The Plane of Focus is a Pane of Glass
Recently somebody who is learning photography asked me to explain plane of focus, depth of field and quality of focus. I've been taking many photographs for many years, and these were subjects which came naturally to me - they were almost instinctive. I knew what actions and settings on my camera would affect these parameters. but explaining instinct to a beginner needed some care and thought. We photographers take these matters for granted, and know that a twist here or there in the controls would vary the three values.
I mulled the questions over for a while and devised my pane of glass model. I'll try to explain it here.
The Plane of Focus is a Pane of Glass
Pretend that somewhere in front of your lens is a pane of glass. It's a bit wider and a bit taller than the field of view, so you can't see its edges. It's perfectly parallel to the image sensor. That pane of glass is your is your plane of focus. Every object inside the pane is in focus; everything lying outside the glass is blurry. For this model let us largely ignore those objects which are just slightly blurry and assume that the magical properties of the glass somewhat extend beyond it; it has some influence over objects near to it yet not actually inside it.
OK, got that. Describe the glass and its effects
A number of settings on your camera will affect the glass, and the nature of the glass will affect the focus properties of pictures taken through the glass. Let's examine what affects what.
» The focussing ring on the lens, whether manually turned or automatically turned, via the autofocus mechanism.
Focus position - seen upon the lens itself as a distance - affects the location of the glass. At infinity the glass is far far away; let's say it's over yonder, at the horizon. When we focus to 15 feet the glass is 15 feet away - and so on.
» The aperture, whether manually set or automatically chosen by the camera
You'll be able to see the aperture value through the viewfinder or perhaps on the lens itself. Aperture determines the thickness of the glass. At f/2 the glass is thin; stopping down the lens, ie, dialling in higher-numbered apertures, thickens the glass. At f/22 - the maximum for most lenses - the glass is is going to be as thick as it gets. But we need to be careful here: making the aperture very small - the bigger f-numbers - reduces the quality of the glass. This is a phenomenon called diffraction. Most lenses are pretty free of diffraction up to f/11 or so, but anything beyond that will start to introduce diffraction. At the other end of the scale very open apertures, like f/1.2, might turn the glass into bathroom window glass, making pictures taken wide-open, as it's called, pretty disappointing. The glass should always be thickened; no lens is at its best wide open.
So we're beginning to get a feel for this bit of glass. We know how to shift it away from us or towards us, via the focus setting, and we know how to determine how thick the glass should be. Fine, but everything is relative. Set the focus to infinity and set the aperture to f/8 and snap that distant barn; it, and pretty well everything in its vicinity will be in focus. But walk half a mile and stand near the barn, refocus, and take another pic at f/8. Barn: still in focus of course; nearby trees and farm machinery: blurry. We meet a new feature of the glass: as the focus ring is turned towards nearer objects the glass gets thinner, regardless of how thick it was before, at the same aperture. Leave it at f/8 and go up to photograph the hinge on the barn door: the hinge is in focus but the wood it's screwed into isn't. How about that.
So, quite simply stated, the nearer you are to an object the thinner the glass. As you get nearer, thus thinning the glass, you can get some thickness back by twisting the aperture towards f/22. Then you can get the hinge AND its woodwork into focus. But then, huh? Why is the photo so bad? Now everything is blurry! Well, two possible explanations for that. If you went down to f/22 you might be seeing the impact of diffraction, and took the pic through the bathroom window glass. But the more likely explanation is that you dialled in f/16 (having remembered about our friend diffraction) but failed to check the shutter speed. Going to f/16 meant that to get enough light the camera had to take the picture at a punishing one fifth of a second. Very few people are much use at holding a camera perfectly still at anything below about a thirtieth of a second. I personally can hand-hold, usually with sucess, down to a tenth of a second, but I do have steady hands and I've picked up a few steadiness tricks over the years. So we can add another camera factor which will, albeit indirectly, affect the thickness and quality of our bit of glass:
» Shutter speed
No, of course shutter speed in itself won't do a damn thing to the thickness or quality of our glass, but it's a factor which will be taken into acount by the camera when setting the aperture, which does affect the glass. And whie we're adding adjustments, let's also consider:
No, of course ISO will not in itself have any effect on the glass, but back to the barn door hinge. We had the camera set at ISO 100 that day, and thus nicely stopped-down pictures needed impossible shutter speeds. Had we dialled in ISO 1600 we stood a good chance of getting that shot, due to a hand-holdable shutter speed.
Let's look closer: macro (whoops, no pun intended!)
When we come down to macro photography we're moving into the realms of specially manufactured glass, glass so thin it could shatter into a million pieces in a puff of breeze. Stick the camera on a tripod (perhaps we should add a tripod as yet another factor?), mount your 100mm macro lens and try to focus on the fly you killed with a quick spray yesterday. A tripod means that the shutter speed doesn't really matter, so it's all about aperture. So dial in f/16, avoiding diffraction. The glass is the thickness of a piece of paper; you can get one leg in focus, nothing else. Move the fly, now you can get one eyelash (if they have eyelashes?). The truth is, you cannot get any worthwhile fly-focus; the glass is just too thin, and there's absolutely nothing you can do about it. If the fly is very important to you then you must use focus stacking techniques, in which you, in effect, take lots of pics, each with the glass in a different place. You'll find a lengthy illustrated piece abut focus stacking right here at CallMeAlan.
So, to summarise.....
Focussing moves the glass back and forth; aperture thickens or thins the glass - but so does focus, because the glass automatically gets thinner as we move in closer. We can thicken the glass by reducing the aperture, but then we collide with our shaky hands, so we bump up the ISO. Or get a tripod! It's all a balancing act, fiddling around with settings hoping to achieve just the desired glass thickness at just the right place.
A final word, a final technique
Right at the beginning I said of the imaginary piece of glass that "It's perfectly parallel to the image sensor." True, true. But what if we could tilt the glass? We could get both the distant barn and the corn growing in the field right in front of us, plus everything in between, in excellent focus! What if we could get just one poppy, 175 feet away, in perfect focus, but NOTHING ELSE! WE CAN! I will leave you with some homework. Get onto Google or Wikipedia and read up about tilt/shift lenses.