How to test the real guide number of a hotshoe speedlight?

Discussion in 'Lighting Equipment' started by sunnym|1, Dec 17, 2014.

  1. I have bought a hotshoe speedlight recently. I think its max. guide number is not as real as marked. Is there an easy way to figure out the real GN?
  2. Not one hotshoe speedlight has a true GN as claimed by its maker. They're all, without exception, at least one stop less powerful than claimed. Doesn't matter if the flash is made by Canon, Nikon, Metz, Sunpak, Vivitar or whoever. Maker's GNs are basically all lies.
    "Is there an easy way to figure out the real GN?" - Yes, buy yourself a flashmeter Sunny and take a reading with it 1 metre from the flash in a dimly lit room. Whatever F-number you get is the GN. Always best to double check by taking a reading at 2 metres distance as well. That should be two stops lower. The beam of the speedlight needs to be fairly accurately pointed at the incident dome of the meter even to get within 1 stop of the maker's rating.
    BTW, my above statement about false maker's claims is backed up by measuring dozens of different makes and models of flash with 3 different makes of flashmeter. The meters all agree well within 1/3rd of a stop with each other, and none of them confirm the stated GN of a single flashgun. One stop under is the norm. Most top-of-the range hotshoe speedlights will top out at a GN of 28 (m/100 ISO) with a 50mm head setting. Second tier models deliver a GN of 20 to 22.
  3. "I think its max. guide number is not as real as marked."​
    Every flash I've owned and tested, both with a flash meter and empirically, has performed within 1/3 EV of manufacturer's ratings at full output. It's possible some do not meet the nominal specs, but I haven't had any disappointments. From my Nikon SB-800 through various Vivitar and even a relatively ancient Canon AB-46, all have tested and performed right at or very close to nominal specs. Even an old Sears brand flash I bought in the 1970s and which survived being dunked in a river during a canoe trip in the 1980s still tested at full nominal output per my Minolta flash meter when I tested it a few years ago.
    "Is there an easy way to figure out the real GN?"​
    Sure, just set the camera exposure settings - aperture and ISO - appropriately for the nominal flash GN and shoot it at a standard distance. Google around for the methodology - there are many sites with this posted.

    Shoot raw to minimize the influence of in-camera JPEG adjustments - some cameras offer quite a bit of help to fix under and over exposure, so JPEGs may not accurately indicate flash accuracy. Open the raw files in a viewer than doesn't automatically default to the in-camera JPEG settings. If you shoot a white wall or primarily light subject and the histogram is a single hump, more or less evenly distributed, and centered or slightly biased toward the right, the flash guide number is pretty close to spec.

    If you use a flash meter be sure to use the flat diffusion disk, not the dome. The dome is often recommended, even in flash manufacturer's instruction manuals, for most practical applications - portraits, etc. But some manuals, including those for Minolta flash meters, will also specify that the flat diffusion disk is recommended for evaluating the guide number - it minimizes the effect of room reflections. The dome diffuser can underestimate the GN significantly - not quite a full stop, but it's not unusual to be 1/3 or more off from nominal.

    The only flash I own that deviates significantly from spec is an old Olympus PS-200, originally intended for the Olympus 35 RC compact rangefinder. The PS-200 nominal guide number is conservative and actually appears to be 1/2 EV or more brighter than nominal specs, estimating empirically on my Nikon D2H. My flash meter battery is dead at the moment so I can't test it to be sure. Some guesswork is involved because the PS-200 GN was presumably based on the 42mm lens on the Olympus 35 RC, and the GN chart on the flash is estimated for ASA (not ISO) 80-100 film.

    Some of these older cameras used a clever distance based "auto" flash exposure adjustment system with all-manual fixed output flash units. Adjusting the focus ring also adjusted the aperture to suit the distance for the fixed output flash.

    Nikon's current high end flash and dSLR systems have a clever GN flash mode that helps minimize variations that can occur even with the best TTL flash with white or black backgrounds, strong backlighting or subjects against deep shadows.

    As I wrote in another thread on a similar topic:
    A 1/3 deviation from nominal specs is possible, considering acceptable tolerance for manufacturing. A full stop deviation is unlikely. Reputable manufacturers like Nikon do not pull guide numbers out of thin air or out of magician's hats. Guide numbers are determined according to ISO standard 1230:2007; and ISO 2827:1988. If Nikon deviated significantly from the ISO standard, their manual flash calculator and GN flash methods were be useless. In practical application they're actually quite accurate.
    As with ISO determination of film speed and sensitivity of electronic recording media to light, the standards specify methodology, which manufacturers follow in order to meet the ISO standards.
    And as with film speeds, sensor sensitivity, etc., there may be some variations from nominal specs, particularly in real world applications that don't involve lab testing methodology per ISO standards.
    But when flash units from Nikon and other reputable manufacturers are tested with accurate flash meters or according to the ISO methodology, the nominal guide numbers are generally within acceptable tolerances. But if you're not photographing test charts it may be appropriate to adjust the flash and/or camera exposure settings to suit a given scenario.
    I've tested every flash I own, from the oldest Canon AB-46 and Olympus PS-200, through the Nikon SB-800, and a few Vivitars and other off brands. All tested within 1/3 EV of nominal specs, both with a flash meter and in empirical evaluation of the film and raw files.
    But the appropriate flash exposure for, say, a document on white paper may not be ideal for a gray tabby cat against a dark carpet. The guide numbers aren't incorrect. They're just reference points, starting points from which we determine the ideal or preferred flash exposure.​
  4. Here is the procedure that's been in every book on flash photography since about 1950:

    1) Put your flash on a stand or on your camera with the camera on a tripod.

    2) Sit a person (or stuffed animal or anything of average tones) on a stool or chair.
    3) Measure and adjust the distance so that the flash is exactly 10 feet from the subject.
    4) Set your ISO at a standard full-stop number like 100, 200 or 400. (This is so it will be easy to do the math to translate to other ISOs later; in the film days you chose a film of a given speed rather than setting ISO)
    5) Make a series of index cards with f-stop numbers from 2 to 32 corresponding to the f-stops on your lens.
    6) With the flash at full power and the flash and camera on full manual, make a series of exposures starting with f/2 (or the widest aperture on your lens) down to f/32 (or the smallest aperture your have. Put the card in each picture to help keep track.
    7) Ideally, do all of this in an average sort of room, rather than one with all white or all black walls, so that your exposure is based primarily on the flash and not the reflections or lack of reflections from the walls.
    8) In the old days, develop the film normally and look at the negatives (or better yet slides) to see which frame you like best as the "correct" exposure. Today, look at the images on your LCD (or better yet on a properly calibrated computer monitor) and decide which you like best, perhaps with the help of the histogram.
    9) Once you know which f-stop gave "correct" exposure, multiple times 10 (the 10 feet your carefully measured out) and that's your guide number for the ISO you were using. For example, if f/8 was the best and you were at ISO 100, then the flash unit's ISO 100 guide number is 80. For ISO 200 it is 160, for ISO 400 it is 320 and so forth.
    10) In the future, measuring the distance from the flash to subject and dividing it into the guide number will give you mathematically perfect exposure. (Although you might still need to open up a touch for dark subjects or close down for light subjects.)

    This is the way I did it before I could afford a flash meter, and the way many, many photographers have done it for decades.
  5. It seems to me folk are over-complicating it. Guide numbers are inherently simple things. Assuming you're using the guide number in metres (ie. not a guide number measured in feet), the guide number is simply the aperture that you would use to get correct exposure firing the flash at full power, at 100 ISO, at an object located 1 metre away from the flash.

    So the easiest and quickest way to measure it is to do just that. You can get at least to get within half or perhaps quarter of a stop or so accuracy very quickly indeed (it should only take a few seconds to do this) just place an object, preferably something with mid-tones, one metre away from your flash. Set the flash to full power, and your camera to 100 ISO and to the highest recommended sync shutter speed available to cut out ambient light (better that the room doesn't have bright daylight). Start by setting an aperture where you expect your guide number to be (typically somewhere between f11 and f36). Take a picture with the aperture set to somewhere around where you expect the guide number to be, if it's correctly exposed then that's your guide number. If it isn't, make an adjustment and try again (and make sure you allow plenty of time for the flash to fully recharge).
    If your lens doesn't stop down to f32 or thereabouts, modify the above by placing the object at 2 metres, and multiply the aperture you end up with by two to get your guide number. Eg. if the flash gives the correct exposure at full power using f16 at 2 metres, then the guide number for that flash is 32 (or at least very close to that).

    One wrinkle to watch out for, is that many manufacturers (Nikon, Canon) exaggerate their flash number by giving the power at the centre of the frame when the flash is zoomed (if your flash has a zoom head). To find the 'real' guide number with the flash zoomed out to a more standard setting, make sure you've set it to somewhere around, say, 24mm.
    Another trick is that guide number is often exaggerated by measuring it in a small white walled room which reflects light back into the subject. Do you tests in a larger room, preferably without white walls, but be aware that in a small white room your flash will gain a lot of power.
  6. "...many manufacturers (Nikon, Canon) exaggerate their flash number by giving the power at the centre of the frame when the flash is zoomed (if your flash has a zoom head). To find the 'real' guide number with the flash zoomed out to a more standard setting, make sure you've set it to somewhere around, say, 24mm ... Another trick is that guide number is often exaggerated by measuring it in a small white walled room which reflects light back into the subject. "​
    They're not exaggerating, just simplifying by stating the guide number at a given focal length setting for zoom head flashes - usually a middle setting of around 50mm, for zoom heads with positions ranging from 28mm or "wider" to short tele. But they also provide complete information tables specifying the nominal guide numbers at every position for the zoom head. The information isn't hidden, it just takes a little effort to find the info for comparing various units on equal footing. And the nominal guide numbers at various zoom head settings appear to be within acceptable tolerances when I checked them with a flash meter.
    " a small white room your flash will gain a lot of power."​
    A small white room only affects the distribution of the flash reflected off the ceiling and walls. It can't increase the power of a flash aimed directly at the object/subject. It may affect visual perception of falloff at the periphery.

    Again, as with film speed and sensor sensitivity, flash output and guide numbers are arrived at through standard ISO methodology. Reputable manufacturers like Nikon and Canon aren't pulling figures out of magician's hats. When real world scenarios and practical applications differ from the ISO methodology, sure, results may appear to differ.
  7. The information isn't hidden, it just takes a little effort to find​
    I wasn't suggesting the information isn't available, just that you have to dig for it, and manufacturers like to give the impression that their latest flash is getting more and more powerful by quoting high sounding guide numbers. So for example, if you look at Nikon's release about the SB900, they claim: "The SB-900 offers a powerful guide number of 48/157.5 (ISO 200, meters/feet)". Which may well be correct, but they are quoting the guide number at ISO 200 rather than the more standard ISO 100, because that gives the impression of a higher number to a casual reader who is perhaps more used to hearing guide numbers in the 30's.
    Again, further down the same page, they tell you a bit more, that the guide number is: "34/111.5 (ISO 100, m/ft), 48/157.5 (ISO 200, m/ft)" - but they don't tell you what zoom settings that is at. If you dig around the internet, you'll find out that is at 35mm zoom. A non-zoom flash will usually have a wider spread, because it has to cover standard lenses to at least 24mm, so you may find a fixed head flash that has a smaller guide number but is actually more powerful than the SB900. All of which you can find out by digging - but you do have to dig.
    A small white room only affects the distribution of the flash reflected off the ceiling and walls. It can't increase the power of a flash aimed directly at the object/subject.​
    Actually it can - and does. Though it depends on the test distance from flash to the subject. At one meter it may not make a noticeable difference, but if you are testing at 3 or 4 meters away the light reflected off walls can easily add a stop or so to the exposure falling on the subject - not just filling in shadow areas.
  8. You can test it without a flashmeter as a good flashmeter isn't very cheap. Providing you accept the following conditions you can test the GN without the meter.
    1. You have a digital camera.
    2. You have to trust that the ISO you set on the camera is the actual ISO the camera has.
    3. The aperture you set on the lens is the actual aperture.
    With that you mount the flash on the camera. Place a gray card at several distances from the camera. Set the aperture according to your calculation based on the claimed GN and distance. Took those picture. Examine the file and see if the gray card is indeed medium gray. If it's acceptable to you then you can use that GN. If not you can use a higher or lower (most likely lower GN) and repeat the test.
  9. What Simon described is the same thing I said, just doing it with meters instead of feet.

    In terms of zoom heads, etc., Lex is correct that they are not exagerating. It's just that flashes have become more complicated. In the past, a flash didn't zoom, so the procedure Simon and I described only had to be done once. But with flashes that zoom, you have to do it for the zoom setting you plan to use, which can mean doing it at least three times (wide, normal, telephoto, more if the flash has more zoom settings than that).

    Even with older non-zoom flashes, if you were going to calculate the guide number for using it with an umbrella or softbox or diffuser that would be a different number than just straight flash.

    Not wonder so many people like TTL. :)
  10. The ISO 200 guide number is often used by default
    because that's the base ISO for many digital
    cameras. It isn't intended to be misleading, but
    rather to be helpful to folks who are only
    quickly browsing the specs. Nikon and other
    manufacturers also offer tables and factors for
    converting guide numbers for other ISOs.

    And dog bless TTL flash. I never really liked
    using hotshoe flash before Nikon got it right
    with the SB-800 and other iTTL/CLS models. It's
    much less a chore now, and easier to get good
    results quickly under rapidly changing

    But I still use manual and guide number flash for
    situations where I have time. It's often more
    consistent, whereas even the best auto flash may
    vary a bit between exposures.
  11. Thanks guys! I would like hear your thought and suggestions.

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