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Old July 30th, 2009, 09:44 AM   #1
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Luminance of a flash..?

Maybe a bit off topic, but I wonder if anyone knows, approximately, how many luminance a standard on-camera flash has? I'm trying to set up some matching lights for a 5D shoot and my Google search gave me nada.

Any help appreciated.

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Old July 30th, 2009, 07:20 PM   #2
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The amount of light a flash produces is described as a Guide Number.

The Guide Number (GN) is equal to the distance (in feet) from the flash
to the subject, times the f/stop, assuming a gain setting of ISO100. The
higher the GN, the more powerful is the flash and the longer it will throw.

For example, a GN of 100 will expose for f/2 at 50 feet, or f/4 at 25 feet.
At f/5.6 it will reach to roughly 18 feet. Again this assumes ISO is at 100.

f/stop = GN divided by distance (or) distance = GN divided by f/stop

Currently there are three models of Canon SpeedLite flash units:

The 580EX II, about $500, GN 190
The 430EX II, about $330, GN 141
The 270EX, about $160, GN 72 / 89

The 580EX II will give you f/4 all the way out to nearly 48 feet when it's
set to full power (it can also operate at lower power settings). There's a
distance scale on the back of the flash which can be set to meters or feet.

There's an online Guide Number calculator which will let you input a
number of variables such as desired exposure, desired ISO, distance,
Guide Number, and power setting:

OCPA's Flash Calculator

Here's a very handy article about using Canon SpeedLites:

Flash Photography with Canon EOS Cameras - Part II.

Hope this was somewhere along the lines of what you were looking for,
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Old July 30th, 2009, 10:07 PM   #3
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Quote:
Originally Posted by Peer Landa View Post
Maybe a bit off topic, but I wonder if anyone knows, approximately, how many luminance a standard on-camera flash has? I'm trying to set up some matching lights for a 5D shoot and my Google search gave me nada.

Any help appreciated.

-- peer
The luminance will vary wildly depending on the flash and camera settings. It especially varies in the default ETTL mode which uses feedback from the flash.

Also, you should be able to find out more than you ever wanted to know about photography and flash from wikipedia.
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Old July 31st, 2009, 06:13 AM   #4
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What happen'd to Chris Hurds answer that was here last night... that was a great site
he posted...
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Old July 31st, 2009, 08:07 AM   #5
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I pulled it because I didn't think it answered what Peer was looking for. Restored now, but I should probably move it to our Still Photography forum.
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Old July 31st, 2009, 10:37 AM   #6
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Quote:
Originally Posted by Chris Hurd View Post
I pulled it because I didn't think it answered what Peer was looking for. Restored now, but I should probably move it to our Still Photography forum.
No no, your post was (as always) highly appreciated -- besides, my question relates very much to video. Not long ago I built my own LED lights for my old XL2 (http://www.dvinfo.net/conf/1023794-post4.html) and currently I'm working with another 5DMk2'er who thinks those lights are "friggin' too bright" and wanted to know what kind of juice they actually produce. Since I only know they are 600 lumens, we tried to find a reference point to onboard camera flashes.

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Old July 31st, 2009, 04:59 PM   #7
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This post is very relevant to something I have been trying to accomplish with both stills and video. I sometimes shoot fire artists and want to get stills and video with the fire and artist both exposed properly. What typically happens is that the artist is barely lit and the fire overexposes so it loses color and definition. I went out and tried lighting them with a little fill light to get them more visible while lowering the exposure to get detail in the fire. Of course, my fill light is nothing compared to the fire on the artist so it did nothing. I then tried some stills with a flash last night and the little flash I got is really not powerful enough. Knowing the facts on flash will help me know how much flash for stills and continuous light for video that I need. With the 5Dii, I am finding that I want to learn both still and video and use whatever is appropriate from moment to moment.

BTW, a little tip I have is to gel the flash down with two layers of CTO to get it closer to the color of the fire. This helps it seem more like the artist is lit by the fire.
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Old July 31st, 2009, 05:29 PM   #8
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Hi Marcus, seems to me that you should experiment with first- or second-curtain flash techniques (slow sync flash) to see if they will produce the the effect you want. First-curtain fires the flash at the beginning of a long exposure while second-curtain fires the flash at the end of a long exposure. Some examples:

1st-curtain: Shutterbug: Flash Tips

2nd-curtain: http://photos.jpgmag.com/196131_19846_10c6b8a028_p.jpg

Some good reading on 2nd-curtain slow sync flash: http://digital-photography-school.co...low-sync-flash
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Old July 31st, 2009, 07:12 PM   #9
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That is what I am already trying and it is even in the links you first posted. Unfortunately, the flash I have is not strong enough to overwhelm the flames even for its short duration. Armed with this new information about the guide numbers, I can try a few new tricks and at least know what to look for in a new flash.

I thought of something else that may be interesting and actually work. I'm thinking of trying a DJ strobe for video. With a 1/30 shutter and keeping flare out of the lens, I should be able to get some interesting video with a smoothly continuous flame and a stroboscopic artist/dancer. What is the strength of a strobe light if it puts out 750W? Does a strobe source put out about 4x as much light per watt like a fluorescent puts out 4x the lumens as tungsten? If so, a 750W strobe would be as powerful as a 3000W tungsten and still run from a battery since it only would fire at intervals.
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