Will the flash be enough?

[dphoto]

Hey all,

Here's a situation I find from time to time... I'm about to take a picture of someone posing. I use Tv mode since I'm holding the camera and people tend to move (including myself!). Let's say I've got the time set to 1/125. There's not enough light, so the 300D displays the widest aperature available on the lens as a flashing number (let's say f/4). I pop my flash up because I know I'm going to need it (from the cool flashing number :D). The only way I know of to know if the flash can produce enough light for the shot is to take it and check the histogram.

What I'm wondering is this: the ETTL is going to do the computation to set the flash's power anyhow, so why can't there be some sort of indicator on the camera to let me know that this shot has enough light? It would have been possible, right? Is it there and I'm just missing it? Oh, I just did a google search and found out that the guide number for the flash is 43 (13 metric). So at ISO 100 and f/4, that tells me that as long as I was within 10 feet, the flash was going to be enough. Hey, I think I just answered my own question.

Anyhow, I was just curious, that's all. It came to me the other night when a group of people quickly assembled and I thought that I might only get one shot off. I thought... I have no idea if this thing can produce enough light... I didn't want to preemptively jump to ISO 200, so what can you do? I just took the picture at ISO 100 and it came out fine. Whew! :D I should have done the quick guide number computation. :D

Thanks for any input,
-Deva

[photoguynorth]

The E-TTL does the computation based on a pre-flash. It might be possible to do an indicator based on this, but the only way to do a pre-flash withouit taking a picture is using the * button. It's probably easier at that point just to take a picture, and make sure the green light comes on the flash (assuming it's a Canon flash).

[dphoto]

Yeah, you're right, you do need a pre-flash. The * button would do it, but like you said, at that point just take the picture. :D

As for the green light, I was talking about the built-in flash. But I do have the 550EX as well, and the green light will tell you. But better than that, when you have the flash set to use ETTL, before you take the picture, there's a little graph showing you how far the flash will reach. It does that light guide number computation for you.

As for the built-in flash, it would be cool if it gave a similar graph. :D

-Deva

[photoguynorth]

You could just fake a graph with the built-in flash - that shows 'not very far'. :)

[robertwgross]

Back in the old, old days, when I had a different camera and a different flash, I liked to shoot nighttime shots of wildlife, like black bears. Late at night, with very little time to set up a shot, and very light work light around my camera, it became very difficult to go through the quick calculations necessary for this. So, I made myself a chart.

I started with my favorite lens and its wide open aperture (you can repeat this for how ever many lenses you deal with). The chart shows distances in feet or meters (take your pick) that could be shot with ISO 100 at that aperture, then another part for ISO 200, etc., up to the highest ISO that could be expected for use. Of course, this is a simple chart, since it is calculated from the guide number of the expected flash unit (GN divided by aperture yields distance). These little charts don't need to be much larger than a postage stamp and can be affixed to your camera case, flash unit, or any place you will find handy.

Then my last step was to stake out my "shooting range" each night. I would place one rock marker at ten feet, one at twenty feet, one at thirty feet, etc. If I guessed the wildlife was between twenty and thirty, I could see where that distance fell on my chart, and then I fired away.

Presently, I use an external flash unit, so the distances are dramatically longer.

---Bob Gross---