How do the ISO, f-number of the aperture and focal length of the lens affect the guide number?

Discussion in 'Lighting Equipment' started by sunnym|1, Dec 22, 2014.

  1. Somebody told me that all of the ISO, f-number of aperture and focal length of the lens can affect the guide number of a flash. However he didn't tell me how. Anybody can help me? Thanks a lot!
     
  2. Googling "flash guide number" brought up a number of pages with explanations better than I could give:
    http://www.scantips.com/lights/flashbasics1c.html
     
  3. ISO is sensitivity of the medium, but the focal length of the lens and the laperture the lensis settohave nothing at all to do
    with the guide number (which is one way to guess at how much light a flash is putting out) unless the flash is a smart
    flash with a built-in zoom function and is connected to the camera.

    Frankly I find guide numbers to not be useful. Even less useful than watt-second ratings.

    Manufacturers use them but usually when they state a guide number it is with the flash at full power and the flash head
    zoomed to a long focal length.
     
  4. As Ellis said, only the ISO affects the Guide Number. Aperture is used to determine the flash distance from the GN, and the focal length of lens has no effect whatsoever, unless the flash has a zoom head. With a zoom head the GN changes slightly as the coverage angle of the flash changes - but probably not as much as is stated in the flash maker's manual.
    I'd disagree that GNs aren't very useful. Provided you know what the real GN of your flash is, it enables you to estimate how far the flash will reach for a given aperture and ISO. Or to get an idea of what aperture to set as a starting point. However, the real GN will almost certainly be nothing like the one stated by the flash manufacturer. Anything over 36 (in metres @ 100 ISO) is almost certainly pure fantasy.
     
  5. By "having the flash head zoomed to a long focal length" what I mean is that many if not most modern hot-shoe mount
    flashes have a sliding deeply curved reflector behind the flash tube. By sliding it closer or further from the flash tube they
    can concentrate or widen the beam angle, the angle of illumination. When you concentrate the light over a narrow angle
    the light is brighter -and hence produces a higher guide number, then it will if set to a wide angle.

    With monolights and pack and head systems, for the most part*, beam angle is determined by choice of reflector that is
    mounted on the flash.


    While these days many people use a hot shoe mount flash off camera, they were designed to be mounted in a camera's
    hot shoe and so they are still marked with a focal length equivalent instead of a beam angle.

    *the most prominent exceptions in pack & head and monolight designs are the Profoto Acute, Pro, flash heads., and to a
    lesser extent the D1 and B1 Monolights. Profoto's brilliant reflector mounting system allows you to change beam angle
    not only through choice of reflector but also by sliding the reflector along the body of the flash head. Following Profoto's
    lead , in the past Speedotron Blackline 202VF and 204VF heads had a similar method of achieving the same effect in a
    limited way, and the now defunct Balcar company made a zoom head that moved the flash tube.
     
  6. I have been a pro photographer for over 17 years and I have never used GN. I use what all other pros use.. a light meter. That is how you measure the light. In the old days of film if you did not have a light meter one did not have the luxury of a display on the back of the camera so one had to rely on physics and GN. Today all you have to do is take a picture and see if the flash was enough or not. The f-stop plays an important part in this as flash exposure is measured by Fstop. A telephoto length lens can help reduce the angle of flash coverage needed by a flash.
     
  7. That's because you're a pro! I was certainly a beginner at age 10 with a manual flash (actually bulb flash so it was expensive) and of course no flash meter the GN was something I relied on.
     
  8. Thanks to all! I am a beginner and eagerly want a comprehensive understanding of the guide number.
    "With a zoom head the GN changes slightly as the coverage angle of the flash changes - but probably not as much as is stated in the flash maker's manual". My hot-shoe mounted flash does have a zoom head (24mm-105mm). The flash manual has a guide number chart for ISO 100. By setting flash to 24mm zoom, 1/1 power, GN is 28 meters. By setting flash to 28mm zoom, 1/1 power, GN is 30 meters. If I set flash to 85mm zoom, the same 1/1 power, how can I calculate the guide number?

    If set the flash to 105mm zoom, ISO 100, GN is 56 meters. Then setting the flash to ISO 200, keeping the same 105mm zoom, GN will be 78.4 meters which is 1.4 times as big as ISO 100. Right?
    "Anything over 36 (in metres @ 100 ISO) is almost certainly pure fantasy." Real?
     
  9. "If set the flash to 105mm zoom, ISO 100, GN is 56 meters. Then setting the flash to ISO 200, keeping the same 105mm zoom, GN will be 78.4 meters which is 1.4 times as big as ISO 100. Right?"​
    Right inasmuch as the maths goes. 56 times 1.4 does equal 78.4, but experience tells me you need to be very cynical about maker's given Guide Numbers. The maker's GN of 56 will almost certainly turn out to be nothing like that in actual use.
    As Michael says, the pro way is to use a lightmeter. And by measuring many, many speedlights with a variety of meters, and verifying the results in practise, I've drawn the conclusion that makers' given Guide Numbers are nearly all one stop too optimistic. I'm betting that your supposed GN of 56 @ 100 ISO and a zoom setting of 105mm is really closer to 40, or even less. Try it. It's easy and doesn't cost anything with a digital camera that shows a histogram. And once you've established a true GN, then you can make your own GN chart.
    "If I set flash to 85mm zoom, the same 1/1 power, how can I calculate the guide number?" - You can't! There's no easy way to relate the zoom setting to an increase or decrease in GN.
    As I said above; do your own tests and make your own GN chart. Which incidentally will be pretty much useless when you advance enough to take the flash off the camera or start to bounce it.
     
  10. As I said above; do your own tests and make your own GN chart. Which incidentally will be pretty much useless when you advance enough to take the flash off the camera or start to bounce it.​
    But as Micheal said, today most of us shoot digital so we can just shoot, review and adjust.
     
  11. "But as Micheal said, today most of us shoot digital so we can just shoot, review and adjust." - But that wasn't what the OP asked about Bebu.
    I still think it doesn't hurt to know the real GN of your flash gear. You can use that knowledge to get a rough idea of which/how much gear to take to a location shoot. Something you can't do remotely by chimping with a DSLR. If you know you're going to be 15 metres away from the subject and shooting with a 200mm lens at f/4 (to get sufficient DoF), there's little point in trying to light it with a flash that only has a true GN of 20. Not unless you want to pump up the ISO and then risk getting contaminated colour from ambient lighting.
    You might also want to try "painting with light" at night. Popping off 20 or more flashes over a period of 10 minutes just to get a trial exposure that might be several stops out isn't the most efficient way to go about things.
     
  12. I have 2 flashmeters but asking everyone who uses flash to have one is a bit of a stretch.
     
  13. On December 23, Ellis Vener said.. "in the past Speedotron Blackline 202VF and 204VF heads had a similar method of achieving the same effect in a limited way.."
    Ellis, Speedotron Blackline 202VF and 204FV flash heads are still being manufactured and today, just as in the past, they still offer that feature.
     
  14. I use guide numbers all the time and find them vital. And I'm a pro. Shoot review and adjust only works if you have the chance to take the picture several times. Using a light meter only works if you have time, and (if it's an incident meter can get an assistant to where the subject is). And anyway, the meter will only tell you what you already know. For a given ISO and flash power (which outside, will typically always be full power), the meter will always tell you the same thing at a given distance. So why use it in typical predictable situations?
    I tend to use the guide number (actually my own one that I've come to through practise) so that I know in advance what aperture I need to set for a given distance. Typically for street phography, I might know for example that if the subject is going to be 3 metres away, and I'll be using ISO 400, I will need to use, say, f11, to get the right flash exposure. This will be absolutely consistent and bang on every time. Much more reliable than TTL etc. (and doesn't have the time delay associated with TTL). If I change to 800 ISO as it gets darker, I just have to open up the aperture one stop. Easy. If the subject is 4 or 5 metres away, I just open up an extra stop. If it's 2 metres away, close down a stop. It's all so fast and accurate and consistent.

    Really recommend getting to grips with them.
     
  15. p.s. not dissing light meters especially in the studio, where different reflectors etc. make the guide number approach largely redundant, I am rather thinking of a small camera flash situation, especially outside.
     
  16. I don't do the math formula when it comes to guide numbers but yes I do shoot in manual flash mode most of the time. Like many other photographers (pros) who use speed lights or any other type of portable flash in manual, I get a feel for the flash and know what it will do at different settings. Knowing the rule of inverse square law helps me to know when to double the flashes power or decrease it. The best advice is to pick a distance most used and set your flash then when you double the distance you increase flash by 2 stops and visa versa. Anything in between I use the 1/3 settings
     
  17. That's pretty much how I work Michael. Another way of saying it is I create my own guide number (ie. aperture) for a given typical situation (preferred ISO). One doesn't always have to use 1 metre at 100 ISO as a guide number. In practise, once you've done it a few times, you just know that it's f11 at a typical working distance with a particular flash, and you don't even have to do any calculation - you already known your 'guide number' (= aperture) for a given situation - and tweak a stop or two either way as the subject gets closer or further.
     

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