JBTaylor Loc: In hiding again

No, what you described is almost how it works with high speed sync in which the flash must light during the entire time that the slit of a shutter moves across the sensor above the flash sync speed. But even that is done by a series of smaller bursts of light. That is a recent, well maybe 10 years old now, innovation. Below the X sync shutter speed, it is as I described and that is how electronic flash has worked since it was invented and the X-sync protocol established.

The whole point of X flash synchronization as has been that the shutter opens to expose the entire sensor. The flash lights for a fraction of the time that the shutter is open then the shutter closes. That is why changing the shutter duration below the maximum X-sync speed changes the ambient light exposure without affecting the flash exposure.



I agree. Check out http://en.wikipedia.org/wiki/Flash_synchronization and hopefully you will understand.


To say the following is incorrect.

Weddingguy Loc: British Columbia - Canada

I think you'll find the answer in this short video.

http://www.youtube.com/watch?v=S3IQFayyyoU&feature=share&list=UUB_kHWYQcoJjepTkO96BxCQ

Wabbit Loc: Arizona Desert

Correct Doc ..... dats what I've been sayin .....It's not a burst of light ..... the light turns on, your camera sets the exposure and takes the picture, then the light goes out ..... it happens so quickly that to the human eye it appears to be a flash of light ..... that's why accurate sync speed is important .....

JBTaylor Loc: In hiding again

:thumbup: :thumbup:

bugguy Loc: midwest

"Thanks" for all the replies! They have been very helpful. I belive I was misunderstood when I used the example of adding flash when I shoot indoor basketball. I WOULD NOT USE flash in this situation. I only used that as an example for an A vs. B comparrision, (these are the settings I currently use, what would I change "IF" I were to add flash in this situation?). Any how this is some of what you hoggers have taught me: 1) the slower the shutter speed the more the ambient light will be introduced into the image. 2) do not exceed my camera's flash sync speed, which for the Sony A77 is 250 3) just like in non flash situations I can shoot in either A/M/S modes. Figure out what works best for me. I generally shoot in A mode. Even when I shoot indoor sports vs S mode. 4) I need to research more about front curtain vs rear curtain modes. 5) We all spend too much time with this :-D

fantom Loc: Colorado

Excellent explanation. You have stated in a paragraph what some books take a chapter to explain. The main difference is that your description of the process is concise and understandable. Thanks

bugguy Loc: midwest

"Thanks" for the link weddingguy. Excellent video(s). I'm going to try shooting indoor flash in manual mode and make fine adjustments with flash compensation. I'm also thinking of ordering a lite scoop.

Wall-E Loc: Phoenix, AZ

Why, have YOU ever used a full size flash at a basketball game?

CajonPhotog Loc: Shreveport, LA

:-) :thumbup: :-) Your welcome.

sirlensalot Loc: Arizona

Ditto nekon on this one. Prefer manual.

Peekayoh Loc: UK

Surely that's not at all what happens, no modern DSLR can "set the exposure" by this method. What you describe is an obsolete system of "off the film" metering for flash exposure not suited to a DSLR due to specular reflections off the Sensor and as a consequence, a pre flash pulse of light has to be employed to determine exposure. The sequence you describe only works by presetting the exposure using manual flash control.

selmslie Loc: Fernandina Beach, FL, USA

Wabbit, you are totally wrong!

You need to learn about how flashes and strobes work. I suggest you keep quiet until you do.

JBTaylor Loc: In hiding again

You are right. Maybe Wabbit is thinking of the old film TTL system where the sensor reading off the film plane tells the flash when to cut power to the flash tube. On the other hand, he says "thru the lens metering has nothing to do with it" and "through the lens" is what the initials TTL stands for.

I thought he might have meant High Speed Sync (HSS) because he said, the light comes on, the camera takes the picture, then the light goes off. I interpreted "Camera takes the picture" as the time from when the shutter opens to the time it closes. Only FP (focal plane) sync works that way. HSS can kinda be thought of working like FP sync but technically isn't the same. In normal non-HSS flash photography below the X-sync limit, the duration of the flash must begin after the camera has started to "take the picture" and end before the camera finishes "taking the picture". Specifically from the time that the front shutter curtain has complete moved out of the way of the sensor and before the rear curtain begins to cover it up.

selmslie Loc: Fernandina Beach, FL, USA

I don't think Wabbit understands the issue at all.

Your statement about low speed sync is correct but incomplete. During the time the shutter is completely open for low speed sync, the flash duration for TTL is determined by the aperture and distance. In other words, the flash is cut off once enough illumination is detected. This could be a very short burst (maybe 1/100,000 sec) for close subjects at wide apertures. It might also be much longer (maybe 1/1,000 sec) for more distant subjects and at smaller apertures. If the distance is too great and the aperture too small, the flash might simply end when it runs out of energy, resulting in an underexposed image.

For very high shutter speeds, where the shutter is never completely open, multiple short bursts can be used (a choice not available with many cameras). But, since these are necessarily short and repeated, the total power of the flash is divided over many bursts. Because of this, the resulting intensity may be greatly diminished. For example, if your normal sync speed is 1/250, using high speed sync at 1/1000 sec might give you four separate flashes at 1/4 power. At 1/8000 sec you might get 32 bursts at only 1/32 of the flash's maximum output. These values may not be entirely accurate but they are correct in principal.

JBTaylor Loc: In hiding again

Well described. :thumbup: :thumbup: