Calculate exposure time

[taurus]

I have got to write a program to output exposure time from ISO and Aperture settings. Is there a certain calculation to calculate the Exposure Time after the user has entered the ISO and Aperarure values??
If so how?

Thanks
phil

 

[abraxas]

Ouch.

It's not like Ohm's Law. You might try a search on google. I'd hate to steer you wrong.

Here's somethin' tho...
http://www.chem.helsinki.fi/~toomas/photo/ev.html

 

[selmerdave]

You'll need shutter speed as well, unless that was intended to be the output of your program. Really, it's all related so once you can calculate one value you can calculate all the others as one "stop" of difference is the same as doubling ISO, doubling shutter speed or moving aperature one step (with 1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32 being the standard values - mulitply each by root 2 to get the next). Just be careful of the direction, more light means more exposure. Can't tell you how to calculate that first value, not sure how you would express the light level.

Dave

 

[Hertz van Rental]

Shutter speed, aperture and ISO are all Logarithmic series.
In each moving up one or down one is halving or doubling the exposure.
f5.6 admits twice the amount of light into the lens as f8.
f11 admits half as much light as f8.
ISO50 is twice as sensitive to light as ISO25 but only half as sensitive as ISO100.
And so on.
So if you have a fixed value (ISO) the aperture and shutter speed are reciprocal - that is, if you increase one you decrease the other by the same amount.
ISO change will affect the overall (total) exposure thus:
ISO100: 1/30@f8 = 1/[email protected] = 1/15@f11
And these all give the same exposure as:
ISO200 1/60@f8 = 1/[email protected]
Because of this relationship between aperture and shutter speed EV or Exposure Values are often used. All the above exposures for ISO100 being identical could be expressed by the use of just one EV number.
EV0 is equivalent to 1s@f1 for any ISO.

 

[castrol]

Aaaand kablewy...there went my head.

 

[Garbz]

Search for Exposure Value on Wikipedia

 

[taurus]

hmm yea makes a bit more sense. thanks.
and wikipedia did the trick as well, give the formula for it:

EV = log((N^2)/T)

WHERE N = aperture and T=Exposure time.
But one question. is ISO the same as shutter speed (exposure time)???

Does that look like it?'


haha and if u think ur head went boom, well i have to program this as well

 

[selmerdave]

No, ISO is the sensitivity of the film or sensor. ISO 400 needs one quarter the light that ISO 100 needs for the same exposure. That difference could be made by a shutter speed 1/4 as long or an aperature two stops smaller (for ISO 400).

Dave

 

[RacePhoto]

No, ISO is the sensitivity of the film or sensor. ISO 400 needs one quarter the light that ISO 100 needs for the same exposure. That difference could be made by a shutter speed 1/4 as long or an aperature two stops smaller (for ISO 400).

Dave

Also, check out the web for the inverse square law which helps explain why you need twice as much to get half the gain.

I look at ISO as somewhat of a constant. Probably back from film days, when what you had in the camera was "IT", or carry two bodies.

Generally when someone is shooting something, there isn't a need to be changing ISO all the time. If you pick an appropriate ISO, they you would only need to deal with two variables. Speed and aperture.

When you change the ISO, the whole scale just slides up or down on one portion. The relationships between shutter speed and f-stop, stay the same.

 

[taurus]

so like the formula above, theres nothing like that for calcualtions exposure time?

If not do u have a general algorithm (procedure) to get the exposure time after ISO and aperture have been presented?

thanks

 

[Hertz van Rental]

Oh, come on people. It's easily worked out from the above information.

Exposure = Illuminance x Duration

Or to put it into a proper formula using the standard symbols:

H = E x t

The unit of illuminance is the lux.
Time is in seconds.
Exposure is therefore given in lux seconds.
If you look up manufacturers data guides the sensitometric data is always expressed in lux seconds. But if you Google you should find a conversion formula.

*EDIT*
I just picked up my Weston light meter and the dial does all the calculations for you. We just need to convert what it does into a program and we are there

 

[Hertz van Rental]

I've just found the manual for my Sekonic meter.
It has a 'Lumidisc' attachment that turns it into a Lux meter - with a conversion table for Lux to EV.

 

[taurus]

yea but wat if i have aperture and ISO, wat do i do to calculate Exposure time?

 

[Hertz van Rental]

The ISO of a film defines the amount of light required by that film to record a specific density with standard development.
This information is provided by the characteristic curve of the film. The film is given a series of accurately controlled exposures (normally by means of a sensitometer). The film is then processed under controlled conditions and the resultant series of densities measured (using a densitometer) and the results plotted to give a graph. This graph shows a great many things.
Changing the developer or any of the variables in processing will result in a different graph.
One of the uses of the characteristic curve is to determine the films average ISO.
Armed with this information it is possible to work out the absolute exposure required by a particular film and this can be used to calculate aperture and exposure time - which is what light meters do. But they take account of local illuminance. That is to say, they measure the amount of light present in a scene and use this information in conjunction with ISO data to calculate aperture and shutter speed.
In absolute terms EV0 = f1@1second for all film speeds. So it should be fairly easy to calculate one term if given the other. ISO plays no part in this.
But such a calculator would be useless in practical terms as it bears no relationship to light levels. You need to have the input of a light meter giving information on the light levels in order to have the data needed to calculate exposure for practical purposes.
For example:
EV0 = f1@1s
EV7 = f11@1s
The difference is 7 stops (and 7 EV steps)
But this information is useless unless you know that the illuminance level of the subject is dictating the use of EV7 to get the correct exposure. Using EV0 under the same lighting conditions would result in a 7 stop over-exposure.
To put it more simply, in bright sunlight you will get a large EV number. In a darkened room you will get a low EV. The overall result in both cases will be identical - that is to say, the film will receive the same exposure. But in bright light the exposure only needs to be brief.
Bottom line: unless you have a lux meter somewhere in there you won't be able to do the necessary calculations.

 

[taurus]

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

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

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

The above information is part of my program i have to do.
Looking at taht how would u say i get exposure time then. would it be say if ISO=6 and APERTURE=f1.2 then EXPOSURE TIME=1/4000???

Or wat?