Exposure Calculator?

[LittleMike]

I try to get out and do long-exposure star streak photos every chance I get, but there is always one thing that has kind of annoyed me. I have no idea how to figure out how long to leave the shutter open I can usually guess pretty close, but it still leaves something to be desired. I have tried to adopt a suggestion from Sw1tchFX which has been by far the most beneficial thing I've done yet, but I still just can't seem to figure out what "math" I need to do to make it really work.

Crank the ISO to 25600, you'll have to use 3200 (Hi 1).

Set your shutter speed between 5-30 seconds.

I usually leave the aperture close to wide open.

Now the first shot's going to look like sh*t, but that's ok. We just want to get our ballpark exposure without waiting 15 minutes, just to find out we got it wrong.

So now that we have our base exposure of 8 seconds ISO 25k, f/2, now we just have to crunch the numbers down to the same exposure at ISO 200ish and stopped down a little more!

Easy.

PERFECT! Nailed it right on, so now all we have to do is some basic 3rd grade arithmetic and do the same thing.

Once you do it a few times, night photography is easy photography.

So I guess my question is, what math would I need to do in order to figure out a longer exposure, assuming I had already done a shorter one at a high ISO/large aperture? Or to make it easy, is there some kind of an app I could download that would figure it out for me :lmao:

Just for reference, here's the post I'm quoting Sw1tchFX from: http://www.thephotoforum.com/forum/...otographing-night-star-trails-lightening.html Scroll down to post #10. Also, my goal is not to simply catch star streaks, but to correctly expose the foreground as well. Similar to the shots Sw1tchFX has given as examples.

Thanks in advance!

 

[Eclivic]

I do 11-22F and Shutter speed depends on the results you are trying to get. I stay 100 ISO to limit any noise.

You get about 1/8 inch of trail per 30 seconds

 

[LittleMike]

Let me rephrase my question a little differently...

Say I just took a shot at ISO 25,600, f/4, 6 seconds, and it came out exposed correctly. How would I determine my exposure if I then changed my settings to ISO 100, f/8, ??? seconds/minutes?

 

[hydkaran420]

Let me rephrase my question a little differently...

Say I just took a shot at ISO 25,600, f/4, 6 seconds, and it came out exposed correctly. How would I determine my exposure if I then changed my settings to ISO 100, f/8, ??? seconds/minutes?

as far as i know if u decrese the aperture size by one stop you need to increase the the time the shuttr is open by one stop to compensate. this way u would get the same exposure. its the same case with iso as well.

i dont have my cam handy, with it i could have gave u an example.




i am just a noob, feel free to ignore my cooments or correct me.

 

[889Media]

You have to learn the word stop and what that does to your exposure. Stop is a relative number that you use on all your settings that changes your exposure. 1 full stop up is doubling your values, one stop down is cutting it in half. You can also stop up/down values by 1/3, 1/2 etc...

Say you take your test shot with that settings you posted, and you changes your ISO down to 100. You have then stopped down your ISO by 8 or 9 stops. (you can do the detailed maths your self as I am too tired to think) 25,600 cut in half is 12,800...cutting that in half i 6400 etc...and you keep stopping down till you get to your desired values. To compensate for this, you have to change your other settings the same number of stops.

If you want to keep your F-stop, this leaves you with the shutter time. This you will have to increase to compensate for downed ISO. So just start stopping up, by doubling your 6 second initial value the same number of times as you stopped down the ISO. 6 x 2 = 12, 12 x 2 = 24 etc. If we say we stop up 8 times, you should end up around 1536 seconds, or about 25 minutes 36 seconds.

I am still learning this, so there could be an easier way...but I think this should get you started.

EDIT: most DSLR cameras can be set to change the Stops by 1/2 increments or by 1/3. So if you do not want to do the math, you can just change the values with your wheel; knowing that if you turn it 3 clicks you have 1 Stop (if camera is set to 1/3 increments) or 2 clicks (if camera is set to 1/2). And just count the stops after every time you pass the given number of clicks!

 

[LittleMike]

You have to learn the word stop and what that does to your exposure. Stop is a relative number that you use on all your settings that changes your exposure. 1 full stop up is doubling your values, one stop down is cutting it in half. You can also stop up/down values by 1/3, 1/2 etc...

Say you take your test shot with that settings you posted, and you changes your ISO down to 100. You have then stopped down your ISO by 8 or 9 stops. (you can do the detailed maths your self as I am too tired to think) 25,600 cut in half is 12,800...cutting that in half i 6400 etc...and you keep stopping down till you get to your desired values. To compensate for this, you have to change your other settings the same number of stops.

If you want to keep your F-stop, this leaves you with the shutter time. This you will have to increase to compensate for downed ISO. So just start stopping up, by doubling your 6 second initial value the same number of times as you stopped down the ISO. 6 x 2 = 12, 12 x 2 = 24 etc. If we say we stop up 8 times, you should end up around 1536 seconds, or about 25 minutes 36 seconds.

I am still learning this, so there could be an easier way...but I think this should get you started.

EDIT: most DSLR cameras can be set to change the Stops by 1/2 increments or by 1/3. So if you do not want to do the math, you can just change the values with your wheel; knowing that if you turn it 3 clicks you have 1 Stop (if camera is set to 1/3 increments) or 2 clicks (if camera is set to 1/2). And just count the stops after every time you pass the given number of clicks!

Thank you! That was exactly what I was looking for.

 

[889Media]

Can also mention, just in case you did not get it by reading the last post;

If you would have changed the aperture, this would also affect the amount you would have to change your shutter speed the same way. If you stopped down the ISO by 8 stops, and closed your aperture by one stop - this would add another stop for you to compensate for at shutter speed. But f-stops can be tricky! E.g. an f-stop value of 8 is not letting in half as much light as 4 does. In other words 8 is not one full stop from 4 etc.

1.4 2.0 2.8 4 5.6 8 11 16 22

On all these numbers, the numbers next to another number is the same as one full stop. This means that 2.8 is one full stop from 4 and let in double the amount of light as 4 does, and 5.6 is one full stop from 4 - letting in half the amount of light. But even here you can still use what I told you about 1/3 or 1/2 increments. When I change my aperture from 4; it goes from 4 to 4.5 to 5 to 5.6! This lets me know that 5.6 is one full stop from 4 because I use 1/3 increments.

 

[ndwgolf]

Try shooting in Manual and look at the exposure meter - -------I-------+ when you change either of the settings (ISO, Shutter, aperture) the meter will also change and hopefully correctly expose the picture for you

 

[889Media]

That work in some cases, but in many cases do not work as well. In many situations the cameras built in light meter falls short. If you take a picture and find it under- or overexposed, you have to make changes and take another shot. When shooting at shutter speeds of 5, 10, 30 minutes or longer, the one thing you do not want to do is risking to take a shot that is no good and having to shoot again.

Taking a test shot with faster shutter speeds, and knowing how to compensate for the changes you make after the test shot - is the best way to calculate how your setting should be to get the desired exposure after the shutter closes 10 minutes later.

 

[ndwgolf]

^^^^^And you can do all that with the meter......................well you can with a Nikon

 

[Dao]

Read more about Exposure Value (EV)

Take a look at this link
Exposure value - Wikipedia, the free encyclopedia

According to Wikipedia, EV 0 corresponds to an exposure time of 1 s and a relative aperture of f/1.0

Now, scroll down to EV table (EV as an indicator of camera settings) from the link listed above and take a look at it.
From that table, you will find the EV values (the 1st column on the left). Let's take an example, you want to take a photo of a full moon. The EV for the full moon, according to wikipedia, is EV 15 (ISO100 based)

Then look at the EV table where EV = 15

If you read across the row, you will see to get EV15, you can use aperture f/5.6 and shutter speed of 1/1000 with ISO100, or 1/500 if aperture is f/8. As you can see, both settings give you EV 15. The only different is 1/500 double the amount of light enter the camera. In order to keep the same exposure, we need to make the aperture (the hole) smaller so that it only allow half the amount of light entering the camera at a given second. So at the end, the total amount of light hitting the recording medium is the same.

People usually refer doubling or reducing half the amount of light a stop of light. So jumping from f/5.6 to f/8 is reducing the amount of light by 1 stop, while lowering the shutter speed from 1/1000 to 1/500 is increase the amount of light by 1 stop. As long as the increase cancel out the decrease, you have the same exposure, or say, same EV.

Once you understand that relationship, then you can factor the ISO (ASA) value. (Light sensitivity) Of course, ISO 200 is one stop brighter or EV +1 than ISO100

 

[LittleMike]

^^^^^And you can do all that with the meter......................well you can with a Nikon

I can use the meter for my high-ISO test shot, but since the long-exposure is on Bulb, it has no way to know when I'll be closing the shutter. Do Nikons read minds?

 

[ndwgolf]

^^^^^And you can do all that with the meter......................well you can with a Nikon

Do Nikons read minds?

Mine does

 

[LittleMike]

^^^^^And you can do all that with the meter......................well you can with a Nikon

Do Nikons read minds?

Mine does

I need to get me one of those! :lmao:

 

[JG_Coleman]

I really wanted to pin down an equation for this scenario, so I crunched numbers for a little while and came up with the following.

Given that:

A = Original Aperture
B = Original ISO
C = Original Shutter Speed
X = Desired Aperture
Y = Desired ISO
Z = New Shutter Speed

Z = ( ( B / A² ) / ( Y / X² ) ) * C

Now... this clearly isn't the most convenient formula to try and crunch in your head. However, if you've got a calculator on your phone, it might come in handy.

This appears to be correct in principal, though I haven't experimented with it extensively. Any corrections are welcome.