Sunday, July 27, 2008

Double Header: Part I, What's Up With My Meter?

I received a Flickr mail from a reader asking a question that I get often enough that it deserves a public response. The question is basically as follows.

When I put my flash on my camera, or have my pop-up flash up in commander mode, or have my SU800 in the hotshoe my exposure meter still says I am way underexposed. What am I doing wrong?

The answer.....Nothing.

This is really pretty simple. Your camera does in fact have a light meter built into it. It is a reflective meter though. Meaning it can only measure the light which it currently can see through the lens. Your flash units are not firing therefore there is no light for the camera to measure. Now comes the follow up question.

But I am using TTL and the camera knows how much power to send to the flash unit. So why can't my meter show that?

The camera does in fact figure out the flash intensity for you. But to understand why your meter can not show this you have to understand how TTL works. So here is a basic break down.

1) You press the shutter.
2) The camera fires a TTL preflash. Meaning it flashes before the shutter opens. This preflash is a known strength. I don't know what that strength is, but for the example let's just say it is 1/16 power.
3) The camera's meter is timed to read this preflash at the exact moment it fires through the lens (Hence the name TTL).
4) Depending on distance and modifiers the light hitting the subject will be a given strength. Your subject may be underexposed or over exposed depending on that factor plus.....
5) Your camera's aperture and ISO settings determine how much of that light reflecting off of your subject reaches your camera's sensor.
6) At this point the camera does some basic math. It says, "I know the preflash was 1/16 the image I now see is under exposed by 2 stops (just an example here). so I need to make the flash power 1/4 to properly expose the subject.
7) The camera then fires the remote flash at 1/4 timed when the shutter opens.

OK, John what does that have to do with the price of eggrolls? Well, your camera only knows all of those variables once you commence the shutter release process, and make an exposure. The camera's meter has no way to know flash distance or modifiers being used until you snap the picture. If you are in manual mode the camera has no idea what your remote flashes are set to power wise on top of the other unknowns.

So what do I do you ask?

Use your meter to set your ambient light levels where you want them. Nothing new here, basic exposure 101. Then set your flash power to properly expose for your subject or let TTL take over if you trust it.

But John, that's the whole point.....How do i know what is a correct flash power?

Practice and learn. You can use charts. But practice makes perfect. Be methodical in making sure you know that if you move that flash twice as far back you know what happens. David Hobby talks about making a cheat sheet for your flash units over on the Strobist Blog.

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