learning flash distance from subject

[cpw]

Hi there

I have been practising with my canon 5d mkII and my 580EX to understand the distance you need to be from the subject to get the correct flash exposure.

I have used ETTL mode with on camera flash and bounced the flash off of walls to hit my subject (a mannequin) to create some pleasing results.

I have learn't the relationship between the shutter and the aperture so that I can increase or decrease the ambient or flash output as I wish.

The thing I don't get just yet is how do you become efficient at knowing exactly (or there abouts) how far or what settings will work in any given situation on manual controls?

e.g. I walk into a church and the lighting is dim with some window light coming through in the background.

I pose the bride in the aisle where there are walls either side to bounce flash from.
At this point I would like to be able to say to myself that e.g. F/4 at 1/15sec will be perfect side lighting on the bride considering how far the walls are away in this situation. Right now I wouldn't know what settings to use for this.

I think at the moment I can rely on ETTL to provide great results but I wouldn't be able to arrive at those results without it which bothers me. Is it really just practice that will make you understand it or does anyone have a good way to learn this please?

I love using off camera flash where ETTL is not an option so this is where i'm heading in the near future. Good exposure first/second time around so I can get the shot and move on

I hope I have put this is a good enough way for you guys to answer
Thanks for reading

 

[Phil V]

Hi there

I have been practising with my canon 5d mkII and my 580EX to understand the distance you need to be from the subject to get the correct flash exposure.

I have used ETTL mode with on camera flash and bounced the flash off of walls to hit my subject (a mannequin) to create some pleasing results.

I have learn't the relationship between the shutter and the aperture so that I can increase or decrease the ambient or flash output as I wish.

The thing I don't get just yet is how do you become efficient at knowing exactly (or there abouts) how far or what settings will work in any given situation on manual controls?

e.g. I walk into a church and the lighting is dim with some window light coming through in the background.

I pose the bride in the aisle where there are walls either side to bounce flash from.
At this point I would like to be able to say to myself that e.g. F/4 at 1/15sec will be perfect side lighting on the bride considering how far the walls are away in this situation. Right now I wouldn't know what settings to use for this.

I think at the moment I can rely on ETTL to provide great results but I wouldn't be able to arrive at those results without it which bothers me. Is it really just practice that will make you understand it or does anyone have a good way to learn this please?

I love using off camera flash where ETTL is not an option so this is where i'm heading in the near future. Good exposure first/second time around so I can get the shot and move on

I hope I have put this is a good enough way for you guys to answer
Thanks for reading

There's no guideline - you measure it. Then you control the amount of flash with FEC if using ETTL or you set your manual flash power and either measure it or experiment a bit.

There's no right or wrong way, you just have to practice and learn which way works best for you. As long as the results are good and you don't appear to be 'faffing' it doesn't matter how you got there.

 

[HoppyUK]

You just get to know this kind of thing through experience, though before we had all this automation (eg flash bulbs ) the guide number was the only way to make a rough calculation. GN only applies to flash fired direct.

Guide numbers (in your flash handbook, eg GN58/ISO100/metres at 105 zoom head setting for 580EX gun) work on the inverse square law principle, that says double the distance reduces the exposure to one quarter, ie two stops drop.

When bouncing, rule of thumb (very roughly, normal white ceiling) is distance from-gun-to-ceiling-to-subject, plus one stop. That will get you at least ball park.

The most accurate method is a test shot, then chimp and adjust accordingly, either in auto-TTL or manual. That's how I do it, though I'm happy to rely on auto-TTL if there's no time for that, after eye-balling the scene and having a rough idea that the gun won't run out of power.

Using any kind of flash modifier throw all guide number calcs out of the window.

 

[G.K.Jnr.]

You just get to know this kind of thing through experience, though before we had all this automation (eg flash bulbs ) the guide number was the only way to make a rough calculation. GN only applies to flash fired direct.

Guide numbers (in your flash handbook, eg GN58/ISO100/metres at 105 zoom head setting for 580EX gun) work on the inverse square law principle, that says double the distance reduces the exposure to one quarter, ie two stops drop.

When bouncing, rule of thumb (very roughly, normal white ceiling) is distance from-gun-to-ceiling-to-subject, plus one stop. That will get you at least ball park.

The most accurate method is a test shot, then chimp and adjust accordingly, either in auto-TTL or manual. That's how I do it, though I'm happy to rely on auto-TTL if there's no time for that, after eye-balling the scene and having a rough idea that the gun won't run out of power.

Using any kind of flash modifier throw all guide number calcs out of the window.

Hi, Excellent comments answers the question perfectly without any confusion, couldn't have put it better myself.

 

[HoppyUK]

Hi, Excellent comments answers the question perfectly without any confusion, couldn't have put it better myself.

From the above, some very broad brush-stroke points:

- Flash falls off very rapidly with distance (the inverse square law thing).
- Bouncing flash both eats power and increases the distance the light has to travel.
- And in addition, using high speed sync reduces effective flash power by at least two stops, just by turning it on, more likely three stops.

Easy test: set the flash in manual to say 1/16th power, shoot a normal subject/distance and adjust aperture/ISO for correct exposure. Then move back to double the distance, see how much you have to increase flash power to get correct exposure back. Do the same with bouncing.

From shooting close with direct flash at low power, to moving back and bouncing, the flash gun will go from an easy 1/16th power to flat out.

Edit: raising ISO one stop, or reducing lens f/number one stop, effectively doubles flash output.