Calculate exposure time

[taurus]

ya i looked that up but doesnt really help me calculate exposure time from entered ISO and Aperture.
So this formula:
EV = log((N^2)/T)

WHERE N = aperture and T=Exposure time.


Now is the EV the constant or is EV = 16 since its daylight?

 

[taurus]

(note: ignore above msg, i think that formula has nothing to do with mine)

It says:
To get the correct exposure in bright sunlight, at f16

but then later says:
Once ISO and aperture are known, the exposure time may be output

So i dont understand, is it at f16, its 1/ISO, but what if the user enters a different Aperture, then wouldnt it be different?

thanks
phil

 

[Steph]

You need to understand that the exposure depends on 3 parametres:

- The shutter speed
- ISO
- Aperture

Let's say you get proper exposure with the following parametres (following the sunny f/16 rule):

- ISO = 100
- Aperture = f/16
- Shutter speed = 1/125 (closest value to 1/100 from your list).

If you change one of these parametres you will have to change at least one of the others to get the same exposure as before on your film/sensor.

So for example, if you set the aperture at f/11, that is twice as much light entering the lens as at f/16. To get proper exposure you therefore need to half the shutter speed (1/250) or half the sensitivity of your media (ISO = 50).

If that is not clear you need to read about basic exposure to understand the relationship between ISO, shutter speed and aperture.

 

[taurus]

oh ok, so say they enter f4, thats 5 times as much light coming in and they enter ISO of say 400 i would get shutter speed of 1/500 as that is closest to 1/400
is that right?

 

[Steph]

oh ok, so say they enter f4, thats 5 times as much light coming in and they enter ISO of say 400 i would get shutter speed of 1/500 as that is closest to 1/400
is that right?

No!!!!!
You need to read about exposure to understand the relationship between shutter speed, aperture and ISO. It is not very hard to understand but quite difficult to explain on a forum (particularly for me as English is not my first language).

The formula below is what you are looking for to solve your problem (but, please, do go to a library or bookshop to find a book that explains the concept to make sure you understand it).

t=K*N*N/ISO

Where

t=exposure time (shutter speed)
N=aperture (1.2, 1.4, 1.8, 2.0, 2.8, 4.0....)
ISO=sensitivity of film/sensor
K=constant (which will depend on lighting conditions)

Using the f/16 rules (N=16, t=1/125, ISO=100) you can calculate K and replace in the formula and therefore calculate the shutter speed for any combination of aperture (N) and ISO.

 

[selmerdave]

Yes you need to understand the relationship between ISO, aperature and shutter speed, and I would agree that although it's already been described in this thread you should probably find out more about exposure until you understand that relationship fully. Then it will all seem relatively simple and make perfect sense.

Dave

 

[taurus]

oh alright, i will have to anymore, need to right a manual for this lol

ok so let me see if im doing this right. For K i got (1/256) at f16.
now for an example:
FOR:
N -> f2.8
ISO -> 320

NOW:
t = (1/256) * 2.8 * (2.8/320)
t = 9.57 X 10^-5
Looking at my values 1/4000 is smalles at 2.5 X 10^-4, so therefore would my answer be 1/4000?

Thanks again

 

[Steph]

oh alright, i will have to anymore, need to right a manual for this lol

ok so let me see if im doing this right. For K i got (1/256) at f16.
now for an example:
FOR:
N -> f2.8
ISO -> 320

NOW:
t = (1/256) * 2.8 * (2.8/320)
t = 9.57 X 10^-5
Looking at my values 1/4000 is smalles at 2.5 X 10^-4, so therefore would my answer be 1/4000?

Thanks again

The calculations are right. The answer is 1/16000 but that value is outside the range. Maybe in that case you can return a message saying that the required shutter speed is outside the range...

Finally, it makes it easier if for the calcultion of K you use t=1/125 (not t=1/100) as 1/125 is s shutter speed from your list and not 1/100.

 

[taurus]

oh alright yea, i jus thought in one part of my assignment it said this:
"...To get the correct exposure in bright sunlight, at f16, the exposure time should be the
same as the ISO (eg: 1/100 sec for ISO 100)..."

and also since its 1/16000 i have to print to the nearest Exposure time so that would be 1/4000

 

[taurus]

so will it even me possible to get 2 for exposure time?

 

[Steph]

so will it even me possible to get 2 for exposure time?

Well, what do you think? And why?

 

[taurus]

well i guess it should be possible since we are given that. to get real numbers (ie no decimals), i would have to have a large aperture and small ISO. But for me the largest Aperture is 32 and smallest ISO is 6 which gives 2/3 which is no where close to say 2?

 

[Steph]

well i guess it should be possible since we are given that. to get real numbers (ie no decimals), i would have to have a large aperture and small ISO. But for me the largest Aperture is 32 and smallest ISO is 6 which gives 2/3 which is no where close to say 2?

Well as you found out, you can't get a shutter speed of 2s under those bright conditions.

BTW, the bigger the number, the smaller the aperture.

 

[taurus]

ok cool, i guess that would be alright, i asked the tutor and he said "that may be the case", cheeky lol

 

[taurus]

hi again;
i got another formula i made:

Exposure time = (aperture^2)/(15*ISO)

where 15 is EV is bright light

does that seem alright?