Flash power help please

I have a Canon 430EX and a 580EX (both version II I believe). I have wireless radio triggers to use them both off camera and I'm comfortable using them both in manual. As I research and try various lighting setups, I try to do a little sketch, so I can remember the setup for another time. At a flash workshop I attended, the instructor stressed writing down lighting ratios rather than the actual light powers when you get a multiflash setup you want to remember since the ratio between lights is the important part that makes the look of the photo and you may want to reuse that ratio another time.

My problem is that I've realized a particular power will not be equal on my two flashes since the 580 is more powerful than the 430. For example, if both flashes are set to 1/4 power, they wouldn't actually be 1:1 power ratio. If I have one set to 1/2 power and one set to 1/4 power, it wouldnt really be a 2:1 ratio. I'm trying to wrap my head around this and figure out how the power of the two flashes relate. I can't seem to find info on this. Does anyone know how much more powerful a 580 is than a 430 at the same manual setting?

I understand your confusion. I've been there and I think I found my way out of this confusion. Let me see if i can adequately explain it. When talking about lighting ratios you have to consider the perspective. Are you talking about the amount of light emitted from each of two or more lights? Or are you talking about the amount of light reflected off your subject from two or more lights? The latter is the important perspective as the reflected light from a source at 1/2 power will vary depending on how far away that source is from the subject. For simplicity let's consider a two light set up, main and fill. Main light is 45 degrees left of subject and fill is almost on camera axis. Measuring the main light by itself is f8 illuminating the right side of your subject. The fill light is f5.6 measured by itself illuminating the front of your subject. f8 is twice the illumination of f5.6. So that means the right side of your subject is receiving two units of light from the main and one unit of light from the fill (3 units of light) while the left side of your subject is receiving one unit of light only from the fill. This produces a 3:1 lighting ratio of reflected light.

Simple. Right.

Now consider the source. The subject is getting twice as much light from the main as it is from the fill so that's a 2:1 ration from the sources.

Hope that sheds more light on the subject than darkness Pun intended.

I've been down the path you have undertaken. Ultimately after experimenting for a couple of years using a light meter I found chimping the fastest and easiest way to get the ratio I wanted. This is using both speedlights and studio lights. I'll use a light meter to get both my main fill within the ball park, but chimp after that. You've got to be comfortable with what you see in your display, so I spent time experimenting to get my picture style set up so that I was confident of the contrast shown on the LCD. Also, different lighting moods require different ratios, things aren't set in stone.

windpig wrote in post #17731666
I've been down the path you have undertaken. Ultimately after experimenting for a couple of years using a light meter I found chimping the fastest and easiest way to get the ratio I wanted. This is using both speedlights and studio lights. I'll use a light meter to get both my main fill within the ball park, but chimp after that. You've got to be comfortable with what you see in your display, so I spent time experimenting to get my picture style set up so that I was confident of the contrast shown on the LCD. Also, different lighting moods require different ratios, things aren't set in stone.

This^
Whilst making notes helps some people reinforce learning, it's not a universal truth, nor is it simple to create a 'lighting ratio' figure when you're using different models of light. Whilst I'm at it, lighting ratios aren't IMHO universally used.

Having an understanding of how light behaves is far more useful, you'll build that yourself over time.

The 'bible' of creative and technical lighting for photographers is a book called 'light science and magic' and I don't think it pays more than cursory mention of ratios.

A more useful book for you might be 'the speedlighters handbook', again hardly a mention of ratios, but a great book for understanding what can be done with speedlights.

What I do as well. Set up one light at a time. I start with the main and go from there.

The difference between the 430 & your 580 should be roughly 2/3 stop - although when people have actually measured their outputs, though a modifier, trying to keep like for like, they have found the difference to be slightly less than that. A flashmeter gives precise answers and it's also a good learning tool - and then chimp too!

If you do get to obtain the ratios the setting will differ slightly at different locations. I think the previous advice of setting up one light at a time is good advice

Thank-you everyone! This is all very good information.

GeeMack, thank-you for your detailed example; although I understand that distance affects the light hitting a subject, I never translated that in my head to be part of what's affecting a ratio. It makes sense to me now why the reflected light is more important to pay attention to than the output power itself.

Phil V, I have heard of those two books and need to make a point to read them soon!

OceanRipple, thanks for the starting point to help me when I'm trying to set my two flashes in relation to one another...particularly if I'm trying to get them approximately equal in a scene.

Overall, it sounds like I should setup and meter one light at a time, adjust with chimping if necessary, and record info but not expect it to work out exactly the same the next time I do a similar setup. There is so much to pay attention to when lighting...I may need to make myself a checklist!

The best thing to do is to use a handheld lightmeter. There are just way too many variables impacting the final reading. Setting ratios with a lightmeter is a snap.

The L-358 allows you to measure the ratio of the light of one of your strobes as compared to another. So set up your 580 and measure the power of the strobe. Then press memory and AV on the L-358, hold the button down, and fire only the 480. The display will read the difference. There is a chart in the L-358 manual that will inform you as to the ratio.