10D with two 550EX and flash ratio

[william_le]

All, I'm trying to use two 550EX for better lighting. I'm trying to

achieve fill in light with one 550 on the camera and one on the side

of subject to give a more flattering image. I think just one straight

on flash looks plain and flat

 

I understand the channels, I understand master is slave A and I set

the slave to B. So should I be using a ratio A-B and 1-1. That's what

I dony understand, what is 1-1 and 8-1. Which is which. i want the

side light to be 1 stop brighter and the front as fill. I hope you

all can help.

 

I dont a test using 1-1 ratio a-b and the slave was brighter

 

thanks all

[bill_goldman]

You're correct that the master is A and the slave is B. Also, as you state, the ratio is A:B so 8:1 means the master is 8 times brighter than the slave and 1:8 means the reverse.

 

When you say that the slave was brighter than the master when the ratio was set to 1:1, is that perceived brightness of the slave, or the actual effect on the picture?

 

Placement may play a part in achieving the set ratio. E.G., if you set an A:B ratio between 1:1 and 8:1 and the slave is 1 foot from your subject and the master is 10 feet, the camera may not be able to adjust the relative outputs properly.

[william_le]

Thanks for that, so how does the ETTL work with two. They both fire a preflash and the ETTL splits the result 50/50 into both flashes, even if one flash is closer to the camera.

[bill_goldman]

When you set a ratio, each slave fires a preflash; the preflashes are fired in an A,B sequence; the camera meters the contribution from each slave; the camera then computes and memorizes the necessary contribution from each slave to achieve the set ratio and the proper exposure for the subject; when the main flashes are fired and the subject has been exposed correctly, the camera quenches each flash output.

 

When a ratio is not set (i.e., 1:1), both preflashes are fired simultaneously and the camera operates the same as when a single Speedlite is employed, quenching both flash outputs when the subject has been correctly exposed.

 

The point I was trying to make about placement was based upon the fact that the Speedlite has a minimum and maximum light output under control of the camera. I assume that if you push these limits, the camera can't necessarily achieve the desired ratio. Also, if you do not set a ratio, you can still achieve one by placing one Speedlite much closer or further from your subject because they are firing at the same output. Only if they are equidistant from your subject are they contributing 50/50.

[bill_goldman]

To clarify that last point about a 50/50 contribution, I meant that with a 1:1 ratio, each flash is contributing the same amount of flash power but if they are different distances from the subject, the subject will not receive the same amount of light from each Speedlight since light falls off inversely proportional to the square of the distance.

[walter_strong3]

Bill, both of the flashes would have to be set on "Manual" in order for that to work that way. If one is set on "Auto" then the distance does not matter (as long as it is within the range of the flash).

[william_le]

Hold on here, Im confused now. What did you mean by the manual comment. I so far understand that, with ratio on set 1:1, both flashes would have there own pre flash and both will give 50/50 power each.

 

Without ratio, both pre flashes will shoot together and will flash 50/50 without being able to control the ratio of both flashes

[bill_goldman]

Since Canon has not published details on exactly how the E-TTL wireless multi-flash system works and particularly how it achieves ratio control, my analysis as to what happens, (although I am trying to apply as much logic as I can), may just be conjecture and I may be missing some details or I may be way off base.

 

For instance, exactly how the functions are apportioned between what the camera controls and what the Speedlites themselves control is difficult to say. For instance, when the master fires a preflash, we know that the camera uses that in part to determine exposure for the subject. However, it also is sending signals to the slaves. We also know that flash output can be controlled by setting flash power and also flash duration. In the case of a set ratio, the master may be setting a relative power output level for the slave. I am pretty sure the camera is controlling the flash duration.

 

In the end, it is not necessary to know exactly how all this works; just that it works. Why worry about it? Just take pictures.

[theoldmoose]

Please forgive my quoting, but there seems to be a lot of confusion over what, exactly a 1:1 ratio is. According to the EOS Flash photography article (http://photonotes.org/articles/eos-flash/index2.html#fillflashratios):

 

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Fill flash ratios.

 

The �fill flash ratio� is commonly described in terms of the ratio of ambient light plus fill flash combined, compared to the fill flash alone. Canon EOS gear, however, usually lets you adjust the fill flash in terms of stops of flash output, in either 1/2 or 1/3 stop increments. What�s the relationship between the two ways of describing fill flash?

 

* A ratio of 1:1 would mean that the flash unit is the sole source of light (0 ambient + 1 flash) and therefore you wouldn�t have a fill flash situation.

 

* A 2:1 ratio would mean that the ambient light and flash are at the same level (1 ambient + 1 flash). That basically means 0 stops of compensation given a fairly flat-lit scene, and usually results in rather unnatural looking fill flash.

 

* A ratio of 3:1 means that there is twice as much light from the ambient source as the flash (2x ambient plus 1 of flash). Such a ratio requires a -1 stop fill flash setting on the flash unit, since each stop means a doubling or halving of the amount of light.

 

* A ratio of 5:1 means that there is four times as much light from the ambient source as the flash (4x ambient plus 1 of flash). This is a -2 stop difference. Typically photographers use between 1 and 2 stops of fill flash to lighten shadows without creating a phoney flash-illuminated look.

 

However, the term �ratio� is confusing and seems to mean different things to different people. Sometimes people talk about a 1:1 ratio when the ambient and fill lights are of equal intensity. So a 2:1 ratio might mean -1 stop fill flash and 4:1 would mean -2 stop fill flash. In this case they�re talking more about the light output than they are about the reflected light.

 

The concept of ratios works well in studio situations where you have total control over the lighting. You can turn off the main light and measure the fill lighting with a meter, you can move lights around to vary their strength, etc. But if you�re taking a candid photo outdoors you have no such control. You can hardly turn off the sun, and automated TTL flash is going to have its own ideas as to what constitutes correct lighting.

 

For those reasons I prefer not to deal with ratios at all for non-studio flash photography but just in terms of the number of stops compensation used by the flash. Note that the term �ratio� is also used in flash photography in conjunction with multiple flash setups, particularly multiple wireless units in the case of wireless E-TTL flash.

 

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So, the difference in light output you may be seeing may be due to a misunderstanding of what it means to set a 1:1 ratio (or not -- I may have missed the mark, here, completely).

[william_le]

Dave, we are talking about ratio's between two flashes, not flash and ambient. Thanks all for everyone's input. It was very helpful.

[theoldmoose]

Ah. Forgive me. Not having played directly with multiple flashes, I spoke out of context (and ignorance). I hope I haven't confused anyone, here. I see that the section I quoted is quite clearly marked 'fill flash ratios'. Mea culpa.