Couple questions about ETTL metering.

[TheBiles]

I just have a few specific questions about using my 580EX II in ETTL mode with my 60D. All questions refer to shooting in manual mode, by the way.

1. Please let me know if I've got this right: When the flash is in full ETTL mode, the power is set to give appropriate exposure to the object that is in focus and the camera's built-in light meter is set to give you the settings for correct background exposure. The manual was very vague on this, and I wanted to make sure I understood what exactly was being metered.

2. Related to the first question, but how much do I need to pay attention to the camera's light meter? When I'm shooting indoors in a fairly close environment, I've had fine results so far just setting the shutter speed at something that prevents camera shake (1/100 or so) and letting the flash do the rest. Since the rooms are small enough, the background is well-illuminated by the flash (especially when bounced off the ceiling). Otherwise, it seems like the built-in light meter is telling me to expose the image for just as long as it would without flash (wouldn't this cause the subject to be over-exposed?).

3. Does the flash take into account the bounce angle when determining power? I know I can hear it "thinking" when I change the angle, but I wanted to make sure it would be giving me more power for a flash bounced off the ceiling than it would for one fired dead-on.

4. Is there any way to view the power of the flash after a photo was taken? The EXIF data just says something like "flash fired, compulsory."

 

[adamgasson]

The first aspect to consider is what mode you're shooting in on the camera. If you're in aperture or shutter priority with no exposure compensation the camera will expose for the scene in front of, with or without the flash attached.

So if you're shooting in those modes you need to use a bit of exposure compensation on the camera (for example shooting -1 stop will allow the flash to fill in the other stop of light). Bear in mind that the exposure compensation will also lower the flash power, so you need to further compensate for that (either by dialling up the power on the back of the flash - press the centre button and rotate the dial clockwise to do this - or by using the flash compensation button on the camera).

As for your questions:

1- As far as I can tell the distance is relayed to the flash via the lens and the camera will use this to try and expose for the distance if possible. Canon's flash system doesn't seem to work as well as Nikon's - although ETTL II is a vast improvement on the original ETTL. Unfortunately a scene of dark subjects will still overexpose and a scene of light subjects will underexpose. Again use your compensation buttons to help overcome this.

2- I think that's kinda covered in the first couple of paragraphs regarding the metering. I would say though that if you're not bothered about keeping ambient light I'd shoot in manual mode (on the camera) and set the shutter and aperture to what you want. If you want ambient do a test shot, in manual mode, until you get the ambient you want, then put the flash on. If you're bouncing off the ceiling remember that you'll probably need to increase the flash compensation (and also take into consideration the white balance as a lot of ceilings aren't perfect white!)

3- The 'thinking' is the flash head zooming back and fore and not any kind of calculations. In a TTL mode the pre-burst should indicate that more light is required but I've typically found that you need an extra stop of flash power in a regular home (depending on ceiling height).

4- Not that I'm aware of. If you shoot the flash in a manual power mode I think it records that data somewhere in the EXIF.

It's well worth playing with the manual settings. In the long run you'll get more reliable results and, over the years, I've found TTL has a nasty habit of blasting out a full power flash when it's not connected properly (or just bored and wants to blind your subjects!).

Hope that makes a little sense!

 

[lensflair]

I'm a Nikon shooter but I think I may still be able to answer some of your questions.


1- With the Nikon system the camera does use distance information based on the focus points along with the preflash to determine how much power to put out. BUT if you put the flash into a bounce position it doesn't use the distance info anymore, only the preflash. To test this set camera on a tripod and take a shot of something with flash (non bounce). Put the camera into manual focus and try pulling focus back to min distance and out to infinity taking shots at each extreme. You should see your subject gets underexposed as the flash puts out less power when it thinks the subject is closer, and the opposite for focused at infinity. Now do the same test with the flash in a bounce position, if all the shots are the same flash exposure then it is dropping the distance info in bounce position and using the preflash only to determine flash output. This is how I would expect it to work but you will have to check for canon.

2- in TTL flash modes, the flash is using a pre flash to determine how much power to put out, you can change your aperture and shutter speed and the flash will adjust itself (within its capabilities) and still give you the same exposure. Use flash exposure compensation to adjust flash to taste. Now the question about the light meter is interesting because with the Nikon system it watches your meter and actually puts out more power if your meter says you are underexposed, which is why using exposure compensation on the camera body can reduce flash output (the EC biases the meter). I'm almost positive this isn't the case for canon and I wont go into it anymore because it can get confusing to say the least.

3. I don't think the bounce angle is used, how would the camera know what angle your light is going to hit your subject and how many surfaces its bouncing off. As Adam syas its the zooming of the flash head, you can see it if you look carefully. The pre-flash is used to determine flash output. In theory it shouldn't make any difference but the Nikon system also often requires a bit of extra flash power dialled into the FEC.

I really like TTL when I am using the flash on camera, either bouncing it or using it for fill outside. I always have the camera in manual mode and ride the FEC dial on the flash. As soon as I take the flash off camera I switch to manual mode though.

You will find a ton of useful info on TTL flash here http://neilvn.com/tangents/

 

[Somelier]

As I see it from using my 7D plus 580 EXII, the camera measures the amount of light, from both ambient and flash and cuts off the flash when enough light has reached the sensor.

I use aperture priority and, depending on what I am shooting, I set the shutter mode when using flash to suit the situation - eg when I'm shooting flowers, I want full balance between ambient and flash so set the mode to allow fully automatic shutter speeds, but if I'm taking pictures of subjects that may be moving, I set the shutter mode with flash to "auto 60th to 250th".