TTL Flash question

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Hello y'all:

I've done some search on photo.net and I still have a few question regarding TTL flash and using flash diffuser.

My questions are:

1) When I am using a flash diffuser on a flash head (say I lose about -1 stop of light) in a small dim room do I need to compensate the flash to +1 since I am losing -1 EV due to the diffuser?

I am confused b/c if the TTL flash technology shuts off only when enough light reaches thru the film then I shouldn't worry about the lose of light due to the diffuser right? Assuming that the flash is the main source of illumination and the subject is fairly close.

2) When using fill-in flash, I assume the camera automatically computes the flash to ambient light ratio for a perfect exposure. However, the recommended -1/2 or -1/3 for fill flash is that -1/2, -1/3 of the camera's computed flash power or -1/2, -1/3 of the full power of the flash?

Thank you for helping.

-- Confused (bulletinmaker@juno.com), May 06, 1999

Answers

Since you didn't mention which camera system you have I'll only answer question 1. If your camera has TTL flash the diffuser will not effect your exposure except that will cut down the range of your flash. Meaning that you don't have to set flash exposure compensation. Despite what you put in front of your flash the system will fire the flash up to it's full power or until it measures that enough light has come through the lens then shut it off. The flash can only put out it's maximum output so if you are too far away from the subject it will be under exposed thus you'll have to either go closer or open the aperture more or remove the diffuser from the flash.

-- Gary Wilson (gwilson@ffca.com), May 06, 1999.

Fill compensation is usally specified in f/stops of compensation applied to the flash. Most TTL flashes using fill mode will tend to add a pronounced flash light on the main subject. So, a negative compensation is suggested to reduce the pronounced flash lighting. A compensation of -1/2 or -1/3 stop on the flash will reduce the pronounced flash lighting, by extending the exposure time, adding more ambient light to the image.

-- Joe Boyd (boydjw@traveller.com), May 06, 1999.

Hello. I am using the Nikon system.

Thank you for sharing, as it clarified some confusion points for me. However, I would still like to know if:

In P, A, S mode when using fill-in flash and dialed compensation, is that compensating from the camera determined falsh power? How about in the M mode?

Thanks again.

-- (bulletinmaker@juno.com), May 06, 1999.


As long as you keep your flash in TTL mode, its light output is controlled by means of TTL metering on your camera, ie no matter if you shoot in P,A,T or M. Therefore your dialed-in flash exposure compensation is taken off the camera-computed off-the-film exposure value for the flash. As you see, flash and ambient exposure in modern cameras are pretty independent.

-- Dariusz Szpunar (dszpunar@flash.net), May 08, 1999.

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