Forum: Pentax DSLR Discussion
01-07-2014, 05:41 AM
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Exactly. I find that even when using a flash with a high guide number of 120 (ft) that when bouncing I can't use some of the smaller apertures that I could with direct flash.
There are at several things going on that lower the amount of light hitting the subject:
- Diffusion and reflectivity of the ceiling (and bounce card if using one)
- Increase in flash to subject distance.
As to the latter light output is subject to the inverse square law - output = 1 / distance * distance ( one over distance squared) thus:
distance to subject direct flash = 6 output = 1/36
distance to ceiling + distance to ceiling to subject = 4 + 4 = 8 output = 1/64. A higher ceiling where total flash to subject distance of 10 would give an output = 1/100.
It can be done but be aware if you go outside the maximum flash distance you won't get good results and that practical subject to camera distance can be much less than direct flash.
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