Ok, I can see I am either confusing you or boring you to death.   But we are nearly ready to make use of all this background knowledge you have accumulated.  I want to explain how you decide which camera mode is best for your scene, but before I do that there is a couple of other things to do with aperture size and shutter speed that you just HAVE to know.

It’s a uphill slog but we are getting there.  First, lets look at shutter speed.

The tortoise and the hare

There are reams of mathematical calculations that the avid photographer will want to absorb and digest in order to understand the next bit, but I’m not a maths teacher – in fact the idea of telling you about the various formulae and ratios is making me feel a little queasy.

So let’s not go there.  But in simplifying the next two important photographic principles, I’m leaving a lot of important technicalities out, so I apologise in advance to those who know more about it than I do and think I’ve over simplified.

Going back to my tortoise and hare; if you want to take a picture of a tortoise, especially one that isn’t moving, then you are probably going to have ALL day to press the button.   And remember that little shutter thingy inside your camera that exposes the film to the light? It can leisurely open  – up until the exposure meter says “Oy, enough!” and it closes again.  But photographing the hare is a different matter – it’s going to whip past you in a second, and that shutter is going to have to be a bit nifty if it’s going to catch the hare legging it across your lawn.    If we let the shutter go at its leisurely pace, it will actually move all the way across the camera’s field of vision as you capture the shot, and all you will see is the blur as it passes by.   No, if we want to capture the hare so we can actually see it, we need to freeze the motion.  And we do this by capturing just a split second of the time the hare is passing the camera, rather than the whole trip.

How do we set this time?  Well let’s for a change work backwards from the best equipped cameras to the basic cameras – it will make more sense.

The shutter on all cameras can be set to various speeds; that is, the length of time the shutter is open and exposing the film or sensor to the light.  Some cameras set this automatically, and some you are able to select this length of time yourself.  Unsurprisingly this is called the shutter speed setting, and can range from a number of seconds (commonly 2 or 4 seconds) to a very small fraction of a second – commonly 1/2000th of a second, while some cameras can go even faster, 1/4000th or even 1/8000th.   Imagine that for a moment.  If I said, “open a window, count to 4 seconds and then close it instantly”, you would do a reasonable job, but there would be a bit of time wasted getting the window open, and a bit more closing it afterwards.  So you either end up with 4 seconds fully open plus a second or two partially open, or you don’t get a full 4 seconds at open, because some of the time is taken up opening and closing the window.   So now, imagine me saying I wanted you to open it 4,000 times faster – Instead of four seconds, I want it open for just 1/1000th of a second.   Impossible to do right?   So imagine the precision and technicality that goes into a camera that can not only open and close it’s shutter in 1/1000th of a second, but can do it over and over again and get the same exposure, time after time.  That’s impressive.

If you imagine how far a running hare can move in 4 seconds, imagine how far it could move in 1/1000th of a second.  Not very far at all, and it would be virtually frozen in time.

So here is a simple rule of thumb.  Fast moving images need fast shutter speeds to prevent blur.  The faster the subject is moving across the camera’s “field of view”, the quicker shutter speed you will need to stop it in its tracks without those tell tale blurs.

Here is a rough guide to recommended shutter speeds:

Getting the light right

Ouch – I’ve just remembered that important rule of reciprocity.  If I change the shutter speed, don’t I have to change the aperture size to compensate?

Yes I do indeed, because think of it this way.   If 1 second lets in the correct amount of light for your aperture, then 1/1000th of a second only lets in 1/1000th of the light needed, so you need to let in the missing 999/1000ths of light somehow, otherwise your piccy is going to be VERY dark!

You can do this manually on more advanced cameras- setting the shutter speed manually and then changing the aperture step by step until your meter says the exposure is OK, but, before you go looking for the meter manual, what about trying this…

 

SHUTTER PRIORITY!   Yes, I knew all that background info would come into play eventually.  Remember I said that with shutter priority, you get to choose the shutter and the camera sorts out the aperture?  Well, this is why it’s there.   So, you’re taking a picture at the Formula 1, you need to make sure the camera never chooses a slow speed which would result in blurred cars (which it may do if you left it in Program or Auto), then put the camera in S mode, set the shutter to 1/1000th using the camera’s controls and the camera will sort out the exposure itself.

In compacts, you don’t get  quite that much control, but you do normally have picture of a running man, which in effect tells the camera to set the fastest shutter speed it can given the maximum aperture and amount of light available to it.  In fact if you look closely, the dial above also has a program mode for fast moving objects, designated by a logo of a runner (next to the flower bud)

Perception of speed at distance

Why is it a plane which is in the distance and travelling at 500 mph appears to crawl across the sky, but they seem to scream past like lightning when they are landing, even though by then they are doing a mere 140 mph?

This is very hard to demonstrate without loads of pictures and diagrams, which I’m trying to avoid, so let me put it to you this way.   Imagine you are sat in a comfy armchair across from a patio door on the other side of a darkened room.   From where you are sat, you can see in the distance the whole fence line at the bottom of the garden and much closer, the small area of patio directly in front of the door.

Now, imagine you are looking out into the garden and a squirrel casually skips across your fence from left to right.  You see it appear into view on the left hand side of the fence, quickly crossing from the left hand side of the door frame to the right hand side, before it disappears out of view.   Let’s say it takes about 10 seconds to cross from left to right.  You happen to have a camera on your lap.  Now, as soon as you see the squirrel appear on the left of the window, I want you to just pick up the camera, point towards the window and ‘click’.    Odds are, if you can still see the squirrel, you will have caught it in your image.

Now, lets do this again but have the squirrel run across your patio, right in front of your door.   It appears on the left, but it’s passed by the door before you’ve even managed to lift the camera.  Damn – must be quicker next time.

The squirrel hasn’t changed speed, so how come you got it in the first shot when it was way off, but you missed it when it’s closer?

Ok,  One diagram then.   Let me draw out what has just happened – in a VERY rough and inaccurate fashion!

It’s all about angles and distances.  But that sounds like maths, so let’s give that a miss. Hopefully you can see from the diagram above that the further something is from you, the wider view you have of it.  So you might be able to see the whole fence, but you can’t see the whole patio as the view closer to you is cut off by the edges of the door frame.  That width of view is know as the ‘field of view’.

And If you measure the distance between the edges of your field of view close up on the patio, you will see that the width of field of view is much narrower close up than it is further away.  In fact, at a distance the field of view on my diagram is over six times the width of the field of view closer to the viewer (count the orange arrows, which are all the same length).

So put simply, the squirrel only has to travel one arrow width close up to completely traverse your field of view.  And that takes no time at all.   But at a distance, you can see the squirrel has to cross six times that distance to cross your field of view.

So to freeze motion in a moving object, you need a faster shutter speed the closer the moving subject is to you.   

Ok, you know the drill now.  Let’s consider this in our camera selection:

Do you want to just let the camera try to guess the scene (fully auto basic cameras), do you want to at least give it a clue by selecting “Running Man” to warn of fast moving subjects (most compacts)  or do you want to be able to fully control the shutter speed and take the exact shot you are after?   (Some bridge / All DSLR cameras?)

Next – The magic of the aperture…

To be continued