sportyman140 Loc: Juliette, GA

I shoot with a Canon T3i, and use Yongnuo YN 565EX ETTL Flash. I am really trying to learn to use Manual Flash to become proficient at portraits inside and outside. I believe in the old saying KISS (Keep It Simple Stupid). That is what I try to live by with my photography. I would like to be able to keep it simple for my Flash Photography and not be fumbling around wasting time. I then can create a distance chart for my flashes

My question is this, manual for my Flash says my GN is 58 or am I totally confused. If I am trying to figure out the proper distance of Flash from subject by the Formula of the GN
GN = DISTANCE X APERTURE
Could I just reverse the formula and do: Distance = GN/Aperture

Example:
(Chart) for a zoom of 35 and a power output of ¼ @ ISO of 100 19.5(m)/64(ft)

an f/8 Aperture
64/8 (f/8) = 8ft
Distance = 8 feet

CajonPhotog Loc: Shreveport, LA

Your math does not appear to be quite right. The bottom expression:

an f/8 Aperture
64/8 (f/8) = 8ft
Distance = 8 feet

64/8 would = 8.......so 8x(f8) would = 64 ......unless f8 stands for 1/8. which is it?

Frank T Loc: New York, NY

Most manufacturers give guide numbers based upon 100 ISO. If you use a different ISO, you need to adjust your guide number proportionately.
Meanwhile, it's a division problem, not a multiplication problem. You know your distance, you know your guide number, you don't know your f/stop.
The formula is:
guide number, divided by distance equals f/stop.
Also remember to check and see whether the guide number is for feet (USA Standard) or Meters (Everyone else standard)

rpavich Loc: West Virginia

The ultimate "keep it simple" method is Wizwow's rope light meter.

No math involved and very useful.

http://www.lighting-essentials.com/small-strobes-knowing-your-flash-unit/

sportyman140 Loc: Juliette, GA

f/8 = 8

jenny Loc: in hiding:)

GN divided by dist. = fstop
GN divided by fstop = dist.
fstop X dist. = GN

BHC Loc: Strawberry Valley, JF, USA

GN is 58 not 64
58 ÷ 8 = 7¼'

Howard5252 Loc: New York / Florida (now)

[quote=sportyman140]I shoot with a Canon T3i, and use Yongnuo YN 565EX ETTL Flash. I am really trying to learn to use Manual Flash to become proficient at portraits inside and outside. I believe in the old saying KISS (Keep It Simple Stupid). That is what I try to live by with my photography. I would like to be able to keep it simple for my Flash Photography and not be fumbling around wasting time. I then can create a distance chart for my flashes

My question is this, manual for my Flash says my GN is 58 or am I totally confused. If I am trying to figure out the proper distance of Flash from subject by the Formula of the GN
GN = DISTANCE X APERTURE
Could I just reverse the formula and do: Distance = GN/Aperture



My question is this, manual for my Flash says my GN is 58 or am I totally confused. If I am trying to figure out the proper distance of Flash from subject by the Formula of the GN
GN = DISTANCE X APERTURE
Could I just reverse the formula and do: Distance = GN/Aperture


The answer to your question is yes. In any equation, things can be rearranged. IF you have any doubt how ... substitute some numbers, like 3x12=36

jenny Loc: in hiding:)

Yeah,Konfuzed!!! Momma used to tell me "haste makes waste"
REVISION and big apology.....
The above equations I wrote as GN should be ISO !!!
If ISO is 100, and you want to shoot at 25' 100/25= f/4
If ISO is 200 " " " " " " 25' 200/25= f/8
etc. Didn't realize I had written GN yesterday instead of ISO,my visit to UH was brief and interrupted so please if you shoot me let it be with a lens and nothing else

GoofyNewfie Loc: Kansas City

This is the reason I bought a flash meter. :lol: :lol:

My monolights are rated in watt-seconds and grids, umbrellas, sotfboxes all modify the output.

jenny Loc: in hiding:)

Well,yes,GoofyNewfie,no reason not to take advantage of technology,but a specific question was asked. Op may consider flash meter later if pursuing portraiture seriously. For less precise metering though,knowing the equation is sort of handy for quick estimate of aper/dist. whether straight on or bounced,and we don't know the equipment being used that people like to use sometimes.I keep,but don't use, a camera that does not have TTL metering,and regularly thought out exposure before using flash straight,bounced, or with umbrella.Everyone was led to think "GN" in this thread yesterday and I got drawn in despite some pressing matters needing attention.
If a situation requires f/1.8 lens and someone has a maximum aperture of f/4,it is necessary to do some thinking...

sportyman140 Loc: Juliette, GA

OK Jenny,
Equipment is : Canon T3i, Sigma 70-200mm f2.8, 24-70mm f/2.8, 10-20mm f/3.5,

(FLASHES) Yongnuo Yn 560_II (manual), Yn 565EX (ETTL), (4)-Yn622C's Tranceivers, 1 YN Battery Pack for Flashes

(2)- 36" shoot through umbrellas, (1) 24" x 36" Reflective- Rectangle Softbox, (1) 30" Octogon Reflective Softbox, (1) 24"x24" softbox;

(2) 10ft high Heavy duty Stands with (2) 7ft long Boom arms.
Plenty of sand bags (10).

I think I am taking my learning process very serious Jenny, I ask questions because I want help from others and besides the only stupid question is the one never asked. I know I am new to the world of flash but i am learning quickly and will keeps going. I know I am a long way from having my studio together that i want but you have to learn first.

GoofyNewfie Loc: Kansas City

Looks like you're loaded for bear!
If you're using your flashes with modifiers, the GN goes out the window until you can figure out the light loss with each modifier. I was half-way kidding about getting a flash meter (I hate math) as previous posters gave you answers and links for your question, but I'm leaning toward the serious side now. GN's are usually overrated anyway, at least they were when I started out. Your mileage may vary.

jenny Loc: in hiding:)

Yes you are obviously serious Sportyman,and I agree with Goofynoof..he is NOT goofy. Portraiture with all that equipment is a whole different game from daylight. As Goof- said,mfrs.have been a bit optimistic in labeling their flash ratings in the past and probably still are. Let me get a little more specific here:
You need to know the capacity of a flash when you buy it, so they are rated @ ISO 100. What happens if you buy a dinky one and need more power..up the ISO,the flash has a higher GN#.
The rating is always for a "normal" angle of lens.Wide or tele setting will affect distance.
Any accessory flash will plainly show you are now carrying your own bit of light and illustrates the Inverse Square Law of light which states,"The intensity of light varies inversely with the square of the distance from the source."
So distance to subject becomes critical.
So your approximate GN/aper=distance,and GN/dist.=aper,then obviously aper X dist.= GN...approximately..at ISO 100.
I applaud your plan to make a chart,but recommend some autoflash help.If you studio space is limited as it is for many of us you can work out something you use regularly.
I just took a couple shots of older accessory flashes to illustrate that they have dials to set for various dist/aper/ISO...which is what you'll be working with, not GN so much..So wrote this long reply and now find no way to send pics.,hope that shows up when hitting the send,button, haven't sent a pic. in a year! Hope I've been of some help at least...

jenny Loc: in hiding:)

server/computer/somebody won't let me add a 2nd pic.