Speedlite vs. On-Camera Flash Distances

Is there a way to compare effective flash distances of on-camera (70D) vs. the various Speedlites? I need to be able to shoot parades at night - est. a distance of 20-25 feet - without 40 sets of red-eyes staring back.

Thanks,

Tim

If your camera-to-subject distance is 10', and the off-camera flash is aimed at the subject from 10' off to one side of the camera, the straight line distance from flash to subject is 1.414x farther, or 14' from the subject.
If your camera-to-subject distance is 20', and the off-camera flash is aimed at the subject from 20' off to one side of the camera, the straight line distance from flash to subject is 1.414x farther, or 28' from the subject.

If I understand your question correctly, you are comparing the built in flash to an external on-camera flash. With the taller Canon 500-600 flashes, I don't think you will get much red-eye. If you do, you can put the flash in red-eye reduction mode which fires a burst of pre-flashes to constrict the pupils. The built in flash will likely have insufficient power at 20-25 feet.

I wouldn't worry about the camera to subject distance at all, all I would worry about is the flash to subject distance. It is the flash to subject distance that defines the level of illumination, and for OCF the camera to subject distance is irrelevant. Simply use the GN for the flash along with the flash to subject distance to calculate the required f/number. As long as you know the maximum flash to subject distance that is required, along with your maximum or prefered f/number, it should be able to tell you if your proposed flashgun will be powerful enough to do the job, even if you are going to use off camera ETTL II metering. Going with off camera ETTL II metering all you would then need to worry about is staying within the maximum subject to flash distance, which for moving subjects may be a lot easier than trying to calculate manual flash exposures. Off camera ETTL II flash would be even easier if you then wanted to add a second light to the setup, either positioned on or off camera.

Alan

Thanks all. I'd be looking at an on-camera Speedlite. Just trying to figure out which one will do the trick at a distance of 20-25 feet.

Try this online flash power calculator:

http://cpn.canon-europe.com …flash_power_cal​culator.do

(You'll need to scroll down a bit to get to the calculator)

The 70D's inbuilt flash has a guide number of 39 feet or about 12m at ISO 100. The guide numbers of Canon's speedlites are in the name, expressed in metres, e.g. 430EX (any version) => 43m at ISO 100, 580EX => 58m at ISO 100.

Short answer is that is precisely what the Guide Number is; a relative power rating that you can compare flashes with.

Understanding Guide Numbers

This one goes into more details

Any of them will do, just raise the iso and open the aperture as needed. What kind of parade are you shooting? If there is sufficient lighting you may be able to shoot it at a high iso and fast aperture, using the flash for fill rather than key. It will make for more dramatic images, assuming it's not too dark.

Good info - thanks all. A few night parades a year - through a very dark school parking lot / drive.

Remember that the guide number is calculated under the assumption that you are indoors, and make use of reflections from the ceiling etc. You can expect less range outdoors. Using the flash outdoors in low/no light will thus also discharge the flash more each time you fire. When I've been shooting night sports, I've used the 580 EX or 600 EX-RT with the CP-E4 additional battery pack. If you have time to wait longer between shots, it's not needed.