Aperture and flash

Joined
Aug 16, 2007
Messages
2,546
Location
Denmark
I have just seen a very fine pic with flash used in daylight - it was a very fine pic, but he used F. 6.3, and I asked him to do it at F. 5.6 or lower (bigger hole) to get a better BOKEH, but he said, that he could not do that, because then his camera (Canon) would loose its ability (do not know the exact name in english, but as a runner, that can not run anymore)

I do not understand his answer ?

Why can you not set the f.stop lower (bigger hole), then the shutter would be quicker, and he said his camera could not cope with that (he shot at iso 50,m and it is one of the pro cameras from Canon)
 
Joined
Apr 3, 2006
Messages
5,614
Location
Texas
Camera shutters have a "maximum shutter sync speed" when used with flash. Your D700 is 1/250 second and your D40 is 1/500 second (maximum shutter sync speed with flash - spec chart in rear of camera manual). The shutter setting simply will not go any faster if the flash is present and ready (turned on).

The typical exposure in sunlight is "Sunny 16", meaning for bright sun, f/16 at shutter speed of 1/ISO, which for example often means f/16 at 1/200 second (assuming ISO 200 for D700 or D40). The f/6.3 must have been in shade or cloudy conditions instead of bright sun, as bright sun needs f/16 at 1/ISO shutter speed.

Since the shutter cannot sync flash faster than the 1/250 second maximum sync speed, this means we are stuck at f/16 1/200 second (in bright sun, with flash). For example, f/11 1/400 or f/8 1/800 second are equivalent exposures (equal to f/16 1/200 second), which yes, it is normally very possible in bright sun, but when the flash is used, the shutter cannot go faster than the 1/250 maximum shutter sync speed.

Camera models like D80 to D700 have the Auto FP mode (D700 menu E1), where a faster shutter speed can be used with flash, then you might be able to use f/2.8 at 1/6400 second with flash. D40 does not have that option, but some Canon cameras do have it. This Auto FP mode is called HSS (High Speed Sync), but it is implemented by the flash unit itself changing to shine a continuous light (for the duration of the focal plane shutter travel), so that sync speed is no longer an issue.

http://www.scantips.com/lights/flashbasics2.html#fp has more.
 
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