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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 24 Jun 2010 (Thursday) 06:59
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Help With My F-Stops

 
Cosha
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Jun 24, 2010 06:59 |  #1

I have a set of Calumet Gen 400's...

Some of you may have them but to others that dont the power is controlled digital ranging from:

#10 - #60 and each time you up the power it goes up one value...

For example if the setting is on 10 and i press the button 5 times, i get 15

I'm having a hard time figuring out what number is what?

I have had a look and this is what i figure but not sure if thats correct at all!

Full Power = Number 60
1/2 Power = Number 30 (As 1/2 of 60 = 30 Loss Of 1 F-Stop)
1/4 Power = Number 15 (As 1/2 of 30 = 15 Loss Of 1 F-Stop)
1/8 Power = Number 7.5 (As 1/2 of 15 = 7.5 Loss Of 1 F-Stop)
1/16 Power = Number 3.75 (As 1/2 of 7.5 = 3.75 Loss Of 1 F-Stop)

Am i on the right track here, as here i dont use 1/2, 1/32 etc.. i use 00.1 decemal

I Dont have a flash meter to work ths out, but i think halfing the power = 1 full F stop

Ok So say number 60 = full power and that is F22 and i want to shoot at F8 i have to loose 3 Stops of light

So i would set my lights too 7.5 (even though my lowest value is 10)?

Some one shoot me...:(


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dmward
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Jun 24, 2010 07:14 |  #2

Two flaws in your reasoning.
A) Digital, European, control each click is 1/10 of a stop. So, in an ideal world 60 is full power, 50 is 1/2 power, 40 is 1/4 power etc.

B) The Genesis line have a control consistency problem. The range from 60 to 10 should be 5 stops, it is usually only about 4 stops. and its not consistent from 60 to 50 to 40 etc.

Since you don't have a light meter use your camera and a white paper towel.

Crumple up the towel and place the light 11 feet from the towel at full power. Set the camera to something like 1/160, F11 at ISO 100. Take a shot, use the blinkies for highlight clipping as an indicator. If they are blinking stop down to F16 and make another shot. If they are not blinking open up 1/3 stop and make another image. The F stop just below where there are blinkies is the optimum exposure at full power. Now beep the light down 10 steps to 50 and repeat the blinkies routine.

It will take a bit of time but you will end up with the exposure for each 10 steps on the light power scale.

BTW, the reason for placing the light 11 feet from the subject; If you move it in to 8 feet the exposure will be close to one stop brighter at the same power setting. Moving it back to 16 feet will make it one stop less.

So, at the end of the process you will know what F stop you need with the light at different distances and power settings. Easy to make a reference table.

Ideally you should do this outside, or at least in a big enough space to minimize the influence of walls and ceiling with reflecting light.


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Jay ­ T
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Jun 24, 2010 07:16 |  #3

I think 60 is full 50 is half 40 is 1/4 30 is 1/8 etc


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Jay ­ T
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Jun 24, 2010 07:17 as a reply to  @ Jay T's post |  #4

and in between are 1/10 stop


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Cosha
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Jun 24, 2010 07:31 as a reply to  @ Jay T's post |  #5

Thanks for the Info guys!

I totaly confused myself a little huh :rolleyes:?

I always have blinking hightlights on so that was a good start!

1x mono light at 60 = Full power bare flash
11ft away (I'm guessing the reason for that is the Sq Inverse law)
Find out where full power is on my camera (I guess should be F16 - F22)
This will give me my F-stop number full power?

So there is 10 x 1/10th stops between 60 and 50?

60
59 - 1/10th
58 - 1/10th
57 - 1/10th
56 - 1/10th
55 - 1/10th
54 - 1/10th
53 - 1/10th
52 - 1/10th
51 - 1/10th
50 - 1/10th

That is a loss of 1 Full Stop in perfect conditions?

Sorry if this is a totaly noob question i have spent a little while looking formyself but im getting confused -

Id rather get this down with out a flash meter as if i get a meter and it breaks down on my ill be at a loss!


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dmward
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Jun 24, 2010 07:38 |  #6

Right, each click is 1/10th stop.

So, for your testing go from 60 to 50 to 40 etc.

The 1/10 stop clicks are for fine tuning the exposure. At least that's what I use them for. And, considering that each click may or may not be a uniform 1/10th stop it ends up being trial and error.


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Cosha
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Jun 24, 2010 07:44 as a reply to  @ dmward's post |  #7

Thanks for clearning that up, sounds better in my head away now!


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Wilt
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Jun 24, 2010 11:32 |  #8

Cosha wrote in post #10418124 (external link)
1x mono light at 60 = Full power bare flash
11ft away (I'm guessing the reason for that is the Sq Inverse law)
Find out where full power is on my camera (I guess should be F16 - F22)
This will give me my F-stop number full power?

Genesis 400 has GN160 (ISO100), according to information just provided to me by someone else in another thread. GN160 / 11' = f/14.5 at full power. If you put the light at 5.6', that would require f/28.6 at full power.

GN160 at full (0EV), GN 113 at 1/2 (-1EV), GN80 at 1/4 (-2EV), GN56 at 1/8 (-3EV), GN40 at 1/16 (-4EV), GN28 at 1/32 (-5EV)


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Cosha
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Jun 24, 2010 14:36 |  #9

Wilt wrote in post #10419263 (external link)
Genesis 400 has GN160 (ISO100), according to information just provided to me by someone else in another thread. GN160 / 11' = f/14.5 at full power. If you put the light at 5.6', that would require f/28.6 at full power.

GN160 at full (0EV), GN 113 at 1/2 (-1EV), GN80 at 1/4 (-2EV), GN56 at 1/8 (-3EV), GN40 at 1/16 (-4EV), GN28 at 1/32 (-5EV)


Wilt thanks for the info!

GN = Guide Number?

So at full power @ 11ft i should get F14.5

;) Ill buy a flash meter


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dmward
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Jun 24, 2010 15:28 |  #10

That's a reasonable starting point.
Not sure if that is with or without the standard reflector.


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Wilt
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Jun 24, 2010 16:16 |  #11

Cosha wrote in post #10420286 (external link)
Wilt thanks for the info!

GN = Guide Number?

So at full power @ 11ft i should get F14.5

;) Ill buy a flash meter

Oddly enough, with speedlights, few of them from any manufacturer are as bright as they claim per their provided Guide Number! I don't claim that studio light claims are any better than their speedlight brethren for realism!!!


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Help With My F-Stops
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