Differential focus — as used by the masters


In my second look at what photographers can learn from the master portrait painters, I want to look at a portrait of a Genoese nobleman, by Bernardo Strozzi, 1581-1644, oil on canvas. Like the other paintings in this series, it's in Birmingham Museum & Art Gallery, and must not be reproduced for commercial purposes.

Here's the image:
This is quite different from the portrait of John Ash which I looked at last time. In fact ? although this is just a crop from the painting, this is very like a modern photographic portrait in many respects. First, Strozzi isolates the sitter from his environment. It's interesting to notice that many painters choose to isolate by setting the sitter on a very dark, near black background (though never entirely black), whereas photographers often choose a white background. You can' see it here, but there is more to this picture, where some environmental background is introduced.
Second, the lighting is very carefully controlled. The face seems entirely three dimensional, almost leaping off the page (if faces can be said to leap). Like the Dr John Ash portrait, the painter has chosen to use short lighting, with the shadow on the side closest to the viewer. By this method, Strozzi is also able to create an almost perfect triangle, from forehead to chin, with the neckline completing the shape ? a clever use of a simple geometrical shape which makes instant sense to the eye. This also emphasises the fashion of the day, as indicated by the choice of beard.
Third, and most important for us today, is the way Strozzi uses differential focus to bring our complete attention to the eyes, while withdrawing attention from the body. We tend to think of differential focus as a result of lens choice ? long lens, wide aperture. The reality is that painters have been using it for centuries. This is because of the way the eye builds up a picture of a face. Nobody stares at the same portion of the face for long. Studies have shown the way the eye darts around, returning with greatest frequency to the centre of interest. In faces, this is the eye. The memory of the face, then is always sharpest around the eyes, as we've spent the most time focussing on them.
Because he is painting, Strozzi can have everything in utterly sharp focus, or have as much defocus as he likes wherever he wants it. In fact, the use of less focused areas is highly sophisticated, because one thing you do when you are studying someone for a portrait, as a painter, is see absolutely everything in sharp focus. Strozzi goes beyond this (as did many great painters) to produce the effect of a spontaneous view, despite the many long hours it would have taken him to paint this.
Back in the old days of manual focus and film, the eyes were always razor sharp in all of my portraits. I used to focus on the eye with the split-prism focusing aid, and then recompose the shot, very often at the camera's maximum aperture. When I moved to auto-focus digital cameras (I never used auto-focus on film) I noticed that I never got the sharpness on the eye that I was used to on film. This is largely because autofocus doesn't happily lift its centre of attention from something as soft as the eye, preferring the hard contrast of the eyelashes, eyebrows, nose or even the teeth. I've seen no end of images presented to me for comment where the moustache or beard, or stubble, are razor sharp and the eyes are soft. I improved this a little with my D2X by fitting a split-prism focusing screen, but haven't yet got round to doing this on the D3.
One of the most common questions on the Glamour & Portrait, Commercial & Studio Photography forum is 'how do I soften skin'. There are lots of ways of softening skin, but none of them really work unless the eyes are sharp. An image in which everything looks soft is simply a soft image ? in the old days we would have put that down to poor camera technique or poor darkroom technique. Differential focus, as epitomised in the Strozzi painting, is about having some parts of the image tack sharp, while other parts are soft.
I recently posted some suggestions for post-processing about an image that someone had asked for advice on, and someone else came back and said I had 'robbed the original image of its authenticity', and made it look like any other image from a fashion magazine. I didn't reply, as I don't really think it helps to get into an argument with someone on this kind of thing. Certainly, an image shot with no particular attention to differential focus, and then no post-processing to amend this, will retain a certain 'candid freshness', but, equally, it will never look any better than a snap-shot.
People often ask for advice on softening skin because they want the airbrushed magazine look, but, looking again at Strozzi's image, it's clear that this is something which predates magazines and fashion PR (though not, of course, fashion). The truth is, that when you talk to someone, you don't stare at their skin. A digital image captures far more skin detail than you would ever see, but, unless done carefully, far less eye detail. If you are romantically close to someone, you will still see the eyes as sharp, because you can see the reflections of distant things in them (in the same way that objects in a mirror do not become blurred if you get close to the mirror), but the skin and everything else will be very soft indeed, especially if the lights are low. Hard skin, soft-eyes is an artefact of the camera. The photographer, in general, should work to represent through the image what the eye sees, and the eye is not a static optical device, but a roving one, driven by the brain, focusing on what the brain finds most interesting.
This brings me back to Strozzi. There was no Photoshop in Strozzi's day, but, in a very real sense, every image was a Photoshop image, because all were built up by the will and skill of the artist, not captured by a shutter opening and closing. Strozzi will have spent far longer perfecting this image than anyone ever does in Photoshop. And yet, this is a highly 'authentic' image, much more so than the slightly self-aggrandising image of Dr Ash from last time.
How to apply all of this to an image today? Most lenses perform best a couple of stops down from their maximum aperture. A portrait photographer may well do better shooting at f4 (on an f1.8 prime lens) in order to maximise chances of getting the eyes really sharp. This, of course, limits the amount of blurring provided by the lens, but, seriously, it's a lot easier to introduce postprocessing blur than it is to introduce sharpness.
If I'm really working hard on an image, I will do three things. First, assuming the capture is good, I will want to refocus the eyes. For this I use a plugin called FocusMagic which is a true deconvolution filter. In other words, it refocuses, rather than just sharpens. I'll run it on the entire image, and then I'll run it a couple of times more on the eyes alone, until just before the point that they would be oversharpened. I want to be able to read the reflections in the eyes, as well as see the irises clearly.
I'll then run some kind of skin utility, usually Portraiture. I'm not doing this to produced an airbrushed look, but simply to overcome the camera artefact that skin is captured too sharp and eyes are captured too soft. I may go on to do some actual softening or blurring on the rest of the image ? for example introducing lens blur on clothing and other elements. Remember that the camera will take everything in sharp focus which is in the same focal plane, whereas your eye creates the sharpest focus on the things in which it is most interested. Depending on the camera angle, you may find that buttons, a necklace or a collar are as sharp as the eye ? this is not 'authentic', but a result of the difference between camera and human vision.
Finally, I'll use light, in much the same way that Strozzi does, to dim areas which are unimportant, but to brighten areas on which the viewers eye would concentrate. Precisely optically, the eyes, being set deeper within the skull, are generally more in shadow than the rest of the face. But we don't notice this in real life, because when we look at them, our pupils dilate to let more light in. When we flick to the rest of the face, the pupils contract very slightly. If you've ever taken a picture of someone in strong sunlight, without the benefit of fill-in flash, you'll recognise the disappointment of all the eyes being in deep, dark shadows. This can be fixed easily in Photoshop, using dodge and burn brushes.
Does this make the image less authentic? I would argue that it makes it more authentic, because it is more like what I see, and less like what a machine sees. A portrait, with the implication of collusion between sitter and artist, is never a photojournalistic picture of the world 'as it is'. Rather, it's authenticity comes from the fact that it presents the person as they really appear to be, with all their characteristics as we experience them.