The Portrait Lighting Ratio Convention

Lighting ratios for still photography express the difference in reflected bright ness on the highlight side of the face vs the shadow side. A ratio of 2:1 indicates the highlighted side of the face reflects twice as much light at the shaded side. The shadow side is always expresses as the value "1" because the convention was established in the early days of portraiture on negative film where the practice of placing the fill near the camera axis where it is "neutral" and illuminates both side of a full-face view evenly while the key light from the side only hits some parts of the face. To understand the ratio math you should also understand that under the convention the key light is assumed to overlap even fill.

Lighting ratios in a studio setting are usually made with an incident meter. An incident meter reads the light hitting the subject, not what is reflected and recorded by the camera. An incident meter is placed near the face of the subject and aimed at each light separately. The strength of the light, not how much is reflected from the face is measured. Thus if key and fill are both the same strength the incident readings will be identical. The reflected ratio is inferred from the incident readings based on the underlying assumption here is that fill is on the camera axis illuminating the entire front of the face and the key overlaps it like this:

H:S
1:1 one unit fill hits both sides of face
1:0 one unit key hits left side only
===
2:1 H:S ratio

The key added to fill appears 2x as bright as the shadow side. If measured with a spot meter the highlight side would be found to be one f/stop brighter. Thus this arrangement of lights in still photography is referred to as 2:1 highlight/shadow ratio.

When key is 1 stop (2x) brighter than fill, the ratio is computed this way:

H:S
1:1 one unit fill hits both sides of face
2:0 two units key (2x intensity as fill) hits left side only
===
3:1 H:S ratio

The key added to fill appears 3x as bright as the shadow side. If measured with a spot meter the highlight side would be found to be 1-1/2 stop brighter. Thus this arrangement of lights in still photography is referred to as 3:1 highlight/shadow ratio.

Those ratios will always produce the same results. In time your eye recognizes what a metered 2:1 or 3:1 looks like from the modeling lights and in the photo and on the screen and prints.

How Canon A:B ratios are different

In a Canon ETTL ratio 2:1 means the A group flash is 2x brighter, based on measurement of reflected light than the B group. In the EX system the Master flash connected to the hot shoe defaults to Group A, requiring the Slave flash to be placed in Group B for the wireless ratio magic to work. So a 2:1 A:B ratio would make the fill on the camera 2x brighter than the off camera key light, a recipe for muddled flat lighting.

Canon adds even more confusion in the way it illustrate wireless ratios in the flash manuals. This duplicates the illustration from the top of page 33 of the 580ex manual which I have reproduced below:

Note how the "A" and "B" flashes are each on opposite sides of the face. That's how the engineers at Canon apparently think a face should be lit. But that's horrible lighting for a face and not even very good for paper towels! Note the dark band down the middle in all of the test shots below taken with three 580ex in the configuration show above:

Canon doesn't assume key will overlap fill when computing its lighting ratios!

The Canon system computes ratios by first firing "A" group only, then "B" group only, then both together and then comparing the reflected brightness.

A:B
1:0
0:1
===
1:1 So if both sides reflect the same amount of pre-flash Canon calls it "1:1"

In that configuration either an A:B = 2:1 or A:B = 1:2 will produce shadows and highlights which are similar to a conventional overlapping pattern of two indentically powered lights:

CANON
A:B=1:2
1:0
0:2
===
1:2

or

A:B=2:1
2:0
0:1
===
2:1

Both produce 2:1 ratios,
but it is unflattering split lighting with a dark line down the center!

Conventional overlapping of equal key and kill
H:S
1:1 one unit fill hits both sides of face
1:0 one units key (same as fill) hits left side only
===
2:1 H:S ratio

For CANON's 2:1 ratio one group will reflect 2x light off the subject as the other. The TTL metering figures out how much power it will take from each group. The off camera flash creating the highlights must output 2x more power for 2:1 and 8x more for 8:1. We never actually know how hard each flash is working relative to its maximum power until one reaches its limit and no amount of FEC can correct the exposure

With manual overlapping flash equal power = 2:1 ratio. Both flashes operate at the same level and recycle time and output relative to the unit's maximum can be selected and control.Neutral fill is not only a more efficient, it also produces much more flattering lighting.

Canon apparently thinks we should buy a third 580ex or ST-E2 controller to trigger the off camera A and B flashes. The more logical and economical arrangement with the Master (Group A) on the camera or bracket which I recommend in my multiple flash tutorial (shown below) is not shown in the 580ex manual or others that I am aware of.

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When two Canon flashes are used as "A" Master (Fill) on a camera bracket with the "B" Slave (Key) off camera we enter territory the Canon system ratios can't comprehend. Remember those engineers who designed it and people who wrote the manual apparently assume A and B will be creating highlight and shadow separately without overlap. You may be thinking, "But A and B might not always be used in an way that they overlap!" That's true and there is no problem with that creatively, but we can't compute lighting ratios if they don't, at least not in the conventional way they have been used for 150 years.

Canon isn't going to change its system, so if we learned lighting ratios with conventional studio lights and a hand held meter, we must relearn ratios on Canon's terms when switching to wireless TTL. If we start with the Canon TTL system and then graduate to manual studio lights, or simply use the Canon system in M (manual) mode then we will need to learn to compute ratios the old fashioned way.

Canon "New Think" is only necessary for TTL. When used in wireless manual power mode the Canon flashes are like any other manual flash, but with one significant difference. In wireless M mode the master still sends visible coded control pre-flashes to the slaves which will prevent the use of a hand held meter or other types of non-EX series optically triggered slaves.

Using any TTL system is like shooting a gun with a sight which hasn't been adjusted for distance and windage. We aim at the bulls-eye but if the shots all wind up high we just adjust the aim point (i.e., EV for ambient or FEC for flash) a bit and start hitting the target dead center every time.

With a single flash all we need to worry about is FEC (Flash Exposure Compensation). We take a shot at FEC = 0, "chimp" the LCD and histogram. If the light reflecting from the scene is fooling the TTL metering of camera we add more or less flash via FEC. When using two flashes in an A;B TTL configuration we still must use FEC to get the highlights exposed correctly, and then control the amount of shadow detail using the A:B ratio we dial-in.

It doesn't matter what the A;B ratio numbers wind up being compared to conventional H:S ratio numbers. We just need to know from experience what 1:1, 1:2, 1:4, and 1:8 actually look like on a face and which are best suited for babies, young kids, teen age girls or boys, young women, mature women, executive portraits, character studies of knarly old guys, etc. We can get that experince in about 30 minutes by shooting a face or other test scene with all the ratios from 8:1 to 1:8 and comparinging them side-by-side. That is the first thing I did when I got my pair of 580ex and my test is shown below:

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You will note that the A:B ratios 8:1 to 2:1 look similar. That is because I used the Group A Master as fill on my Stroboframe camera flip bracket (connected to the hotshoe with the TTL extension cord 2). In neutral fill configuation like that the "A" fill will overpower the "B" key light until both are indentical in power where the hit the subject. At A:B = 1:1 the highlight/shadow difference is noticable and as the ratio increases to 1:2, 1:4, and 1:8 the TTL metering, once I had zeroed in on the correct amount of overall FEC, did a very good job of keeping the highlights correctly exposed as the shadows become darker. I would encourage all Canon flash system users to perform a similar test.