Depth of field and the importance of achieving critical focus

Reference polar bear and Noctilux. Nikon D700, AFS 60/2.8 G Micro

Your plane of focus determines more than just what’s sharp and what isn’t: it directs the eye of the viewer to the subject, too. The shallower your depth of field, the more exaggerated this becomes – to the point that with a very shallow DOF lens, a close subject and far background, you really won’t be able to identify what the background actually is.

Jazz time. Leica M8, 35/2 ASPH

All the more important that you choose the right plane of focus, then.

From a compositional point of view, the obvious thing would be to ensure all of your subject in focus – or is it? Not necessarily – sometimes you need just enough of it to be identifiable, and the rest can be merely a suggested impression. Take the eyes in a portrait, for instance. You don’t need to have the ears and hair in perfect focus, too. Less obvious is the need for context: today’s photographers seem to be fixated on achieving ever shallower depth of field, not knowing that in the past, achieving sufficient depth of field was a huge struggle that required the use of camera movements, long exposures, enormous lights and tiny apertures (I don’t know if any readers remember or have heard of Ansel Adams’ f.64 club – so much for ‘f8 and be there’). Part of the problem was the insufficient image quality of smaller media; the other part of the problem was the psychological conditioning of observers and viewers: before photography there was drawing and painting, and neither of those had any such thing as bokeh.

Rain and traffic. Leica M9-P, 50/0.95 ASPH

I used to be one of those chasing ever faster lenses – for the simple reason that earlier digital cameras were frankly pretty crap when it started to get dark – higher ISOs were all but unusable. If you couldn’t use an tripod (reportage work, for instance) then you had no choice but to accept grain (compromising image quality), use a flash (compromising ambience and context) or use a faster lens.

Fortunately, we now have sensors good enough that we don’t always have to shoot at f1.4 the minute it gets dark; we can do so because we like the look, but not because we have to – and that’s important, because it means we can choose just how much context* to include in the shot. Moral of the story: think very carefully about how much of the image needs to be in focus to tell the story effectively: too little, and you lose context because the subject has no stage; too much, and you lose focus and isolation, because the subject may not longer be as separated from it environment.

*By ‘context’, I mean background that isn’t subject. If your entire shot is subject, then there’s no context, of course.

At the crossing. Nikon D700, AFS 85/1.4 G

This brings me to the second portion of this article: focus. It might sound obvious, but ensure your subject is in focus. The non-obvious bit is that there’s in focus, and in focus. The difference is obvious at the pixel level: are you seeing single-pixel detail (or near enough, to the resolving limits of your lens/ sensor combination)? If you’re not, go back and try again.

It matters for two reasons: there’s nothing more annoying than looking at a slightly OOF image, and you yourself will probably look at the photo later and think ‘if only…’; and then there’s the ever-increasing pixel density that makes it more and more difficult to achieve critical sharpness at the pixel level. The higher the resolving power of a camera, the more finely geared the focus mechanism has to be in order to achieve critical focus. In the film days, this wasn’t so much of an issue because the ‘sensor’ had some thickness to it – the emulsion was of a fixed depth – however, with digital sensors, this ‘depth’ is all but nonexistent – so an image tends to snap in and out of focus fairly abruptly, with very little acceptable transition zone – a zone that gets increasingly narrow as your depth of field decreases.

Just enough context. Leica M9-P, 35/1.4 ASPH FLE

The reality is that with modern lens and camera designs, camera shake aside, your ability to focus has by far the greatest impact on the resolution and perceived sharpness of an image. It’s actually much more difficult to achieve critical focus with a fast wide lens than a fast long one – reason being that the transition zone between in and out of focus is far more gentle; an increasing reliance on contrast detect AF rather than phase detect AF does’t help, either.

Fortunately, there are many things you can do to maximize your chances of getting a positive hit. I’ll go through them for each of the common camera types.

Afternoon cigar. Leica M9-P, 50/0.95 ASPH

Compacts

1. Half press to focus, and gently release. You’d be surprised how many people just jab at the shutter button and wonder a) why their camera is slow and b) why images are blurry. The former is because the camera will take some time to find focus; the latter is because either it didn’t find focus properly (the electronics told it to trip the shutter when focus was acceptable or because of the jerk caused by the finger-jabbing motion and recoil.
2. Pick a high contrast subject. All compacts (except the Nikon 1 and some Ricohs, but those are all hybrid systems anyway) use contrast detection to determine when the image is in focus or not. The lens is racked back and forth until the portion of the image under the selected AF box reaches maximum contrast; this may or may not correspond to what you’ve selected. So, to be safe, select a subject with good contrast.
3. If in doubt, stop down a little – but not too much. Stopping down gives you more depth of field to cover focusing errors; on a compact with very short real focal lengths, you’ll find that f4 (or f5.6 at maximum for larger sensored cameras or longer focal lengths) is probably enough to ensure everything is in focus; any more than that and you’ll be seeing a reduction in sharpness due to diffraction.
4. Turn off that AF assist light. Not only is it hugely annoying and conspicuous at night, I find that it also tends to give a lot of false positives.
5. For moving subjects, continuous AF is nearly useless. The only way to reliably hit a moving subject is either to rely on your extended DOF and hope single AF is quick enough to nail it, or use trap focus by pre focusing on a certain point, and waiting for your subject to arrive.

Crossing. Nikon D3100, AFS 85/1.4 G

DSLRs

1. I repeat again: half press to focus, and gently release. The same applies for DSLRs and compacts.
2. And again: pick a high contrast subject. Phase detect AF sensors also like high contrast subjects – preferably something with either horizontal or vertical lines, depending on the orientation of your AF sensors.
3. Don’t let the camera do the choosing. Pick your AF boxes manually. If you don’t, you have no idea what the camera will focus on – it’s generally the closest thing in the frame, but that might not be what you want it to do. Especially if you’re shooting with a very shallow DOF lens.
4. Use continuous AF mode. Again, especially for shallow DOF lenses, a few millimeters of subject movement or photographer movement to or from the subject can affect sharpness greatly – to avoid this problem, use continuous AF mode and only lock focus if you’re absolutely certain you’ve got what you want. (A popular alternative is to put AF-ON on a separate button. I personally use that button to lock focus, because generally it’s easier to use one finger to activate AF and shoot off the shutter button alone, rather than two) And without saying, you HAVE to use continuous AF on moving subjects; make sure the focus box always tracks the subject.
5. Check your focus calibration. Set up the camera on a tripod, with an angled static target (I recommend the LensAlign) that lets you see if you’re focusing on your intended point or not. Adjust your AF Fine Tune settings if you have them, until you can reliably achieve critical focus. Do this for every lens you’ve got.
6. Get your mirror aligned. If you’re manual focusing a lot, you’ll want to get your mirror aligned so that what you see in the finder is the same as what the sensor sees – what this effectively does is adjust the rest position of the mirror so that the distance from mount to sensor is exactly the same as mount to focusing screen, over the whole frame.
7. Use a split prism or micro prism screen. Remember the half-circles in the middle of manual focus film SLR viewfinders? Those were focusing aids called split prisms, which let you see very quickly if something was in focus or not – all you had to do was turn the focusing ring until the verticals aligned. Why they’re no longer standard equipment on DSLRs these days is beyond me. Why they’re no longer available even as accessories, except for some third party companies, is even more mind boggling. The standard screens have a very narrow scatter angle, which means that there isn’t much difference in brightness between slow and fast lenses – great for kit zooms, but not so great for determining critical focus with very fast lenses; they lack the required snap. If you don’t believe me, try using the DOF preview button at f1.4, f2 and f2.8 – you’ll probably see no difference in the viewfinder.
8. Don’t trust the focus confirmation aids. The dot (Nikon) or beep (Canon) is somewhat misleading because there’s a range to ‘acceptably in focus’ – and the extremes of that range are pretty wide. It’s a guide only, not an absolute.
9. Use magnified live view and a tripod for absolutely critical images.

Untitled. Nikon D700, AFS 28-300/3.5-5.6 VR

Rangefinders

1. The most critical portion of the focusing system is the rangefinder. It is impossible to emphasize how important a perfectly aligned rangefinder is – especially when using fast lenses. Remember also that you have no way to visualize the focus plane. So: make sure your rangefinder is properly calibrated for the lenses you use – each lens will be slightly different because of cam wear, alignment and a whole load of other factors. Do not attempt to do this yourself, because there are four adjustment points – one each for near and far limits, one for travel, and one for vertical alignment. Not knowing which one does what can result in a bit of a disaster. Since there’s a feeler cam that always interacts with the focusing cam on the lens, the calibration can, and does, drift after a while, so check it annually or so if you use the camera frequently.
2. Clean the RF and VF windows! Dirt + smudges + oil = low contrast = difficulty in focusing.
3. Use a magnifier over the viewfinder if you’re using longer lenses. This helps by enlarging the central portion of the frame that contains the RF patch (at the expense of the outer portions) – you’ll find that since the RF patch occupies a large portion of the frame, it’s difficult to see exactly what’s in focus and what isn’t without the magnifier.

Waving to the stars. Nikon D700, AFS 85/1.4 G

Finally, remember the higher the resolution camera you’re using, the more critical focusing becomes – don’t go out for an upgrade of either camera or lenses until you’re sure that you’re getting all you can out of your current one. MT

For readers in Kuala Lumpur (and Singapore, when I’m there), I offer a Rangefinder Calibration Service for Leica M cameras.

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