Reasearch (http://timothyallen.blogs.bbcearth.com/2009/02/24/time-lapse-photography/)
Once you’ve framed your shot with the camera firmly on your tripod, you’ve now got to decide how much interval to leave between each frame. I’ll use the sun and cloud examples since they both need different intervals. Basically, timelapse movies run at between 24 and 30 frames per second. British TV runs at 25 frames per second, so that’s the figure I use since my time lapses are designed for inclusion in a TV programme. At 25 frames per second (fps), 250 exposures will produce 10 seconds of movie, so if you want a 10 second clip on TV, you need to set your intervalometer to shoot 250 frames. If you can’t specify that on your intervalometer, you must instead calculate how much time it will take to shoot 250 frames and then time your sequence with a stop watch (eg. at a rate of 1 frame every 3 seconds, it will take 250 x 3 = 750 seconds or 12.5 minutes to complete a 10 second time lapse film).
Put simply, the shorter the interval between each frame, the more slowly the action will move in your film. Using the shadows and clouds example again – clouds will almost definitely tend to move much faster through the sky than a shadow cast by the sun will move across a landscape. So, we normally set a shorter interval between shots when time lapsing clouds, and a longer one when shooting the sun’s path across the sky. Practically speaking, this means roughly 1 to 15 seconds for clouds (depending on the wind speed), and 30 seconds and above for the sun’s movement. These are my generalizations of course. There are no hard and fast rules governing time lapse intervals. The interval you set is totally up to you and how fast you want the action to move in your clip.
It’s worth mentioning that if you are shooting an unfamiliar situation it is often a good idea to shoot your time lapse with a shorter interval than you may expect. This way, you will have the option to remove images from the final sequence at a later date if you decide in hindsight that the action looks better speeded up, remembering to make your new interval length divisible by your original one in order to make the post production simple (eg. if you expect a 4 second interval but you’re not certain, shoot with a 2 second interval and then afterwards you can easily revert back to the 4 second interval by removing alternate images from the sequence. Remember though, that the exposure length you choose for a 2 second interval will not necessarily produce smooth and realistic movement in your 4 second interval sequence).
There is one final thing to say about interval length. All digital cameras need a certain amount of time to process and download image data to the memory card. Cameras contain their own RAM memory, called the buffer, in which images are temporarily stored before being written to the card. Having a buffer affords the camera the ability to shoot frames rapidly in succession but there is a limit to the number of files they can handle before the memory fills to capacity and the camera automatically disables its shutter in order to give the processor time to free up more space in the buffer.
If you are shooting a time lapse with a short interval length (eg. 1 second) then at a certain point your camera’s processor may struggle to keep up with the amount of data that it needs to continuously write to the memory card. The result will be a slight pause in shooting every now and then which will distort the continuity of your sequence. Similarly, if you are shooting RAW files, long exposures (eg 1 minute) or pictures containing excessive detail then your camera’s buffer will also need more time to process the image data. To combat this problem you have a few solutions. Firstly, reduce the resolution of each frame by decreasing Image quality in your camera’s menu options. Secondly, invest in a higher quality memory card that can handle faster data transfer rates (written as MB/s or ‘megabytes per second’ on the card). Or lastly, you can upgrade your camera to a better model which has a higher buffer rate.
So, since we’ve decided upon the interval and length of our clip, we now need to talk about our exposure. In the beginning, set your camera to its fully manual mode – you need to fix both your aperture and shutter speed so that each frame is exposed identically. I’ll talk about auto exposure time lapses later on and also the problem of time lapse flicker in a manual mode when we get onto the more refined aspects of time lapse photography. For now, we’re using only manual settings with the auto focus and auto white balance functions in our camera disabled.
Deciding upon the aperture setting depends on two things – how much depth of field do you want and how fast (or slow) you want your shutter speed to be. The latter of these two will probably be your bigger concern in the beginning. Shutter speed in time lapse films is a very important factor. If you shoot with a fast shutter speed (eg. 1/100th second), the movement in your final film will tend to look less realistic than if you shoot at a slower speed (eg. 1/2 second). If you want to know what I mean, play any action movie on your computer and then freeze frame it during a fast moving sequence. You will notice that all the action is blurred when you view only one frame. However, when you run a sequence of blurred images one after the other, the illusion of movement is created and the blurring is ignored by your eye. Of course, any static objects in that sequence will remain pin sharp no matter how long the exposure as long as the camera is completely still on the tripod.
Roughly speaking, your shutter speed should be just under half that of your interval. So, for a 3 second interval, a 1.3 second exposure is great. At longer intervals this rule ceases to apply, so for example, when shooting the stars at night, a 30 to 60 second exposure is usual depending on the star cover (at ISO 800, f2.8) with a 5 second interval between shots for super realistic movement. If you expose too long, the path of each star may begin to appear as a line in your picture and can look a little unrealistic. However, if it’s an interesting effect that you’re after, then expose as long as you want. To be safe, when you are shooting anything moving relatively slowly (eg clouds) for the best results, use a 1 to 2 second exposure for short clips. If your shot has people or cars moving through it then it is imperative to use this kind of exposure length otherwise your moving objects will appear as ‘blips’ on the screen, creating a very unrealistic time lapse. As a general rule of thumb, if you are leaving a long interval between each frame (eg. 30 seconds or above) then it is extremely important to use a slow shutter speed in order to make the movement of natural objects run smoothly between frames. Bear in mind also that time lapses containing people don’t generally work very well with large intervals even if your exposure is long. Experience will be your best teacher here and as you practise you will discover what works best for you.
Shooting longer exposures in daylight will require you to set your ISO to the smallest value (ISO 50 or 100), and if you still can’t achieve a slow enough shutter speed, you will need to use the neutral density filters I mentioned earlier. My ND 3.0 filter will add 10 (f) stops to a lens. In reality, this means you can easily shoot a 3 second exposure at f2.8 in bright sunlight.
As you get more particular about the results of your time lapses, you may want to shoot at wider aperture settings in order to eliminate dust marks from your final images and so reduce the amount of post production necessary. Remember, at f22 your camera will bring quite a lot into focus including the marks on your lenses and filters. Using ND filters will solve this problem if it arises.
Great! We’re now ready to shoot our time lapse."
(http://timothyallen.blogs.bbcearth.com/2009/02/24/time-lapse-photography/)
