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FORUMS General Gear Talk Flash and Studio Lighting 
Thread started 31 May 2009 (Sunday) 18:43
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Does 200W/s is sufficient to power up large modifiers?

 
SkipD
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Jun 02, 2009 08:15 |  #31

abdul10000 wrote in post #8034356 (external link)
If I cut the distance of light to subject by half does that reduce my exposure by 1 stop? For example, if the light is at 10ft and I bring it closer to 5ft, will that reduce my exposure from f16 to f11?

That would be fairly accurate for a small light source.

If you are using a large umbrella or softbox, however, then there would be much less change in exposure when moving the lights.

The inverse square law (which your assumption is derived from) truly applies only to a point source of light.


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abdul10000
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Jun 02, 2009 08:25 |  #32

SkipD wrote in post #8034394 (external link)
That would be fairly accurate for a small light source.

If you are using a large umbrella or softbox, however, then there would be much less change in exposure when moving the lights.

The inverse square law (which your assumption is derived from) truly applies only to a point source of light.

Thanks, can a 7" reflector be considered a point source?

http://www.alienbees.c​om/specs.html (external link)
Assuming so, Alienbee's specifications link states that using the B1600 (640ws) with a 7" reflector at 10ft and iso100 gives f22 exposure. Which means if the light was to be brought closer to the subject (more like 2.5ft which is the normal distance with portrait hard lighting) the exposure will go down to f64.

Using a light with a 5 stop range the exposure can be increased to a maximum f11. With a 6 stop range the exposure will go up to f8. Sounds like 600Ws is a little too much power for hard lighting, especially if the goal is to achieve f4-5.6?




  
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Jun 02, 2009 08:31 |  #33

With apologies for continuing the off-topic part of the discussion, why Ws and not Joules?

regards,
/alan


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bobbyz
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Jun 02, 2009 08:42 |  #34

abdul10000 wrote in post #8034439 (external link)
Thanks, can a 7" reflector be considered a point source?

http://www.alienbees.c​om/specs.html (external link)
Assuming so, Alienbee's specifications link states that using the B1600 (640ws) with a 7" reflector at 10ft and iso100 gives f22 exposure. Which means if the light was to be brought closer to the subject (more like 2.5ft which is the normal distance with portrait hard lighting) the exposure will go down to f64.

Using a light with a 5 stop range the exposure can be increased to a maximum f11. With a 6 stop range the exposure will go up to f8. Sounds like 600Ws is a little too much power for hard lighting, especially if the goal is to achieve f4-5.6?

If you need AB1600 and want to shoot at f4-f5.6, you should check out X1600. It is like Ab1600 and AB400 in a single light. There is a switch to go to lower power mode. That way you can go to really low power so that you can shoot at f2.8 and ISO100 if you need to.


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SkipD
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Jun 02, 2009 08:42 |  #35

abdul10000 wrote in post #8034439 (external link)
Thanks, can a 7" reflector be considered a point source?

http://www.alienbees.c​om/specs.html (external link)
Assuming so, Alienbee's specifications link states that using the B1600 (640ws) with a 7" reflector at 10ft and iso100 gives f22 exposure. Which means if the light was to be brought closer to the subject (more like 2.5ft which is the normal distance with portrait hard lighting) the exposure will go down to f64.

Using a light with a 5 stop range the exposure can be increased to a maximum f11. With a 6 stop range the exposure will go up to f8. Sounds like 600Ws is a little too much power for hard lighting, especially if the goal is to achieve f4-5.6?

No, light from a 7" reflector does not act like a point source of light would (relative to the inverse square law). It is closer to acting like a point source of light, though, than light coming through a 30"x60" softbox would be.

You really need a light meter which can measure light from flash sources rather than trying to do math to establish your exposure settings. I highly recommend the Sekonic L-358, used in incident mode, for the purpose.

Your last paragraph is quite confusing. I don't quite know what you're getting at. If you can explain your thoughts better, maybe we can help with an answer.


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ebann
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Jun 02, 2009 08:54 |  #36

alan_potter wrote in post #8034464 (external link)
With apologies for continuing the off-topic part of the discussion, why Ws and not Joules?

regards,
/alan

Here... enjoy and let's be done with it! :lol:

1 joule is equal to:
1×107 ergs (exactly)
6.24150974×1018 eV (electronvolts)
0.2390 cal (gram calories or small calories)
2.3901×10−4 kcal (kilocalories, kilogram calories, large calories or food calories)
9.4782×10−4 BTU (British thermal unit)
0.7376 ft·lbf (foot-pound force)
23.7 ft·pdl (foot-poundals)
2.7778×10−7 kilowatt-hour
2.7778×10−4 watt-hour
9.8692×10−3 litre-atmosphere
1×10−44 Foe (exactly)
Units defined in terms of the joule include:
1 thermochemical calorie = 4.184 J
1 International Table calorie = 4.1868 J
1 watt hour = 3600 J
1 kilowatt hour = 3.6 × 106 J (or 3.6 MJ)
1 ton TNT exploding = 4.184 GJ
Useful to remember:
1 joule = 1 newton × 1 metre = 1 watt × 1 second


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abdul10000
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Jun 02, 2009 08:55 |  #37

bobbyz wrote in post #8034511 (external link)
If you need AB1600 and want to shoot at f4-f5.6, you should check out X1600. It is like Ab1600 and AB400 in a single light. There is a switch to go to lower power mode. That way you can go to really low power so that you can shoot at f2.8 and ISO100 if you need to.

Yes! that one interesting mono, how does it achieve that with two capacitors in one mono?




  
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abdul10000
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Jun 02, 2009 09:03 |  #38

SkipD wrote in post #8034512 (external link)
No, light from a 7" reflector does not act like a point source of light would (relative to the inverse square law). It is closer to acting like a point source of light, though, than light coming through a 30"x60" softbox would be.

You really need a light meter which can measure light from flash sources rather than trying to do math to establish your exposure settings. I highly recommend the Sekonic L-358, used in incident mode, for the purpose.

Your last paragraph is quite confusing. I don't quite know what you're getting at. If you can explain your thoughts better, maybe we can help with an answer.


Sorry I just realized that I was not detailed enough. Regarding the last paragraph, what I meant is that if you dial a 5 stop 600ws mono to its lowest power range it will give you f11 exposure at 2.5ft and iso100. Using a 6 f stop mono at its lowest setting will give f8.

Of course as you pointed out a 7" reflector is not exactly a light source so those f stop figures are most likely wrong. Probably the real f stops are a little higher.




  
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danielyamseng
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Jun 02, 2009 09:48 |  #39

abdul10000 wrote in post #8034621 (external link)
Sorry I just realized that I was not detailed enough. Regarding the last paragraph, what I meant is that if you dial a 5 stop 600ws mono to its lowest power range it will give you f11 exposure at 2.5ft and iso100. Using a 6 f stop mono at its lowest setting will give f8.

Of course as you pointed out a 7" reflector is not exactly a light source so those f stop figures are most likely wrong. Probably the real f stops are a little higher.

abdul10000, 600w at it's lowest setting still yeild f11 at 2.5ft at ISO100? At 50ws(min power), it's still able to produce such a result? What if a fullbody softbox and Octabox was used?




  
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Jun 02, 2009 10:07 |  #40

The light is a virtual 'point source' from a 7" reflector if the distance between the object and the source is more than about 3x the diameter of the source, if I remember correctly; the farther away, the more it becomes a virtual point source. Also the 7" reflector does not behave like a true point source because its light is not all emitted in all directions, but it is somewhat 'focused' in its directionality. For short distances relative to the size of the source, lights may not strictly follow the Inverse Square principle. Inverse Square is generally applicable to lamps used in shallow reflectors, but not when deep reflectors are used, and is not applicable to the illumination provided by a spotlight'. Focused beams, like theatrical spots, also do not obey the Inverse Square principle. A softbox is not a point source because the distance to the subject is 'small' in proportion to it's size.


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phr0ze
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Jun 02, 2009 10:20 |  #41

I'm getting .215hps out of my AB400. Should I be concerned?


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Jun 02, 2009 10:29 |  #42

phr0ze wrote in post #8035063 (external link)
I'm getting .215hps out of my AB400. Should I be concerned?

I'd start feeding that horse some oats if I were you.


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bobbyz
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Jun 02, 2009 10:30 |  #43

abdul10000 wrote in post #8034578 (external link)
Yes! that one interesting mono, how does it achieve that with two capacitors in one mono?

capacitor bank switch from what I understand.


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Jun 02, 2009 11:18 |  #44

shooterman wrote in post #8035106 (external link)
I'd start feeding that horse some oats if I were you.

bw!


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abdul10000
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Jun 02, 2009 12:09 |  #45

danielyamseng wrote in post #8034847 (external link)
abdul10000, 600w at it's lowest setting still yeild f11 at 2.5ft at ISO100? At 50ws(min power), it's still able to produce such a result? What if a fullbody softbox and Octabox was used?


Check this out:

http://www.alienbees.c​om/specs.html (external link)

The AlienBees B1600 (640Ws) with 1) 7" reflector 2) at a distance of 10ft from the flash meter 3) at iso100 4) powered at full power provides an exposure at f22.

Since the B1600 is a 5 stop power range monolight if you dail it to lowest power level 1/16 or 40Ws and do the same test you will get an exposure of f5.6.

Now To simplify my next example assume a 7" inch reflector acts as a "point source". In this second example keep all settings from the previous example and only change the light distance to 2.5ft from the light meter. Since the light is now closer by 2, exposure will move down from f5.6 to f11.

As others have pointed out a reflector in not technically a "point source" so my guess is that the exposure would be higher than f11, perhaps as high as f8. So for hard light application, a 640Ws unit seems too much. A 160Ws monolight would be 2 stops lower so it can provide f5.6 exposure at its lowest power setting or perhaps f4.

Hope this helps and if I did get anything wrong please correct me.




  
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