Using flash in an auditorium

[danisha_bryant]

<p>Last night I shot a Christmas concert in a decently lit auditorium (the audience wasn't in total darkness) and was using flash until I learned they were video taping the show so opted to shoot without the flash. The results were unexpected in that my shots with flash were very warm and "glow like" whereas those without flash were actually brighter and cooler - and clearer too! My question is, how is this possible without my changing any settings (shooting in S mode at 400, AWB, Single Shot, AF-C, AF area and a Carl Zeiss 16-35mm f/2.8 Zoom) and technically less available light (with the flash off)? Thanks.</p>

[jcoffin]

<p>At a guess, you were probably far enough away from the performance that your flash wasn't providing much of the light for the exposure. The light balance, aperture, shutter speed were all adjusted assuming the exposure would come from the flash -- and when it didn't, the result wasn't particularly great.</p>

<p>With the flash turned off, the camera ended up adjusted for the light really being used for the exposure (correct white balance, and if you were using auto-ISO, probably a higher ISO to get a faster shutter speed), so things came out better.</p>

[dave_redmann]

<p>Let's see:<br>

<em>shooting in S mode at 400</em><br>

Do you mean shutter-priority auto-exposure with a shutter speed of 1/400 s, or so you mean sensitivity set to ISO 400, or what? Because that description is not clear to me. Look at the EXIF data on the pictures; what are the shutter speed, aperture, and sensitivity? Also, how far away were you from the subject(s)?</p>

<p>But basically, my top suspicion is the same as Jerry's: the flash didn't have enough power to illuminate the subject at the distance in question. You don't say what camera you were using, or whether you were using the built-in flash or an accessory flash, and if so, which one, and how it was set. But the typical built-in flash has a nominal guide number of about 12 m, which means that at ISO 400 at f/4, realistically, once the subject is more than about 15 to 20 feet away, the flash doesn't have enough power; use f/8 and your maximum distance is more like 10 feet. On the other hand, if you were using a shutter speed of 1/400 s, the only flashes that can work with that and a Sony or Minolta SLR will be using "High Speed Sync", which greatly reduces their power--basically, the flash has a lot more power than the built-in flash, but using it the way you may have done negated that advantage.</p>

 

[danisha_bryant]

<p>Jerry, that makes sense. Thanks. Dave, thanks for your questions. To clarify, S = shutter priority mode at ISO 400 and yes, it was the built-in flash at too far a distance to make a difference other than some extra catchlights in the eyes of the performers. I was at least 60 feet from the stage and the lens auto focused. I don't have access to the photos at the moment so not sure what the shutter speed was, but will be sure to check and refer to your comments. I have an external flash that I haven't tried yet and your advise will help give me more perspective now, when I do get around to using it. (The camera by the way is the a580. Sorry.)</p>

[jcoffin]

<p>From 60 feet away, a flash would have to be quite powerful to provide adequate illumination. The flash built into the A580 has a guide number of 39.3 in feet at ISO 100. Using ISO 400 doubles the guide number, so you have a guide number of 78.6. A guide number works like: maximum distance = guide number / aperture. Assuming you were shooting at f/2.8, that would mean your maximum distance was 78.6/2.8 = ~28 feet. At f/4, it would be 78.6/4 = ~20 feet.</p>

<p>Guide numbers are something of an approximation though -- the rating on the box is usually based on the assumption that you're using the flash in a place around the size of a normal room in a normal house, so a subject is lit not only by light coming directly from the flash, but also partly by reflections off the walls, floors, and ceilings. In a large auditorium, that generally won't happen (enough to notice anyway). In a case like this you generally need to de-rate the guide number by something like 30% to get roughly the real maximum distance. That means your real maximum distance in this case was probably somewhere around 20 feet (14 feet at f/4).</p>

<p>I should point out one more detail: the power you need from a flash is proportional to the square of the distance to the subject. That means to reach the 60 feet to your subject you didn't just need a flash that was 3 times as powerful -- you actually needed one that was about 9 times as powerful.</p>

<p>You can also look at things from another direction. To get coverage from that flash at that distance you could also increase the ISO. In this case, you'd need to use around ISO 3200 or 4000 for that flash to provide proper exposure at that distance.</p>

[phil_derosier]

Danisha, why are you even fooling around with ISO 400, anyway? Most modern cameras perform very well at 1600, and

above—which should be a starting point, or 'default' ISO, at least for indoor work.

[danisha_bryant]

<p>Phil, just to get as crisp an image as I can, I always go with the lowest ISO possible. But I will try a higher ISO and see what results I get.</p>