Calculate exposure time

[bit827]

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

 

[taurus]

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

 

[table1349]

Dear Moderators, Please start an Algebra Sub forum.

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

 

[Steph]

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.

 

[taurus]

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

 

[taurus]

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?

 

[selmerdave]

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

 

[taurus]

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?

 

[selmerdave]

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

 

[taurus]

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?

 

[Sideburns]

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.

 

[selmerdave]

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

 

[taurus]

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

 

[selmerdave]

???

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

 

[taurus]

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)?