Originally Posted by yelvertoft
(Post 567)
Figuring that we’ve probably got some people lurking here who have just got a “serious” camera for the first time, and may be unsure about using manual exposure mode, I thought I’d put together an article.
Firstly, it’s worth remembering that for all the modern cameras fancy features, you are essentially still only changing 3 things to get the exposure right:
1. The aperture – how big the hole in the lens is
2. The shutter speed – how long the hole is open for, and
3. The “film” speed (ISO number) – adjusting how sensitive the sensor is
The best analogy I’ve come across for all this is that it is like filling a bucket from a tap. Getting a bucket full to the brim is equivalent to getting the exposure right. Not filling the bucket is under exposure, overfilling the bucket so that it spills is over exposure.
The aperture is the flow rate of the water from the tap, tap on fast = wide open aperture.
The shutter speed is how long the tap is on for, tap running for a long time = slow shutter speed.
The ISO number is a measure of how big the bucket is, small bucket = high ISO number of your film or digital sensor.
In the days of film, you could only change the bucket for a different one when you changed films, now you can change the size of the bucket with every shot. However, you can see that there are a multitude of different ways of filling it. For a given size of bucket you can have the tap on very fast for a very short period of time, you can have the tap on very slowly for a very long time, or you could have anything in between.
So, enough about filling buckets with water, how does this translate to photography?
Your lens has a range of adjustment to set the aperture, these set the size of the hole the light can pass through. These are marked as f-numbers. The standard sequence of f-numbers runs as:
1.4 2.0 2.8 4.0 5.6 8.0 11 16 22
This seems an odd sequence of arbitrary numbers, it isn’t, but that’s another thread for another day. The important thing to remember for the moment is that the smaller the number, the bigger the hole. The bigger the hole, the more light is able to pass through it.
Your camera has a range of shutter speeds, the amount of time the shutter is open for. The standard sequence of shutter speeds is:
8seconds, 4sec, 2sec, 1sec, 1/2sec, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000
Modern cameras will often have setting in between the figures listed above for both aperture and shutter speed but I’m trying to keep things simple for the moment. You can see that the sequence of shutter speeds listed above, each setting is (near enough) double or half the speed of each of its neighbours. So, in this sequence the shutter is open for half or twice as long. If all other factors are equal, there will be half the amount of light getting through to the sensor for each increment along this sequence from left to right.
It may not seem like it, but the sequence of f-numbers listed above is also a sequence where each step will allow half the amount of light through if read from left to right, or twice the amount of light through for each step from right to left, i.e. f/8.0 lets through twice the amount of light as f/11 or half the amount of light of f/5.6.
Your camera has a range of ISO speeds, typically:
100, 200, 400, 800, 1600, 3200
You can see that each number is again, double or half that of its neighbours. Again, your camera may have other setting between those listed but bear with me. Each of the numbers in the sequence from left to right will need half the amount of light to achieve the correct exposure.
So, lets start to join up the dots. If you use your camera in program mode, it may tell you that it wants to use ISO200, f/5.6 and 1/60th of a second. You can change any of these settings when using manual mode and still get the exposure right, you can get that bucket brim full in a variety of different ways. Bear in mind that in manual mode, the camera’s meter will advise you of its opinion of the exposure, usually by indicating a negative figure e.g. –1.0 if it thinks you are using a combination that will result in under exposure or a positive number e.g. +1.0 for over exposure. For each whole number indicated over or under exposed, you need to move one step along in any one of the sequences listed.
Looking at the three sequences listed above, for our combination of ISO200, f/5.6, 1/60th, if we keep the ISO the same at 200, we could also achieve the correct exposure by doubling the exposure time to 1/30th and closing down the aperture to f/8.0. This can be continued along the sequence(s). Or, we could go the other way and set 1/125th second shutter speed and open up the aperture to f/4.0. If your camera has a "program shift" feature, you are effectively running along the line of acceptable combinations of aperture/shutter speed.
If we were to adjust the ISO figure to 400, the sensor would be twice as sensitive, so only need half the amount of light to get the correct exposure. If this were the case, we could keep the shutter speed the same at 1/60th and close down the aperture to f/8.0, or keep the aperture the same at f/5.6 and double the shutter speed to 1/125th. I hope you can see from this that in manual mode, you can change any of the variables, typically changing any two from the set of three, and still get the same exposure.
Why would you want to do this? There’s another few new threads there………..
I hope someone out there finds this useful.
Duncan.
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