When i-ttl flash works, and when it won't

i-ttl works here, with one flash bouncing to the left of these meeting attendees.

Now that I have a set of Radiopoppers to play with, I’ve been thinking quite a bit about how ttl and i-ttl flash works with my Nikons. I’m going to assume that the criteria are roughly the same with Canon flashes and cameras, though I don’t use Canon.

Specifically, I want to do more with the techniques that I already have under my belt, and I want to add in a bunch of new stuff as well. From my own experimentation (read: screwing up pictures that would otherwise be okay) to even breaking down and re-reading my instruction manuals, I’ve come up with a few general rules that help me work and also give me a point of departure for when I want to try new stuff.

One little notation that I read in the instruction manuals for my D200 was that I-ttl flash will not work when the meter was set to “spot metering” mode. As I rarely ever spot meter anyway, except to maybe double-check a problem area in a photo, I thought, “umm, okay, I wont have to worry about that.” But it turns out that I sometimes set up my lights so that one or more of them is lighting just a small section of a photograph, maybe the equivalent of nearly the same number of pixels that might be read if I were spot metering, and it’s not infrequent in those situations that my i-ttl exposures have been off the mark.

Basically, when you’re working in i-ttl mode, your camera is doing all the metering for you when the pre-flash fires and is “deciding” how much power your remote flashes should be putting out based on what the matrix metering program tells it. Even though matrix metering is pretty fancy, I think it is largely trying to accommodate for a handful of fairly common photographic situations. For instance, it looks at about top one-third or fourth of your image area and “guesses” that this is probably the sky, so if it’s bright it lets it stay bright and doesn’t skew the entire exposure by stopping down.

Your meter understands this situation: bright sky at the top,
darker stuff below, subjects reasonably prominent in the frame.

Similarly, it’s going to assume that your main subject is going to be a pretty big blob somewhere in the lower two-thirds of the photo area, and it may take up more room than that. So it decides that’s the area you want properly exposed, and after sampling light values from a bunch of different spots it creates an exposure plan for you. If you’re shooting on one of the ‘automatic’ modes like Program or Aperature Priority it just sets the camera for you, or if you’re on Manual it tells you whether your current settings are over or under or just right.

Where does this take is, flash-wise?

In my case I really like setting people off from the background with a bit of an accent light, which is to say a light that just gives a bit of lighter definition around the edges of a subject so that they stand out from the background. This light can come from behind and above, as it often does in movies and in almost every case where you see a TV anchorperson sitting at a desk, or it can come from the side, or from directly behind on the same level with the subject. Most often them this is called a backlight, but most of the time I use it from the side. That’s how I roll.

the "accent" flash coming from the right is a little hot, working on i-ttl

Because this light is only going to expose a very small area on my subjects, it just isn’t something that can be metered accurately. Most of the time if I try to set that accenting flash to ttl, it ends up putting out waaaaaay too much light.

..that's because it is illuminating a very small area of the photo,
shown here in red, and the camera's meter just doesn't get enough information,
overall pixel-wise, to make an accurate exposure from that flash.

So I can’t really rely on those side flashes to expose properly when they're at extreme angles unless I’m shooting manually, and then I have to watch my light-to-subject distances. As people get closer to the flash, the light striking them is going to be brighter than when they’re further away. I’ve tried instructing the accenting flash unit to underexpose by a stop or two using i-ttl, but that still is a crapshoot because the camera’s just not getting enough information from the light bouncing off the subject, in most cases. In some cases you can set the flash on it’s Automatic setting (AA on SB-800’s triggered remotely) but I’ve never trusted flashes set on automatic. You don’t know what they’re looking at and they are more easily fooled by some of the common metering pitfalls (a bride in a big white shiny dress dancing with a guy in a dark black tux, for instance) than most cameras are.

This is why, for instance, in the RadioPopper manual they say you’ll have better results setting your remote flashes on manual than using them in i-ttl mode. By “better” I think they must mean “more consistent.” Your flash is always going to know what 1/4-power is, whereas your camera may have a lot of trouble picking out one little point in the picture that you’re lighting up and determining a correct i-ttl exposure.

And that is why it’s a very good idea to get to know how much light your flash puts out when you set it at the various fractions of the full manual setting. Here’s a tip: it doesn’t matter what you set the ISO at on the flash. If it’s on manual, it’s putting out the same power whether you set that at 100 or 1600. Take some test exposures at 1/2, 1/4, 1/8, and so on until you’ve become VERY familiar with them. Use a flash meter or just chimp it on your screen to see what the results are. Once you’ve figured out that your SB-800 puts out f/8 at ten feet at 1/4-power when your CAMERA (not the flash -- it’s dumb) is set on ISO 400, you can extrapolate from there what happens when you move to a higher or lower ISO on the camera. Knowing this will save your bacon again and again.

Mmmmmmmm, bacon..

When I’m shooting a wedding reception, the trend seems to be to turn the lights in the room down so that you can barely read the dials on your camera, particularly once the dinner is over and the dance floor opens. Maybe that’s so nobody sees just how awkward the dancing can be. These are the situations where I’m using multiple flash setups, and where I’ve had a lot of trial and a lot of error with i-ttl. Until I get my D3, I’m reluctant to jack up my camera’s ISO up to above 1000 so that I can shoot at faster than 1/4th-second in these situations. I’d rather face up to a lighting challenge than have to deal with a lot of sensor noise after the fact.

1/8th of a second at ISO 1000, lit with flash to brighten the main subjects.
I will shoot this slow once in a while, but don't like to do it throughout.

In what situations does i-ttl succeed?

It seems to do very well in those times where you have enough available light to be able to make a decent exposure and you can use your flash to simply complement the ambient.

flash to the right is just filling the subjects here.

And it works pretty well where it’s your main light, illuminating enough of the subject that you can get a good matrix meter reading.

the camera's meter can read a large area of the frame here
lit by flash, and can deliver a pretty accurate i-ttl exposure.

And it seems to do quite well when the subject that you’re lighting fills up a goodly part of your frame, like at least as much as 1/4th of the photo area and preferably more. That's the case with the first picture in this posting, where the light from the flash is illuminating enough of the subject area to give a proper exposure.

I'm sure I'll have many more chances to discover when and why it fails, and from that I expect to learn when I can really use it to full advantage.


Aaron Oster said...

Very thoughtful & useful post! I use an SB-28 with a D50. Though I have a pretty good feel for using it on camera
in manual, I've been considering getting an I-ttl flash for added flexibility. In a wedding or event situation, how often do you use TTL flash and ride the FEC? That seems to be what a lot of pros do.

Thomas said...

Hi, Aaron..

It's pretty rare that I switch to TTL when I'm working a wedding, unless it's a situation where I have a fair confidence that I'm not giving my meter a complicated area to read.

So, maybe when the bride is prepping in a hotel room I might, but probably not during a reception (when you're in a big dark room, photographing a woman wearing a bright white dress dancing with a guy in a jet black tuxedo.)

Most of the time, even in "uncomplicated" situations, I go for manual flash because I know it's going to put out exactly the same power level each time. That way, it doesn't matter if the bride turns toward me during portraits (thus reflecting more light back off the front of her dress and making my camera think the flash is bright enough before it really is) or turns to the side (and reflects less light from the larger dress surface.)

Once you get a feel for how much light your flash is putting out on manual, you get pretty comfortable picking a setting and running with it, and with Nikon's i-ttl control you can always just adjust it from the camera if it's not what you want.