CHDK Shutter speed... a reminder that I hate maths!

So I have frazzled my brain a lot and with thanks to 89rafa from YouTube for explaining the basics of the maths to me on my original video, I now believe I have an understanding of the way in which you can calculate the shutter speed in CHDK when you are multiplying the shutter speed by fractions. For a loser in maths like me this is a real headache but I hope I can simplify it the best I can to make it clear for others too.

So the easy part of overriding shutter speed with CHDK is taking the value factor (number you multiply the shutter speed by) as a single number and working out how many seconds you want your shutter speed. This would be something like:

Shutter speed: 60
Value Factor: 1

1 x 60 = 60 leaving a 60 second shutter speed.

Not too tough right?

This is the way the shutter speed is calculated in "EV step (Exposure Value step) shutter speed enum type" in CHDK. However when we use the "Factor shutter speed enum type" it gets harder to calculate.

We change the bottom number to a fraction and it gets more complicated.

Here is a simple version...


Shutter speed: 5 (seconds)
Value Factor: 1/10 (One tenth of a second)


Here we are multiplying 5 seconds by one tenth of a second. The easy way to do this is to divide 5 by ten which leaves us with 0.5 of a second, or.. half a second.

As another example...


Shutter speed: 50
Value Factor: 1/10

We are still dividing the top number by the fraction so if we divide 50 by 10, we get 5. So the shutter speed is 5 seconds.

And so on and so forth. So in those examples we were changing the shutter speed and always dividing the number by ten.

However, we can also change the fraction and this is where things switch around and the shutter speed can be lower than the number dividing it.

As an example...


Shutter speed: 5 (seconds_)
Value Factor: 1/100 (One hundredth of a second)

Here we are dividing 5 seconds by one, one hundredth. The way to figure this one out is to calculate how many 5's there are in one hundred, in this case the answer is 20. This number then goes into a fraction as we are dividing 5 by 100 and the answer is 1/20th of a second.

It's a lot to take in but once you get used to it you can figure the maths out. Here are some ways to remember how to calculate it...

If the shutter speed number is smaller than the bottom fraction number e.g SS 5 VF 1/10 then you are calculating a faster shutter.

If the shutter speed number is larger than the bottom fraction number e.g SS 50 VF 1/10 then you are calculating a slower shutter.

If you are keeping your value factor at a certain fraction, then all you have to do is move the decimal point. For example here I am always using the fraction of 1/10 so I am always dividing by ten:

1/10 x 5     =  0.5 of a second
1/10 x 50   =     5.0 seconds
1/10 x 500 =     50.0 seconds

So if I were using a Value factor of 1/100 I would move the decimal point two points over instead of one.