Trying to learn. Please help

Started Aug 17, 2013 | Discussions thread
WFulton
Senior MemberPosts: 2,607
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Re: Trying to learn. Please help
In reply to _Jon_, Aug 20, 2013

_Jon_ wrote:

I am using a nikon d7000 with an sb-700 off camera. My flash is about 3 feet away from my subject, and my camera and flash are both in manual mode with spot metering. ambient exposure is f/5, 1/80 at iso 2000. So, I increase the shutter speed to 1/250 to reduce the ambient exposure by two stops. If I set my flash in manual mode at the lowest power setting, 1/128, the flash is way too intense. If I put the flash in TTL mode, the exposure is perfect.

Your flash is too strong at 1/128 power, but you are using ISO 2000 and f/5 at 3 feet?

Turn the ISO down, or stop your aperture down.

The Guide Number for ISO 2000 is 4.5x greater than the ISO 100 chart.

I don't know your flash zoom value, but lets assume 24mm, to have a number.

SB-7000 manual page H-24 has the DX GN chart, showing 1/128 power 24mm zoom is GN 8.2 (ISO 100).

ISO 2000 is 4.5x, or GN 36.9 for ISO 2000.

You said 3 feet, so that is GN 36.9 / 3 feet = f/12.3 expected exposure.

You are using f/5... which is 2.3 stops overexposed.

1/250 and f/12.3 will underexpose ambient by more than four stops.

If you set ISO 100, the GN is 8.2, and divided by 3 feet is f/2.7.

So 1/128 needs a little more GN, but not 2000.

I really would not have expected TTL could go that much lower than 1/128, but you seem to show that it can.

More about GN at http://www.scantips.com/lights/flashbasics1c.html

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