This post is the first of a series, all to do with a challenge that I’ve set for myself. The idea is to write 26 posts in total, each with a title based on a different letter of the alphabet, all of them being informative about photographic topics, and, I hope, especially helpful to those of you who want to get more out of your photography, and not just stay in the ‘fully automatic program’ mode!
As I type this, I’m not sure how frequently I’ll get them done, but we’ll see… OK, here we go with the letter A!
The word aperture means an opening or a hole. In the context of photography, the opening in question refers to a hole in an opaque component which is built into a lens. In most cases, and certainly in lenses used with cameras like DSLRs, this component is in the form of a diaphragm made of several blades (typically 5 or 7) arranged in a circle. These blades are adjustable by a system of pivots, springs, levers and so on (obviously all very tiny) so as to vary the size of the approximately circular hole left in the middle – all a bit like the way the pupils of your eyes change in size. This adjusts the light-gathering power of the lens. Clever, eh?
Now we need to address a particular issue: how should we define the size of the opening? Can we just say that it’s set to 3mm, 6mm, or whatever? Well, unfortunately, that gives us a problem; the resulting light-gathering power (which is what we’re really interested in) isn’t determined by the absolute size of the opening, but its size as a factor of the focal length of the lens. Now, I’ll add a couple of things straight away: one, we’ll discuss focal length of lenses in detail another time, so stand by on that for now, and secondly, the bit in italics isn’t quite true, but it’s close enough for now, OK?
Right: now, in practice, the aperture size is typically way smaller than the the focal length, so it’s sensible to think of it as a fraction. If,say, the focal length of our lens is 100mm and the opening is set to 12.5mm in diameter, the aperture setting would be 100 divided by 12.5, or in simple algebra, f/8. And because saying ‘f over eight’ or whatever, is a bit of a mouthful, we just say ‘f eight’ or ‘f sixteen’ or whatever.
So now, a whole lot of things, that puzzle many newcomers to the practical science of photography, start to make sense. Let’s look at some of them, in the form of questions…
1. Why is, let’s say, ‘f eight’ a bigger aperture than, say, ‘f sixteen?
These are fractions, remember! Just as an eighth (⅛) is twice as much as a sixteenth (1/16) so f/8 is twice as much as f/16. Easy to see, now, isn’t it?
2. I know that the sequence of f numbers goes 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32…
Yes that’s right. Go on.
And they tell me that each increment represents a change in light-gathering power equal to halving it, so f/11 will give me half the light-gathering of f/8…
But 8 times 2 isn’t 11! Explain, please.
Ha! I thought we would get to that. This is a very important point. Remember, those ‘f numbers’ relate to focal length and diameter. BUT the amount of light that will get through the hole isn’t a function of its diameter, but its area, which in turn relates to the square of the diameter! So twice the diameter will give four times the light. To get just twice the light (or half as much) the factor for the diameter must be the square root of 2, which is about 1.4 (1.4 x 1.4 gives you 1.96.) Now look again at that sequence. Makes sense now, doesn’t it?
3. Why do photographers call a change of one aperture increment ‘one stop’ and refer to setting a small aperture as ‘stopping down?’
Well, nowadays, cameras can adjust the aperture setting using electronics and some extra mechanical bits and pieces. But before this, aperture was set by moving a ring on the lens. And so that you knew what you were doing (even without looking) this ring moved in clicks, from one detent point to another (a bit like the setting knob on a lot of modern washing machines.) Hence the term ‘stop’. This isn’t all that long ago, by the way.
But you’re OLD, Phil…
Yeah, yeah, no need to rub it in. When I need help filling in the form to get my bus pass, I’ll let you know, OK?
I have only two more things to add, for now. One issue is that if we use the lens as a viewfinder, as any DSLR does, we need as much light-gathering as possible when viewing. To allow this, the control system in the camera keeps the lens at maximum opening until the shutter button is pressed to take the shot. Only at this point is the aperture reduced to the chosen setting by the electronic and mechanical systems in the camera. And the final question is, if high light-gathering is good, why do we bother with an adjustable aperture system at all? The answer to that will be discussed in another post. Watch out for the letter D!
In this picture of the back of a lens (click on it to enlarge it) you can see the diaphragm blades and (just) the hole in the middle. Also, to the left of the optical part, you will notice the little sticking-up bit that connects with the camera to work the adjustment of the blades.
Thank you for reading. Please put feedback – or questions – in a comment!
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