Beginner Understanding flash?

Carl Hall

Carl Hall

Messages
3,817
Name
Carl
Edit My Images
Yes
Hello everybody, I’m trying to get my head around shooting with a flash as I’m generally a landscape shooter so flash is something I’ve had very little experience of! I think I’m finally getting somewhere and I just want to check if my understanding is in the right area.

Say I’m shooting a portrait of someone outside. I want a reasonable DoF so I’m shooting at f/8. I meter the background and it tells me the correct exposure for f/8 at ISO 400 is 1/250 sec. Am I right in saying (I hope so, because this will be a lightbulb moment for me!) that I set my camera at f/8 and 1/250, using ISO 400, which correctly exposes the background, and then change only the flash power to correctly expose the subject?

To expose the subject I’m using a flash which has a guide number of 141ft. My subject is at 9 feet from me, giving me an aperture to use of f/16. To get back up to my f/8 do I just half the power of the flash for each stop that I up? I.e GN of 141 = 1/1 power at f/16 or 1/4 power at f/8? This is of course for ISO 100, so my actual power to use is 1/32 for ISO 400?

If I’m totally wrong, any guidance or help is appreciated!

I have a few more questions afterwards, but I think it’s best to get my head round the basic bit first :LOL:

Many thanks,

Carl
 
First answer: use ETTL or iTTL and let it take care of the flash power.

I haven't checked your maths, but you have the right idea.

Just make sure you stay under the sync speed. If you can't, then you'll need to read up on high speed sync too.
 
Phil V said:
First answer: use ETTL or iTTL and let it take care of the flash power.

I haven't checked your maths, but you have the right idea.

Just make sure you stay under the sync speed. If you can't, then you'll need to read up on high speed sync too.
Click to expand...

Thanks Phil, it's good to know I'm on the right sort of track :)

The camera that I'll be using is an RB67 which doesn't support ETTL modes unfortunately (actually it doesn't even have any electronics whatsoever), so it's manual mode all the way for me! It also uses a leaf shutter which I believe will work with flash at all speeds, although the max shutter speed is only 1/500.

Thanks for the help :)
 
Yes, you're on the right track, and you are approaching it from the old-fashioned craft perspective rather than the "take a thousand shots and see if any of them work" perspective, which isn't a criticism.

Yes, your RB67 will synch at any shutter speed, and using 1/500th will give fairly good control of ambient light.
As you're shooting on film, I strongly recommend that you use either
1 .A digital camera
2. A flash meter
3. Polaroids
to measure/calculate the contribution made by the flash - don't expect the guide numbers to be anywhere near correct at any time, let alone when you aren't getting light reflected from ceilings, walls etc.
 
Carl you're on the right track, and understanding it this way will stand you in excellent stead, especially when the time comes where you don't have TTL :D
Just pay attention to the ISO that your flash GN is rated at (Normally 100) as Mike points out, else you'll be two stops out ;)
 
Garry Edwards said:
Don't expect the guide numbers to be anywhere near correct at any time, let alone when you aren't getting light reflected from ceilings, walls etc.
Click to expand...

In addition, if using speedlights then the flash zoom reflector position will make a big difference to the real guide number. For my YN560III it's something like 4 stops between min and max zoom.
 
OP, definitely on the right track. I agree with Phil that ETTL/ITTL are nice, but I think it helps to have a real understanding first. So I would advice buying a light meter and going manual, at least until you have the hang of things like the Inverse Square Law.

Best of luck!
 
Last edited:
It also depends on what you are trying to achieve, if it's to balance the exposure to match the background lighting then what the op suggests is a good starting point.

If you just want to add fill in flash to lift shadows for a back lite subject then a setting of 1/3 to 1/4 less might be more appropriate.

As the op is using a none TTL camera I would suggest it might be worthwhile looking into getting an old flashgun with a good auto range like the old Metz 45. This setup worked will in the film only days so would still work well today.

Sometimes technology advances just make simple things more complicated, just because they can.

Sometimes simple is the way to go.
 
Mostly I prefer to under expose the background by a stop or two which will help your subject stand out. But it depends on the look you want.
 
It mostly depends on what you want to achieve.
In bright sun usually the aim is to fill the shadows.
whilst in shady conditions or back lit ones, you have the option of setting the flash to be the main light.

As you are using film you really need to get your hands on digital flash meter. (Even bottom end ones are incredibly accurate)
Guide numbers work very well indeed in small bright interiors, however they are hit and miss in other situations.

I spent many years as a professional using fill flash by rule of thumb. You very soon learn what power your set up provides, in terms of f stop, at a given distance.
 
Last edited:
You must log in or register to reply here.
Back
Top