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11/21/2006 12:06:02 PM · #1 |
Seems like I should know this, but I guess everything I've done until know has been by trial and error. How do you guys measure exposure for nighttime shots? Do you have light meters? I'm just not sure how to determine shutter speed once you get outside the range of the camera's meter. I realize a few seconds won't make a huge difference when you're exposing for 5 or 10+ minutes, but still...
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11/21/2006 12:14:12 PM · #2 |
The light meter on my camera does not work well in low light. I use a trial and error approach, often using that on my film camera.
On a dSLR you can do a high speed test, use the maximum ISO the camera can offer, open the aperture completely, then do trial and error shots until the exposure is right. You can then do a long exposure on low ISO and stopping the lens down as needed by just figuring out how many stops compensation you must make. |
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11/21/2006 12:24:41 PM · #3 |
Originally posted by drewseph:
Seems like I should know this, but I guess everything I've done until know has been by trial and error. How do you guys measure exposure for nighttime shots? Do you have light meters? I'm just not sure how to determine shutter speed once you get outside the range of the camera's meter. I realize a few seconds won't make a huge difference when you're exposing for 5 or 10+ minutes, but still... |
I think it really depends on the scenario. If you have some lights in the photo (i.e. a cityscape or something), chances are the exposure won't be TOO far outside of your camera's metering capabilities (30 seconds or so?). Of course that's drastically affected by your aperture and ISO. If you're doing a much longer exposure (like a landscape by moonlight), you probably want to factor in if there are stars visible, if you want star trails, or if you want the stars to look like stars. If you want the stars in a shot to show up as stars (i.e. points of light), chances are you don't want to make the exposure much longer than 30-45 seconds. The only way to really accomplish that is to open that aperture wide and up your ISO as high as you can (1600?). If star trails aren't really a big factor, I usually just take a number of test exposures at my desired aperture and see which shows up best on the LCD. So for me, I guess it's less science and more guess-and-check. ;) |
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11/21/2006 12:27:59 PM · #4 |
Originally posted by cutlassdude70:
if you want star trails, or if you want the stars to look like stars. If you want the stars in a shot to show up as stars (i.e. points of light), chances are you don't want to make the exposure much longer than 30-45 seconds. The only way to really accomplish that is to open that aperture wide and up your ISO as high as you can (1600?). |
Just don't confuse this with the "star filter" effect, which requires a small aperture and long exposure and has nothing to do with celestial stars. |
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11/21/2006 12:29:13 PM · #5 |
I use the meter in my camera.
I'll set it to the highest ISO, widest open aperture and take a meter reading. Then I just use reciprocity to work out the correct exposure for the ISO and aperture I want to use.
I.e., double the time, each time I half the ISO.
Double the time, each time I close down the aperture by one stop.
When I'm shooting for star trails, I aim to get back to about ISO 400, f5.6 or so.
When I'm shooting for 'star effects' around point sources of light, I'll work back to the most stopped down aperture my lens has, and ISO 100
Then I just plug the resulting time into the timer remote on my camera and let it go. (Remembering to compensate the initial metering, if you want the blacks to actually be black, rather than a 18% grey like the meter initially evaluated)
You can also shoot the high ISO/ wide open aperture version to help you work on the composition/ exposure (histogram) then, once you've got all that dialed in, stop down, change the ISO and calculate the correct shutter speed.
Message edited by author 2006-11-21 12:31:48.
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11/21/2006 12:45:13 PM · #6 |
Originally posted by Gordon:
I use the meter in my camera.
I'll set it to the highest ISO, widest open aperture and take a meter reading. Then I just use reciprocity to work out the correct exposure for the ISO and aperture I want to use.
I.e., double the time, each time I half the ISO.
Double the time, each time I close down the aperture by one stop.
When I'm shooting for star trails, I aim to get back to about ISO 400, f5.6 or so.
When I'm shooting for 'star effects' around point sources of light, I'll work back to the most stopped down aperture my lens has, and ISO 100
Then I just plug the resulting time into the timer remote on my camera and let it go. (Remembering to compensate the initial metering, if you want the blacks to actually be black, rather than a 18% grey like the meter initially evaluated)
You can also shoot the high ISO/ wide open aperture version to help you work on the composition/ exposure (histogram) then, once you've got all that dialed in, stop down, change the ISO and calculate the correct shutter speed. |
do you set it to the highest iso because it has the highest light sensitivity and should record the light correctly?
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11/21/2006 12:50:01 PM · #7 |
Originally posted by achiral:
do you set it to the highest iso because it has the highest light sensitivity and should record the light correctly? |
I set it to the highest ISO because my camera meter starts to just flash 30s when the exposure time is greater than 30s. Same with using the widest open aperture for metering. I'm just trying to get a workable shutter speed from the camera meter, to begin calculating the correct shutter speed at the ISO/aperture I want to use. It is all based on the fundamental principle of reciprocity. Many texts on photography assume that you've only really got two variables to work with. Aperture & Shutterspeed. Double one, half the other. But for digital exposure, we have 3 variables that you can trade off on a per picture basis. ISO, aperture and shutter speed. Again double any one means you can half one of the others(or go up/down by one stop) and have the same effective exposure.
Using the highest ISO (to meter with - not to shoot) means I'll pretty much always get a shutter speed in the 1s-30s range, that I can then work with to come up with a correct exposure at a lower ISO (to actually shoot at)
Using the highest ISO to shoot a preview also means I get that composition preview in the shortest amount of time. No point waiting around for effectively a 'polaroid' that you only want to use as a rough guide.
E.g., Suppose at F4, ISO 1600, I get a shutter speed of 8 seconds.
then, it is exactly the same exposure as all of these:
F4, 16s, ISO 800
F4, 30s, ISO 400 <- okay, really 32s, but you can start being a bit rougher if you like at longer exposure times.
F4, 60s, ISO 200
F4, 120s, ISO 100 <- now start moving the aperture up in full stops
F5.6, 240s ISO 100
F8, 480s, ISO 100
F11, 960s, ISO 100
and so on. No doubt I've screwed up one of the full stop aperture changes that someone can point out ;) but you get the idea.
The advantage is, if I tried to meter at F11, ISO 100 (because that's the aperture and ISO I really want to shoot at - my camera would meter it as >30s, blink and not really shoot it properly)
Message edited by author 2006-11-21 12:56:43.
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11/21/2006 03:26:37 PM · #8 |
Originally posted by Gordon:
F4, 16s, ISO 800
F4, 30s, ISO 400 <- okay, really 32s, but you can start being a bit rougher if you like at longer exposure times.
F4, 60s, ISO 200
F4, 120s, ISO 100 <- now start moving the aperture up in full stops
F5.6, 240s ISO 100
F8, 480s, ISO 100
F11, 960s, ISO 100 |
I'm showing my ignorance here probably, but I thought 1 stop equaled a doubling of the F-stop number. f/4 is one stop wider than f/8? f/11 is a stop and a half difference from f/4?
This is the part that makes my head spin, so concise explanation is welcome. |
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11/21/2006 03:32:23 PM · #9 |
Multiply each aperture number by 1.4 and you'll get the next stop, approximately.
1.4 x 1.4 = 1.96 or 2.0
2.0 x 1.4 = 2.8
2.8 x 1.4 = 3.92 or 4.0
4.0 x 1.4 = 5.6
You get the idea 8o)
BTW, f/4 is two stops wider than f/8, don't forget about f5.6. 8o)
Message edited by author 2006-11-21 15:33:57.
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11/21/2006 03:33:21 PM · #10 |
| I guess and make a couple samples, then pick the one I like :) [I'm serious] |
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11/21/2006 03:37:38 PM · #11 |
Originally posted by _eug:
I'm showing my ignorance here probably, but I thought 1 stop equaled a doubling of the F-stop number. f/4 is one stop wider than f/8? f/11 is a stop and a half difference from f/4?
This is the part that makes my head spin, so concise explanation is welcome. |
It is based on the halving the size of the aperture. However, the actual F-stop numbers are based on the square root of the value.
x approx sqrt(x)
1 1
2 1.4
4 2.0
8 2.8
16 4
32 5.6
64 8
This all works out because you are actually halving or doubling the size of the circular opening at the front of the lens. These values are a ratio based on the diameter of the opening and the focal length of the lens. (so these values end up being linked via pi*radius squared
There are plenty of articles that go into the maths properly on how the f-stops are derived - but the simplest thing to do is just learn the list of full stops and remember it.
1 1.4 2.0 2.8 4 5.6 8 11 16 22 32
Then know that when you go up or down on this list, you've doubled or halved the exposure. Then to get the same actual exposure, you do the opposite to the ISO or shutter speed.
Twice as much light getting in - then let it in for half as much time.
Message edited by author 2006-11-21 15:39:53.
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11/21/2006 03:46:40 PM · #12 |
Originally posted by Gordon:
Originally posted by _eug:
I'm showing my ignorance here probably, but I thought 1 stop equaled a doubling of the F-stop number. f/4 is one stop wider than f/8? f/11 is a stop and a half difference from f/4?
This is the part that makes my head spin, so concise explanation is welcome. |
It is based on the halving the size of the aperture. However, the actual F-stop numbers are based on the square root of the value.
x approx sqrt(x)
1 1
2 1.4
4 2.0
8 2.8
16 4
32 5.6
64 8
This all works out because you are actually halving or doubling the size of the circular opening at the front of the lens. These values are a ratio based on the diameter of the opening and the focal length of the lens. (so these values end up being linked via pi*radius squared
There are plenty of articles that go into the maths properly on how the f-stops are derived - but the simplest thing to do is just learn the list of full stops and remember it.
1 1.4 2.0 2.8 4 5.6 8 11 16 22 32
Then know that when you go up or down on this list, you've doubled or halved the exposure. Then to get the same actual exposure, you do the opposite to the ISO or shutter speed.
Twice as much light getting in - then let it in for half as much time. |
Coming from a math background, this makes perfect sense. |
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11/21/2006 03:48:43 PM · #13 |
| I'm extremely appreciative of all your insight, Gordon. You mentioned remembering to compensate the initial exposure to make the blacks true black. I've heard about this 18% gray metering thing, but how do you determine how much to compensate? |
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11/24/2006 10:48:44 PM · #14 |
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