Guide Numbers Studio Flash

Discussion in 'Beyond the Basics' started by Stevedevil, Feb 25, 2007.

  1. Stevedevil

    Stevedevil TPF Noob!

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    Ok So I am looking at purchasing some studio flash equipment, I have posted a thread before and had some really good responses, but the thread kinda lost its way.


    SO I started reading up a lot on studio flash equipment and most equipment have references to Guide Numbers

    And found some usefull reading

    Understanding Flash Guide Numbers
    The guide number of a flash is a measure of its light output. The higher the guide number, the greater the intensity of the flash. Guide numbers are determined experimentally. A flash unit and subject are set up and exposures are made at a variety of f-stops. When the best exposure is determined, the guide number is calculated from the distance and the f/stop used as follows:
    GN = f/stop x Flash-to-subject distance
    More powerful flash units with higher guide numbers have a greater range, have faster recycle times, and make bounce flash more effective.
    Guide numbers are usually given for ISO 100 speed film. Faster ISO settings will require less light so you can shoot at a greater distance. For this reason, you need a different guide number for each ISO setting. Increasing the ISO film speed by a factor of 4 will double the guide number. When film speed doubles, the guide number changes by a factor of 1.4X; when film speed is halved, the guide number changes by a factor of 0.7X.
    In addition to being an indicator of a flash’s power, guide numbers also can be used to calculate aperture settings and subject distances when using the camera and flash on manual mode as when you are using a flash not designed to work with the camera.
    • To calculate the f/stop needed, divide the guide number by the distance to the subject.
    • To calculate the maximum flash range, divide the guide number by the selected or largest f/stop.
    When making these calculations, there are a few things to be aware of:
    n It is the flash-to-subject distance that you use, not camera-to-subject. This is the same when using on-camera flash but not when using flash on a cable or a slave flash.
    n Guide numbers are usually given for both feet and meters so be sure you use the right one in your calculations. The differences can be substantial. For example, a guide number of 12, when using meters as a unit of measure, is the same as a guide number of 39 when using feet. Conversions between a guide number for feet and a guide number for meters are:
    From meters to feet: GN(ft) = GN(m) x 3.3
    From feet to meters: GN(m) = GN(ft) x 0.3
    The 995 has a guide number of 10 (m) or 33 (ft) at ISO 100. Its lens has a maximum aperture of f/2.6. Its ISO range is from 100 to 800. Using these characteristics of the camera, you can calculate the following:
    • The maximum flash range in feet at ISO 100 and f/2.6 is: 12.7 feet
    • The maximum flash range in feet at ISO 800 and f/2.6 is: 34.8 feet
    • The maximum flash range in meters at ISO 100 and f/2.6 is: 2.6 meters
    • The maximum flash range in meters at ISO 800 and f/2.6 is: 7.1 meters
    My question would be Are GUIDE NUMBERS any real use when looking at purchasing equipment.

    If so Are the higher Guide Numbers "BETTER" than lower when it comes to choosing which equipment.

    Also would lens f stop be an advantage with lower Guide Numbers as have just got a 50mm f1.8
     
  2. fmw

    fmw No longer a newbie, moving up!

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    Yes guide numbers are very useful for making a purchase decision. Few commercial strobe manufacturers provide them, however. Usually they will provide a "typical aperture" figure to go along with the watt/second rating which is input power not output power. Higher guide numbers are always better. You can reduce the output power of the strobe below the guide number figure but you cannot increase it beyond the guide number.

    Studio shooters don't use guide numbers as a rule because things get complicated with light modifiers and multiple flash heads. We normally just use a flash meter.
     
  3. Stevedevil

    Stevedevil TPF Noob!

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    I noticed the Aperture rating as well and their range is quite varied, I believe that these readings are set on full power 1M away from meter set at 100ISO, but most do not mention the shutter speed set for the meter reading, I believe its 1/125..

    Again this is direct light and this, to me, is fairly unimportant as I use a fast 50mm f1.8 lens and the worst case I saw were Bowens 125w which gave a Guide of 41, but an aperture of f16, where an Interfit gave Guide 30 with f45!!!

    So my confusion gets more with regards to the guide.....
     
  4. fmw

    fmw No longer a newbie, moving up!

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    The shutter speed is immaterial. It matters only for purposes of synching the flash units to the shutter, not for exposure. f45 is enough to get good depth of field with a 4X5 view camera and a longish lens. It is not an uncommon aperture in studio photography. You wouldn't want to be using f1.8 for much of anything in a studio. The DOF is simply to shallow to be useful. We aren't often focused to infinity in the studio.

    125 W/S is not very powerful and a GN of 41 or 30 is weak - weaker than any of my portable flash units - even my ancient Vivitar. If you get more powerful flash heads you won't need so many of them. You would want about 4 times that kind of power at a minimum per head.
     
  5. Stevedevil

    Stevedevil TPF Noob!

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    I have been looking at getting 400 - 500w Heads x 2, with an additional 250w later on, In UK the most popular are Bowens & Elinchrom ( 500 Bowens & 400 Elinchrom ) they both come with the option of Battery Packs, and come as a kit with umberellas ( silver, white & Gold ) 2 No soft boxes, Tripods etc

    Questions I will ask as you seem to be the man to answer is the main advantages of their standard ranges, to their Digital Ranges, as I can only see an advantage if you need a particular setting on the camera as to adjust on the Strobes....

    Can I not use a Test Card and use RAW and adjust all photo's taken to reach the correct k value. ( In Photoshop )

    The main reason that Im quite fussy is that £ 1500-00 ( $2500 ) is quite an outlay for something if it wont fit the job...

    Also Being a Novice, I need something that will grow with me, and that I wont outgrow to have to spend more and more

    What do you think...

    Steve
     
  6. EOS_JD

    EOS_JD TPF Noob!

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    Steve
    I'm looking at the Elinchrom D-Lites. Was thinking of the 400ws strobes but am at the moment undecided between the 400's or the 200's.

    I have a pretty small space and the 400 is probably overkill but as always stated you can always reduce the power by use of modifiers etc but you can't increase it!

    So 400s are still likely to be my favoured ones. then it's a background system and light modifiers - which ones to get! Reflector and grid and barndoors seem interesting additions and a good background stand to hold muslin or perhaps even vinyl.

    What sort of space do you have for a studio Steve?
     
  7. Stevedevil

    Stevedevil TPF Noob!

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    I have 4M x 3M and have not decided whether to go long and not as wide or wide but less depth if you know what I mean, I am booked on a Portrait course in June and this will give me a good Idea of not just the Camera Settings, but also positioning of the lights, How many, Wattage, also the backdrop layout etc, as you say there is So much to look at, and so many questions to ask,

    But sometimes questions are not met with the type of reply your looking for, but thats another thread I have been reading on here.

    I did read though that the Dlite 2 & 4 Kits seem a little plastic'y and there is an issue of usage with overheating, but it was probably from a Bowens enthusiast, they did get the thumbs up in Digital Photo magazine though.

    Its not the sort of thing that you can try before you buy, hence this thread to see some options that may help point me in the correct direction.
     
  8. fmw

    fmw No longer a newbie, moving up!

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    Of course. Photoshop is the best way to adjust or change white balance for any image.

    The monolights I use personally are made by Bowens (a British company) and are very reliable. Four of them have been in use by me for 12 years and a fifth one has been in service since 2002. I've replaced a few flash tubes over the years (they do wear out) but I've never had a failure of any sort. Elinchrome also has an outstanding reputation among photographers. Get whatever appeals to you. Both are excellent.

    A trio of 400 w/s heads (1200 w/s) is probably enough for portraiture and more than enough for small product photography. Most portrait photograhers have 2500 w/s packs distributed among however many heads they need for the shoot although I've seen a few using 1500 w/s packs. I think the 400 w/s monolights would do OK for traditional portraits and perhaps even full figure portraits but you wouldn't have full depth of field flexibility. If you need to shoot at f22 or f32 to get the DOF you need they may not do the job. Mine are 750 w/s each (one is 350 w/s) and I've never felt hampered by lack of power. That gives me a total of 3350 w/s and that's pretty good flexibility.

    Remember the light modifiers like umbrellas or soft boxes reduce the light output by a ton. Buy all the power you can afford. Trust me, more is not too much.
     
  9. Stevedevil

    Stevedevil TPF Noob!

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    Would you think it more worth my while investing in 2 x 750w for starters?? as a Kit they still come with all brollies and a softbox, also ( as they appear quite a bit cheaper ) get the Gemini's as the DX range is quite a bit more,

    Does it matter, as long as the output is adjustable, why go DX ???
     
  10. fmw

    fmw No longer a newbie, moving up!

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    I'm sorry, I'm not familiar with the model numbers. My units are Calumet Travelites which are made for Calumet by Bowens. Calumet is a major supplier to professional photographers here in the U.S. and have been for a long time. Their studio flash equipment is all Bowens like their view cameras are all Cambo.

    More expensive units are usually more reliable. They are made with better components. Also they are more efficient. The watt second is a measure of input power to the flash tube, not output power. Some units produce more light (higher guide number) than others and are therefore are more efficient with the same input power. The guide number or anticipated aperture are measures of output power.

    Inexpensive units may also not be serviceable. If you can't replace a worn out flash tube then you have throw the whole unit away when the flash tube weakens. Flash tubes are expensive but not nearly so expensive as a complete monolight. You want to be sure the units are serviceable. More expensive units are more rugged. This is important if you use the units on-site. In the studio they don't need to be so rugged but they do need to be reliable.

    Would buying 750 w/s units be better than buying smaller ones? Of course. Buy all the power you can afford. It is easy to fall into the trap of deciding to buy a powerful main light and then buying lower powered units for fill, background and hair. One day you will use the lights for something other than a formal portrait and wonder why you scrimped on the power.

    I'm not in a position to tell you what you should or shouldn't buy. You should spend your money, not me. I'm just trying to provide some things to think about. Best of luck.
     
  11. Stevedevil

    Stevedevil TPF Noob!

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    I appreciate the feedback, I will look into this a lot further yet, but now I can look a little up the Power Range as the units will be used at various locations therefore will need to be sturdy enough to be carted and put up and down.

    Would still like to know the difference between the Digital & Standard ranges

    So hopefully someone will post
     
  12. Jazz

    Jazz TPF Noob!

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    Steve - fmw has generously given you some great information. I am a studio photographer, and would like to help too. I see your last question but I'm afraid I don't understand what you mean. Can you explain the question a little? What do you mean by standard and digital ranges?
     

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