Studio flash, have I got this right?

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Ok I have just taken delivery of the two head Lencarta set, with radio trigger and softboxes :D I'm really pleased with them and looking forward to the steep learning curve I have just set myself.

I'd be grateful for some clarification to ensure I've got the basics right. The guide number of the lights is 99 giving an aperture of f11 at 9ft.

So, if I half the power on the heads I would need an aperture of 5.6 to get the same level of exposure. I can half the power either on the flash head or by moving the light back 9ft.

So conversely if I change the aperture I need to adjust the power on the head or move it further away.

Sorry if this is stating the obvious I just need to make sure I've understood correctly.

My next question is that I assume I would use one light to light my model and one to provide some fill. or background lighting. Do I need to account for both flash heads when calculating my aperture or power of flash?

I assume that the fill flash will be of a lower power to correclty expose the b/g.

Have I got this right or am I on an even steeper learning curve than I thought:shrug:

Thanks for your help folks(y)
 
No (in a word), not right.

The light fall off is inversely proportional to the distance between the surface and the light source.

To calculate the fall off for moving the lights you need to use the inverse square of the distance.

An example may make this simpler. If you are lighting an object from 10 ft at f8 and then move the lights to 20 ft you would get :-

(10x10)/(20x20) ie. 100/400 or 1/4 the light

To get half the light you need to move the light back the equivalent of the square root of 2 multiplied by the original distance ie. 10ft x 1.4 = 14 ft.

You can check this using the initial equation.
(10x10)/(14x14) = 100/196 = 1/2

there is a slight error which will make no difference in real use as the actual square root of 2 is 1.414 not 1.4 (for practical purposes just use 1.4)

So, to half the light by moving the light move it back by 1.4 times the original light to subject distance. To double the light move it closer to 0.7 times the original distance.

Half the light is one stop so reducing the light by one stop from f11 requires one stop more on the camera to get the same exposure ie. f8. Turning the light down on the flash head by half is usually one stop ie from f11 to f8.

A more detailed explanation is here:

http://www.alienbees.com/manuals/bluebook.htm

(the first reference I could find there may be others which are better or easier to read)

Hope this helps

John
 
look up gary edwards on here, you will find it very good
 
John is right - sort of - but the inverse square law (optical) is concerned with a theoretical point source of light (or other form of energy such as heat) and studio lights are not point sources, so they don't follow the inverse square law in strictly linear terms. But this only becomes a major difference if the light source is really large.

But to get back to your question, adjust the power of the light to change the exposure, not the distance from light to subject. If you move the light further away it will become smaller in relative terms and therefore harder, so set the light at the distance that gives you the effect you want and then adjust its power to give you the lens aperture you want. You can also adjust your ISO setting if necessary, but it's always best to shoot at the lowest ISO you can.

As for the effect on exposure of combining lights, the only correct position for a fill flash is where it will illuminate all of the subject as seen by the lens, which means that it has to be on axis with the lens, either above or below it but never to one side. Because of this the fill flash will always affect the exposure.
 
A great way to learn lighting is to shoot with the camera tethered to a computer. That way, you immediately see the image on the computer monitor not on the tiny (in comparison to the monitor) LCD display on the back of the camera.

A good tutorial on shooting tethered as well as many other tutorials on lighting can be found at:

http://www.prophotolife.com/video-library/
 
goes off to look for the drowning not waving emocation!

Thanks for putting me right folks(y) I'll go off and do some more reading and experimenting... then I'll come back with more simple questions. ;)
 
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