flash exposure question

greenspun.com : LUSENET : Large format photography : One Thread

i just starting using flash with my large format equipement. here is my question. i know that the sync speed is not important with large format lens since they are leaf shutters. but here is the question. if i use a flash meter and get a f stop of say f22, if i place the shutter speed at 125th of a second, the amount of ambient light entering the camera is different than if i set the the shutter speed at 1/2 second for example. my question is, how do i determine the shutter speed to set the flash so that the ambient light exposure and flash exposure is correct? thanks

-- howard schwartz (howards@ameritech.net), February 28, 2002


For a "balanced" look you need the ambient light and the flash to each provide 1/2 the light. If you have a flash that will give F22 (ignore the shutter speed) you need to adjust your shutter speed to whatever will also give f22 for the ambient (1/2, 1/8,...whatever). If you're using negative film you can stop there - you'll get slight overexposure which is a good thing under the circunstances. If you're using transparency film you would close down an additional aperture stop to avoid burning out the highlights. For a better rendering with all films a reduced popwer "fill flash" is a better option. With this you determine the exposures as before but slow the shutter speed by one stop to allow the ambient to overpower the flash by one stop, then close the aperture by one stop to balance the exposure to the ambient and effectively cut the flash power by 1/2. You can carry this even further for a compensation of 1 1/2-2 stops depending on the subject, the situation and your taste. And you can of course compensate in the other direction also to knock the background down or eliminate it.

-- Wayne DeWitt (wdewitt@snip.net), February 28, 2002.

I determine what the shutter speed should be based on what I want the final iamge to look like. In some situations I wantthe flash to be mostly "invisible" in the over all look of the image, in others I want it to be more pronounced all the way to the point of excluding any traces of ambient light.

Now if you are menaing balancd one thing that will help is to use a meter that can read both ambient and flash simultaneously or let's you put into a memory function the flash reading and then seperately read the ambient light and see the numerical relationship between the two readings as you change shutter speeds.

-- Ellis Vener Photography (ellis@ellisvener.com), February 28, 2002.

No one has mentioned it but depending on the circumstances you may encounter 'ghosting', that is a double or faint second image in addition the first image, which sometimes occurs with slower shutter speeds.

-- Jonathan Brewer (lifestories@earthlink.net), February 28, 2002.

I did not mention that 'ghosting' tends to come into play as open up your lens, and tends to be less of a problem as you close down.

-- Jonathan Brewer (lifestories@earthlink.net), February 28, 2002.

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