Could someone please explain flash ratios

I don't think so if using ittl - the amount of each group is automagically calc'd during the pre-flash so there's no fixed relationship between each group - at least that's how I think it works!

you can set manual output power for each group, then you know the ratios

Sil

 

Yes. You adjust the lighting ratio using EV just as you suggested. a One EV difference between two groups is 1:2, and two EV difference is 1:4, etc. iTTL adjusts the exposure keeping these ratios fixed between the flashes. Sometimes due to subject luminance, reflectance of the surroundings, direction of the light, etc. it is necessary to use additional EV correction to adjust the overall exposure, but if you keep the same difference between groups the ratios will be preserved.

 

that's interesting, I guess thinking about it it must work that way I have never tried it like that,. manual power output just seems easier to understand for me

Sil

 

I believe that the way multiple flash iTTL works is as follows. During the pre-flash sequence, the master sends each flash commands in succession to fire at reduced power (probably twice each at slightly different power levels according to the algorithm described in one of Nikon's patents describing CLS). The camera uses its flash exposure sensor to measure reflected return from each flash individually, and then computes power required for "normal" exposure using these values along with the current EV compensation settings. Finally, the master sends commands for all flashes to fire at these predetermined levels for the actual exposure. Therefore, the output levels of each flash can be different depending on their distance and orientation to the subject, the desired EV compensation settings (i.e. lighting ratios), etc. However, when one or more flashes reach their maximum available output these ratios, and the desired exposure may not be achieved, and those flashes will blink their ready lights for a few seconds to alert you to the problem.

I do not believe that the algorithms for balanced (TTL-BL) flash mode are used when controlling multiple flash via wireless CLS. This is likely because one of the key variables used in the balancing act, the flash to subject distance, is unknown with wireless flash. You can, of course, still balance manually using shutter speed to control ambient exposure and flash EV compensation to control the flash exposure.

This is all based on my reading, testing and experience using multiple flash iTTL. Unfortunately Nikon does not supply much detail concerning how CLS works, and thus leaves it up to us to figure out how to use it effectively. Fortunately, with digital it is easy to try a few test shots and determine how things work in any particular situation.

 

I had no idea there were multiple test shots being fired. In fact, I never thought about how CLS worked in TTL mode. I just knew it worked.

 

GVK,
I never thought about the EV difference the way you presented. Thanks! I think I will have better control now in TTL. Say on the Flash Menu you set
Group A : TTL +1 EV
Group B : TTL -1 EV
A difference of 2 reflecting a ratio of Group A:B = 1:4
My question is what if you set the flash compensation via the button on the left of the lens mount and turning the dials to -2 (for discussion purposes).
Does this mean that you are asking TTL/CLS to do a -2 flash compensation then what it gets from the internal metering but do a 1:4 ratio between Gp A and B?

Thanks

 

Maybe someone with a Fuji S5 Pro (or D200, that I believe is similar) can answer regarding setting flash EV compensation on the camera. On my D2X the camera's normal EV compensation affects both the flash and ambient exposure. There is no built in flash, and thus no flash specific compensation control on the D2X. I use an SU-800 that allows me to set mode and EV or power level for each of three flash groups.