Hi and welcome again to one of My Photo tutorials. My name is Rob Barron and I am taking you to various different basics of using SlR’s in order to do your photography.

This is my SLR. This is a Canon 40D, but you might be using a different one. It does not matter. I bet all of what we are going to talk about today will apply to any digital SLR that you are using. In fact what we are talking about today will apply to any SLR and even a lot of compacts.

Here, I have got some fruit, two nectarines, two oranges, and two peers. Okay. Now, if I talk about them as nectarines, I have only got two. I have only got two pairs. I have only got two oranges, but I have got six pieces of fruit. Agree? Of course you do. You can see them here, right.

Now, what we are going to be talking about today is reciprocity. No! Do not go running away. That sounds complicated, but it is not and my purpose today is to show you exactly what that means and how important it is for using your camera.

So, let us go back to fruit just for a moment shall we. We have got two peers. We have got two oranges. We have got two nectarines. But let us just leave the oranges out for a moment. And the peers, okay they belong to you. The nectarines, these are mine, okay. Now, if I say to you hey fancy swap. I mean you have got two pairs, I have got two nectarines. How about you give me your peer? Well you are not to kin on that because now I have got three pieces of fruit and you have only got one piece of fruit. That is not very fair. I say, well look, how about I will give you one of my nectarines instead. Is that okay? You would say okay, yes that is fine, right. Okay, now you have got a nectarine and peer and I have got a nectarine and peer too. So, we both got two pieces of fruit. So, what we have done is we have reciprocated each other. I took a peer from you. You took a nectarine from me. We are both happy because we still have an equal amount of fruit. Nothing has changed except the type of fruit that we have got. You might think Rob has lost it. This has got nothing to do with photography. It has.

Let us take them and call them something else. I know this look like fruit. I know they look like peers, but we are going to call them these shutter speed. And these two nectarines, we are going to call them those apertures. And these two oranges, we are going to call those ISO. Yes, I know that is a catchy name, but ISO is on a camera, it is what you should have called film speed and we could call it the sensor speed now, okay. It is how quickly how the sensor responds to light, okay.

So, we have got ISO, we have got apertures and we have got shutter speeds, okay. If we talk about shutter speeds and things like this in this way which we do not, we could say “look, I would tell you what. I will take one of your shutter speeds and you will have one of my apertures and we have still got the same thing.” You might think “I am a bit confused.” Well, I understand. So how about instead of talking about fruit, we talk about the real thing. Is that okay? Right. Let us put this to one side. There we go. I will eat you later. The shutter speed, the aperture and the ISO, they come in three different scales and you are looking at those on your screen right now. You can see that the aperture goes from a big wide open aperture F1.8. You can actually go even wider than that, but there are not many lenses that have those except for very expensive ones. So we will stick for practical purposes with F1.8 and then we go to F2.8 and then F4, F5.6, F8 and so on down the scale. And each one of those steps which all look a bit different, but each one of those steps is called a stop, okay. So when we go from F1.8 to F2.8, we say we are going down one stop. Down, not up? No, it is down because remember the aperture when we go up in number is getting smaller, so we are going down, okay. With the shutter speed, we go in 130th, 160th, 125th, 250th, 1500th and so on. And again, you will notice that 30th to 60th, yes that is the double the speed which is half the amount of light because it is only open much quicker. So it is open half as long, therefore we are getting half as much light through it, okay.

So, again these are called stops. So from 30th to 60th is one stop. From 30th to 125th, that is two stops, 60th, 125th and then down to 250th, that is another stop. Now because we are calling them stops instead of shutter speeds and apertures, it is just like having fruit. We can talk about going opening up a stop, going down a stop. Upper stop and aperture, down stop in shutter speed and so on.

How does that affect our photography? Well, now let us have a look. Let us have a look my camera screen here. This is my information screen on the camera and this is going to give us some information. Now, right here we can see the shutter speed and you will see it says 125 that is 125th of a second. The next number over here is 8.0 that is the aperture, the F-stop. So this is F8. And down here, you can see we have got a number 200 and that is my ISO speed, the film speed or sensor speed that we were talking about. So those are the three numbers. The other number up here, you do not worry about. That just tells me how many focus I have got remaining on my memory card. So those three numbers are the most important numbers when it comes to choosing my settings for taking a photograph.

Now, some of these can be done for you and if you want to understand more about that have a look at my Av mode and my Tv mode videos and you will understand more about those. 125th of a second at F8, 125th of a second, if was to half that amount of time and make it 125th of a second, I would need to then allow twice as much light to come through the aperture if I want the same amount of light to fall on the center because the shutter speed is now only open for half as long. And so I am going to reciprocate by opening the aperture up more. I am going to go to F5.6 which is one stop up. I have now doubled the amount of light coming through the aperture and I have half the amount of light going through the shutter because it is only open half as long. What is actually happening then is I am putting the same amount of light through to the center, but in a different balance. If want to have a shallow depth of field, then I will go to a wide aperture. If I want to be the depth of field, I will go to a narrow aperture. But if you want to know more about that that is already there, a video tutorial of mine called understanding depth of field. Do have a look at that.

So, what we need to understand is this principle of opening the aperture more, we can cut down the shutter speed and vice versa. And we can do the same thing with the ISO. The ISO is making the film or the sensor more sensitive to light. If it is mores sensitive to light, it is responding more quickly, therefore I can reduce the amount of light I am actually pushing through the lens and it will still have the same effect the same. And if you look at our chart, you can see the way the lines are going to show that they are reciprocated. Open one close another, you have the same thing and you can adjust any two of these to give you these different effects. So you could adjust the ISO against the shutter speed or you could adjust the shutter speed against the aperture or the aperture against the ISO. You can choose whichever way suites the kind of picture that you are taking at the time.

Do look at my other videos because that will help you to understand some of the other principles that we have talked about. But I hope now that you can understand what reciprocity is all about. Thanks for watching. See you again soon.