The power of light - comparing apples with apples?

Hi all, This topic may have been covered before and I have read a couple of posts trying to establish the "power" of certain speedlites, but I am looking at a three way comparison.

Comparing Speedlites with Studio Strobes with Continuous Lighting.
Speedlites are given a guide number as a rating of power, Studio Strobes in watts/second (w/s) and many continuous lighting setups in straight watts.

How can these be compared side by side? Does a 300W light produce the same level of light as a 300w/s Strobe? How does the 550/580 compare with say a 300w/s strobe?

Any ideas thoughts or inspirations?

Yes and no.
My simple explanation is...
300W continuous light during 1 second will give you same amount of light as 300W/s flash.
The MAJOR difference is a flash will give you this amount of light instantly.
So when your exposure is 1/60s for example, your continuous light gives you same amount of light as 5 W/s flash.

I have heard that 580 is one stop weaker than 300 W/s flash roughly.

Ah that explains it well. That also explains why any continuous light setups I have used seem to perform so poorly for all the light and heat they throw off.

If you are indeed attempting to compare apples to apples, you must have a basic understanding of the unit "apple".

Actually the amount of heat produced is a lot closer to the fundamental definition of what a Watt actually is, that a light output.

You might want to check out the actual definition of a Watt. A Watt is a joule/second. A joule is a measurement of energy. A Watt is the classic form of electrical power (power is defined as energy per unit time).

Heat is a form of energy. And in the case of a common filament light bulb, the principal form.

So, a Watt is a certain amount of energy per unit time.

Watts can be converted directly into "horsepower", so light bulbs rated in Watts, could also be rated in horsepower (multiply the Watts value by 0.000102 to get the approximate horsepower value).

So, a Watt is not actually a unit that measures light output.

And, studio flash units are often measured in Watt-seconds. That “-seconds” part makes this unit quite a different kettle of fish (note that it is not a Watt/second, or not a Watt per second) .

But, enough for now, but you can see that these things make direct comparisons difficult.

But in general for still photographic purposes, flash units produce a huge amount of light, even compared to very large continuous lights like 1000 Watt units.

This is why electronic flash is so popular. A lot of light in a little package. And a package that produces a grand amount of light, with a much smaller proportion of heat.

Enjoy! Lon

Give a look at this article from the AB makers. Maybe you get some answers
there: http://www.alienbees.c​om/manuals/bluebook.ht​m

Good luck.

A. There is NO DIRECT CONVERSION of the units of electrical charge stored or used (watts, watt-seconds) vs light output 'indicator' (Guide Number).
B. And there can be no DIRECT COMPARISON of units of electrical charge as an indicator our light output!

Units can be very different in the efficiency of converting electrical charge to light output...the cable efficiency to the bulb from the capacitor can be different, for example, so that although the stored electrical charge might be identical, the conversion to light at the flash tube might be better in one than in the other. If you go to the hardware store, you will even see that two 100w bulbs (different manufacturer, or even different models of 100w bulbs made the same manufacturer!) outputs different amounts of Lumens (light) So NO measure of energy is a direct indication of light output!

Units can be very different in directing the light from the flash tube to the subject, too! Although two units might produce identical quantities of light at the flash tube, efficiency of the reflective surface as well as fundamental shape of the reflector (or absence of the reflector) causes different amounts of light to appear at the subject.

The ONLY way to try to directly compare multiple electronic flash units is to stand a certain distance with a flashmeter and measure the output from each unit...and that comparison becomes an invalid comparison once you alter the reflector on any single unit. And there is NO WAY to compare electronic flash to a continuous source...the electronic flash outputs light for a finite period of time, whereas the continuous source continues to output light for as long as your shutter is open!

Begging to differ, I think there ARE ways to compare them from the standpoint of photography, as long as we understand the variables involved. Moreover, I think it’s important to understand HOW to compare them, since both can be used to illuminate a subject such that we can capture an image with the same camera at the same exposure settings. Flash and continuous light sources are often used to illuminate the same image.


One common scenario involving a mix of light sources is fill flash in sunshine. Suppose a neutral gray target meters at 1/200, f/11 in the sun. A flash unit with a guide number of 165 feet would provide the same illumination at a distance of 15 feet (165/11). But if you use 1/50 and f/22 (an identical ambient exposure), that same flash unit would provide the same illumination at only 7 ½ feet.


Flash and continuous light sources can be compared, but only if we understand how shutter speed and distance affect the comparison.


Now, if the OP wants to know how many watts of tungsten light would be required to provide the same illumination as a couple good studio strobes at the same distance and 1/200 shutter speed, the answer would be: more watts than the electrical service in your house can handle.

Yes, but...

In the same scene I could take ambient light readings of (for example) 1/30 f/22, 1/60 f/16, 1/125 f/11, 1/250 f/8. Then assuming full power manual output from a flash with GN150, I could provide same intensity level with f/16 only if I stand at 9' away, or at f/11 only if I stand at 13.6' away... if I stand farther away, I cannot make the exposure with same brightness as ambient. So what does that really prove?!?!?! It proves only that same intensity is possible at certain specific combinations of f/stop and distance for flash, assuming I use specific combinations of f/stop and shutter speed for the ambient light!

If I know that f/11 is possible at 9' away with flash, and if I can meter a continuous light (e.g. hot light) as f/11 30 seconds, what does that prove?!?!? It proves only that same intensity is possible at certain specific combinations of f/stop and distance for flash, assuming I use specific combinations of f/stop and shutter speed for the ambient light!


With both of these cases, what enlightenment does the OP have to the original topic question?

Hopefully, he now understands that continuous and flash light sources can only be compared at a given shutter speed. So before he can determine the suitability of a certain continuous light source, he must first determine what shutter speed he can live with. If he does not yet have this enlightenment, then I guess I didn't explain the concept very well.

Curtis, I hope we have both done a good job answering the OP, that you cannot directly compare the lighting sources from numbers alone, but you can intermix the lighting for suitable photos, if you control ALL of the variables...f/stop, shutter speed, ISO, power output from variable output devices, light-to-subject distances, kinds of light modifiers in use (reflectors, softboxes, etc.)

Geez Wilt, you make it sound so complicated!

Driving down a street in traffic, with pedestrians crossing at crosswalks and jaywalking is no less complex, yet people manage to do that, too. It simply takes practice and then all the elements seem simple...in BOTH cases!

That's a beautifully simple explanation, avoids all the complications I usually get into before I give up.

Gerry@Rick wrote:
That's a beautifully simple explanation, avoids all the complications I usually get into before I give up.

It might very well be a "beautifully simple explanation" and easy to understand. But, it is also very likely not accurate.

Actually if it is accurate, it is only due to wild chance.

There are approaches to buttress my opinion, here are just three (most likely there are more).

Watts are not a measure of light intensity
Flash units are rated in Watt-seconds, not Watts per second (or Watts/sec as indicated in the post)
A Watt-second (or even a Watt/second) is not a measure of light intensity

So, in an "apples to apples" comparison. This comparision looks someting like "zebras to a Bach concerto"

Enjoy! Lon

Gerry@Rick wrote:
That's a beautifully simple explanation, avoids all the complications I usually get into before I give up.

As Londuck infers, the above statement is only true purely by chance...when the first 4 planets from the sun are aligned along an imaginary plane in space!

You cannot even state with certainty that two 300w/s studio lights from two different manufacturers can have the same light output! The *only* thing you know is the storage capacitors hold the same amount of charge --assuming they are the same voltage (which is probably a wrong assumption)!

Nor, as I said earlier, do you know that a Sylvania 300w lightbulb produces the same amount of light as a GE 300w ordinary lightbulb as compared with a GE 300w Longlife lightbulb!