Lumen/Watts/Wattsec. - how to compare light output

this doesn't answer all questions but some.
watt seconds

Thanks for the links. It does explain a bit, but I am still totally clueless how to compare the three categories of light sources across the range and to each other:
- regular tungsten and energy saving lamps / measures in Watts and Lux or Lumen
- dedicated flashes / measures in Guide numbers
- strobes / measures in Wattseconds or Lumenseconds

What I would like to know is something like "the 42am produces in one thousandth of a second light like 5 or so energy saving light bulbs do having XX Watts output", or "the Alienbees 300 Ws produce light like XX 58am at full power " and so on... anyone else or am I the only one who cares about this

Edited by tomiZG - 30 March 2010 at 23:23

I think you need to rephrase your question to include what you're trying to figure out. For instance, why would you want to compare these?

The properties of light make these three light sources all significantly different KINDS of light. They all behave differently.

If you want to compare hot lights to strobes... don't. Get as powerful as you can, and put a dimmer on it. Again, there are different efficiency bulbs, different types, and different sizes. There is no number to compare with, and the properties are totally different.

BTW I just wrote an essay and deleted it because I was getting confused

tomiZG wrote: Now I want to know, can I start (for the beginning) with maybe 8 energy saving bulbs (for example 8 equal HQ Osram x 24 Watts bulbs) that produce the equivalent of 880 Watts floodlight, and is this the same amount (not quality, I know) of light coming from let's say 2 300 Ws strobes?

I don't think so. I've never measured it, but instinct tells me that the strobes would be much more powerful -- I feel sure that you would need a much shorter exposure using the strobes than you would using the energy saving lamps.

I really feel that you are trying the wrong approach here. I don't think you are ever going to get the direct comparison you want, and even if you did, what then? I can't see what use the information would be. As someone has already said, you would be better advised to get a light meter -- one that measures both ambient and flash.

I would be shooting the products through one of those lastolite cubes and thought of putting many energy saving lamps around. WB can be fixed in PP anyway. For starters. Is this a stupid idea?

Not stupid, but perhaps not the best way of doing it. For a start the energy saving lamps will probably be fluorescent, perhaps surrounded by some sort of warming 'filter'. Fluorescent WB is, in my experience, the most difficult to correct and if there is a warming filter as well I don't know what the result might look like. You could end up with considerable colour difficulties. Flash or tungsten are better options.

If you are using a light cube, flash is probably the better option. But a light cube is not really the best way of shooting products. The light is bounced around the cube, giving you a flat, somewhat uninteresting result. I use a cube for photographing ebay products, but for clients' products I generally use carefully placed strobes and light modifiers. Or sometimes I photograph objects in daylight; it depends on what works best for the product and the intended use of the photograph. I use my cube only in desperation, if nothing else is working for me.

The only way you could consider comparing flash output to continuous light output is to also include a shutter speed. But what speed? Flash exposures occur in less than 1/1000 second, generally using a shutter speed of around 1/200 second or slower. Do you also want to make your continuously lit exposures at 1/200 second? If so, you'll need some pretty intense continuous light. Consider that even a fairly humble flash unit can compete with the sun for flash fill purposes. But you don't need to shoot at 1/200 second, do you? If your subject is stationary you could even shoot it by moonlight with a very long exposure.

For the purpose you describe - product lighting - I too recommend ordinary tungsten lights (and nowhere near 880 watts would be needed unless you're lighting something like an automobile) or some mainstream portable flash units. Nothing huge is needed.

If you are going to do some product photography, my first advice is - get a good light meter!

I did. I bought a second hand Minolta Flashmeter V, which I am EXTREMELY pleased with. It has an unbelievable accuracy and repeatability - better than 0.1 EV!

These Minolta meters are now made and sold by Kenko. Sekonic also makes good light meters (professional photographers did prefer the Minolta meters, though).


I understand your confusion when trying to compare the light output from different light sources. Firstly, these figures denotes different physical quantities. It is as if you would compare e.g. speed for one car with the power (horse power) with another. Although there are relations, it is not enough information if you want to compare these two cars.

This will be the same if you would try to compare the input to a tungsten lamp (watt) with the input to a strobe (wattseconds). The former is power, the later is energy. You need more information. Then there is very different efficiency with the transformatiion between the input electrical energy and the output light for these different sources.

Further, the size and type of light modifiers - reflector, spot, barndoor, honeycomb, softbox etc has a HUGE impact of the resulting light output. There will be large differences between the evenness and the angle of usable light from different brands.

This means that you will not have a fixed conversion factor between Guide numbers and Ws either. That factor would vary wildly between different flashes and strobes. Actually, even the same guide number from two flashes from different manufacturers can give markedly different exposure in practical use!

On top of that, for commercial strobe products, the market department may sometimes "polish" the figures considerably. One manufacturer may quote e.g. 100 Ws for one of its strobes, while another may claim 300 Ws for an identical unit!


I have measured the light output from two different cheap 300 Ws strobes. Nr 1 managed to put out 300% more light than Nr 2. However, the stronger one had a very uneven light field, while the weaker one was even and nice.


Some final information. Flashes/strobes generates far more light than continuous light lamps and bulbs. A 300 Ws strobe will, at a typical flash time of say 1/1000 seconds, unleash a peak power of 300.000 watt. Now we can see that there will be a huge difference between an 880 W floodlight and an 880 Ws strobe.

With this information you may now be able to deduce the practical operation conditions for these different light sources.


Continuous light (bulbs and lamps):
Often hard to get enough light. Exposure times will often be whole seconds, which sometimes may cause problems with shaking unsharpness, if the table and tripod isn't sturdy enough. If they are, this may otherwise be cheap and quite OK.

Continuous light may quickly build up untolerable heat at the objects/models.
Energy saving and fluorescent lamps have a non-continuous colour spectrum, which may make it impossible to get a decent white balance.
However, with continuous light it is of course easy to see how light and shadows will fall.


Flashes/strobes:
No problem to get enough light, often the contrary! You need a strobe where you can diminish the light output at least 5 EV steps PLUS neutral density filters (according to my experiences)!
With a curtain shutter you may usually be able to use a shuttertime of around 1/160 seconds or shorter, depending on camera, and with a leaf shutter its shortest time (usually 1/500 seconds). The flash duration will usually be 1/1000 seconds or shorter, hence no problem with shaking unsharpness.
WB is usually simple. However, cheap China strobes may not have particularily consistent light output and colour balance from flash to flash, and even more so when you change the power settings, so you may have to check the WB every time you do this if you need good quality. I have found Profoto strobes to be extremely consistent both with light output from flash to flash and colour temperature when changing power settings (but their price will be 5-10 times as much as the cheapest China strobes, I think). However, second hand units can be had for a much more modest cost. And they will work forever!

With strobes you will have no problems with heating of objects or models.
It may be harder to perceive the light and shadows, though. However, some flashes and all strobes I have seen have had some kind of modelling light, which will give you an apprehension of what the outcome will be.


Edit: While I was writing, some did it quicker than me! :-)

Edit 2: Corrected some spellings and some words.



Edited by Swede101 - 31 March 2010 at 16:44

Mink wrote: ....... surely trial-and-error is a practical alternative to a flash/incident light meter?

Up to a point, yes; and I sometimes do use that method . But a flash meter is extremely useful when you are using several strobes to illuminate a subject. It helps you gauge very accurately the effect of each individual strobe.