using built in flash over 200 feet

Started Feb 11, 2013 | Discussions thread
tom Senior Member • Posts: 2,724
Re: using built in flash over 200 feet

Olovskie wrote:

tom wrote:

Lets assume she was using an A77 (I know you said canon, but lets just play with it.)

GN of the built in flash is 12m (at ISO 100). For us metrically challenged that is a range of 39.36 ft at f/1.

So lets use a realistic f-number of f/5.6. At ISO 100 that is a range of 7 ft (assuming all the exposure is from the flash).

Let's change the ISO. At ISO 800 that is 56 ft. At ISO 1600 it is 112 ft.

If the distance is dubbled you need 4-times more light.

Aperture (f) = square root (ISO/100) x GN / distance.

With f/5.6 at ISO 800 the distance is 20 ft and at ISO 1600 28 ft.

Since it was a cream colored lens, lets try f/4 -- at ISO1600 it is 157 ft

In this case the distance is only 39 ft


Writing too fast I forgot the sqrt of ISO/100. Thank goodness the GN slide computers on the flash.


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