canon speedlites and guide numbers

Sigma 10-20mm f/4-5.6 EX DC HSM for Canon

01/06/2008 09:58:03 AM · #1

i'm trying to figure out how to use the 550ex guide number calculator with the flash set to slave and in manual mode.

the only way i can get it to come up is if master/slave mode is turned off. which seems pretty useless to me.

the book doesn't really cover the subject outside of a chart. but even that doesn't seem to provide all of the info. gives you distance and power so you can calculate the guide number at a given power and distance, but not the aperture. which is what i am after.

i don't mind doing the calculation in my head but i don't know how to get from those three variables to the ideal aperture.

ideally i'd like to calculate the flash max distance from my chosen aperture and a given flash power setting.

any clues?

the way i figure it works is at

thanks.

//strobist.blogspot.com/2007/12/guide-number-your-free-flash-meter.html

Message edited by author 2008-01-06 10:24:42.

inshaala

Canon EOS-7D

01/06/2008 10:05:00 AM · #2

Whenever i am presented with a situation like this i follow the rule of trial and error and visual judgement. Just try one setting, and if you dont like it change it. Beauty of digital there - no cost in mistakes, only time. Also - over time you will probably get a feeling for what is about right for the situation. Sorry i couldnt be of exact help.

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

01/06/2008 10:26:08 AM · #3

well that's what i have been doing, and i am trying to be more precise about it.

and therefore save time.

trial and error gets to be a pain when you are working with multiple lights - for instance.

say i want to shoot at f:8 and i want a flash power of 1/2 at ISO 100 1/200th. roughly how far will the flash need to be from the subject to properly expose ?

fstop * distance = GN

so GN/fstop = distance.

is the GN the power setting on the flash?

Message edited by author 2008-01-06 10:38:55.

PGerst

Canon EOS-20D

01/06/2008 11:26:45 AM · #4

This article is what prompted me to post this question.

Though, knowing the guide number did help me to figure out the power of one of my cheapo slave units. I basically calibrated my speedlight by measuring the median value on the histogram. Then, calculated the guide number of the slave flash based on the median value from that flash. Worked well.

Otherwise, not sure how much utility that article from the strobist was.

Canon EF 100-400mm f/4.5-5.6L IS

01/06/2008 11:43:38 AM · #5

it doesn't just have to be used with direct hard light. you'd just need to do a couple tests to see how many stops you lose with a shoot through or a bounce back modifier. that is gonna be a constant for each type of modifier.

awpollard

Canon EOS-5D Mark II

01/06/2008 11:43:58 AM · #6

Watching this thread as I just picked up the 580EX II, I can trigger my 420EX with it and get lots o light in a scene but all those #'s in the LCD is hurting my brain. Would be nice to know about Guide numbers but I never really got that down.

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

01/06/2008 11:46:45 AM · #7

i didn't think this was gonna be such a tricky question. sort of thought i was just missing something simple.

does everyone just use e-TTL? i really don't like the auto metering especially with multiple lights. and the ratios ( with the st-e2 )are tied to e-TTL as well as far as i can see.

why would canon hide the guide number calc from me in the situation where it is most usefull.

i am considering switching to PW's and nikon strobes. but i really don't want to spend the $$.. ( and yes i know there are cheaper alterantives, but i like quality build and reliable output )...

Message edited by author 2008-01-06 11:50:36.

PGerst

Canon EOS-20D

01/06/2008 12:03:15 PM · #8

I use eTTL at times. Others, I'll dial in a flash exposure. I generally fire a few test shots and adjust accordingly.

Nullix

Canon EOS-5D Mark III

01/06/2008 12:13:10 PM · #9

This is where (I think) Canon does a bad job. Their flashes are missing same bells and wistles.

Of course I'm only using my 550ex in manual mode being triggered by Hong Kong wireless setup.

I can only assume, when you set the master in manual, the slave just picks up the settings of the master. You can set the ratios, but I think that's about it.

There should be a way to have indepentent setting on each, but I haven't experimented with it yet.

-- Fellow Strobist

01/06/2008 12:34:28 PM · #10

both the speedlites are slaves to an st-e2 at the moment.

i set both speedlites to manual. and can dial in the power and zoom settings. the st-e2 will fire them fine. but i want a quantatative means to set the power of the speedlites. not guess and guess again, and again. i want to be able to get a good working control on the 1st fire of the lights, and maybe make one adjustment to get the exposure spot on.

and yes i'd agree - canon cheaps out on something simple for what reason other than to force a canon shooter to switch to a nikon lighting scheme...

Message edited by author 2008-01-06 12:38:06.

Nullix

Canon EOS-5D Mark III

01/06/2008 12:51:37 PM · #11

Originally posted by soup:

i want to be able to get a good working control on the 1st fire of the lights, and maybe make one adjustment to get the exposure spot on.

Not a good answer, but maybe you should get a lightmeter.

Doesn't solve your problem, but helps. I'll watch this thread to see if anyone figured it out. I'd like to know to without guessing too.

I just guess at the flash setting, set the lightmeter to my camera's sync speed, take a reading with the lightmeter, then adjust accordingly (usually to get f8 for DOF).

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

01/06/2008 01:11:53 PM · #12

i'll figure it out... ;}

i think...

SamDoe1

Canon EOS-350D Rebel XT

01/06/2008 01:58:13 PM · #13

My Sigma has a setting where you can input the aperture and the ISO you're using and will expose properly based on that. Does the Canon have something similar?

This might be helpful to you.

Edit to add: For the most part, I just use ETTL and/or trial and error. I've gotten pretty good at guessing the right power setting on the first go and then tweaking as needed.

Message edited by author 2008-01-06 13:59:11.

awpollard

Canon EOS-5D Mark II

Canon EF 100-400mm f/4.5-5.6L IS

01/06/2008 02:30:09 PM · #14

Originally posted by SamDoe1:

The 580ex II has a Manual Flash mode that works as you mention.

ED: Great link btw... that is what I need (kind of a flash for dummies)

Message edited by author 2008-01-06 14:31:58.

01/06/2008 03:13:01 PM · #15

it comes down to the way i was reading the chart in the 550ex manual.

take your power setting, and look up what zoom you have the speedlite at.
find the 'guide number' based on those settings, and use GN/distance = f:stop. to rough out your aperture.

it works.

seems to be off by a tad - but that's due to rounding i think.

the shoot through umbrella kills @3 stops. the bounce back umbrella kills @3 stops also. keep the aperture you're happy with knowing this, and adjust the flash power instead ( if possible ). if that still doesn't work - move the lights.

same results as far as histogram is concerned ( keeping in mind the difference in the source of the light ( hard, soft, and soft )):

zoom of 105mm GN for 1/8 is 19.5

19.5/4 = @ f:5 ( i upped it a stop ).

550ex @ 1/8power - -w no modifier - source 4' from subject - camera ISO 100 - f:6.3 - 1/200th
550ex @ 1/1power -w bounce umbrella - source 4' from subject - camera ISO 100 - f:6.3 - 1/200th
550ex @ 1/1power -w shoot through umbrella - source 4' from subject - camera ISO 100 - f:6.3 - 1/200th

work with that chart enough and i think i'll have a mental burn that will let me not have to refer to the chart.

Message edited by author 2008-01-06 15:36:54.