Guide Number Flash with Modern Cameras

I think @BeBu Lamar wants a camera like those old rangefinders, where you have a ring to dial in your guide number and end with a mechanically computed distance appropriate aperture setting.

I don't know any such (modern!) camera. Talking Pentax / Samsung, Leica M, elderly Fujis & EOS. - Maybe something else exists?

Correction: "Whatever takes a medical Nikkor"

Something like this? Link Understanding Camera Flash Guide Numbers, plus GN Calculator

Sandy I don't want to learn about the GN I want to implement it in a modern camera.

Jochen, yes similar to those cameras of old but rather a mechanical system it's easy to do it with software.

Perhaps I would ask Andrew as he may be able to obtain Nikon source code for the firmware and make a change for me. I don't know how much he would charge me for that. Don't think a couple stacks of pancakes would do.

A tip of the hat to the bloke who conceived the Guide Number! Clever – by trial and error, ascertain the f-number that delivers a satisfactory exposure, multiply flash-to-subject distance, and voila! – You have the Guide Number. This humble method takes into account the complicated association that light falls off with distance following a law in physics called the Law of the Inverse Square. A practical way to express this law – double the distance lamp-to-subject, and the arriving light falls off arriving at only 25% of its former strength. How can the humble GN perform such complicated math? The brightness of the flash declines based on the square of the distance. Conversely, the light that will traverse the lens aperture decreases per the square of the f-number. Thus both are tamed by simple division. In other words, if the guide number is known, the f-number is calculated by simple division (distance ÷ GN = f-number).

If you long for the good old days, we fumbled a lot, but eventually got the job done. Nowadays, most rely on the logic of the modern camera; we call it TTL (Through-The-Lens metering). This modern marvel evaluates the light reflected from the subject from the flash. This measurement is done in just milliseconds at the beginning of the flash exposure. The camera’s logic determines the correct exposure under actual working conditions. The TTL system is likely about 1000% more accurate than the venerable GN method. Who of us wants to drive a stick shift with clutch car? Only those who reminisce, but when you get a sprained knee, you wish you had an automatic transmission.

I have seen the equivalent of guide numbers implemented in Nikon flash units. Dial in the ISO and you get a table of distance v f/stop. As an alternative, many of these flash units had an Auto mode. You dial in ISO, and a sensor in the flash determines the exposure based on light reflected from the subject. The latter works very well on a manual camera, and works better on a Hasselblad than ostensibly coupled flash units.

When I use studio flash units, I use a flash meter (Sekonic 708) to measure the ratios and exposure, with radio triggers. In the day, I taped a dope card to the flash head.

I have not seen a modern camera which adjusts the aperture based on the distance and a guide number. That doesn't mean it couldn't be done, but there is little incentive to do so. If you need an on-camera flash, the best solution is one designed to operate with that camera, or built-in.

Is there any DSLR or MILC with a flash guide number mode? That is once you have determine the true GN of the flash you simply enter the GN into the camera and it would automatically set the aperture based on the focus distance and ISO?

Years ago several companies made interchangeable lenses with this feature built in, including Nikon (GN Nikkors), Sigma (Flash Auto), Topcon (GN-Topcors) and Zeiss (Contarex Blitz Planars). You set the flash guide number on the lens, and as you focus the aperture is adjusted automatically. All these lenses can be used on some modern digital cameras with suitable adapters.

There was a 45mm f/2.8 GN Nikkor that showed (coupled?) the aperture needed as the focus ring was moved. They're rare and consequently pricey for an average-performing Tessar type lens.

AFAIK, there's no camera that does what you want. However, why would such a function be needed today? Aside from the fact that all flash makers' guide numbers are just lies and out by at least one stop; many flashes have auto-aperture mode, which works extremely well as long as the detector cell is kept pointed at the subject.

Use of the Guide Number (should you be naive enough to believe it) went out with the Ark. Thyristor control of flash power and said AA mode saw to that.

There was a 45mm f/2.8 GN Nikkor that showed (coupled?) the aperture needed as the focus ring was moved. They're rare and consequently pricey for an average-performing Tessar type lens.

 

AFAIK, there's no camera that does what you want. However, why would such a function be needed today? Aside from the fact that all flash makers' guide numbers are just lies and out by at least one stop; many flashes have auto-aperture mode, which works extremely well as long as the detector cell is kept pointed at the subject.

 

Use of the Guide Number (should you be naive enough to believe it) went out with the Ark. Thyristor control of flash power and said AA mode saw to that.

I said in the first post, I would have to find out the true GN. It's simple with a flash meter. If I want to fill flash on a person face it would be as accurate as a flash meter but much quicker. TTL or any technique that measure reflected light wouldn't be as reliable.

I said in the first post, I would have to find out the true GN. It's simple with a flash meter. If I want to fill flash on a person face it would be as accurate as a flash meter but much quicker. TTL or any technique that measure reflected light wouldn't be as reliable.

- Auto aperture mode can easily be used for fill flash; you simply set the aperture on the flash wider than the actual lens aperture. That way the flash gives less light than is needed for a full exposure.

Example: You're shooting a subject that needs an ambient (lit side) exposure of f/8 at whatever shutter speed. So you set the flash to f/4 as its auto-aperture, which automatically gives you a fill 2 stops lower than the key - simple!

Life gets more complicated if you use a brighter fill (artificial-looking IMO). A one-stop-under frontal fill will add significantly to the key light, and therefore you'll have to adjust the lens aperture downwards. This goes equally if you simply use a guide number calculation. However, there will be some degree of automatic adjustment if AA mode flash is used. Not fully IME but certainly closer to a correct exposure than simply whacking a guide-number's worth of extra light at the subject.

At least my SB-800 has a "reverse GN mode" where-if I remember right-it adjust the flash output based on distance information(from D lenses) and the selected aperture. It's not QUITE what you're looking for, but I've used it in certain situations and found that it works well. If Nikon can do that, one would think that they could keep the flash at a constant power level and adjust the aperture based on the distance(like the GN-Nikkor).

As far as the GN-Nikkor goes-I don't know where the "rare" perception comes from. It was the least expensive Nikkor F-mount lens back in the day, and was fairly popular-especially for flash photography. At any given time, there are a handful of them on Ebay, and you probably won't pay over $200 for an AI version. Mine is a "hacket" AI conversion, and I paid around $150 for it-it's in nice condition other than that. I understand that the cams CAN wear when the GN feature is used, although I've yet to see one advertised where the feature didn't work. BTW, I think most folks are buying it these days for the fact that it's the second smallest F-mount lens(behind the 45mm f/2.8 AI-P) and most folks don't use the GN feature. Also, it's worth mentioning that the focusing ring turns in the "wrong" direction to make the GN feature work.

At least my SB-800 has a "reverse GN mode" where-if I remember right-it adjust the flash output based on distance information(from D lenses) and the selected aperture. It's not QUITE what you're looking for, but I've used it in certain situations and found that it works well. If Nikon can do that, one would think that they could keep the flash at a constant power level and adjust the aperture based on the distance(like the GN-Nikkor).

 

As far as the GN-Nikkor goes-I don't know where the "rare" perception comes from. It was the least expensive Nikkor F-mount lens back in the day, and was fairly popular-especially for flash photography. At any given time, there are a handful of them on Ebay, and you probably won't pay over $200 for an AI version. Mine is a "hacket" AI conversion, and I paid around $150 for it-it's in nice condition other than that. I understand that the cams CAN wear when the GN feature is used, although I've yet to see one advertised where the feature didn't work. BTW, I think most folks are buying it these days for the fact that it's the second smallest F-mount lens(behind the 45mm f/2.8 AI-P) and most folks don't use the GN feature. Also, it's worth mentioning that the focusing ring turns in the "wrong" direction to make the GN feature work.

It's actually a good idea although some how I think the distance information from Nikon lenses is not reliable somehow. I never saw it in the EXIF data or somehow display it and in the case of the GN mode of the SB-800 you have to enter the distance into the flash. It can't transfer the distance information from the lens and camera.

You have the Guide Number

Excellent summary!

And, as said, there're plenty of excellent reasons why it is no longer used except by the occasional enthusiast trying to put old equipment back into us.

And, as said, there're plenty of excellent reasons why it is no longer used except by the occasional enthusiast trying to put old equipment back into us.

I find it interesting that they've only ever really been used for on-camera lights...which I guess is the strength of the GN system, but it also gets into a lot of guesswork and estimation if you're bouncing. "Auto" flash-(I often call it Auto Thyristor since I learned and used it extensively on Vivitar 283s and other Vivitar equipment before I moved into more advanced strobes) is still a wonderful system that works pretty reliably. I know that back in the D1/D100 era, many folks got better results using it as opposed to the awful D-TTL/SB-28DX system on these camera. I should mention the fact that the SB-28DX is otherwise a solid flash-it's just that D-TTL is a terrible implementation. I've had good results with eTTL(Canon) and iTTL(Nikon) but I still think plain old film TTL may be better-although of course the advantage to digital is that you can adjust on the fly.

Flashbulbs make it REALLY fun as the full GN chart list different GNs depending on the reflector size and finish, plus of course the shutter speed for focal plane shutters(or at least on long-burn bulbs like the GE #6). Flash with bulbs gets REALLY fun on the Nikon F since you have three or four different flash sync settings(not just a simple M/X). You can use a #6 bulb on the "green dot" sync setting at all "green" shutter speed, but of course the GN changes according to the shutter speed. I have one of the "official" Nikon flash guns, and fortunately the calculator dial does help make some sense out of these also.

In any case, I've never seen a studio strobe that was rated in terms of GN. I always figure that they just assume you're going to stick a modifier on it and have it pointed in any given direction, plus use it with more than one. Then, of course, back in the old days you'd use a flash meter and probably confirm with a Polaroid. Now I don't bother with a flash meter if I'm shooting digital, and if I'm using film under the strobes I'll confirm what my flash meter says with digital. I'm sitting on a half dozen packs of in-date FP-100C but prefer to save them for things other than checking flash exposure/lighting.

I find it interesting that they've only ever really been used for on-camera lights...which I guess is the strength of the GN system, but it also gets into a lot of guesswork and estimation if you're bouncing. "Auto" flash-(I often call it Auto Thyristor since I learned and used it extensively on Vivitar 283s and other Vivitar equipment before I moved into more advanced strobes) is still a wonderful system that works pretty reliably. I know that back in the D1/D100 era, many folks got better results using it as opposed to the awful D-TTL/SB-28DX system on these camera. I should mention the fact that the SB-28DX is otherwise a solid flash-it's just that D-TTL is a terrible implementation. I've had good results with eTTL(Canon) and iTTL(Nikon) but I still think plain old film TTL may be better-although of course the advantage to digital is that you can adjust on the fly.

 

Flashbulbs make it REALLY fun as the full GN chart list different GNs depending on the reflector size and finish, plus of course the shutter speed for focal plane shutters(or at least on long-burn bulbs like the GE #6). Flash with bulbs gets REALLY fun on the Nikon F since you have three or four different flash sync settings(not just a simple M/X). You can use a #6 bulb on the "green dot" sync setting at all "green" shutter speed, but of course the GN changes according to the shutter speed. I have one of the "official" Nikon flash guns, and fortunately the calculator dial does help make some sense out of these also.

 

In any case, I've never seen a studio strobe that was rated in terms of GN. I always figure that they just assume you're going to stick a modifier on it and have it pointed in any given direction, plus use it with more than one. Then, of course, back in the old days you'd use a flash meter and probably confirm with a Polaroid. Now I don't bother with a flash meter if I'm shooting digital, and if I'm using film under the strobes I'll confirm what my flash meter says with digital. I'm sitting on a half dozen packs of in-date FP-100C but prefer to save them for things other than checking flash exposure/lighting.

I actually never use the GN. But I think it's useful to automate it but not doing the calculation yourself or determining the distance yourself. I guess at the power of the flash I have to set and adjust it from there. The pictures that I sent you of the A-1 paint loss I took that setting the my flash on tripod, bounce it to the ceiling. Set the 55mm f/3.5 micro to f/11 and set the flash at 1/4 power and I got that shot. Exposure is good enough don't you think with the first guess.

I actually never use the GN. But I think it's useful to automate it but not doing the calculation yourself or determining the distance yourself. I guess at the power of the flash I have to set and adjust it from there. The pictures that I sent you of the A-1 paint loss I took that setting the my flash on tripod, bounce it to the ceiling. Set the 55mm f/3.5 micro to f/11 and set the flash at 1/4 power and I got that shot. Exposure is good enough don't you think with the first guess.

- Jeez Bebu, you do seem to like making things difficult for yourself.

AA (or auto-thyristor) mode works on or off camera. All you need to do is ensure the flash sensor remains pointed at the subject. IME it works near perfectly every time - much more accurately and reliably than over-complicated and over-engineered i-TTL.

Point the flash into a reflective brolly with the AA sensor towards the subject; it works.

Put the flash on camera and bounce it off a wall or ceiling with the AA sensor towards the subject; it works.

Take it outdoors and use frontal flash as fill; it works.

Put a flash with AA mode on a film camera; it works.

On a digital camera set to A or M mode; it works.

AA mode just works! And it needs very little fiddling with camera or flash settings, and very little brain power.

If you don't already have a flash with AA mode, they can be picked up from around the equivalent of $5 US. Worth it to pay extra and get a Nikon SB-24, 25, 26, 28, or 800 though IMO. They're reliable, powerful and have a decently short recycle time, plus a good range of settings. Pick of the bunch would be the SB-25 for me. The SB-24 has a more limited manual power range, while the SB-28 onward made it inexplicably awkward to change ISO speed, and the SB-26 appears to be a bit of a rarity.

PS. 200 bucks for a crappy little f/2.8 Tessar? I don't care if it is a trendy 'pancake' shape, or even calculates your tax return for you; that's too much!

PS. 200 bucks for a crappy little f/2.8 Tessar? I don't care if it is a trendy 'pancake' shape, or even calculates your tax return for you; that's too much!

It beats $400 for the 45mm f/2.8 AI-P.

Granted, if one really wants a manual focus Nikkor "pancake" the bargain is the 50mm Series E f/1.8 at around $50-75. It's optically good enough that Nikon still uses the same formula in the 50mm f/1.8D. Of course, it is the largest of what I'd consider Nikon's 3 "pancake" lenses, but it's certainly not huge(it's a lot smaller than the 50mm f/1.8 AI-S or 50mm f/2) and is lighter than either of the 45mms.

It beats $400 for the 45mm f/2.8 AI-P.

 

Granted, if one really wants a manual focus Nikkor "pancake" the bargain is the 50mm Series E f/1.8 at around $50-75. It's optically good enough that Nikon still uses the same formula in the 50mm f/1.8D. Of course, it is the largest of what I'd consider Nikon's 3 "pancake" lenses, but it's certainly not huge(it's a lot smaller than the 50mm f/1.8 AI-S or 50mm f/2) and is lighter than either of the 45mms.

- Sorry, but I really can't see the attraction of 'pancake' lenses (stupid name aside). I lug around either a D7200 or D800, and a cm of extra lens protrusion and a few grams more weight make almost zero difference to the handling... apart from having a decent sized focus grip that doesn't hide under the prism overhang.

Now if somebody made a full-frame 16-80mm zoom with a constant f/2.8 aperture that stuck out no further and was no heavier than a 50mm f/1.8, then that might be useful.

- Sorry, but I really can't see the attraction of 'pancake' lenses (stupid name aside). I lug around either a D7200 or D800, and a cm of extra lens protrusion and a few grams more weight make almost zero difference to the handling... apart from having a decent sized focus grip that doesn't hide under the prism overhang.

 

Now if somebody made a full-frame 16-80mm zoom with a constant f/2.8 aperture that stuck out no further and was no heavier than a 50mm f/1.8, then that might be useful.

Let alone that they look ugly on a D800. The 50mm f/2 or f/1.8 is a lot cheaper and I doubt that the 45mm f/2.8 AI-P is any sharper.

Let us get back to the Guide Number for a moment. It is just that, a GUIDE. That number is determined for a specific situation, usually for a defined room size (whatever that happens to be, they never tell you) and a specific reflectivity (a room where every surface is 18% grey or whatever they choose.) A smaller light colored room would give you overexposure, a larger room with dark walls would give you underexposure. Go outside at night and you are lucky to get much light on your subject at all by GN calculations.

I don't know what you shoot with; as a matter of fact I honestly don't know whether you shoot at all, or whether you just post on various fora.

A lot of my very early professional work was shot with a 200 Ws Matador all manual flash with three power settings. In those days you lashed it to your Hasselblad, which for reception work typically had a Distagon 50 attached to it. A Distagon 50 at 2.5 meters (that's about 8 feet, but I grew up metric) would nicely cover a couple or a small group, and at f/8 you had sufficient DOF for some distance misjudgment. To determine the power level you found what seemed similar to a more or less typical reception venue room. You tape a large grey scale target to a wall, put your flash on a stand 8 feet from the wall and connect it to the Blad with a long PC cord. Then you put a Sonnar 250 on the camera and shoot the target at various power levels and apertures while keeping track of which frame is which. Develop the film and examine the frames. Even though the event will be shot on color negative film it was best to shoot the target on slide film because of its narrower latitude. Then you do the math to account for different ISO sensitivities and then you find the optimal distance/aperture combination for. Then you largely ignored your conclusion and used a power level that would technically overexpose by 1 to 1 1/2 stops. Now, remember, you are shooting color negative film here, and that tolerates overexposure really well, underexposure, not so much. If the Maitre'd hadn't been turning the room lights down to save on the electric bill (he called it making the event more romantic) you could drag the shutter to get some of the background to register on the film.

Now I have an experiment for you. That is predicated on you having some digital photographic equipment, actually using it, and really know how it works.

- Set your camera to JPEG only. No RAW!

- Set your ISO manually, I suggest 200.

- Set your Speedlite to manual power (only Speedlites had guide numbers.) I suggest you start with 1/8 power.

- Set the camera to manual exposure, shutter speed 1/160 sec.

- Go into your kitchen, photograph a victim 6 feet away. Adjust flash power until you have what seems like a correct exposure; look at the histogram! Keep all settings!

- Take subject into bathroom, again shoot at 6 feet.

- Go outside after dark, again shoot at 6 feet with the same settings as before.

- Compare the results. You will likely see significant illumination differences on the subject in the different scenarios.

- Congratulations! Hopefully you have now realized why the industry has abandoned guide numbers.

Photographic technology moves forward. That doesn't mean that all practitioners have to move at the same speed. There are still people out there shooting film on view cameras, using light meters and bellows factors tables to determine their exposures. Everyone moves at their own speed.

When I bought my first SLR TTL light metering had just been introduced. People a few years ahead of me were still using hand held light meters. Soon we got automatic exposure, then multiple metering patterns. And with program exposures we got different values depending on the focal length in use. Then we got TTL flash exposure determination, eventually with multi segment metering. Finally we got hit with the digital revolution and also radio controlled multi flash capabilities.

All of these represent advances. You may not use them, you may not like them. But every last one of them has been embraced in the marketplace. And I have happily gone along for the ride. Unless I am doing studio work I set my canon camera to P, Auto ISO, and my Speedlite (and even my compatible studio strobes occasionally) to ETTL II. Obviously, if I don't like what I see I will adjust, or even revert to manual control. This approach is working very well for me. That, however, doesn't mean that it will work for everyone else. Work whichever way you feel will give YOU the best results!

Well, there's a 'guide', and then there's the madey-uppy, stupidly inflated numbers that speedlight manufacturers publish.

You'd have to photograph the subject in a mirror-lined cupboard before the published GN gave an anywhere near correct exposure.

It would be nice if the ISO actually published the methodology for GN determination - without demanding a ludicrously high fee for the specification.

It would also be nice if there was some consistency between makers that allowed direct comparison of flash 'power'.

Is there any DSLR or MILC with a flash guide number mode? That is once you have determine the true GN of the flash you simply enter the GN into the camera and it would automatically set the aperture based on the focus distance and ISO?

Some flashes says in its screen the f number once you have focused the lenses ¿Does it not help?

Some flashes says in its screen the f number once you have focused the lenses ¿Does it not help?

No!