# Calculate exposure time



## taurus (Sep 3, 2007)

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


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## abraxas (Sep 3, 2007)

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


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## selmerdave (Sep 3, 2007)

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


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## Hertz van Rental (Sep 3, 2007)

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 = 1/60@f5.6 = 1/15@f11
And these all give the same exposure as:
ISO200 1/60@f8 = 1/125@f5.6
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.


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## castrol (Sep 3, 2007)

Aaaand kablewy...there went my head.


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## Garbz (Sep 3, 2007)

Search for Exposure Value on Wikipedia


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## taurus (Sep 4, 2007)

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


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## selmerdave (Sep 4, 2007)

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


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## RacePhoto (Sep 4, 2007)

selmerdave said:


> 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.


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## taurus (Sep 4, 2007)

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


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## Hertz van Rental (Sep 4, 2007)

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


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## Hertz van Rental (Sep 4, 2007)

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.


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## taurus (Sep 5, 2007)

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


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## Hertz van Rental (Sep 5, 2007)

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.


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## taurus (Sep 5, 2007)

> 2) Prompt and get the camera&#8217;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,
> ...



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?


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## Hertz van Rental (Sep 5, 2007)

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.


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## taurus (Sep 6, 2007)

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


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## Hertz van Rental (Sep 6, 2007)

taurus said:


> 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.


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## selmerdave (Sep 6, 2007)

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


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## taurus (Sep 6, 2007)

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


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## Steph (Sep 7, 2007)

taurus said:


> 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.


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## selmerdave (Sep 7, 2007)

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

Dave


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## Hertz van Rental (Sep 7, 2007)

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.


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## selmerdave (Sep 7, 2007)

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


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## Hertz van Rental (Sep 8, 2007)

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.


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## taurus (Sep 9, 2007)

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.
> ...



i cant understand


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## selmerdave (Sep 9, 2007)

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


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## taurus (Sep 10, 2007)

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


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## taurus (Sep 11, 2007)

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?


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## Steph (Sep 11, 2007)

taurus said:


> 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.


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## taurus (Sep 11, 2007)

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?


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## taurus (Sep 11, 2007)

(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


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## Steph (Sep 11, 2007)

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.


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## taurus (Sep 11, 2007)

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?


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## Steph (Sep 11, 2007)

taurus said:


> 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.


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## selmerdave (Sep 11, 2007)

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


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## taurus (Sep 12, 2007)

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


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## Steph (Sep 12, 2007)

taurus said:


> 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:
> ...


 
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.


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## taurus (Sep 12, 2007)

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


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## taurus (Sep 12, 2007)

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


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## Steph (Sep 12, 2007)

taurus said:


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


 
Well, what do *you* think? And why?


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## taurus (Sep 12, 2007)

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?


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## Steph (Sep 12, 2007)

taurus said:


> 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.


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## taurus (Sep 12, 2007)

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


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## taurus (Sep 13, 2007)

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?


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## bit827 (Sep 13, 2007)

Hey

I think there is another formula for this but I need to verify that it is correct

According to the article on exposure values there are two equations that can help us find the solution if used together:

One of them was stated alread : 
EV = log(base2)((N^2)/t)

Now that the conditions are brightly lit then using the equation

EV(s)=EV(100)+log(base2)(S/100)
where S is the ISO value
and EV(100) is the constant 16(because of brightly lit conditions)

We should be able to calculate the EV(s) and then substitute it into the first equation and then find the 't' value which is ultimately what we need.

I would like to know from the pro photographers if this formula makes much practical sense or not. Coding this would be no big deal


Thanks


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## taurus (Sep 13, 2007)

so if we have a different iso and aperture how would it look?


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## table1349 (Sep 13, 2007)

Dear Moderators,  Please start an Algebra Sub forum. 

My head hurts.  I'm going to get a pain reliever.


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## Steph (Sep 14, 2007)

Taurus, do yourself a favor and find a book which will explain about exposure and the relationship between ISO, shutter speed and aperture! You will get all the answers you want from this forum but they will be useless if you do not understand anyhting about exposure in the first place (as your posts seem to suggest). I don't want to seem rude but it is not the answers that are important but the thinking you put behind solving the problem: that is called learning.


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## taurus (Sep 14, 2007)

ok jus as two examples:
1) ISO = 200, APERTURE = f5.6, EXPOSURE TIME = ????
2) ISO = 500, APERTURE = f11, EXPOSURE TIME = ????

just so i can play around could someone help by filling in what exposure time would be, i can the try figure the rest out
thanks


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## taurus (Sep 14, 2007)

ok iv been doing some research. Now what i understood is shutterspeed (exposure time) is simply 1 over the ISO (1/ISO).
Ok now the problem is relating it to Aperture. Now this is what i 'think':

say i have iso of 250 which is shutterspeed of 1/250.
now say aperture is f8 which is two(2) stops up (with bright light f16 as reference)?
so would that mean my shutterspeed goes down by 2 so now becomes 1/1000?


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## selmerdave (Sep 14, 2007)

No, you have a lot more reading to do.  Actually, just a bit of reading as it doesn't take a lot of text to explain it but a fair amount of understanding left to do.  If you understand what each of aperature, shutter speed, ISO and light level *mean* and how they relate to each other, none of these things would be questions as they would be obvious.  Don't think in terms of equations, think in terms of plain words.  If you can explain in plain words why all three are factors on exposure at a given light level, and how they inter-relate, you probably understand it, and if you can't you probably don't yet.

Dave


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## taurus (Sep 14, 2007)

ok this is what i understand:

ISO is the sensitivity of the film. So a faster shutter speed means less light can hit the film, now if the aperture is doubled then the shutter speed would have to be halfed. And in my case i use f16 (bright light) as the reference point??

Does that seem right?


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## selmerdave (Sep 14, 2007)

Yes, it does.  Actually, when I reread your previous post, I didn't realize that you were starting with f16 assuming bright conditions, apologies.  I thought you were suggesting that shutter speed by definition is 1/ISO irrespective of light or aperature.  You do seem to understand the basics and using the sunny 16 rule as a reference point you can see how it all works under other lighting conditions as well.

Just remember that were talking about making equivalent exposures, and while 1/250 @ f16 is the same exposure as 1/4000 @ f4 (assuming light level and ISO are constant), other aspects change such as depth of field.  But since you understand the other stuff you probably already caught that too.

Dave


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## taurus (Sep 14, 2007)

yea i did, about the change in the field view, background blurred a bit etc etc. so basically there isint really a simple formula where u slot in aperture and iso and can get shutter speed?


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## Sideburns (Sep 14, 2007)

taurus said:


> yea i did, about the change in the field view, background blurred a bit etc etc. so basically there isint really a simple formula where u slot in aperture and iso and can get shutter speed?



Not at all.  You have to meter the light levels in the picture.

How much light enters to the sensor ultimately depends on how much light is available.  The other settings you can input are affected by the light level.

Light can change, and therefor would change all of your settings.

You can use a formula to change the settings if you know all 3...but you can't find a third if you only know 2.  Unless you have a light reading....which you don't.


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## selmerdave (Sep 14, 2007)

taurus said:


> yea i did, about the change in the field view, background blurred a bit etc etc. so basically there isint really a simple formula where u slot in aperture and iso and can get shutter speed?



Yes you already have it and have used it, if it's understood that we're talking about sunny 16ish lighting conditions.  You know the relationship between them, and changing light levels just involves changing the values the same way changing aperatures does with everything else constant.  Back in the dark ages of the film era ISO was constant once you loaded your film so that wasn't a variable from frame to frame, but with DSLR's it's possible to vary it as much as the others.

BTW, the difference between f16 and f4 is more than "a bit", but when you do some shooting and experiment with it you'll find that.

Dave


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## taurus (Sep 14, 2007)

ok yea, i just cant think of a formula for it


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## selmerdave (Sep 15, 2007)

???

Steph gave it to you in post #35 and you used it in post #37.  All you have to do for different light levels is adjust it appropriately, and not go outside of the maximum values for practical photography.

Dave


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## taurus (Sep 15, 2007)

But im not sure if its the right thing. Is K in my situation (bright light f16) always equal to 1/256?

when i tried the calculation i couldnt get an exposure time of 1/2 with different values of ISO and Aperture. 
just doesnt seem to give right answers. eg:

Iso = 500
aperture = 4

exposure time = (1/256)*4*(4/500) = 0.000125
shouldnt the answer me 0.005 (1/200)?


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## selmerdave (Sep 15, 2007)

taurus said:


> But im not sure if its the right thing. Is K in my situation (bright light f16) always equal to 1/256?
> 
> when i tried the calculation i couldnt get an exposure time of 1/2 with different values of ISO and Aperture.
> just doesnt seem to give right answers. eg:
> ...



Hmm, where did you get 1/200 from?  At f16 under those conditions, t= 1/500 (1/ISO), so f4 being 4 stops more light to get the same exposure you would need a shutter speed four doublings faster (1/8000), which is exactly the answer you got.  For a shutter speed of 1/250 you would need a _smaller_ aperature than f16 by one stop, AKA f22. Not many cameras have shutters that can shoot 1/8000 (perhaps 5 models in production) so basically with an ISO of 500 you can't expose properly at f4.  Similarly and at the other end of the spectrum an exposure time of 1/2 is going to make for an aperature smaller than is available on cameras, so that is also not possible.  What is possible with an ISO of 500 and bright sunny conditions are settings such as f16 1/500, f22 1/250, f11 1/1000 etc.  That is why under bright conditions people usually use slower film (ISO 100, ISO 50) or similar settings on a digital camera.  Likewise at EV 7 in a dimly lit room you are not going to be able to get a shutter speed of 1/500, it would require an aperature significantly larger than the limits of manufacturing and design.  It's good to know those limits:

Aperature - f1.4 to f32
Shutter speed - 1/4000, no limit on the other end other than practicality once it gets to hundreds of hours.
ISO - for film 25 to 3200, digital 100 to 3200

Those are for generally what is available today although not necessarily at the same time (I'm not aware of any lenses with a maximum aperature of 1.4 _and_ a minimum aperature of 32), but if we're not concerned about the specific lens being used at a given time something within those parameters should be possible.

Dave


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## Sabir (Sep 25, 2007)

Hi, This post is   very informative, however I would like some specific information. If someone   can help me then please send me a private message. Best Regards,


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