one thing i need to get my head round is what 1 stop is in regards to iso/aperture
how much can i change the if i close the aperture by one stop what is the corresponding iso increase to get that 1 stop back
When you adjust an exposure by 1 stop that means that you have doubled, or halved, the available amount of light that reaches the film or sensor. Doubling the light, or increasing, the exposure is called opening up - halving the light, or decreasing the exposure, is called stopping down.
Unless you're stopping down, or opening up, for artistic reasons it's normal to apply a compensatory exposure change if you want to
maintain the same exposure. For example, if you decide to increase the shutter speed to catch a moving subject you have stopped down (less light is reaching the sensor) and so you must compensate by opening up the exposure via the aperture or the ISO setting (allowing more light to reach the sensor). In this way the light is balanced and so a perfect exposure can be achieved.
So what is a stop? It's easier to consider shutter speeds first. Starting with a speed of 1 second, it's clear to see when the light is halved or doubled:
1s 1/2s 1/4s 1/8s 1/15s* 1/30s 1/60s 1/125s* 1/250 1/500
Notice that each shutter speed setting is either half, or double, the length of time of the settings on each side (* or close enough that it makes no difference). So 1/60 seconds is twice as long as 1/125 seconds but it's half as long as 1/30 seconds. Changing the shutter from 1/60 seconds to 1/125 seconds has doubled the available light so you have opened up by 1 stop. Changing from 1/60 seconds to 1/125 seconds has halved the amount of light so you have stopped down by 1 stop.
ISO numbers refer to the sensitivity of the sensor (or film) and are also fairly easy to understand as most of them are single stops:
25 50 100 200 400 800 1600
Again you can see that the numbers are half, or double, that of the numbers either side. In other words, your sensor is twice as sensitive with an ISO of 400 then it is with an ISO of 200, whereas ISO 100 is half as sensitive as an ISO of 200. If you change your ISO setting from 200 to 400, you are doubling the light that the sensor can record so you have opened up by 1 stop. Change it from 200 to 100, and you have halved the light that the sensor can record, so you have stopped down by 1 stop.
Things might appear to be trickier with your aperture settings because the f/numbers may appear to be confusing. However all becomes clear once you appreciate that the number doesn't represent a physical size - it's a ratio from which the size can be calculated. I'll explain how they work later but, for now, all you need to remember is that the f/numbers marked on most lenses are given in single stops:
f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22
So, just as with the shutter speeds and ISO settings, f/5.6 allows twice as much light to reach the sensor as f/8 does, but only half as much light as f/4 does. Changing the aperture to f/4 has increased (opened up) the available light by 1 stop whereas changing it to f/8 reduces (stops down) the light by 1 stop.
Hopefully you can already see the answer to your question now, but I'll make it even clearer. If you open up your shutter by 1 stop, then you stop down either the aperture or the ISO by 1 stop -
to maintain the same exposure. If you stop down your shutter by 3 stops, then you must open up the aperture or ISO by 3 stops -
to maintain the same exposure. Whatever exposure change you apply to one setting, you must apply the
opposite change in another setting -
to maintain the same exposure.
Here's a quick example. Your camera has measured the scene and suggests an exposure of 1/60s at f/4 on your ISO setting of 200. However, your subject is moving quickly so you decide to increase the shutter speed to 1/125s. You have actually stopped down by 1 stop, so to maintain the same exposure, you need to open up the aperture by an equivalent 1 stop, i.e. you must change it from f/4 to f/2.8. But what if you want to keep the original aperture setting? In that case, to maintain the same exposure, you must open up the ISO by changing it from 200 to 400.
You can also combine the settings. Assuming your camera suggests the same exposure - 1/60s at f/4 with an ISO of 200. You decide that you want much more depth of field so you stop down the aperture to f/8 which is 2 stops different from the original exposure. Because you've forgotten your tripod, you are going to risk hand-holding the camera but you know that 1/15s won't work. So you open up the the shutter by just 1 stop, to 1/30s, and you also open up the ISO by 1 stop to 400. Once again, you have balanced the exposure.
If you do nothing else, but memorise those single stop numbers, you will be well on your way to understanding how to balance any exposure.
But what about all those other numbers you see?
Most cameras are designed so that you can make adjustments in half- or one third-stops. So, depending on the complexity of your camera model, you might see shutter speeds of:
1/30 1/45
1/60 1/90
1/125
These intermediary numbers are half-stops - 1/45s allows half as much extra light as 1/30s but prevents half as much light as 1/60s.
Or shutter speeds like:
1/30 1/40 1/50
1/60 1/80 1/100
1/125
These are third-stop settings. 1/50s allows two-thirds as much light as 1/30s but only one-third as much light as 1/60s.
Some lenses might also have half- or one third-stop f/numbers marked on them although it's more common to see these indicated in the viewfinder on digital cameras:
f/2.8 f/3.2 f/3.5
f/4 f/4.5 f/5
f/5.6 f/6.3 f/7.1
f/8
Once again, these are one third-stops with the full stops marked in bold. So you can make adjustments to the aperture, or shutter, in smaller increments and also make the same complementary adjustments to the shutter, or aperture, in the same small increments. So, using the earlier example, opening up your shutter speed by one third stop from 1/60s to 1/45s requires you to stop down the exposure by one third from f/4 to f/4.5
to maintain the same exposure.
ISO settings are usually given in single stops but it was relatively common in pre-digital days to buy film that was measured in one third-stops:
50 64 80
100 125 160
200
So that hopefully explains what all the various numbers you can see displayed in your camera actually mean. More importantly, it should help you to understand how you can make exposure adjustments in one setting but then balance them up in another setting
to maintain the correct exposure. You should now have figured out that, if you close (stop down) the aperture by one stop then you must open up the ISO (or the shutter speed) by 1 stop.
Why are f/numbers so funny?
There's no need to read this bit if you don't want to...but here comes the simple science. I mentioned earlier that f/numbers are just a ratio. This ratio is a way to calculate the size of the aperture (and therefore the amount of light reaching the sensor) for any lens. The calculation is quite simple and there's a big clue in the way we write the
f/
number down: the aperture diameter = the
lens focal length /
f/number. So a 50mm lens with an f/stop of f/1.0 is f (focal length) / 1.0 or 50/1 = 50mm. There, wasn't that easy? With an f/stop of f/2.0 then it's f (focal length) / 2.0 = 25mm. An f/number of f/1.4 is 50 / 1.4 = 35.7mm
This way of calculating an aperture size by using the f/number ratio holds true for any lens.
The more astute readers might now ask ahh, but why is f/1.4 a single stop number when the diameter isn't half the size at f/1.0, or double the size at f/2.0? Which is a good question with a relatively simple answer. Unfortunately, unlike halving a shutter speed or an ISO setting, reducing the diameter of the aperture doesn't half the amount of light. That's because an aperture is a circle so it's the
area of that circle which actually determines the amount of light reaching the sensor. Again, this can all be calculated, so bear with me while I keep it simple with three examples:
You'll remember from school that the radius of a circle is pi * r squared, or 3.14159 times the radius of the circle multiplied by itself. Here's an example using the same 50mm lens at f/1.0:
diameter = 50 / 1 = 50mm
radius = 25mm
r squared = 625mm
area = 3.14159 * 625mm =
1963 sq. mm
With an f/number of f/2.0:
diameter = 50 / 2 = 25mm
radius = 12.5mm
r squared = 156.25mm
area = 3.14159 * 156.25mm =
491 sq. mm
With an f/number of f1/4:
diameter = 50 / 1.4 = 35.7mm
radius = 17.85mm
r squared = 318.87mm
area = 3.14159 * 318.87mm =
1002 sq. mm
So, as you can see, the area of an aperture at f1.4 is almost half the size of the area with an aperture of f/1.0 so this is a single stop. Similarly f/2.0 is almost half the size of the area at f/1.4 so this is the next single stop. And that's why all the other f/numbers are single stops...f/4, f/5.6, f8, f/11 and so on. The area of light reaching the sensor at the specified ratio is half or double the amount of light for the next ratio, irrespective of what size of lens you are using.
It's actually possible to calculate the f/number in the field by reversing all of these calculations. For example:
Your original aperture was f/4 with a 50mm lens and you have opened up the shutter by 1 stop - what aperture do you need to use to compensate in order to
maintain the same exposure?
You know that the diameter is 50/4 = 12.5mm so the area is 3.14159 x 6.3mm squared or 124.7 sq. mm with an f/number of f/4.0. Dividing that area by 2 (you need to half it to reduce the light by 1 stop) we get 62.3 sq. mm which we can now convert into an f/number:
62.3 / pi = 19.85 mm
Square root of 19.85 = 4.45mm radius
Diameter of aperture = 4.45mm x 2 = 8.91mm
50mm focal length / 8.91mm = f/5.6
Wow, who would've guessed that? I wouldn't suggest you really do this in the field as it's far easier to just use the markings on your lens, but you should now understand how f/numbers work and how to calculate them if you ever need to.
May have to look into getting that lens after bag + tripod etc. 
At the minute I've got a lens hood, tripod, bag, rocket duster and uv filter on my shopping list. I will prioritise everything but the tripod for now as I feel those are more necessary items. I've got an extra battery and both are 100% charged, I may buy another battery and compact flash card. What's a good bag for kit?