KG

For Canon setup (7D+580EXII), when using ETTL, sometimes I wonder how much more available flash power I have in a given situation.

For example, last night, I was shooting semi-outdoors at night. There was a high dark ceiling, so bouncing off of it wouldn't work.

I was far away from my subjects, so I cranked up ISO to 800, set aperture to f/4, and used ETTL for the flash metering.

Since the flash was the only reasonable source of light around, I set shutter speed to max sync speed of my body 1/250 and left it at that.

The shots came out OK, not too bright not too dark.

All the time while shooting I kept wondering if I could either make aperture smaller or use lower ISO.

Of course, the obvious answer would be to try changing the settings and see if the speedlite can compensate by outputting more light.

But this got met thinking. Is there some way to see either for FE lock or immediately post shot with ETTL(2), what power level is/was used?

It would be nice to know, for example, that for a given shot in specific circumstances, the camera needed, say, 1/4 power of the speedlite.
If that were the case, then I would know that I have two more stops of flash light at my disposal.

And if on the other hand, speedlite was already operating at max power, then I would know that I'm at my limit.

Does anyone know if such information is stored/displayed anywhere in camera or on the speedlite unit?

larrycumba

It is my understanding the brightness of the flash is constant. What is controllable is the duration of the flash.

rpavich Loc: West Virginia

That is true.

What you could have done was to open up the aperture and the flash would have more "reserve" juice.

I also might have lowered the shutter speed to gain more ambient light .

PS: I'm not sure what size room you are talking about but even one speedlight is amazingly powerful and can really do the job even in a large room.

Check this blog entry out where Neil does just that.

http://neilvn.com/tangents/2012/02/01/bounce-flash-photography-and-inverse-square-law/

KG

That's right. I guess my choice of words wasn't the best.
But the question remains. How can I figure out if my flash is at its limit of if I have room for more light output when using ETTL?

For example, I'm shooting at 1/250, f/4, ISO 800, with ETTL flash for some specific scene. The shot came out OK. Now I want to lower the aperture to f/5.6 (while keeping ISO the same, and shutter speed having no effect -- no ambient light). Will my flash unit be able to output twice as much light or was it at its peak already? Any way to tell?

rpavich Loc: West Virginia

No idea.

CaptainC Loc: Colorado, south of Denver

I think not. The ideal way would be with a flash meter, but that will not work for a flash in TTL mode - the required pre-flash is what the meter will see and that is useless.
The only way would be to keep reducing the exposure with the aperture and seeing where the flash can no longer deliver a decent exposure. Now the Nikon flash will blink the red light (the ready-light) in rapid succession when the unit fires at max power. This is to let you know that it can put out no more light - do not know if Canon has the same feature.

KG

Hmm, I'll check the manual to see if Canon Speedlite indicates max power use in some way.

For now, I just switch to manual mode, enter 1/1 power and take a shot. I then look at the histogram to see the difference from the ETTL shot. But this is really not an efficient way.

Alternatively, dialing in FEC +1 or so would show if there is more flash light available for use. But this is also not efficient.

KG

rpavich, thanks for the link. Not exactly what I needed, but I learned something new. Great read.

Cornishpete Loc: Illinois

And thanks from me too rpavich. I think I have now managed to get my head around something that was so counter-intuitive.

George Kravis

Look in the manual for the 580EXII, page 54, and under the heading of Guide NO. at ISO 100, you will see tables of flash output in GN's. If you're not aware, you get the aperture for acceptable exposure by dividing the GN by the distance you wish to cover. Note that the GN increases proportionately with ISO.

Guy Johnstone Loc: Ocean Shores WA

Yes The Guide No. is the only practical way to know your flashes capability. A meter would work if, like most flashes of its type it has a full power manual setting. But then all you would be doing would be establishing a guide number anyway. A brief web search turned up a guide member of 190 for your flash. What that means is that if you are 10 feet from an average subject, Camrea set at 100 iso, with your flash at maximum output, your f-stop would be 19. Seems pretty high for a hot shoe flash. No matter once you know the guide number you can figure your maximum f-stop from any given distance with some simple multiplication and division. The systems been around for 120 + years, and is totally applicable today.

GoofyNewfie Loc: Kansas City

My old days are coming back to me- amazing how much I've forgotten. Thanks for the brain jolt.

Guy Johnstone Loc: Ocean Shores WA

yes well not long ago I could have given him the multipliers to figure f-stop/distance changes. Can't remember now. I figure if he cares enough to look them up he will be more likely to remember them.

But as for me "it is for my mind that I fear most".

twowindsbear

Would you please define 'ETTL,' 'FE,' 'FE lock,' and 'ETTL(2).'

Thanks

twowindsbear

One way to tell if you're getting max output from your flash is to compare recycle times. Use fresh batteries, set the flash to manual, fire the flash & see how long it takes the flash to recycle. Say it takes 10 seconds. If your flash is recycling substantially faster, then you have more flash output available.