I recently put these words in context for a friend, who was looking for a quick comprehensive guide on how (and why) to manually prepare his camera. Maybe you get something out of this, too:
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“So if I would break it down, maybe also for a future blog of mine for my website, I’d probably put it like this
(and I keep reminding myself that sometimes having ANY shot can be better than none at all, so everything comes with a grain of salt, always):
Capturing light has a technical and an artistic side, which play closely together.
The physics are fixed – we have shutter speed, aperture (or F-stop), and ISO or sensitivity.
Then of course you have the three dimensions in which you build your shot:
What to get into the frame, how close you stand from it, and where you guide the eye in the Z-axis by using focus. Focus I imagine as being a “wall” or pane, that you manually push along the Z-axis back and forth via the focus ring (or, you let the auto focus help you push it).
a) ISO, or “sensitivity” of the camera sensor.
The sensor in digital cameras can amplify the incoming light for processing, making dark scenes brighter.
The more you amplify light though, the more “noise” or grain you will add to your picture.
So it makes sense to try to have the ISO as low as possible always (“native” ISO being mostly 100).
Of course, having a camera with a good sensor helps producing good images up to 3200 ISO at least, while always trying to set the lowest possible value, in evaluation to b) and c):
b) Shutter speed, or how long will your sensor be exposed to the light you chose to get through your lense.
Taking photos handheld with too slow of a shutter will blur your pictures, because humans do not have really steady hands (except for surgeons maybe). The slower you set your shutter speed, the worse the blurring will be.
With a tripod (or flash, but that would be unfair at this point) you can get away with slower speeds / longer exposure times of course. But if you have moving objects or subjects in front of your lens, you will then see them move, i.e. appear blurred then of course.
As a rule of thumb, when you are going handheld your shutter speed should be at least [1/focal length].
I.e. when you have a 30mm lens (or a zoom lens set to 30mm), your shutter speed should be at least 1/30th of a second (better 1/60th, but you will find out yourself). Is your lense more tele, like say 85mm, you would need at least 1/85th of a second (or the next higher number your camera offers you).
c) Aperture, F-stop, or “how big is the diameter of the tunnel my light has to travel through”.
Here it gets a bit more interesting.
Technically, the more light you get through the lens and then onto the image sensor, the more options you have.
That means if you have good and sharp lenses with big maximum aperture (=F-stop) of 2.8 or greater (the biggest known to man, or woman, being 0.95 at the moment), the “faster” this lense is. Fast, because it allows you to set your shutter speed faster, while getting the same amount of light onto the sensor.
No wonder lenses producing sharp images at big apertures are more expensive, as the optical system needs to be made to perfection.
You will notice however, that any lense that you shoot with “wide open” / with the maximum aperture (=smallest aperture number), will give you a relatively shallow depth of field. Meaning, the “wall” or pane of in-focus area in your picture will be thinner in depth, the bigger the aperture you shoot with.
For example, say you are shooting a portrait with a lens around 50mm focal length, your subject is 2,5 meters away, and you have an F-stop of F1.8 (which is a rather big aperture, good on you ;)
Then it will be hard to get the depth of the head all in focus, because your “depth of field” will be around only 5cm at your current settings.
You kinda have to choose between eyes/nose, or ears being sharp, since your focus wheel will push an only 5cm thin wall of “in-focus” through the room, along the Z-axis within your shot.
You then would probably change the aperture to something like 2.8 or even 3.5, and by closing down the opening of your lens you will increase this “wall of focus” in depth, allowing you to capture the persons’ head entirely.
All the while the rest of the image closer to you and also behind your subject remains comfortably blurred (can you spell “bokeh”).
You don’t need a super fast shutter here, your subject is sitting or standing, so 1/100th will be great for the job.
And when you close the aperture further down, almost everything in the entire depth of your shot will be in focus (sidenote: Avoid apertures smaller than F11 as a rule of thumb, as other optical problems will occur then ;).
Also keep in mind, that this “wall of focus” or “depth of field” gets thicker, the further away your subject is from the sensor of the camera.
And here you already see the trade of, in the third dimension or “Z-axis”.
Everything to be in focus needs closed down aperture, which then needs more light to compensate, which I either get from long exposure times / long shutter speeds, and/or higher ISO settings.
(Or flashes, studio lights,… but that is off the camera point for now :-).
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