advertisements downwards, in TV and film as well as stills. We are all now visually trained to accept and appreciate images which have only tiny areas of critical sharpness. The days of stopping down to f/32 and using camera movements to get a plate of food sharp from front to back are long behind us (though restaurant displays around the Mediterranean still depict the exact contents of what you can order, pin sharp, down to the number of prawns!).

So, have a go - start using unfocused detail creatively, and enjoy exploring the qualities of the lenses you own.