Converting a guide number to f-stop or vs.

[villiam_shahvosian1]

Hi everyone here it goes my light meter @10' distance from a light

source shows, lets say f-11 4/10. How i convert the given f-stop to a

guide number. Or i see a guide number of 150 in feet. How do i convert

that to a f-stop. I know the equation and all that but you can't just

tell me for the first one you have to multiply 10 to f-11.4 (10'*11.4)

because that won't come out right as far as i know. So what is the deal?

[steven_muncy]

The deal is decimal f/stops. They aren't marked on lenses so we tend not to think in those

terms. I'm sure some of the newer cameras with TTL flash can actually use f/stops in 1/

10th increments but most can't.

 

Most lenses run the typical series with only the maximum opening being out of order such

as a 70-210 F3.8. That .8 is not the same as the .8 from a decimal reading light meter.

 

11.4 (decimal reading) is actually f/13 so 10 x 13 is 130 GN.

 

GN of 150 at 10 feet is f 15 which would be f/11 plus 8/10 stop.

 

Remember that guide numbers are just that. A little testing should give you a very good

personal guide number that works with your equipment and methods.

[villiam_shahvosian1]

Thanks Steven, but I was hoping if you could of give me some equation to which you go to GN 130 or to f-11 plus 8/10. But before anything this is how I do it but I just want to make sure that I am right. For example for f-11 4/10 simply I multiply f-8 to square root of 2 to get to the next f stop which comes to f11.314 then I multiply f11.314 to square root of 1.4 (because of 4/10) and that comes to 13.386?. and then multiply to 10 which ends up to GN 133.8 or for guide number 150 I first divide 150 to 10 that equals to 15. Second I divide 15 to f-11.314 (why 11.314? I can see that 15 Is between f-11 and f-16) which comes to 1.325. Finally the square number of 1.325 comes to 1.75 and the nearest number would be 1.8 so GN 150 is equal to 11 plus 8/10. Am I on the right track or is there more simpler way?

[steven_muncy]

That's esentially correct. As long as you understand the doubling/halving of each

exposure step it always has a sq rt of 2 in the equations. I'm really wondering why you

need such precise calculations.

[steven_muncy]

Also found this. It should take care of most of your needs.

 

http://www.riversidecardiology.com/fstop/f-stop.htm

[jim_gifford]

Villiam,

 

<<For example for f-11 4/10 simply I multiply f-8 to square root of 2 to get to the next f stop which comes to f11.314 then I multiply f11.314 to square root of 1.4 (because of 4/10) and that comes to 13.386?. >>

 

Yes, yes, of course. But the unfortunate thing is, I tried to set f/13.386 on my Nikon and wound up at f/13.387 by mistake and the shots were all a little dark, drat the luck.

 

So I have introduced the TLAR correction factor into my calculations. TLAR is an acronym for "That Looks About Right." If the math starts with an indication of f/11.4 I just skip all those other calculations entirely, set f/11 on the lens and, with mouth and tongue held Just So for proper upper body muscle pretensioning, give that aperture ring a bit of a nudge toward f/16. That's it, most of the time.

 

If it actually snicks into the f/16 detent, I have to hold mouth and tongue Just So again and go the other way, until the ring rests between f/16 and f/11.

 

That TLAR system should work most of the time for you, unless you are calculating the optical characteristics of the next generation of NRO photoreconnaissance satellites or something. And think of all the graphite you'll not be spraying out of the rails of your slide rule.

 

Be well and have fun,

[helenbach]

Jim,

 

Don't you have the Nikon AMV-1/4 (AMV-4/1 in the USofA) vernier aperture control unit? They come up on eBay every now and then. I use mine all the time now that I've determined that the true speed of Tri-X is ISO 387.26 (+/-100).

 

Helen

[peter zisson]

Villiam,

 

Here is my understanding of the whole guide number "thing." I may be wrong, but I am not uncertain. <grin>

 

The basic equation for the use of the GN is GN / distance = f stop. Note that since the flash duration is very short, shutter speed is not considered.

 

This would all be a piece of cake if the flash manufacturers' marketing departments were not involved. After all, a higher GN is better, right?

 

Where marketing gets involved is in how the GN is calculated. The first thing to consider is the distance units. My Metz 45 CT-4 has a GN of 45 (hence the model number). This doesn't sound like much until you realize that the distance units are meters. The Gn in feet is 148, a much more satisfying (to the marketing types) number.

 

The next factor is the angle of coverage. My Metz (and I use the Metz as an example simply because I have one and can refer to the manual as I type this - no endorsement intended) specifies its GN "at 35 mm." This means that the light output covers the same area as seen by a 35 mm camera with a 35 mm lens. If I use a 80 mm portrait lens the GN, for f-stop calculation purposes, does not change. I am photographing a smaller area, but some of the flash's light is lighting areas outside the edges of my picture.

 

Recently, with the advent of auto-zoom flash units, I am seeing advertized GN's of x at y mm, where y is the longest zoom the flash can cover. For example, say Brand X offers a flash with a GN of 150 (feet) at 105 mm. Is this flash as powerful as a Metz 45 series? Well, if you are shooting with a 105 mm lens, yes, but there is a catch. Remember that the Metz has a GN of 148 @ 35 mm. If I put a 35 mm lens on my camera and use Brand X's flash the head zooms out to light the whole area covered by the lens. The effective GN for Brand X's flash now becomes 17!! The calculation for this is:

 

150 / (105/35)^2

 

The third factor in setting the GN is the speed of the film used for the calculation. So far, I personally have not seen any flash manufacturer specify a GN based on other than 100 speed film, but that is not to say it could not happen. Since the reason for the GN is to tell you how much light to let pass to the film plane, based on distance to subject and camera aperture, the GN will change with different film speeds. For the Metz, the GN (feet) for 200 ISO film is 296; for 50 ISO film it is 74.

 

Confused yet? <g>

 

So here is how I use the GN (on the rare occaisions where I actually set the flash for manual exposure). Subject at 20 feet, lens 85 mm. Lens is longer than 35 mm and the flash doesn't zoom, so GN 148 is still valid. 148/20 = f 7.4. Set camera for f 8 - close enough. But wait! I'm shooting Velvia 50, which is one stop slower than ISO 100, so I set the lens to f5.6 and take the picture. Sure, the film will actually be underexposed a little, but if the shot is that critical I would bracket anyway.

 

Happy shooting.

 

Peter

[villiam_shahvosian1]

Thanks Peter you are very right about the angle of coverage of the flashes and good advise how to calculate them, and Steven this question rose because of deciding which studio flash to buy. Strobes come with different GN and finlly i came up with this calculation to compare strobes. I didn't decide to be so precise but that's how this equation does but if there is any other equation that is simpler by all means please let me konw.