Where to focus

Autofocus on modern cameras is simply fantastic technology, but still it has limitations and under certain conditions I will revert to manual focus.  The more you use (and therefore practice) with your camera and lenses and review your images after each shot, then the chance that the odd shot will be a bit soft diminishes.  Unless you are a beginner becoming familiar with your equipment, it should be apparent when difficulties could arise, or at least when we need to pay more attention to focusing.   Even then, there can be the occasional human error or an imperceptible movement with your otherwise sturdy tripod that can produce an out-of-focus shot.  

With my go-to Nikon D750, it clearly favours good light in providing easy, reliable and quick focusing; quite typical of most cameras of course.  But there is nothing more frustrating than arriving on location to find conditions in your favour and capturing what you expect to be a great shot, only to find later it looks a bit soft.  Post processing will not be a fix.  Unless the scene and light are changing very quickly and you can’t afford the time to make the necessary checks, then arguably this situation should not arise as it should be detected on location and adjustments made. I personally have difficulty using the depth of field preview facility; yes, becomes it produces a fairly dark image when stopped down, but perhaps my eyesight isn’t as good as it once was.   Live view is incredibly helpful allowing you to interrogate the shot at a magnification of 100% and checking the sharpness front and back, and if necessary adjusting the aperture and perhaps position of the camera if the foreground is soft.  If this is checked each time, then shots should all be acceptably sharp.

Returning to difficulties poor light and/or low contrast during those favoured moments of the day, i.e. around dawn and sunset, my camera will sometimes set off ‘hunting’ for focus and will occasionally fail altogether, and at other times it will apparently fix on focus only to find later that it was not accurate.  So it is quite obvious, at least with my Nikon that particular care needs to be taken in these conditions, and checks using Live View are essential.  If in doubt, and should conditions impair clarity when viewing the rear screen (for example very bright days and then a hoodloupe can be helpful), I will try a number of shots trusting that the camera and my controls will capture at least one acceptably sharp image.  A little bit last resort perhaps.

Returning again to practice or simply knowing your equipment and its particular ‘sweet’ spots will greatly help slickness of control, and getting it right first time.  One technique used by even professional photographers is simply to focus at ‘infinity’, but in the knowledge of the distance when features closer to the camera begin to show signs of softness.  So set up the camera on a tripod focus at infinity take a shot and magnify the image to 100%, and progressively examine parts of the  image getting closer to your camera.  The distance that the image begins to soften will be apparent and record this distance. Knowing that the foreground is beyond this distance, as it so often will be, allows you to focus at infinity knowing the image will be acceptably sharp from front to back.     

The photograph above of starling murmurations around the old Brighton pier, is an example of where focusing technique is simple and with plenty of contrast in the structural framework to assist focusing despite the dim light. Here simply focus on the structure, which is at infinity in any case, and with no foreground to worry about sharpness, will be excellent. The small depth of field in the shot was no problem for apertures starting at f5.6 and reducing further as the light faded and with the ISO being cranked up to to ensure a fast enough shutter speed so that the starlings weren’t just a black fuzzy cloud. It was a constant juggling act.

Linked to the above, is the question where to focus when the foreground is reasonably close?  For a given scene such as the one opposite, I am quite sure photographers would individually chose different locations with spot-focussing. Probably brought about by contradictory advice.  There seems to be a fixation with the hyperfocal distance and recently published books and articles in magazines still reference the hyperfocal distance as if it is the technique to be adopted. It’s almost as if every article about focusing must bestow the virtues of hyperfocal focusing, because that is what readers expect despite the fact that it in my experience it is rarely used, and does not automatically produce good sharpness front to back.

In fact, hyperfocal distance focusing produces an acceptable level of sharpness at infinity which in terms of acceptance can be noticeably a bit soft, and that’s the problem.  I don’t want to touch on circles of confusion! Many years ago I remember taking out on location a steel tape to measure the hyperfocal distance accurately (probably in connection with the above photograph) only to find the foreground was pin-sharp and the background was slightly blurred; arguably with such close foreground I should be considering focus stacking. The slight softness is in fact an artifact of the hyperfocal distance which has been calculated on the premise that 0.03 mm of background blur will occur. This may be too high for today’s high resolution cameras and monitors. So hyperfocal distance focusing will not produce the best possible focus. For many situations, if you know your equipment well enough and experienced/practiced enough then you instinctively know where to focus; at least the concept of optical physics behind hyperfocal distance focusing is important to appreciate.

To overcome some of the shortcomings of hyperfocal distance focusing and in attempting to produce equal ‘front and back’ sharpness some professional photographers advocate double-distance focusing - locate the closest feature in your shot that you want in focus and estimate its distance, double this distance, and focus there. It’s more reliable and accurate than hyperfocal distance focusing and the rule of thumb of focusing a third of the way into the image.