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Old Oct 28, 2010, 11:45 AM   #11
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P.S.

Also note that some filters can reduce light significantly. For example a typical Polarizer will reduce light by about 2 stops, depending on how it's rotated (meaning only 1/4 the light gets through to the camera). So, it's a good idea to remove any filters like that when shooting indoors.
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Old Oct 28, 2010, 3:08 PM   #12
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Let me reinsert the proper GN to aperture to distance chart:
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Old Oct 28, 2010, 9:57 PM   #13
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Quote:
Originally Posted by JimC View Post
The GN of the A300 built in flash is 12 Meters (not feet). If you look at Sony's specs, the GN is 39 feet at ISO 100. ;-)

To calculate flash range, divide the GN by the Aperture. So, if you're zooming in much with one of your lenses (where the aperture is down to f/5.6), then your max flash range would be approximately 6.96 Feet at ISO 100 (GN of 39 feet divided by an aperture of f/5.6).

Then, each time you double the ISO speed, flash range increases by 1.44x.

An easy way to figure it is to remember that each time you quadruple the ISO speed (for example, from ISO 100 to ISO 400), your flash range will be approximately double (for example, if you go from ISO 100 to ISO 400, your flash range would be closer to about 14 feet instead of the around 7 feet that you'd get at ISO 100 and f/5.6).

I'm using f/5.6 because that's the widest available aperture when you zoom in much with those lenses. If you stay on the wider end of the lens, you'll have more flash range. Again, just divide the GN by the f/stop to determine range at ISO 100. Then, flash range increases by 1.44x each time you double your ISO speed.

Also note that if you're using a full power flash (you're at the max range for your ISO speed and f/stop), recycle times will be longer (because the flash capacitor will be fully discharged).

Be careful with a slave flash, as most will not work properly with digital cameras. That's because a modern digital camera uses a metering pre-flash to measure reflected light from your subject before firing the longer flash burst to take the actual photo. Because the shutter is still closed when the preflash occurs, and because most optical slaves will fire with the pre-flash, then the slave will not contribute anything to the exposure. It looks like that particular Zeikos is "digital aware" and can ignore the metering preflash. But, many optical slave flashes are not digital aware.

I'd suggest a more powerful dedicated flash instead if flash range with the built in flash is a problem at the distances you're shooting at (and keep in mind that you can increase ISO speed to get more range from it).

But, I'd ask about any flash you consider, as many dedicated flash units designed for Minolta film cameras will not work properly with digital.


Thanks for the info.
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Old Oct 28, 2010, 9:58 PM   #14
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thanks frank for the gn scale
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