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09-20-2020, 04:46 PM   #1
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Effect of Flash Zoom on GN

I noticed an advert for a Nissin ND60A-C Flash :
Nissin i60A Wireless Compact Flash ? nissindigital.us
... and was surprised to see that for a compact flash it had a Guide Number of 60 (meters). Then I saw that it had a 24-200 mm zoom head and that was at maximum zoom . At 50mm they say the GN is 39. I think their zoom figures refer to Full Frame equivalent, and they are for ISO 100.

This set me thinking about the relationship between GN and the flash zoom setting. The advert gives this table of relationship :-

24mm GN 27
50mm GN 39
105mm GN 50
135mm GN 54
200mm GN 60

In this table it looks like the GN is proportional to the square root of the zoom "length" (with rough mental arithmatic anyway). But I would have thought it should be linear.

That is because, taking the zoom length to mean that the spread angle of the light is the same* as the angle of view of a lens of that same focal length (FF equivalent), the illumination of a frame-filling target will always be the same as long as the flash zoom length is set to the same as the focal length of the lens being used. With all other things equal (ie F-stop, ISO, and max flash energy) the effective GN will therefore be in linear relationship to that lens focal length and flash zoom length.

Looking at it another way, in the flash formula the GN and distance are linearly related (unlike the ISO number which appears as a square root).

Am I missing something here ?

* In practice it will be a bit more to avoid edge fall-off.

09-20-2020, 07:28 PM   #2
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QuoteOriginally posted by Lord Lucan Quote
Am I missing something here ?
I don't know about the math(s), but guide numbers at longer zoom settings are higher because the beam is focused more tightly. In recent years, it has become industry practice to state GN in the specs at the highest zoom setting even though the flash will seldom be used in that manner.


Steve
09-20-2020, 09:38 PM   #3
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While my Yongnuo YN560 III speedlites don't zoom as high as the Nissin, the numbers might shed a little light.

24mm = 28
50mm = 42
105mm = 58

While the GN at 24mm is similar to the Nissin, that for 105mm is a bit higher. Conventional wisdom is that the flash reflector and lens designs strongly influence both the guide number and coverage/evenness of illumination at the various zoom points. The intent is have similar lighting coverage relative to object frame, but it is hard to say how well that succeeds and whether any expectation of a predictable guide number is warranted based on focal length alone.

...and my Sigma EF610DG Super (just for kicks and giggles)

24mm = 34
50mm = 46
105mm = 61


Steve
09-20-2020, 11:43 PM   #4
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A few more data points from some flashes of mine:

Metz 52 AF-1 38@35mm 52@105mm
Metz 45CT-1 45@35mm

and others I once looked at with only these guide numbers to go by:

Pentax AF 360 FGZII 36@85MM
Shanny SN600FGZ 60@200mm

There is probably a CIPA standard somewhere that defines how to specify flash output, but does anyone use it?

09-21-2020, 12:39 PM - 1 Like   #5
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QuoteOriginally posted by rawr Quote
There is probably a CIPA standard somewhere that defines how to specify flash output, but does anyone use it?
...and even if, CIPA is for Japanese makers.

Guide number is sort of a slippery concept. I don't believe there is an ISO standard, though I just looked at the Wikipedia entry and found its discussion to be VERY comprehensive and pertinent to this thread.

Guide number - Wikipedia

Note: Crop factor is not a consideration and should not be. The focal length aspect of the tables indicates the relative area of illumination only. The actual illuminance for a given distance should be the same regardless of crop.


Steve

Last edited by stevebrot; 09-21-2020 at 12:52 PM.
09-21-2020, 01:03 PM   #6
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The GN page at scantips.com is also very useful...

Understanding Camera Flash Guide Numbers, plus GN Calculator


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09-21-2020, 03:39 PM - 1 Like   #7
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From that article.

"Unfortunately, the optics of flash heads are complex; each manufacture's designs not only have illumination areas that are slightly different, but are the product of differing relative proportions of transmission, diffusion, reflection, and refraction among their optical elements (flash tube, reflector, Fresnel lens, and add-on wide-angle adapter). Accordingly, there is no universal formula for precisely calculating how guide numbers diminish from, for instance, a 105 mm setting to 50 mm or 35 mm settings. "
09-21-2020, 04:00 PM - 2 Likes   #8
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QuoteOriginally posted by stevebrot Quote
In recent years, it has become industry practice to state GN in the specs at the highest zoom setting even though the flash will seldom be used in that manner.
This is one of my biggest pet peeves. Manufacturers are trying to make their flashes seem more powerful than they really are. And since different brands have different max zoom settings, it's almost impossible to do a meaningful side-by-side comparison. My Vivitar 283 has a guide number of 120 (feet) @ISO 100 at its native FOV, which was appropriate for a 50mm lens on FF. With the 135mm concentrating lens attached it goes up to GN174 (feet) @ISO 100. The manual also states an output rating of 2900 BCPS (beam candle-power seconds) that is unrelated to ISO or 'zoom' factor. I wish all units were rated by one standard for output power that was not affected by the sneaky fiddling with zoom factors that modern manufacturers use. As my Dad used to say, "figures don't lie, but liars can figure".

09-21-2020, 07:08 PM   #9
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QuoteOriginally posted by Apet-Sure Quote
My Vivitar 283 has a guide number of 120 (feet) @ISO 100 at its native FOV, which was appropriate for a 50mm lens on FF.
It may be appropriate for a 50mm but has coverage adequate for 35mm. Comparing the Viv 283 to my YN560III at 35mm is 120 vs 127 or 118 for the EF610DG Super. The Viv 283 is very comparable to current product in regards to power.


Steve

Last edited by stevebrot; 09-21-2020 at 07:50 PM.
09-22-2020, 02:50 AM   #10
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QuoteOriginally posted by stevebrot Quote
The GN page at scantips.com is also very useful...
That and the previous link you gave look like some good stuff to get my teeth into. Bed-time reading

QuoteOriginally posted by Apet-Sure Quote
This is one of my biggest pet peeves. Manufacturers are trying to make their flashes seem more powerful than they really are. And since different brands have different max zoom settings, it's almost impossible to do a meaningful side-by-side comparison.
Indeed, You don't need to be Einstein to do the maths for comparison but at least some understanding of what the maker is saying is needed, and I suspect there are many average Joes who look no further than the headline figure ("60" in the case of the Nissin i60A - it is even in the name). There was a time when flash unit names included the GN standardised for a 50mm lens field of view and ISO 100; the Pentax AF160 and AF280T etc for example - even if they were multiplied by 10.

QuoteOriginally posted by swanlefitte Quote
From that article."Unfortunately, the optics of flash heads are complex; ..... Accordingly, there is no universal formula for precisely calculating how guide numbers diminish from, for instance, a 105 mm setting to 50 mm... "
Flash head optics are crude compared with camera zoom lenses. They spill massive amounts of light outside the required field of view in order to achieve an acceptably even level of illumination within it. No doubt the percentage of wasted light varies greatly as the flash zoom angle is changed. For example I have a cheap (but effective) old unit with a zoom head, and the head is just a sliding tube with a fresnel lens and painted silver inside. We cannot expect a precise relationship between flash zoom angle and effective GN.
09-22-2020, 04:16 PM - 1 Like   #11
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QuoteOriginally posted by stevebrot Quote
The GN page at scantips.com is also very useful...

Understanding Camera Flash Guide Numbers, plus GN Calculator
Wow, it's midnight and I have just read through it. I don't think English is his first language, and he repeats himself over and over (I wanted to go back and count the number of times he said "inverse square law" - yeah we got the point ), but it is good, and all you need to know is there. Looks like that site has a lot of other info too.

I think he is saying what I said earlier - that the GN should go up linearly with flash zoom "focal length", but it goes up less than that because of the crude optics of flash units. Here is the crucial part :-
QuoteQuote:
My one regret about the GN calculator is that it doesn't also compute zoom. If the flash zooms, increasing the flash zoom mm number concentrates the power into a brighter smaller beam. We might imagine (if treating its field like focused lens zoom, which it's definitely not), doubling the flash zoom mm should theoretically cover half size dimensions, which is a 1/4 smaller area. The same flash power concentrated into 1/4 area would be 4x brighter intensity, which is +2 EV. Which you'd think the GN calculator could calculate for zoom too, but the flash zoom system does not actually refocus. Speedlight zoom does move the reflector back and forth, but does not change its curve to refocus it. Also the front fresnel lens is molded plastic, which does not refocus with zoom. Most of the range is not focused, and each reflector design may vary individually too.
He makes the point that doubling the zoom focal length (like doubling the camera lens focal length) should concentrate the same light energy on 1/4 of the subject area, raising its illumination by a factor of 4, ie two stops (he uses EV). Plus two stops of illumination would enable the lens f-number to be doubled, which is equivalent to a doubling of the GN.

But because of the crude flash unit optics, in practice, as he goes on to say, the GN at the various flash zoom settings can only be found by experiment, hopefully done by the maker and put in the instruction manual and/or the unit's software. The fact that many of these tables, like the one in my OP and in the Wikipedia article, seem to show a GN proportional to the square root of the flash zoom distance, rather than being linear, is just a co-incidence.

Incidentally I never understood what a "speedli[gh]t[e]" was. I thought it (and "strobe") were American words for "flash unit". But I've learned from these articles that they are brand names for Nikon and Canon hot-shoe flash units

Last edited by Lord Lucan; 09-22-2020 at 04:23 PM. Reason: Clarity
09-22-2020, 05:36 PM   #12
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QuoteOriginally posted by Lord Lucan Quote
Incidentally I never understood what a "speedli[gh]t[e]" was. I thought it (and "strobe") were American words for "flash unit". But I've learned from these articles that they are brand names for Nikon and Canon hot-shoe flash units
Yes...and slang for a full manual flash, something similar to my Yongnuo YN560III with tilt/swivel decent power, and a good range of intensity options.


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09-22-2020, 05:40 PM   #13
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QuoteOriginally posted by Lord Lucan Quote
He makes the point that doubling the zoom focal length (like doubling the camera lens focal length) should concentrate the same light energy on 1/4 of the subject area, raising its illumination by a factor of 4, ie two stops (he uses EV). Plus two stops of illumination would enable the lens f-number to be doubled, which is equivalent to a doubling of the GN.
Well that allows him to invoke the inverse square law. I would have made the point that doubling zoom and distance illuminates the same area at the focus plane. I had initially thought a coefficient could explain your GNs drop off from linear. Might find one by graphing it but it won't hold for other models anyway.
09-22-2020, 07:21 PM   #14
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QuoteOriginally posted by swanlefitte Quote
Well that allows him to invoke the inverse square law. I would have made the point that doubling zoom and distance illuminates the same area at the focus plane. I had initially thought a coefficient could explain your GNs drop off from linear. Might find one by graphing it but it won't hold for other models anyway.
I am of the opinion that flashes don't have focal length come in focal lengths.

Addendum: The zoom settings are labeled as such for convenience sake. What the GN for the 50 setting means is the maximum distance at which a 50mm or longer lens may be used at that setting.


Steve

Last edited by stevebrot; 09-22-2020 at 07:30 PM.
09-22-2020, 07:48 PM   #15
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QuoteOriginally posted by stevebrot Quote
I am of the opinion that flashes don't have focal length come in focal lengths.
I am going off scantips.com.
"We might imagine (if treating its field like focused lens zoom, which it's definitely not), doubling the flash zoom mm should theoretically cover half size dimensions, which is a 1/4 smaller area."

It seems logical to assume the focal length is pinned 1:1 to correspond to area of coverage of said lens focal length.
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