Tuesday, September 7, 2010

Lightning Strikes!

Bad! I've been very bad! Well, OK. Maybe not really 'bad' -- just BUSY! I do think regularly about creating new posts, but everything else seems to get in the way. When I look at the date of my last post, I get all embarrassed!

Some of the most fun photos to take are of lightning. I like to joke with folks about how hard it is to capture lightning. You know, wait, see the flash, press the shutter button. Darn! Missed again! It brings on a chuckle. Obviously, there must some other way to do this. And so there is -- long exposures!

But, there can be a problem with that approach, too. There are three factors that add together to create the proper exposure of a photograph; film (or sensor) sensitivity, shutter speed, and aperture. The sensitivity is the ISO value, such as 100. As the numbers double, the sensitivity increases by a factor of two. But, there is a down side to using larger ISO values, and that is increased graininess of the image. Newer cameras are getting better about this all the time, but it will always be an issue. The shutter speed is the length of time that the shutter remains open allowing light to reach the sensor. This is given as a number of seconds, or as a fraction of a second, such as 1/125 of a second. The longer the shutter is open, the more light will reach the sensor. And, finally, there is aperture. In essence, this is a number that is representative of the diameter of the opening in the lens through which light is admitted to the sensor. This value is usually specified as the letter 'f' divided by a number, such as "f/1.4" or "f/5.6". In technical terms, the aperture number (or f-stop number) is calculated as the focal length of the lens divided by the diameter of the opening. But, don't get bogged down by that. Here is the important thing to remember, every time the aperture value increases by an additional multiple of 1.4, the amount of light that reaches the sensor will be cut in half. Or, to make it even simpler yet, the larger the f-stop number is, such as 5.6 vs. 1.4, the smaller the diameter of the opening, and the less light that makes it to the sensor.

So, here is the deal, to take a picture with a longer exposure time, and yet maintain a correct exposure, requires that you either decrease the sensitivity of the sensor (set for a lower ISO value), and/or shoot through a smaller diameter lens opening (larger f-stop number). Sounds easy, huh?

That's what I thought too. But, recently I learned something. And that is that it is much harder to take lightning shots during the day than it is at night. Why is that, do you suppose? Well, here is what I have figured out. Let's see if I can explain it clearly enough.

First off, I always try to shoot at the lowest ISO value I can get away with, so as to avoid grainy pictures. This means that I stay at ISO-100 as long as I can. Yes, there are reasons to increase the ISO value, but for now, let's just stick with 100. So, that just leaves altering the aperture and shutter speed.

It may seem that the easiest thing to do to capture lightning shots would be to shoot with a very slow shutter speed. My camera allows me to go to as long of an exposure as 30 seconds. Beyond that, I can put the camera into its bulb setting, and, using a remote trigger device, can shoot exposures as long as I like. Such as when I have taken some star photos with exposures as long as 15 minutes. OK, so what's the problem? Well, technically, it has to do with the mathematical integration of all that light energy. But, let's see if we can make that a little easier to understand.

Let's pick an example of wanting to shoot with a shutter time of 30 seconds. Of course, to compensate for that, you'd have to shoot with an smaller aperture (i.e., a larger f-stop number) such as f/22. The exposure will be a function of the total amount of light that hits the sensor. During the brighter hours of the day, to shoot a 30 second exposure would require a really small aperture so as to not over expose the image. The brief little lightning flash, when compared to the overall brightness of the sky/clouds, is so insignificant that it just disappears, and doesn't show up at all in the image. Wow, all that time taking that picture, and the lightning did flash while the shutter was open, but, darn, nothing shows up!?!? Yup.

During the darker hours of the evening or night, you will be able to shoot at a much larger aperture. And, the integration (or collection, or summation) of all that light energy will still produce a correct exposure. But, the amount of light energy that came from the lightning flash, will be a much larger percentage of the total light energy collected from the dark sky, and so it will show up much better in the picture. Whew! I hope that makes some kind of sense! Trust me, it's easier to think through than it is to try and explain it!

So, here are a few shots from a few nights ago. I was very lucky on this particular night, because usually when I try to capture lightning, I may take a dozen or so shots but only come away with a single shot actually containing a strike. I think I got about a half-dozen strikes on this night. Following are the best four.

This first shot came first, while it was still a little lighter out. Note that the exposure was only 8 seconds at F/22. The shorter exposer makes it harder to catch a flash, because you are just guessing when a strike may occur, and triggering the shutter ahead of time. I got lucky in this one a captured a fairly decent strike.

ISO-100, 8-seconds, F/22
In the next shots, note that I've been able to increase the exposure time to 30 seconds, making it easier to capture a strike. Because it has gotten a little later in the evening and has gotten somewhat darker, I've been able to open the aperture to f/18. Note also that the longer exposure time makes the moving clouds streak and blur. That's an effect I rather like.
ISO-100, 30 seconds, F/18

ISO-100, 30 seconds, F/18

ISO-100, 30 seconds, F/18
I'll try to not be so long at getting back to make my next post. Till then, God Bless!!!