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  #1  
Old 14th December 2009
Oak Lea
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Guide Number

Would someone be so kind as to explain what the guide number referrs to when it is mentioned in conjunction with flashes

many Thanks

John
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Old 14th December 2009
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Re: Guide Number

I'll try my best. Basically it's a guide to the power of your flash so you can work out it's effective maximum range. For example an FL-36 has a guide number of 36. I happen to know that this is measured in metres so at ISO 100 you have a range of 36 metres at F1. You no doubt don't have a lens that wide so you simply divide the aperture you want to use into the guide number e.g. F2 gives you 18 metres, F4 gives you 9 metres etc. But then if you change the ISO to 200 you double the range, to 400 quadruple and so on.

Hope this is clear enough.

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Old 14th December 2009
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Re: Guide Number

many Thanks

well explained

John
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Old 14th December 2009
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Re: Guide Number

It's worth mentioning that the FL-36 (and FL-50) has a zoom facility, so the guide number of 36 is the maximum rating when zoomed to cover the view of a 42mm focal length lens (Four Thirds view, equivalent to 84mm in 35mm full frame). On the FL-36 the guide number drops to 20 when the flash is zoomed for wide coverage (12mm Four Thirds, or 24mm 35mm full frame format).

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Old 14th December 2009
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Re: Guide Number

Those of us old enough to remember the days before flash guns were automatic will recall that one divided the guide number by the distance to give the aperture to use. E.g. at 4 metres 36/4 = f/9 which was then rounded to a useful f/8 as the aperture ring only had click stops!

Note also that if you double the ISO setting then you need to multiply the guide number by the usual 1.4.
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Old 14th December 2009
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Re: Guide Number

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Originally Posted by Adagio View Post
Those of us old enough to remember the days before flash guns were automatic will recall that one divided the guide number by the distance to give the aperture to use. E.g. at 4 metres 36/4 = f/9 which was then rounded to a useful f/8 as the aperture ring only had click stops!

Note also that if you double the ISO setting then you need to multiply the guide number by the usual 1.4.
That's effectively what I was trying to say without making it too complicated - guide number divided by aperture gives the maximum distance - guide number divided by distance gives the required aperture. But you're right about the ISO (I was a bit doubtful about my own advice at the time) - you multiply it by 1.4 and NOT 2 as I stated in my earlier post.
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Last edited by meach; 14th December 2009 at 02:59 PM. Reason: Corrected some typos
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Old 14th December 2009
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Re: Guide Number

This information should be made a sticky or something, it's good handy information to get lost in the deep space of this forum.
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Old 14th December 2009
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Re: Guide Number

Thanks for the help guys

John
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Old 14th December 2009
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Re: Guide Number

There's a small, further wrinkle. As Ian stated, the GN is quoted at a particular focal length. The flash channels the light with its zoom facility according to how wide the lens angle of view is. The Oly flashes are quoted at 84mm (their maximum zoom, 35mm equivalent focal length - I will refer to the 35mmm equivalent for the rest of this post). The Metz flashes are quoted at their maximum zoom (105mm). However, to see what the relative power of the flashes are, you need to compare them at the same focal length. For example, the Metz 48 AF-1 has a GN of 48 (at 105mm focal length). At 85mm (the nearest quoted to the 84mm of the Olympus flashes), it has a GN of only 43. So the Metz has a power in between the FL-36 and the FL-50, not very near to the FL-50, as you might imagine. It does, however, have much better cycle times (recharge time between full-power flashes) than the FL-36.

Further complications arise because the flash zooms in steps - fixed focal length steps, not a continuous variation - in order to cover the area you are viewing through the lens, it must choose a focal length as wide or wider than the focal length of the lens. So the FL-50 zooms in steps of 16mm (with diffuser panel), 20mm (with diffuser panel), 24mm, 28mm, 35mm, 50mm, 70mm, 85mm. If you zoomed to 48mm (35mm equivalent, so 24mm in fourthirds land!), the flash chooses 35mm (the next widest below 48mm).

All fine so far...

Except, Metz flashes were designed originally for APS-C and 35mm cameras, which take pictures in a 3:2 aspect ratio format, not 4:3 like fourthirds. Because of this, the light coverage shape of the Metz flashes is more like the 3:2 aspect ratio, not the 4:3 of fourthirds. To prevent dark areas on the image, the Metz zooms a bit wider than what you might expect. The Metz's zoom positions are: 24mm, 28mm, 35mm, 50mm, 70mm, 85mm and 105mm.

So at a focal length of 36mm, you might expect the Metz to zoom to 35mm - it doesn't, it zooms to 28mm. The good news is that it doesn't just choose the zoom position 1 below what you'd expect all the time (thus scattering light to areas beyond the lens can see), it is more like it takes the lens focal length, knocks off a few mm and then chooses the appropriate flash zoom in the way you would expect.

What this means in practise, is that at certain points in the zoom range, in choosing a 'wider' flash zoom than you would expect, the effective GN of the flash is also reduced, so it isn't quite as powerful as would be ideal.

The net result of this is, if I think I would like to get a FL-36R, I would buy the Metz 48 instead - slightly more power (but not as much as you'd think because of the reasons above), better cycle times, modelling light and about the same money (the metz may even be cheaper). If you need power, the FL-50R and Metz 58 are about the same. The Metz has a few more features (secondary reflector and modelling light), but is less user-friendly (buttons and slow-to-access menus vs nice thumb wheels). If you aren't worried about the remote-control triggering, picking up a second-hand FL-50 is the best bet. In all cases, the build quality and user ergonomics are better on the Olympus flashes - but there isn't a massive difference to the Metz flashes. The Olympus flashes zoom a bit wider than the metz ones (even with the built-in diffuser), so if you habitually use very wide angles, the Oly ones are better.

Andy
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Last edited by oly_om; 14th December 2009 at 08:54 PM.
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Old 14th December 2009
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Re: Guide Number

Good stuff Andy,

"The net result of this is, if I think I would like to get a FL-36R, I would buy the Metz 48 instead - slightly more power (but not as much as you'd think because of the reasons above), better cycle times, modelling light and about the same money (the metz may even be cheaper)." - I think it's relevant to add that the 36R is 260g + 2xAA batteries and the 48 is 340 + 3xAA ie it's quite a bit bigger and heavier.

I have been known to take out a 36R over a 50R from time to time (although I prefer all 3)

Just for completeness.
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Old 14th December 2009
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Re: Guide Number

Quote:
Originally Posted by photo_owl View Post
Good stuff Andy,

"The net result of this is, if I think I would like to get a FL-36R, I would buy the Metz 48 instead - slightly more power (but not as much as you'd think because of the reasons above), better cycle times, modelling light and about the same money (the metz may even be cheaper)." - I think it's relevant to add that the 36R is 260g + 2xAA batteries and the 48 is 340 + 3xAA ie it's quite a bit bigger and heavier.

I have been known to take out a 36R over a 50R from time to time (although I prefer all 3)

Just for completeness.

Fair comment. The slow recycle time rather than the weight (it's on an E-3 anyway!) is the deal-breaker for me. If you want to go as light as possible, FL-36 is better.

Andy
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Old 14th December 2009
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Re: Guide Number

Just out of interest...

A long forgotten feature of the Guide Number system, was that manufacturers were able to produce a GN lens. This special lens capability was provided by setting the power of your flash system directly on the lens, which by changing the focus distance automatically adjusted the aperture to produce the optimised exposure.

I have this feature on my Olympus 35RD and would often attach a 20 GN flashgun and set the lens to 20 GN and effectively have automatic flash exposure. As soon as the thyristor controlled guns appeared the system was redundant as the aperture was then set to various fixed values and the gun adjusted it's flash dependant on the distance to a reflective surface...
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  #13  
Old 14th December 2009
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Re: Guide Number

Quote:
Originally Posted by Graham_of_Rainham View Post
Just out of interest...

A long forgotten feature of the Guide Number system, was that manufacturers were able to produce a GN lens. This special lens capability was provided by setting the power of your flash system directly on the lens, which by changing the focus distance automatically adjusted the aperture to produce the optimised exposure.

I have this feature on my Olympus 35RD and would often attach a 20 GN flashgun and set the lens to 20 GN and effectively have automatic flash exposure. As soon as the thyristor controlled guns appeared the system was redundant as the aperture was then set to various fixed values and the gun adjusted it's flash dependant on the distance to a reflective surface...
I guess it's mechanically coupled a bit like the rangefinder mechanism was.

Andy
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Old 14th December 2009
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Re: Guide Number

In good old imperial times the GN was 3.3 times larger than today <g>
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