Thursday, 30 October 2008

[strobist] Balancing Twilight

This is the same setup as everything else particularly the balancing ambient setup. The only thing that we need to take care of is the rapidly dropping ambient.

As usual, we've got the correct ambient exposure at max sync using aperature and our flash lit subject. now we need to care about the ISO setting. Remembering that the higher the ISO, the more noice pontential on your image.

Say we have the situation where we have f8 @ 1/250 with 1/2th flash power @ ISO 200.

As the ambient light drops (think of it at 1 stop at a time), we need to compensate so we can get more shooting time and we START by openning up the aperature AND dropping the flash power (remember, the more wide open, the less flash we need). Remember, aperature affects both the flash AND ambient lit scene so as the open up, we are compensating for the fact that the ambient is dropping. This is why the shutter speed stays the same (for now).

As the aperature controls only the the flash lit portion, we need to compensate for the wider aperature by powering down on the flash to give the flash lit subject the same exposure. So we're now shooting

f5.6 @1/250 with 1/4th flash @ ISO 200
f4 @1/250 with 1/8th flash @ ISO 200
f2.8 @1/250 with 1/16th flash @ ISO 200

When we're wide open, we start dropping the shutter speed -- we can not let any more ambient light to the sensor with aperature so we have to do it via the shutter until we get to unstable (what DH has called hail mary shutter speeds) shutter speeds and then we bump the ISO higher and start again.

f2.8 @1/250 with 1/16th flash @ ISO 200
f2.8 @1/125 with 1/16th flash @ ISO 200
f2.8 @1/60 with 1/16th flash @ ISO 200 (end of the road)

and now we repeat starting at

f8 @ 1/250 with 1/2th flash @ ISO 400
.. dropping aperature, power down flash
f2.8 @1/250 with 1/16th flash @ ISO 400
.. dropping shutter til @ 1/30th
f2.8 @1/60 with 1/16th flash @ ISO 400
.. start stepping ISO

related strobist post: lighting 102 - 3.1 balancing light: twilight, free custom backdrops delivered daily

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