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Archive 2009 · Flash meter in aperture mode
  
 
mat.bastian
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p.1 #1 · p.1 #1 · Flash meter in aperture mode


Is this possible?

I'd like to set my depth of field then bring the light to that point. Is this bass ackward?

My understanding is shutter controls ambient and aperture controls flash exposure. If this is the case, wouldn't it make more sense to let me meter at my chosen aperture?



Apr 02, 2009 at 02:35 AM
BrianO
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p.1 #2 · p.1 #2 · Flash meter in aperture mode


Aperture affects both flash and ambient. Shutter speed only affects ambient (a simplification) because the flash duration is so short that all the light gets in whether shooting at 1/2 second, 1/20 sec., or 1/200 sec.

So, use the reading off your flash meter as a starting point, then get to the aperture you want by using equivalent exposures. If the aperture you want requires a shutter speed faster than your maximum sync speed you'll need to cut the amount of light by using using a lower ISO setting, using ND filters, dimming the lights, etc.

If it's hard to imagine why one affects both types of light and the other doesn't, here's a thought experiment:

Picture a fat guy and a skinny guy trying to get through a small manhole. The skinny guy (dim light) can get through easily. The fat guy, no matter how fast or slow he runs (shutter speed) can't get through. So speed doesn't change the outcome of door size (aperture).

Now picture a door (shutter) that opens and closes really briefly. A slow walking guy (ambient light) can't get completely through the door before it closes on him, but a fast runner (flash) can.

HTH.



Apr 02, 2009 at 02:42 AM
cgardner
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p.1 #3 · p.1 #3 · Flash meter in aperture mode


Indoors setting desired aperture and then bringing the light up is actually a good approach. Set your aperture, start with just your fill and raise it to the point the camera records the amount of detail you want in the shadows (more fill, more detail, softer looking lighting) then turn on the key light which overlaps fill and raise it until the highlights it creates over the base level of fill are about 1/3 below clipping on white textured areas in the brightest part of the light.

If you do things the other way around, starting with key light then adding fill you'll find the f/stop keeps changing as you raise the fill because the fill hits the highlights too making them brighter. If you start with the fill first, then add the key light the key light will not affect the shadows it doesn't hit, and you simply keep raising it until the highlights are correctly exposed at the f/stop you picked. Logical no?

Outdoors the catch-22 is the shutter's flash sync limit (x-sync), typically 1/250th sec. That requires something in the general vicinity of f/11 to correctly expose sunlit highlights.

Hot shoe flashes like the 580ex get around the limit by pulsing the flash at about 40,000 Hz so it acts like a continuous source and shutter controls exposure. It makes shooting at f/2.8 on a sunny day possible, but the shutter will be in the 1/3200th to 1/4000th sec. range.

The other approach is to use ND filtration over the lens to cut the ambient to the point where its so dim the flash provides more light than the sun does. If normal exposure is f/11 and you wanted to shoot at f/2.8 with conventional flash it would require a 1.20 ND (each .3 ND cuts the light by 1/2). That will also require 4-stops more flash power than normal fill at f/11 would.

Chuck



Apr 02, 2009 at 03:20 AM
 

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mat.bastian
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p.1 #4 · p.1 #4 · Flash meter in aperture mode


Thank you both. Very helpful.


Apr 02, 2009 at 11:35 AM
4honor
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p.1 #5 · p.1 #5 · Flash meter in aperture mode


cgardner wrote:
Hot shoe flashes like the 580ex get around the limit by pulsing the flash at about 40,000 Hz so it acts like a continuous source and shutter controls exposure. It makes shooting at f/2.8 on a sunny day possible, but the shutter will be in the 1/3200th to 1/4000th sec. range.


Hi Chuck! I am in the process of getting some lighting gear, and was a little curious about what you said about the 580EX (I have the second version)... Is that what the High Speed Sync means? My camera (XSi) x-syncs only at 1/200 sec., but I still want to cut out the ambient light. Can I do it with my 580EX II or do I need more powerful flashes like the AlienBee B800?



Apr 02, 2009 at 02:30 PM
cgardner
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p.1 #6 · p.1 #6 · Flash meter in aperture mode


4honor wrote:
Hi Chuck! I am in the process of getting some lighting gear, and was a little curious about what you said about the 580EX (I have the second version)... Is that what the High Speed Sync means? My camera (XSi) x-syncs only at 1/200 sec., but I still want to cut out the ambient light. Can I do it with my 580EX II or do I need more powerful flashes like the AlienBee B800?


Click on the WWW button below and look at the last tutorial in the Canon section.

Chuck



Apr 02, 2009 at 05:45 PM





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