Flash

[Luke]

I have a few questions about how flash really works. Bearing in mind i shoot a canon EOS-1, so any technical advice should be given to that.
Okay, as I understand it:
Shutter speed has no effect on the exposure/duration of the flash. How then? does a flash determine how long to flash for?
Okay, just say to make a correct (18% grey) exposure without flash, you need f3.5 at 1/15th of a second.
How would you get your flash involved for a correctly exposed forground- if your priority was to keep the f stop the same? --->>> i imagine it would work like this: you meter (SOMEHOW?) with regards to the flash and then expose with a higher shutter speed - okay, fair enough.
But what if you want the background exposed well too and your flash doesnt have enough range to do it - how then does it work.
These are some basic questions, you can answer them, or, assuming this gives you an idea of what i do and don't know, you can just give general advice/rules.
ALL help apreciated.

 

[ksmattfish]

Shutter speed has no effect on the exposure/duration of the flash. How then? does a flash determine how long to flash for?

For most flash units the actual duration of the flash is like 1/10,000th of a second or faster. Adjusting the flash power doesn't normally affect flash duration in ways we'd notice, it just changes the amount of light emitted.

My Canon 550EX and 580EX flashes have a "high speed sync" mode which does create a longer duration flash so that I can use them at a higher shutter speeds than my camera's normal flash sync shutter speed. As far as I can tell I'm losing a lot of lighting power to get the extra duration.

Okay, just say to make a correct (18% grey) exposure without flash, you need f3.5 at 1/15th of a second. How would you get your flash involved for a correctly exposed forground- if your priority was to keep the f stop the same? --->>> i imagine it would work like this: you meter (SOMEHOW?) with regards to the flash and then expose with a higher shutter speed - okay, fair enough. But what if you want the background exposed well too and your flash doesnt have enough range to do it - how then does it work.

Use shutter speed to control the exposure of the parts of the scene that won't be lit be flash, and use flash power to adjust exposure of the parts that will be lit by flash. If your flash doesn't have a handy flash power calculator built in, you can do the math yourself, and make your own chart.

Guide Number (the flash power) divided by distance to subject = f/stop for "normal" exposure. Usually GNs are listed in ISO 100/feet and ISO 100/meters. Pick whatever you use (feet or meters), do the math for ISO 100, and then adjust according to the ISO you are actually using.

Example: I'm shooting a person in my backyard. They are 10' away, and I want to use f/4 at ISO 400. First I meter the background at f/4 to determine the shutter speed necessary to get the background properly exposed (we'll assume here that the shutter speed I come up with is less than my camera's flash sync speed). My flash GN is 160 (ISO 100/feet) at full power. So 160/10'=f/16 for normal exposure. With ISO 400 this goes up to f/32. Everytime I reduce my flash power by half, I'm losing a stop of light. So to get to f/4 from f/32 I need to reduce power 6 stops, which is setting the flash to 1/64th power. Underexposing from the "normal" flash exposure by 1 stop often makes the use of flash less noticable, so actually I might go to 1/128th power.

Of course modifiers like ND filters and flash diffusers have to be taken into consideration. Also, sometimes it seems that flash manufacturers are a bit generous when estimating GNs, so test and see if you need to add a stop or so to really get the flash power you want.

It all sounds a lot more complicated that it really is. As long as you have a sound understanding of what a stop is, and how to adust shutter, aperture, ISO, etc... in terms of stops, it gets easy with practice. Or get a flash meter.

 

[Big Mike]

If you haven't yet, read this

It's been called the Canon Flash Bible.

 

[Luke]

Okay, thanks for the help guys. I had found that link previously, but I found it less conveniant as there is an abundance of information and I don't actually have an EOS flash at this point to experiment with!!
Okay, so I'm starting to understand, but heres another question:
What's the relevance of a flash sync speed??

 

[ksmattfish]

What's the relevance of a flash sync speed??

A camera's flash sync speed is the fastest shutter speed where the shutter actually opens all the way. Shutter speeds faster than the flash sync are obtained by only opening part of the shutter at a time, and moving it across the film/sensor area. For instance 1/4000th is just a narrow slit that moves across the film/sensor. If you were to use a regular flash at speeds higher than your sync speed, only a portion of the film/sensor area will be exposed by the flash.

If you can find a site that shows common photo problems they'll probably have an example of what this looks like. It used to be more of a problem. Newer, electronic cameras usually won't let you make this mistake.

You don't need an EOS flash. They have nice features, but the only part where being an EOS flash makes a difference is when you are using the automatic modes. If you really want to learn flash you have to do it in manual, and almost any flash will work for that.