Calculate exposure time

[Hertz van Rental]

The ISO range is in 1/3rd stops.
6 - 12 - 25 - 50 - 100 - 200 - 400 - 800 - 1600 - 3200 - 6400
are all 1 stop apart.
The other values are all +1/3rd and +2/3rd
The aperture and shutter speed are all in 1 stop steps.
If you are not going to have 1/3rd stops for those you will have to round up and round down for the 1/3rd ISO steps.

Basically tou are looking at a calculation along the lines of:
ISO*aperture = shutter speed.
If you are not actually using a light meter in the process then the choice of base point is purely arbitrary.
So:
ISO*A = S
ISO+1*A = S-1
ISO*A+1 = S-1
ISO-1*A = S+1
ISO*A-1 = S+1
ISO+1*A-1 = S
ISO+1*A+1 = S-2
where altering the total value on the left will adjust the value on the right by an equal but opposite amount.
If you assign suitable values to each ISO, aperture and shutter speed in order to do the calculation it is then just a matter of translating the answer back to a useable f-number.
Hint: using logarithms for the values will make the calculation easier as it will be merely a matter of addition and log 0.3 is one stop.
Hint: draw up a truth table - aperture down one axis and ISO along the other. Each cell will then give you the correct shutter speed. You can then do a similar truth table for the calculation to see if they match.

 

[taurus]

hmm so exposure time == shutter speed
this stuff is complicated .

 

[Hertz van Rental]

hmm so exposure time == shutter speed

Yes.
It's not complicated in principle.
The total exposure is dictated by the ISO.
The aperture controls the 'brightness' of the light going in to the camera.
The shutter speed controls how long the light is let in for.
These two are manipulated to give exactly the same exposure for every frame.
If there is not much light around then you will need to let a lot of light in to the lens or let it in for a long time.
If it is sunny then you need to reduce the amount of light getting into the lens or only let it in for a short time.

 

[selmerdave]

Taurus,

You seem to be missing one piece of information. You need the light level first before you can calculate anything else. With a given light level, if you have a fixed/chosen ISO you can then determine any combination of aperature and shutter speed. For example, if you are at ISO 100 and you are measuring EV 11 for a light level, 1/15th f11, 1/30th f8, 1/60th f5.6, 1/125th f4 etc will all give the same exposure. With ISO 400 and an aperature of f8, your shutter speed will depend on the light level (ie. EV 13 = 1/125th). If you have a digital camera you could fix a given aperature and shutter speed and then determine the ISO that will make that combination a proper exposure, but again you need to know the light level first.

Dave

 

[taurus]

see i dont have that lux light level thing thats why. ok could someone put it more simple. if you had to write a way to calculate exposure time wuth only aperture and ISO what steps would you take to get exposure time? simple as possible lol

sorry im being a pain

 

[Steph]

you had to write a way to calculate exposure time wuth only aperture and ISO what steps would you take to get exposure time?

You just can't. The exposure time will depend on ISO, aperture AND the amount of light falling on the subject.

 

[selmerdave]

The purpose of exposure is to capture light, so if you don't know the light level all exposure settings are irrelevant.

Dave

 

[Hertz van Rental]

But the purpose of his program is just to do the calculations for adjusting aperture and shutter speed, not for it to work as a light meter. It's not a program aimed at being practical - it's just an exercise.

 

[selmerdave]

But he keeps asking for the other factor. If the program is just to show the relationships of ISO, aperature and shutter speed, I can understand. Then if he had a starting combination it could "calculate" other possibilities. Asking what the shutter speed is if the ISO is 6 and the aperature is 1.4, and it could be anything from 1/4000th if he was shooting a nuclear bomb going off to three hours if he's shooting stars, sounds to me like he wants to be able to calculate an exposure. Anyway, I can't figure it out.

Dave

 

[Hertz van Rental]

I know. I've tried to explain - and even given him some options.
But students often get stuck in closed loops and can't find their way out.
I think he needs to clarify his objectives with his tutor.

 

[taurus]

ok maybe someone can get something out of this, this is what iv been given to do:

1) Prompt and get the user's first name. You should
declare a char array to hold this name and its
size is 20 characters. You must ensure that there
is enough room in the array to hold the name.
Any initial space characters may be skipped.
2) Prompt and get the camera's ISO setting.
The ISO value should be validated for acceptable values:
6, 8, 10, 12, 16, 20, 25, 32, 40, 50, 64, 80, 100,
125, 160, 200, 250, 320, 400, 500, 640, 800, 1000,
1250, 1600, 2000, 2500, 3200, 4000, 5000, 6400
If not valid, an error message should be printed out
and the menu system continues.
If the user's name is known, it must be in the prompt.
3) Prompt and get the camera's aperture or f-stop value.
The aperture should be validated for acceptable values:
f1.2, f1.4, f1.8, f2, f2.8, f4, f5.6, f8, f11,
f16, f22, f32
If not valid, an error message should be printed out
and the menu system continues.
The entered value includes the 'f'.
If the user's name is known, it must be in the prompt.
3) The user may choose a menu number in any order.
4) Once name, ISO and aperture are known, the exposure time may
be output, and the program then finishes.
The output exposure time should be an acceptable value and
be the closest value from the list below:
1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30,
1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8, 15, 30, 60
Choose an informative and professional output format for the entire output!

i cant understand

 

[selmerdave]

Yeah, same problem. Unless it is understood that what you are shooting is at a set light level, it's impossible to know the shutter speed from the ISO and aperature. If that's the whole assigment, I'd speak to the instructor about it. I would imagine the main point of the excercise is to demonstrate the relationship between ISO, aperature and shutter speed, but the last part about having the program "output" a shutter speed from the ISO and aperature without any reference of light level makes no sense.

Dave

 

[taurus]

hmmm ok yea i see, i wonder what he means (the tutor)

 

[taurus]

Iv just clarified something with the lecturer and he said use 'bright light conditions' so thats a constant correct? how would the formula look then?

 

[Steph]

Iv just clarified something with the lecturer and he said use 'bright light conditions' so thats a constant correct? how would the formula look then?

Maybe he was referring to 'the sunny f/16 rule'. Look it up on Google. Basically, under sunny conditions the shutter spped to use at f/16 should be 1/ISO. So if you shoot with the camera set at 100 ISO (or with 100 ISO film) the shutter speed at f/16 should be 1/100th of a second. From there you can work out shutter speeds for other apertures and/or ISO settings.