A little semi-technical post, concentrating on one of the three things that are often at the core of photographic failure*. This should be on my technical page I guess, but it is not, so deal already.
Types of focus
Manual focus (MF) is where you manually move the focussing mechanism (usually a ring around the lens, but sometimes not), or choose a focussing point by touching a screen. Either way, you pick the spot, not the camera.
Manual focus is the best choice when you need focus to be locked on a point, to be very precise, maybe irrationally out of perfect focus (artsy speak there) or you simply do not want/trust the camera to choose.
Auto focus (AF) is usually when you apply pressure to the shutter button and the camera responds by finding a point of focus (which is a very important point). AF is also possible using the photographers eye, screen touching, sometimes simply turning a camera on and pointing it etc.
Auto focus is ideal when you need speed or when manual focus may be difficult.
The choice should be easy, but unfortunately, our reliance on auto focus tends to over-ride logic.
A prime example is street photography. In most cases, speed of composing and shooting is the key to success. Many street shooters need to compose almost instinctively and auto focussing adds one more obstacle. Taking time to focus > recompose > shoot is not quick enough. Also creative use of depth of field, blur, abstraction also matter. Zone focussing is a good starting point which requires manual control.
Using manual-zone focus (pre-set using a distance scale on the lens), the foreground subject was ignored, making the four in the middle more important. Incidentally, that is a Billingham bag over the boys shoulder, which in Japan would have likely set him back an eye watering $6-700au.
For a long time, during early AF dominance, but well before it was actually perfected, manual focus was often difficult, even impossible on new cameras. I remember buying a special screen for my 5D Mk2 just to allow accurate MF with a wide aperture lens, because the focussing screen supplied was tuned to about f4 to 5.6 dof to aide with AF composition. This made use of any wider aperture literally a guessing game in manual focus. This was so important to me, it was one of the main reasons I avoided the 5D Mk3 as it could not change screens.
Ironically, the latest, often AF perfect cameras actually make it easier to manually focus with electronic view finders and tools like “peaking” replacing the need for ground glass or split screens.
Depth of field and focus accuracy
Depth of field or literally the depth of in focus area in an image is dependant on a lot of factors. Aperture, magnification, distance to subject and its relative distance to its background are the main ones as is the “Bokeh” or transition from in-to-out of focus areas produced by any given lens (which is also effected by the above factors).
Depth of field falls mostly behind the point of best focus. Important point, that right there.
This is deep depth of field, easily produced using a semi wide lens (15mm), small aperture (big number like f6.3) and long relative distance from the camera to focus point (vending machine), but short relative distance from the focus point to it’s background.
Shallow depth of field produced even more easily than above by employing a wide aperture (small number like f2) on a long-ish lens (45), used quite close to the focussed subject (the sign), with the background relatively further way.
So, once you have determined your depth of field and focus point, you have taken the lions share of creative control into your own hands. Feels good right?
So, if depth of field is both sometimes limited and highly creative, how do we guarantee it will fall where we want it to.
Below is a simple example of focus being used to shift the sharp and soft areas of an image.
Ok, hold on, plenty to think about here.
If you are using MF, the main determining factor for accuracy, after depth of field selection which will determine the amount of in focus you have to play with, is some kind of focus assistance, like peaking (a coloured fringe around the in-focus bits), coincidental indication (a split or patterned screen that aligns and “pops” clearly into focus), or increased magnification either in the view finder or the rear screen. The first and last are digital innovations, the middle one is old school.
Work out what options you have, then decide on the one that works for you. White peaking is my go-to now or simply magnifying the rear screen for landscape, a laser matt screen was my preference with SLR’s.
Use manual focus when loose composition requires off centre focussing or you are doing slow and accurate projects like astro or landscape work.
For AF, you have to decide on a couple of things.
Single shot or continuous, which is as it sounds;
Single shot holds the chosen focus point until you shoot or re-focus, which is ideal for static subjects, while continuous constantly changes if the subject moves as long as you hold down the shutter button.
Focus point pattern or area. This is the amount of the screen area that is activated to focus. The fewer points or smaller the area, the more accurate your focus, but the harder it is to hit the subject. This can also take the form of touch, eye, animal or face detect focus.
Once selected, the focussing pattern can often be shifted around the screen, but also try to focus in the middle, then while holding the shutter button down (in single shot mode), re-compose and shoot.
This image was taken using a long lens with a group of three small boxes stacked on each other which is roughly “human” shaped. The stack is quickly shifted across the screen using five spaced columns for hard left/right, middle left/right or central area focus. It is accurate, but twitchy, allowing me to shoot a single person in a group, but I accept a higher hit/miss ratio than blanket coverage would allow.
For a shorter lens, I use a slightly “fat diamond” shaped cluster of five points, because a closer subject is often relatively bigger and faster. The finer control from the longer lens if used in the above image would possibly miss the pair or grab the ball.
Often for considered or precise work, a single focussing point is ideal, especially in single shot mode as you can accurately pick your point, then re-compose the image to suit. manual focus may even be better.
Face/eye/animal/vehicle detection can help if available (and trust worthy), but do not use these universally for obvious reasons.
In focus or not?
Actually acquiring sharp focus on the intended point of best focus is assumed, but what if it is not?
This image is in focus by most measures. Sharp from front to the main subject (assume it is the pumpkin head). Depth of field does drop off towards the back, which is fine.
This is the same image a little out of focus (pulled forward for even OOF rendering). The lights become “Bokeh” balls which are high quality thanks to the wide open, circular aperture of the lens (45 f1.8).
This is the same settings, focus pulled further forward, so the Bokeh balls are starting to overlap. None is more “right” than another, it is just up to personal choice.
Ok, so lets re-cap.
For speed and accuracy under pressure, but only if you can control where the subject may be or you have no better idea than the camera, so may as well trust it, use camera AF, with the tightest pattern of AF points you can get away with, but also look to touch or intelligent focus if suitable.
Use single shot mode for static subjects, continuous if the subject (or you) are moving. AF can also be useful in poor lighting situations with AF assistance (focussing on a small light sent from a camera or flash unit).
If time is not an issue, but accuracy and consistency are or you actually want focus to be more abstracted, use manual focus. If you control depth of field well enough, manual focus is also ideal for see-shoot situations like trap focus in sports or street shooting and often in studio situations, where AF can be fooled by low light and plain backgrounds or simply fail.
Hope that helps.
*Poor focus, poor exposure, poor motion arrest, which were the three biggies we used to concentrate on when showing people how to use their compact cameras.