Advice on calculating exposure...

theMusicMan

theMusicMan

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John
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Hi All

Well, I had my first real experience of long exposures last night when I went to a local spot and attempted to photograph the Second Severn Crossing. Lovely light, lovely spot, all the right filters... but heck, doing this isn't just a question of pointing, clicking, and waiting eh!

I have lots to learn, but will certainly enjoy it whilst I do. Anyway... here's a question for the experts out there.

Having not tried my hand at long exposures until now, I discovered that a limitation of my camera is that in Manual mode, the shutter speed will only go up to 60 seconds. Now, this is fine for milky river effects, but unfortunately it isn't quite enough for the milky sky effect that I want too. I do have 'Bulb' mode, where I can keep the shutter open as long as I need, but I have to time it manually - which is fine except I have no idea how to calculate the desired shutter speed or exposure time for these long exposures.

Scenario: lets say I have an exposure of 60 seconds, with an aperture of f8. If I want a longer exposure, and I close the aperture to say f11, how do I work out what the amended exposure should be as my camera only goes up to 60 seconds. I assume there's a chart or algorithm somewhere that will allow me to work this out...?

Any pointers...?

Thanks
 
going from
1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 apertures is a 'gap' of one stop

and one stop corresponds to a doubling or halfing of the shutter speed depending which way you've adjusted the ap.

so in your case you've gone from f/8 @ 60s to f/11, therefore you double the shutter speed, 120secs



:)
 
actually just another pointer... with long exposures the precision of the timing becomes less critical anyway... so don't worry if you're a few seconds over.
 
Hi John

A lot of it is experimenting really although you probably can work it out by setting your exposure then compensating for the loss of light from your filters.

For your scenario of going from f/8 to f/11 that would be a difference of one stop, there would be half the amount of light and you would therefore need an exposure time roughly double what was required at f/8, so it would go from 60 secs to 120 secs. you could time this with a stop watch and use a remote switch to start/stop the exposure in bulb mode.

I think this is correct but there are others on the forum who are far far more experienced at shooting night time landscapes etc and it may not be as simple as i think.

Hope this helps

Andy
 
Excellent advice here guys, thanks a million.

There's loads to learn re long exposure shooting, and I am right at the bottom of that ladder... hehehe. Not a chance will I post my results from last nights attempts... :) It aint as easy as it looks, and really makes you think about everything.
 
theMusicMan said:
Excellent advice here guys, thanks a million.

There's loads to learn re long exposure shooting, and I am right at the bottom of that ladder... hehehe. Not a chance will I post my results from last nights attempts... :) It aint as easy as it looks, and really makes you think about everything.
Click to expand...

No probs dude. Honestly, don't worry about posting your results. The forum is here as a learning exercise for everyone - ok some think it's a dumping ground to show off but they're a minimal number. People might be able to suggest ways to improve or something that you may have missed :suspect:

But don't feel compelled to either.

ps. just to complete the thread for anyone searching...
the one stop increments for ISO are 100 - 200 - 400 - 800 -1600 etc
 
Another thing with long exposures is to keep the ISO as low as possible. to avoid hot pixels etc.
 
Hehe... sure, I have been a member here for a long time, and have posted many shots too. However, in so doing I have set an expectation that I can at least sometimes, take a half decent shot thats just the right side of the cr4p-o-meter. If I were to post any image I took from last nights expedition, the good peeps on here would think I had lost my marbles... :) hehe.

Besides.... Ed Gary has set the bar rather high, don't ya think...? :)
 
If they're really long exposures you probably want to stack multiple exposures instead. Google exposure stacking, it's normally an astrophotography technique.

Quick formula:
New exposure (in seconds) = old exposure (in seconds) x 2^stops.

So say you've got a 1 second exposure and a 10 stop ND filter.
New exposure = 1 x 2^10
New exposure = 1 x 1024
New exposure = 1024 seconds (or about 17 minutes.)

Us IT geeks know our powers of 2 off by heart, but if you just learn the ones for your ND filters you'll be fine. 3 stops is 2^3 is 8, so multiply exposures by 8 to get the new one, etc.

Don't be afraid to round, approximate, etc. You can be off by 10% or so and it'll make no noticeable difference to the exposure.
 
Just been reading an interesting article regarding the Olympus sensors and the Dynamic Range.

Apparantly the best DR is at ISO200 giving an extra 1/3 at the bottom and 2/3 of a stop at the top of the range, with no increase in noise.

Always worth knowing ;)
 
Oh that's great advice there Chris, thanks. Just back in from another evening shoot... about to have a look at the mess now!! :)
 
Unfortunately it's not as simple as just switching apertures and times around mathematically.

With film you have what is called reciprocity failure, where the film becomes less cooperative at longer speeds (in to many seconds or minutes) and doesn't follow the rules. You have to expose for much longer than calculated.

With digital, you will find that calculating mathematically a longer exposure will not give you the desired result, or even a pleasing result. For example, if you were to leave the camera to its own devices and let it calculate an exposure for your night shot, you will find that you will end up with something that is trying to look like daylight. It might look ok (and probably noisy) but it won't be what you want. So you have to do the opposite as with film - expose for less. In the end you will end up throwing in to the wind all your careful calculations and taking several shots at different exposures to achieve the desired result. Which you would do with film as well.

When I do night shots I don't try to work out an exposure, or anything. I keep the ISO on the low side. I select an aperture that will give me the required depth of field, say 8 or 11, and give me some leeway for manual focusing error if necessary. Then I think of a number. It might be for example, 2 secs. A guess, with a bit of experience thrown in. Take the picture. It might look ok. It might not. It's not the end of the world. It hasn't cost you anything. So I will probably take one at say 1, 4, 8. I will have a good spread out of which one will be the one I want.

I do the same when I take pictures of stars - not stacked, or complicated stuff with telescopes. A good range - 30, 60, 90, 120, 150 (until earth rotation kicks in) for example. Look at the screen, see what I get, adjust as required. No exposure calculation. No rocket science.
 
MisterE said:
Unfortunately it's not as simple as just switching apertures and times around mathematically.

With film you have what is called reciprocity failure, where the film becomes less cooperative at longer speeds (in to many seconds or minutes) and doesn't follow the rules. You have to expose for much longer than calculated.

With digital, you will find that calculating mathematically a longer exposure will not give you the desired result, or even a pleasing result. For example, if you were to leave the camera to its own devices and let it calculate an exposure for your night shot, you will find that you will end up with something that is trying to look like daylight. It might look ok (and probably noisy) but it won't be what you want. So you have to do the opposite as with film - expose for less. In the end you will end up throwing in to the wind all your careful calculations and taking several shots at different exposures to achieve the desired result. Which you would do with film as well.

When I do night shots I don't try to work out an exposure, or anything. I keep the ISO on the low side. I select an aperture that will give me the required depth of field, say 8 or 11, and give me some leeway for manual focusing error if necessary. Then I think of a number. It might be for example, 2 secs. A guess, with a bit of experience thrown in. Take the picture. It might look ok. It might not. It's not the end of the world. It hasn't cost you anything. So I will probably take one at say 1, 4, 8. I will have a good spread out of which one will be the one I want.

I do the same when I take pictures of stars - not stacked, or complicated stuff with telescopes. A good range - 30, 60, 90, 120, 150 (until earth rotation kicks in) for example. Look at the screen, see what I get, adjust as required. No exposure calculation. No rocket science.
Click to expand...

You've misunderstood the question. The OP presumably had a "good" exposure at 60 seconds at f/8 but wanted the exposure at f/11. There's only one correct answer in this case.

And if you can do the correct calculations then you don't need to bracket exposures. I can get my lightmeter out, meter off of a specific part of the scene, decide how I want it to appear on the negative/sensor/print and compensate accordingly. No guesswork and I get the results I want. Pretty important when it's blowing a gale and just below freezing in the Scottish Highlands.
 
Blapto said:
You've misunderstood the question. The OP presumably had a "good" exposure at 60 seconds at f/8 but wanted the exposure at f/11. There's only one correct answer in this case.

And if you can do the correct calculations then you don't need to bracket exposures. I can get my lightmeter out, meter off of a specific part of the scene, decide how I want it to appear on the negative/sensor/print and compensate accordingly. No guesswork and I get the results I want. Pretty important when it's blowing a gale and just below freezing in the Scottish Highlands.
Click to expand...

No, I don't believe I misunderstood the question. I'll get back when I can generate some examples to illustrate my point.
 
theMusicMan said:
Scenario: lets say I have an exposure of 60 seconds, with an aperture of f8. If I want a longer exposure, and I close the aperture to say f11, how do I work out what the amended exposure should be as my camera only goes up to 60 seconds. I assume there's a chart or algorithm somewhere that will allow me to work this out...?
Click to expand...

That's the question.

The answer is that you double the exposure when you increase f number by sqrt(2).

8/11*(sqrt(2))=1.03, near enough to 1, so you double the exposure.

The question had nothing to do with how a photo should be exposed, simply how to calculate exposures.
 
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