Of course, you can use a tool such as Focal Point, from OnOne Software. 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…..
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.
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.
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.