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Tuesday, September 29, 2009

Exposure Calculation

Some time back (a few years) I found a website with a terrific exposure calculator on it. BobShotS had this nifty exposure calculator where you could set your lighting conditions and f- stop or shutter speed and get the corresponding measurement for good exposure. It was a really good tool to get new dslr users some starting points for understanding exposure without overwhelming them with the details.

I had made a "cheat sheet" using some of the variables from that calculator and carried it around with me in my bag until I had gathered enough data to create my own and my daughter had asked me about it the other day.

I went looking for BobShotS Exposure Calculator this morning, but sadly it was gone from the internet sometime last year...in fact his whole site disappeared. Bummer. I looked him up on the internet archives and there's a page with the calculator, but with all the files off line it doesn't work anymore. So I set out on a mission to find some others. And there aren't too many out there that were as good as this one. If I could find the guy, I'd consider asking him to sell me his files for it so I could help other beginners.

I did find a few, so I'll link them here (below) for you. There are plenty of printable ones, but I've found that the online ones (though you can't take them with you) seem to give the beginner a better understanding of how exposure works, as opposed to looking at printed charts. I think it's probably because they are interactive and you get to select certain settings that you might be familiar with or use all the time.

Before I list them, today's "tutorial" isn't downloadable, it's simply this article with a few additional instructions and explanations.

These calculators aren't exact, and they aren't really meant to be. They are meant to be a starting point for your own calculations - they give you a place to begin. The only way to really learn is to do some work for yourself.

Everyone has heard of the "sunny 16 rule", but what exactly is "sunny"? A day where the sun is shining right? But sunlight is different under different conditions and in different locations. Early morning sun isn't as strong or bright or even the same colour as bright noon hour sun, nor is late afternoon sun as bright as noon hour sun. Sunlight out on the water (in a boat) is likely to be brighter than sunlight in the middle of the city, or out in an open field. The reasons of course are the reflective qualities of the surroundings. Sunlight in early winter months where there is no snow on the ground will be different than sunlight in the middle of the summer, or sunlight shining on the white snow too. The sun is at different angles - further away in the winter, closer in summer. All these variables make using a single rule for "sunny days" the wrong thing to do.

The right thing to do is take all those settings that you've calculated from online calculators or printed charts and go out on shooting expeditions and collect your own data. Do some seasonal comparisons. Take your "sunny 16" generic settings and go out into a big open field and take a shot using those. Check it in your lcd display. Now change your f-stop up or down and take another. Do that for at least 3 or 4 different stops below the suggested setting and 3 or 4 above. Now go into the city or an area where there are some tall obstructions (like buildings, houses, skycrapers. etc.) and do the same thing. When you get a chance, go do the water one too. You don't have to be on the water in a boat - if you have a lake or beach near you, that will work too.

Do that for all the settings until you have a nice little book of settings.

Yes, it's a lot of work. And will take a lot of time. Chances are though, that after the first few outtings you'll have developed a much better understanding of how exposure calculation works, and not only will you not likely "need" the cheat sheets or calculators anymore, you won't even need to finish the entire exercise. You'll have learned how to judge the light around your for proper exposure.

After all, photography isn't something you learn in a week, or a month or a year. It's a lifetime of learning and adapting, so spending a few months gathering experience like this will stand you in good stead for the rest of your life. It establishes an experimental process that you can put to use in a lot of different ways in your photographic career.

Yes, I know, I know - that's what light meters are for right? Sure, but not everybody can afford a light meter and beginners aren't likely to even understand a light meter (and remember, these tutorials are written with the beginning dslr user in mind). And we all know that the camera can meter the available light for exposure but let me ask you this. How many times have you set the camera on auto or "P" (programmed-auto) mode and gotten a bad exposure? Uh huh. Probably enough times to make learning how to shoot with manual exposure settings worthwhile.

There's nothing wrong with using auto modes in a pinch or in continuously changing conditions - when things happen quickly you don't always have the time to reset all the manual settings on your camera. But when you learn how to control the exposure, you open up a wide range of possibilities for yourself.

The other thing to keep in mind is your own shooting habits and abilities. Just because someone says you should be able to get a sharp shot using an exposure of 1/60 doesn't mean you are going to be able to. Especially if you aren't carrying around a tripod. You need to know the settings that work best for your own physical abilties. Of course, the advent of lenses and cameras with image stabilizers gives you a lot more leeway, but even with those, knowing where you limites lie can stand you in good stead.

Oh, I guess you are all wondering where the calculators are, right? Okay. First up is an exposure calculator from Calculators.org. Out of the ones I found, this one is the closest in functionality to the one that was on BobShotS Photography. The second similar one has been provided free online by DesignMentor Training. If you happen to be a student of theirs, you can even download it.

This next one is not so much an exposure calculator as it is a visual representation of how f-stop and shutter speed work together and affect exposure. It's known as the "sim-cam" and it's been around a while. It's pretty limited in options, but immediately seeing the result on a picture of the differences in settings can be very helpful to beginners. It's been provided by Photonhead.

But before you all go running off to make your own cheat sheets. there's just a couple of other things. There's one fellow on the net who has a downloadable program for exposure calculations - Jon Sachs very kindly has this free download for you, along with a few others you might find of interest.

Then there's an astrological exposure calculator available here at Covington Innovations.


Anonymous said...

"Then there's an astrological exposure calculator available here at Covington Innovations."

It's ASTRONOMICAL. Astrology is a failed religion, astronomy is a science.

Gil Tennant
Lead Astronomer

Anonymous said...

I thought astronomy was a failed science?