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01-30-2014, 10:42 AM   #1
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Flash Question

Hello All,
Regarding a flash's effective range, in the description of the Pentax AF360FGZ II is the text "Guide Number 119 at ISO 100/ft (36 at ISO 100/m)". Is this flash truly effective at 100 feet? I'm sure I'm misunderstanding this description.

Pardon my ignorance on this topic, any replies are much appreciated.

01-30-2014, 11:54 AM   #2
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The guide number is more useful for calculations than showing the absolute limit. That 119ft is at f1, ISO 100 and with the flash giving its narrowest beam. Other variables come into play to determine whether or not it is really effective at that distance, but it can actually reach.

Since you aren't likely to be shooting at f1, each stop beyond that the distance is cut in half. You can get some distance back by raising the ISO. This calculator will help you understand the relationship.
01-30-2014, 11:58 AM   #3
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Yes, but only if you use it with an f/1.0 or faster lens. If you have one of those rare f/0.95 or f/0.75 lenses (not sure if those were ever available in Pentax or m42 mount), you could actually get farther. Of course, most people don't have lenses that fast. So ....

You just take the guide number, and divide by distance, to get the ideal f/ stop at a full-power flash discharge. A more practical calculation is to take the guide number, divide by your maximum aperture (which depends on the lens), to find the maximum effective range.

So, if you have an f/ 2.8 lens, you can reach up to (119 ft) / (2.8) = 42.5 feet.

Another example: The DA 18-55 zoom has a variable aperture, where the maximum aperture is somewhere between f/3.5 (at the widest setting) and f/5.6 (at the most telephoto setting). So your effective range is at most 21 feet at the full telephoto setting, or up to 34 feet at the widest angle setting.

The quoted figure is for a given ISO. You can get a longer reach with a faster ISO of course. Also, the quoted figure is for a given "zoom" setting on the flash head itself.
01-30-2014, 12:07 PM   #4
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Guide Number = distance f to provide enough light on the subject.

So at ISO 100, the flash would be effective on a subject 100 feet away, if your lens were open to f1.2, in theory. You could raise ISO a lot, say ISO 3200. If I counted right, that would allow you to use f6.7.

I actually have a photo in almost those conditions:



I used my AF540FGZ which has more power than the AF360FGZ II (I think about 50%). This was at ISO 3200, f5.6, 78mm with the SMC Pentax-F 70-210mm f4-5.6 lens. I think the deer was more like 50 feet away, with very little ambient light. I don't know how much flash power was used. The photo shows one issue: the flash lights up everything in between you and your subject. Closer objects will get more light because they are closer (inverse square law). In an automatic mode like P-TTL, those objects will fool the meter into reducing flash power. The flash and P-TTL system won't know that your subject was the deer, not the gravel.

So yes, you could probably get the flash to light something 100 feet away, but taking a decent photo at that distance is challenging.

01-30-2014, 12:10 PM   #5
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QuoteOriginally posted by Tanzer Quote
...Also, the quoted figure is for a given "zoom" setting on the flash head itself.
Since I already looked that up, it's 85mm for both the 360 and 540 models, the maximum zoom.
01-30-2014, 12:19 PM   #6
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Thanks for the info everyone. I don't really need a 100 ft. reach, but wanted to understand the specs. before I add this to my wish list.

Hoping to become an active member and Pentax owner in the near future (late Feb./early Mar.)

-JG
01-30-2014, 09:46 PM   #7
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RedWagon - you got the basics, but I did not see where anyone pointed out that zooming the flash head wider will reduce the effective reach. So, if the guide number says 40 feet @ ISO 100 - that is only with the flash zoomed to 85 mm. If you bring it back, the guide number gets much less because the flash must light up a much larger area. There is a page in the manual that can get you started.
01-30-2014, 11:46 PM   #8
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QuoteOriginally posted by Canada_Rockies Quote
If you bring it back, the guide number gets much less because the flash must light up a much larger area.
That's why I hate zoom heads. I know why they exists, but I can't keep all those tables in my head

01-31-2014, 11:36 AM   #9
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Yeah, I know. I carry all my manuals with me. Didn't used to have to in the first days. No fancy adjustments at all. However, I should dig out some negatives from 1962 when I shot Tri-X at 3200 in a hockey rink with a #50 flash bulb. If you hold the print far enough away, you can see they were hockey players. Close up, it was just a gravel parking lot. :LOL:
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