how do i measure output (stepless power)

Simon photo · 2 Feb 2012

Hi there! Right, time to pick some brains. I got myself some ebay flash heads last year (yeah i know but money is tight lol) and with the kit supplied i get some good results, honestly. What i need to know is what power they are putting out at any given setting. They are 250w heads (not sure if that's 250w or ws) the power control is stepless with no increments or markings. Is the power the same but the length of flash longer as you increase the setting or does the output from the flash increase? I have a light meter, polaris. That measures flash, corded flash and ambient light with shutter and f-stop for the latter. Its not been an issue before as if i needed more light i just crank the dial, but now i need to have some sort of measurement or ratio as i am just starting a section of college course that's touching on studio lighting. I could get by without the info but i try to be as in depth as possible.

alexam · 2 Feb 2012

You seem to have all the tools you need already. Set up your strobe unit at a specified distance from your subject and at the lowest power setting, take a reading of the flash with your light meter set to flash. Then increase to maximum power to get the highest setting. You may then be in a position to calculate the settings in a similar way for 0.25 - 0.5 and 0.75 settings to build up a table you can work with at that working distance. If you move the light further from the subject then there will be a proportional reduction, which you can also calculate. Is this what you are looking for?

Garry Edwards · 2 Feb 2012

It's Ws or watt seconds, the fleabay sellers tend to talk about 'watts' because they don't know any better. The power change is achieved by using a potentiometer with these cheap lights, the power does actually reduce (although I would be surprised if it reduces as much as they claim with these lights) and, at the same time, the flash duration becomes longer and the colour shifts towards yellow/orange.

The method Malcolm suggests will work. If you don't have a flash meter you can get a good approximation in lightroom or PS, But don't expect the flash energy to be consistent at any power setting, and especially not at the lower ones. It may well vary by around 50%.

Simon photo · 2 Feb 2012

Yea got a flash meter like i mentioned. Ok, when i meter in flash mode i get a reading in f=value, which is the meters recommend aperture for the given iso. I know you have explained a way of working it out maybe Im being a little dim witted here but i can't seem to figure out how to convert the readings into something i understand. Such as 1/16th or 1/4 that you get as settings on speedlights and built into the camera. Maybe Im missing something obvious here and will be putting my palm to forehead, victor style when all becomes clear.
I really don't get any major shift in colour with differing power settings, i understand where your coming from with that but have had no problems at all, set my wb to 5400 kelvinators and its good to go!
Everyone seems to poo poo the cheaper strobes available but to be honest, Im not a professional, i don't do work on the side and they are purely for my enjoyment (and a bit of college work) only. I mean yeah, i would love a nice set of elinchrome or broncolor lights hooked up to a blad but for now, its the cheap stuff. With guesswork settings lol

Garry Edwards · 2 Feb 2012

OK...
Set it to minimum power, fire the flash and take a reading. Say that it reads f/4.
Now turn it up until it reads f/5.6 - that is twice the power of f/4
Now turn it up until it reads f/8 - that is twice the power of f/5.6 and 4x the power of f/4

etc.

You should now have the info you need to set on the dial. The problem with fleabay flashes is they normally produce a different power with every flash, which makes the whole thing a bit hit and miss

Simon photo · 2 Feb 2012

Ahhh fantastic, thank you. Having only glanced at "good" strobes i haven't had the pleasure of actually using them so am totally green as to what is even marked around the volume (Im sure mine will go to 11)
Only kidding, right, so to explain how much light i am using on say, strobe 1 i say "strobe 1 is set at 4 stops of light" ?
I have been using them for a few hours recently and give consistent meter readings throughout the session. But even at the lowest setting i find them very bright for the space i work in.

Garry Edwards · 2 Feb 2012

so to explain how much light i am using on say, strobe 1 i say "strobe 1 is set at 4 stops of light" ?

I don't really understand why you have to say anything, presumably it's something that you have to write about for your course - but don't worry, if it's about studio lighting and its a college course then the tutors will probably know less than you anyway...
If you need to explain it, this makes sense
"Flash head one is set to 1/4 power.
Flash head two is set to full power so is producing 2 stops more light than flash head one"

And, if you really want to confuse them add
"But because flash head two is twice as far from the subject the amount of light received by the subject is the same from each flash head"

Simon photo · 2 Feb 2012

Yes! Clarity! Haha thank you. I knew we would get there. I will set so with the masking tape and sharpie forthwith! To be fair to my tutor he has been a studio photographer for most of his adult life and is like a breath of fresh air compared to the tutor on my last course, we see eye to eye on most things and both like each others work, but alas the course is based around location this year instead of studio, which is fine as i have a project i have been wanting to shoot for some time now, just think...bob Carlos Clarkes work, white heat.
The whole class was taken into the studio for a quick session to explain about what's what to the uneducated but never went further than hooking up a sync cord and taking a couple of shots of each other, lights were set up ready and backgrounds were out.
I do try to throw myself into things, this is only an evening course, btec3 but any scrap of information i know goes into my work, and not knowing something as simple as what my strobes were set at would leave a definite hole in my work.
Thanks again guys, you saved the day!