# Guide Numbers Studio Flash



## Stevedevil (Feb 25, 2007)

*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&#8217;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


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## fmw (Feb 25, 2007)

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.


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## Stevedevil (Feb 25, 2007)

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.....


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## fmw (Feb 25, 2007)

Stevedevil said:


> 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.....


 
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.


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## Stevedevil (Feb 25, 2007)

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


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## EOS_JD (Feb 26, 2007)

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?


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## Stevedevil (Feb 26, 2007)

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.


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## fmw (Feb 26, 2007)

Stevedevil said:


> 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...
> 
> ...


 
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.


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## Stevedevil (Feb 26, 2007)

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 ???


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## fmw (Feb 26, 2007)

Stevedevil said:


> 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 ???


 
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.


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## Stevedevil (Feb 26, 2007)

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


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## Jazz (Feb 26, 2007)

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


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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## Stevedevil (Feb 26, 2007)

A lot of producers of Studio Flash equipment have Launched a range of So Called " Digital " range, where the announce that the output of the strobe is better suited to Digital Cameras.

I have looked at various pages of information within this site and also the manufactures pages, and all I can see is a price difference

Lets look at the Bowens they offer Digital Esprit DX Heads, at 250w, 500w & 750w, also just the Esprit Gemini Range, again at 250w, 500w, & 750w ( They also go into 1000w )

The kit for 2 Esprit Gemini including all stands, softboxes, umbrellas etc is £769-00,  while the Digital Esprit DX for the same kit ( 500w twin and all stands ) £ 999-00

The site I use for this info is  www.warehouseexpress.co.uk

Under Studio and Bowens

Cheers

Steve


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## Jazz (Feb 26, 2007)

> output of the strobe is better suited to Digital Cameras.


Dang, yet another thing for me to learn.  

I did not know about that Steve.  I shoot both digital and film in my studio with the same power packs, and admit that I have no idea what they're talking about.  Sorry I can't help.  My apologies for jumping in here.


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## Digital Matt (Feb 26, 2007)

I'm not 100% sure, but fairly certain that they are just talking about sync voltages.  Most DSLRs sync at 6v, which is far less than film cameras.  If you are unsure, you can always get a safe sync adapter.


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## Stevedevil (Feb 26, 2007)

No Sync voltage on both are 5v ( even though most people use remotes nowadays I would think )


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## Digital Matt (Feb 26, 2007)

There is nothing that I can think of that is unique to digital that would require special digital strobes, other than the lower sync voltage.  Sounds like a marketing ploy to me.


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## Stevedevil (Feb 26, 2007)

Reading further ( as we do here ) I found that the only apparent difference is that the so called " Digital Range" have a couple of extra features

1)  Infra red remote control up to 10M


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## Stevedevil (Feb 26, 2007)

Ok I have been reading all night

The so called " Digital Range " is so that it has Infra Red Firing built in along with a Remote control to make changes.....

Wow so we go to model with light meter and set off flash, so using seconic 558, I believe you can activate infra red strobes, I push a button and measure with 308 flashmate, 

So then you can set remotely, wow ive had to walk to model to gain light levels so if I then adjust levels remotely, after walkin past lights, will have to take new reading to adjust camera

Please stop me if Im missing something

So now camera set, I hit the button

What has a Digital set up got for the shot!!!


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## EOS_JD (Feb 26, 2007)

Steve
Just one thing. In a small studio space you have I think 750W is overkill.  You'll struggle to get power low enough!

That's why I was between the Dlite2 & Dlite 4s.  Ithink 400W is more than enough.

Regards the setup, 9' in width is narrow but probably workable. If you go for 9' in depth that will not be a good idea.  You have a backdrop then room for subject say 3'-4' away from the backdrop leaves you 5'.  Your lights need to be positioned and then you have to have room to move in.  You'll need to be at least 4' away from your subject just for a head and shoulders so not enough room to go "wide" as you say.  Unless you're going for a different look with a shot with a wide angle lens.


I have 16' x 9' so I know which way my studio will be set up.


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## fmw (Feb 26, 2007)

EOS_JD said:


> Steve
> Just one thing. In a small studio space you have I think 750W is overkill. You'll struggle to get power low enough!
> 
> .


 
Here is my small product light tent.  The two bare heads lighting the tent from just a few inches are 750 w/s Bowens monolights.  I normally shoot at f16.  I have one of the units dialed down to 1/4 power and the other one to 1/8.  They will dial down to 1/32 should I want to use a wider aperture.  I could add light modifiers to shoot at f1.4 if I wanted to.  Not overkill.  Easy to dial down.  Not a problem at all.  In fact, using them dialed down improves their longevity and reliability.  It also allows me to shoot hand held at f16 because of the short flash duration.  At full power, the flash duration is just 1/125.  At 1/8 power it is around 1/4000.  At full power I would have to use a tripod for this small product setup.

I've also used all five of my monolights to handle an indoor shoot of an RV exterior for which I used a parachute canopy for a light tent.  I shot at f11.  Steve may not need to shoot RV's but the extra power never hurts.  If 400 w/s units will do the job, so will 750 w/s units or 1000 w/s units.  I't not suggesting he needs that much power.  I don't know what he needs.  Neither does he.  However, I do know that you can dial down flash heads but you can't dial them up.

At any rate, I'll get off the soapbox and post a shot of the monlights and the light tent.


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## Stevedevil (Feb 27, 2007)

Looks good

On your lights Fred are you only using one as flash as two do not appear to have sync leads??

Or do they fire remotely??


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## Digital Matt (Feb 27, 2007)

They are slaves.  They detect the initial flash and fire.


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## northcoastcamera (Feb 27, 2007)

You should listen to fmw he knows what he is writing about.  With flash there is no such thing as too much power.  You can always reduce the output.  1/16 or 1/32 power is available on all decent strobe systems.


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## rppnj (May 18, 2008)

> 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* )


 
Just purchased Elinchrom 500 Flash System - 3 Heads.  I am looking for the PDF user manual for the Elinchrom 500.  It is not available on their website or anywhere I can find, although it does exist.  Any help would be very much appreciated.


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## bango707 (May 18, 2008)

northcoastcamera said:


> You should listen to fmw he knows what he is writing about.  With flash there is no such thing as too much power.  You can always reduce the output.  1/16 or 1/32 power is available on all decent strobe systems.



I am going to agree and disagree with this statement. 
If you have too much power you can always power the unit down, use scrims, and or use ND gels to lower the intensity of the light.

On the other hand if you don't own scrims or ND gels then you are stuck with the 5-7 or however many stops the unit is capable of losing. 

The problem is cheap strobes.  If you own and shoot with Profotos then you can pretty much use the light at any power level and it will look/act the same. But it doesn't work the same when you are shooting with say AlienBees.  When used at the lowest power they tend to have major color shifts just like your cheap lens might at wider apertures. 

There is a great example over at POTNF that someone did comparing a few different brands of strobes at varying power levels.  Like said earlier in this thread cheaper lights are cheaper for a reason. They aren't build as nicely as the more expensive lights and they can't perform as well.  

So saying that there is no such thing as too much power simply isn't true in all situations.


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## Early (May 18, 2008)

Jazz said:


> 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?


Just so you know!  I found your post very informative.  Thanks!


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