Flash guide number comparisons

[ray_bohn]

<p>I am not new to photography, but new to digital equipment. I am confused about flash guide numbers. My last flash had a GN 110. I think that meant that I had to use F11 at 10 ft with ISO 100 film.<br>

I now have a Olympus E510. The built in flash Guide number appears to 12m. Does that equal to a GN in Feet of 36 or F3.6 at 10 Feet? I hope not, because that means I can't shoot over 10 ft based on the largest aperture opening on my lens, unless I select a higher ISO.<br>

I am looking at the FL36 flash. Based on what I understand, it has a GN of 85 at 10 ft. Is that correct?<br>

Thanks from an old guy.</p>

 

[Alan Marcus]

<p>This camera sports a guide number of 12 for those folks using the metric system. For folks living in Liberia and the USA, (only countries still using the old English measure system). The conversion factor is 3.28 making the guide number for feet =40 of ISO 100. If we choose to use 200 ISO we multiply the guide number by 1.4 thus for 200 ISO the guide number is 56. For 400 ISO the guide number is 80. OK to round values for convenience. <br>

<br>

We use this guide number by estimating the subject to camera distance and dividing this value into the guide number. Say we set our camera to 100 ISO and the subject is 7 feet from the camera. We divide 40 by 7 = 5.6). This tells us to set our camera aperture to f/5.6. Set the camera to 400 ISO the guide number is 80, subject at 7 feet, the aperture setting is f/11.<br>

<br>

Sorry to report that the flash produced by the built-in flash is quite feeble. To get more power, heavy duty batteries and a component called a capacitor must also be heavy duty. We get powerful flash units when they are permitted to get. This built-in flash is typical in power for this camera type. <br>

</p>

[dave_redmann]

<p>I <em>suspect</em> there's an apples-to-oranges issue; if the built-in flash's guide number is listed as 12, that is almost certainly in meters, and equates to 39 feet (close, but not exactly, 36 or 40). If the shoe-mount flash's guide number is listed as 85, that is almost certainly in feet, and equates to 26 meters.</p>

<p>Guide numbers are standardly--indeed, I think used to be always--listed relative to ISO 100. Sometimes now you see them listed for ISO 200 or something, which inflates the number by 1.41x. So a guide number of 12 meters at ISO 100 is the same output as a guide number of 17 meters at ISO 200.</p>

<p>Once you have the guide number in the unit of interest--meters or feet--the math is pretty easy:<br>

f-stop = guide number / flash-to-subject distance<br>

So at ISO 100, a flash of guide number 12 meters or 39 feet, when used 10 feet from the subject, calls for f/3.9; and a flash of guide number 26 meters or 85 feet, when used 10 feet from the subject, calls for f/8.5. Each stop you increase the sensitivity (e.g., ISO 100 to ISO 200) means the effective guide number goes up by 1.41x. So, for example, if your lens has a maximum aperture of f/5.6, the subject is 15 feet away, and all you have is the built-in flash, flipping the equation :<br>

guide number = f-stop x flash-to-subject distance = 84 feet<br>

that's what you need. To bump up the effect of the build-in flash, going from ISO 100 to ISO 400 will give you an effective guide number of 79 feet--close enough.</p>

<p>All of this assumes that any given flash achieves its advertised guide number. Some do, some don't, and the assumptions on which the specification is given do matter (e.g., open space versus inside a small room with white walls and ceiling).</p>

 

[William Michael]

<p>I’ve just looked at the user manuals for both the items of gear that you listed:</p>

 

<blockquote>

<p>I now have a Olympus E510. The built in flash Guide number appears to 12m.</p>

</blockquote>

<p>Yes. The user manual states the GN = <em><strong>12 Metres</strong></em> at ISO100.<br>

As already mentioned – that’s a GN of about 40<em> FEET</em> at ISO100.</p>

<p>*</p>

 

<blockquote>

<p>I am looking at the FL36 flash.</p>

 

</blockquote>

<p>Many of these types of Hot-shoe mounted Flash Units have a zoom function which focusses the beam of the flash more narrowly and also many units have a wide lens which can flip down over the front of the Flash. This model has both. The zoom function of the flash produces a read-out on the display of the flash unit and that is a number to indicate the Flash will cover the scene if you are using the corresponding Focal Length Lens, on the camera. Similarly if you are using a wide angle lens you can flip down the W/A lens on the Flash to ensure the Flash covers the entire scene that the lens captures.<br>

<br>

So, in this case and unlike your IN-camera Flash which has neither a zoom mechanism nor wide lens attachment, <strong>the Guide Number of the FL36 will change depending upon the zoom (or wide angle) setting of the flash unit.</strong><br>

<br>

So although the GN of the FL36 is listed as: GN = 36 Meters at ISO100 - that is the GN at the Flash’s maximum zoom setting and of little use to you if you are using a wide angle camera lens and require the flash to illuminate the breadth of the scene.<br>

<br>

You can download the pdf of the user manual for yourself, but for assistance here is table of the flash’s wide angle and zoom positions (to mimic the FL of the lens used) followed by the respective Guide Number for full power flash. (The Flash’s wide angle and zoom numbers, represent the actual FL of the lenses used on your 4/3rds camera and the GN are in Metres at ISO100):<br>

<br>

FL8/GN12;<br>

FL10/GN14;<br>

FL12/GN20;<br>

FL14/GN22;<br>

FL17/GN26;<br>

FL25/GN28;<br>

FL35/GN32;<br>

FL42/GN36.<br>

<br>

WW</p>