Measuring Light

[iND]

There was an earlier poster who wanted to know how one flash would compare to another.


I often wonder how my 580EX at full power (manual) compares to my speed light at full power.


Sure I can talk about stops of light, but I would like to measure the output in lumens or something


and directly compare it.


I have a sekonic light meter but I would like to know my strength of flash in my arsenal as I plan


my shots.


Ie. What manual power on my 580 EX II equals 1/2 power on my speed light.


Ideas? Certainly there is a meter than measures ambient light and could also measure the flash


output.

[Dave Johnston]

I may be completely in the wrong here, but I think the guide number lets you know how "strong it is."


Someone else who knows more should chime in soon.

[Brendan7]

I believe the guide number is a measurement of at what distance the flash is still effective. Don't compare the numbers as distances (i.e. don't plan on using a 580 to light an object 191' away), use them in comparison to other flashes.

[jcrowe87]

Guide numbers work in ratios. You can compare them to distance, but must also take f/stop into effect. For instance, something with a GN of 191' would project light onto something 191' away at an f/stop of 1.0. You can get some idea of the distance a flash will through by working with simple doubling/halving from there.

[peety3]

Guide numbers work in ratios. You can compare them to distance, but must also take f/stop into effect. For instance, something with a GN of 191' would project light onto something 191' away at an f/stop of 1.0. You can get some idea of the distance a flash will through by working with simple doubling/halving from there.
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...would project light onto something 191' away at an f/stop of 1.0 and an ISO of 100...

[jcrowe87]

People shoot at ISO's other than 100? Wild. [:P]

[Sheiky]

People shoot at ISO's other than 100? Wild. /emoticons/emotion-4.gif






Hmm interesting and it leaves me with another question:


When you have a GN of 58m (580 flash) at f1 iso 100, and you put it at f1.4 (1 stop higher) the range would be 29m, am I correct?


Then is it also reversable, so let's say the same flash with GN 58m would have a range of 116m with f1 iso200? That doesn't really seem correct. I mean at f1 with an iso of 25600 it would have an amazing range which I wouldn't believe. Perhaps it is, but it sounds crazy [A]


Can someone shine or flash a light here? [:D]

[alexniedra]

That doesn't really seem correct. I mean at f1 with an iso of 25600 it would have an amazing range which I wouldn't believe. Perhaps it is, but it sounds crazy /emoticons/emotion-13.gif


This seems weird - The 580 EX II Flash, when mounted on the EOS 1D Mark IV at ISO 102400, will have an effective range of 59392m. That 60 km! [H]



But in all seriousness, I say ditto to that - Can somebody enLIGHTen us?

[Sheiky]

This seems weird - The 580 EX II Flash, when mounted on the EOS 1D Mark IV at ISO 102400, will have an effective range of 59392m. That 60 km! /emoticons/emotion-11.gif


Would be quite cool huh??[H]

[Mark Elberson]

<span style="font-size: 9pt; color: #000000; font-family: 'Verdana','sans-serif';"]Hey guys. This is an interesting conversation. I hope someone with a much deeper understanding than I do will enLIGHTen us too but I'll give it a stab for S's &amp; G's :-)<o:p></o:p>


<span style="font-size: 9pt; color: #000000; font-family: 'Verdana','sans-serif';"]I understand light to obey the inverse square law. What this means is that (all other things being equal) when you double the distance of your light source to your subject you get 1/4 (25%) of its original power. That being said, in order to double the GN (Guide Number) of a flash you would need to quadruple the output. Since with a 580EX you are limited to its GN of 58 in order the only way you can affect its "power" is to adjust either the aperture or the ISO. Since GNs are rated at f/1.0 and ISO 100 we're pretty much limited to changing the ISO. Here's where I take a leap...based on every I just stated, I believe that if you change your ISO from 100 to 400 you will effectively increase the GN of the 580 from 58m to 116m. Make sense??? Someone with more knowledge please correct me where I'm wrong :-)<o:p></o:p>


<span style="font-size: 9pt; color: #000000; font-family: 'Verdana','sans-serif';"]Another way to understand the GN of your flash is if the GN is 58m @ f/1.0 &amp; ISO 100 then to maintain that 58m GN @ f/1.4 you would need an ISO of 200. To maintain that 58m GN @ f/2 you would need an ISO of 400, etc, etc.
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]EDIT:
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<o:p></o:p>
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]Just did some quick arithmatic...if what I said before is actually correct, here is how the GN would react to quadrupling the available ISOs on the ID Mark IV:<o:p></o:p>



<table border="1" cellpadding="0" cellspacing="0" style="border-collapse: collapse; mso-border-alt: dashed #BBBBBB .75pt; mso-yfti-tbllook: 1184; mso-padding-alt: 0in 0in 0in 0in;" class="MsoNormalTable"]
<tbody>
<tr style="height: 12pt; mso-yfti-irow: 0; mso-yfti-firstrow: yes;"]
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"] 58 GN<o:p></o:p>
</td>
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"] 100 ISO<o:p></o:p>
</td>
</tr>
<tr style="height: 12pt; mso-yfti-irow: 1;"]
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"]116 GN<o:p></o:p>
</td>
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"] 400 ISO<o:p></o:p>
</td>
</tr>
<tr style="height: 12pt; mso-yfti-irow: 2;"]
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"]232 GN<o:p></o:p>
</td>
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"] 1,600 ISO<o:p></o:p>
</td>
</tr>
<tr style="height: 12pt; mso-yfti-irow: 3;"]
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"]464 GN<o:p></o:p>
</td>
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"] 6,400 ISO<o:p></o:p>
</td>
</tr>
<tr style="height: 12pt; mso-yfti-irow: 4;"]
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"]928 GN<o:p></o:p>
</td>
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]<span style="mso-spacerun: yes;"] 25,600 ISO<o:p></o:p>
</td>
</tr>
<tr style="height: 12pt; mso-yfti-irow: 5; mso-yfti-lastrow: yes;"]
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]1856 GN<o:p></o:p>
</td>
<td style="height: 12pt; background-color: transparent; border: #d4d0c8; padding: 0in;"]
<p class="MsoNormal"]<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-fareast-font-family: 'Times New Roman'; mso-bidi-font-family: 'Times New Roman';"]102,400 ISO<o:p></o:p>
</td>
</tr>
</tbody>
</table>



<span style="font-size: 10pt; color: #000000; font-family: 'Lucida Console'; mso-bidi-font-family: 'Courier New'; mso-themecolor: text1;"]<o:p>By the way 1856m = 1.15 miles! That's too cool to be true right?!?!?!?</o:p>

[Sheiky]

Very nice Mark! Yes it isn't 60km, but still very impressive!! Ah well since I don't have a f1 lens it doesn't really count for me, but still..


I just kept on thinking about it myself and just had to look it up [:P] And yes you're right!


But then the other way around isn't right as well. What I said earlier:



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When you have a GN of 58m (580 flash) at f1 iso 100, and you put it at f1.4 (1 stop higher) the range would be 29m, am I correct?


That just wouldn't be correct then, wouldn't it? That should be quadratic as well. Or am I missing something important now?


Jan
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[Mark Elberson]

But then the other way around isn't right as well. What I said earlier:



When you have a GN of 58m (580 flash) at f1 iso 100, and you put it at f1.4 (1 stop higher) the range would be 29m, am I correct?
<div>


That just wouldn't be correct then, wouldn't it? That should be quadratic as well. Or am I missing something important now?


Jan
</div>
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I think a GN of 58m @ f/1.0 would turn into a GN of 14.5m (58/4) @ f/1.4 since@f/1.4 the lens would gather half the light of f/1.0 makingthe flash'sability to illuminate a subjectaffective at only1/4 the distance?


Who thought photography was going to be so math intensive!

[jcrowe87]

Photography = Physics in disguise. [8-|]

[Jon Ruyle]

Sure I can talk about stops of light, but I would like
to measure the output in lumens or something


I don't think guide numbers translate into lumens, but rather to lumen seconds. Lumens measure light per square degree per unit time, while guide number measures light per square degree gathered up over time. In other words, if flash 1 flashes with uniform brightness for 1/10,000 second, and flash 2 flashes with uniform brightness for 1/100,000 second but with 10 times as brightly (10 times as many lumens), the two flashes will have the same guide number.


According to this ("http://www.flashbulbs.com/basics.htm), a guide number of 150 is about 7000 lumen seconds.






<span style="font-size: 9pt; font-family: 'Verdana','sans-serif'; color: #000000;"]I understand light to obey the inverse square law. What this means is that (all other things being equal) when you double the distance of your light source to your subject you get 1/4 (25%) of its original power. That being said, in order to double the GN (Guide Number) of a flash you would need to quadruple the output. Since with a 580EX you are limited to its GN of 58 in order the only way you can affect its "power" is to adjust either the aperture or the ISO. Since GNs are rated at f/1.0 and ISO 100 we're pretty much limited to changing the ISO. Here's where I take a leap...based on every I just stated, I believe that if you change your ISO from 100 to 400 you will effectively increase the GN of the 580 from 58m to 116m. Make sense??? Someone with more knowledge please correct me where I'm wrong :-)


<span style="font-size: 9pt; font-family: 'Verdana','sans-serif'; color: #000000;"]Another way to understand the GN of your flash is if the GN is 58m @ f/1.0 &amp; ISO 100 then to maintain that 58m GN @ f/1.4 you would need an ISO of 200. To maintain that 58m GN @ f/2 you would need an ISO of 400, etc, etc.





I'm don't claim more knowledge, but I agree with most of this, though I would say that when you double the distance you get 1/4 the intensity, not 1/4 the power. IIRC, intensity is power per unit area. As you get further away, the light spreads over an area that grows like the square of distance. The power stays the same, but the power per area, or intensity, obeys an inverse square law. (A 100 W bulb is a 100 W bulb no matter how far away you are).

[Mark Elberson]

As you get further away, the light spreads over an area that grows like the square of distance. The power stays the same, but the power per area, or intensity, obeys an inverse square law



Terrific point! I knew I was forgetting something :-)

[nickds7]

Nevermind, somehow I missed the inverse square law had already been brought up.

Anyone know how far a 580exii will reach at102,400? It is still an interesting question. I don't think it will be that amazingly far though.