Off center flash using Sigma EF 500 flash

[bongvr]

Hi all. I'm planning to buy a Sigma EF 500 DG Super flash for my Drebel, but since I read somewhere that the said Sigma Flash only uses one focusing point (center?) unlike the 550ex which can use all 7. Is it possible for me to shoot off center portrait?... another concern of mine is that although the Sigma flash has FEC feature, but the rebel body doesnt have that feature (except for the hack firmware), can I still make use of the flash FEC? Pardon in advance if my questions are basic(maybe dumb?) for some but I'm a newbie and I would appreciate any advice.

I planning to use it for a friend's wedding next week and I'm worried that Sigma wont give me a clear photo if the subject is off center.

[MatsP]

I've got absolutely no knowledge of Sigma's flashes, but as far as I can figure, it's the camera that decides which AF point to use for the flash metering, and the flash has nothing to say in the matter.

As I understand the protocol between flash and camera, it works something like this:
1. Camera meters existing light.
2. Flash pre-flashes, and gives return light reading to the camera.
3. Camera sets up aperture/shutter according to measured existing light and flash light combined (depending on mode, the camera will do different things here, but we'll discuss the details at some other point).
4. Camera opens shutter.
5. Camera tells flash to light up.
6. Camera measures light from flash, and when "enough", it tells the flash to shut off.
7. Camera closes shutter.

Now, there is a possibility that the flash itself does the assesment of "Sufficient light", in which case it's going to be a "full-scene" measurement, not centre AF point, nor any other AF point. This would be a way to simplify the flash and certainly not in line with the ETTL method that Canon uses.

The only sure way to find out what the Sigma flash actually does is to talk to the people at Sigma, I should think. Or buy one and try it out.

--
Mats

[Puncher]

What is meant by the flash only using the center focus point is that the AF illuminator only covers the center focus point , ie. the illuminator doesn't project a big enough target for that to be covered by all the focus points.

[MatsP]

Originally Posted by Puncher

What is meant by the flash only using the center focus point is that the AF illuminator only covers the center focus point , ie. the illuminator doesn't project a big enough target for that to be covered by all the focus points.

Aha, ok, that makes more sense.

I would think that this only matters if it's quite dark, so for most of the wedding photos of the Original Poster, it wouldn't make any big difference.

When it gets quite dark, the camera will need some extra light to help it focus, and at this point you'll get three possible results:
1. The camera focuses correctly on the subject.
2. The camera focuses at something other than the subject.
3. The camera can not focus.

#2 of the above is the "bad case", because #1 is what you want, and #3 will cause the camera to not be able to take the photo.

There are some things the photographer can do to help the situation:
1. Use manual AF point selection. This way, you will know which AF point is active, and if you choose one that isn't in the center, the AF assist light shouldn't make any difference. The greatest chance of #2 happening is when the camera is set for "all AF points", because it will pick something in the middle that is lit by the AF assist light, rather than the person(s) on either side.
2. Use manual focus. This can be hard if the lens doesn't have a distance scale, but if it does, you should be able to "guestimate" the distance even if you can't see to focus yourself.

As a tip, you may also want to try using manual settings for the shutter/aperture. If you're shooting at a wide-angle, you may for instance set it to 1/30, F4 or F5.6, so that you get some DOF in the picture. At a longer lens setting, you may want to shorten the time a bit. Generally, the camera will prioritize for time, rather than give a decent aperture.

You should also be able to use the flash exposure compensation, just like on a 550EX. This is the main reason some DRebel customers are choosing the 550EX over for instance the 420EX.

Best of luck.

--
Mats

[Rune]

Originally Posted by MatsP

1. Camera meters existing light.
2. Flash pre-flashes, and gives return light reading to the camera.
3. Camera sets up aperture/shutter according to measured existing light and flash light combined (depending on mode, the camera will do different things here, but we'll discuss the details at some other point).
4. Camera opens shutter.
5. Camera tells flash to light up.
6. Camera measures light from flash, and when "enough", it tells the flash to shut off.
7. Camera closes shutter.

No, the flash does most of the work. With A-TTL the camera would measure the light hitting the film plane, but that mechanism is long gone with E-TTL where the preflash is used exclusively.

All about E-TTL (and then some): http://photonotes.org/articles/eos-flash/

Note that E-TTL was improved (by using distance information from lens, plus a more average/evaluative reading vs spot reading) in 1D-Mark II and 20D.

[MatsP]

Originally Posted by Rune

No, the flash does most of the work. With A-TTL the camera would measure the light hitting the film plane, but that mechanism is long gone with E-TTL where the preflash is used exclusively.

All about E-TTL (and then some): http://photonotes.org/articles/eos-flash/

Note that E-TTL was improved (by using distance information from lens, plus a more average/evaluative reading vs spot reading) in 1D-Mark II and 20D.

I stand corrected.

--
Mats

[bongvr]

My concern regarding the AF assist light (which correct me if I'm wrong help the focus point to well..focus and meter).....is IF the subject is off-center and I lock the focus on the subject e.g. on the right side... it fires before the actual flash.... when data already read by the camera, but if you RECOMPOSE the framing, the data in the light assist might no longer be accurate.....which means if the newly wed is for example in the rightmost part of the frame, after recomposing the frame, the assist light (using the sigma) will preflash pointing at the center already (e.g. the altar) ....which most likely darker than the subject, making the flash fire more light, possibly "burning" my subject at the right side, especially the bride who is wearing white adding bounce to the already strong light- hence overexposure.

Regarding the Flash Exposure Compensation (FEC)......if the flash unit has that (like the Sigma) and the camera body (300d) doesnt have that, will it work? how or is it automatic?

[Rune]

The AF assist light isn't the issue here. The issue is that you focus and then reframe the shot. Canon's E-TTL system hinges on the active AF point being positioned on the subject.

The solution is FEL (Flash Exposure Lock). Point the middle AF point towards an area of your subject that will (ideally) reflect about 12% light, and press the FEL button. At this point you can recompose and take the shot. Provided of course that the 300D even has the FEL capability...

This all assumes that the Sigma flash unit even supports E-TTL. If it doesn't, then the flash unit is responsible for figuring all this out. As a sidenote: There are those who hail Metz' auto mode as superior to E-TTL in many cases. So a lack of E-TTL capabilities may not be the end of the world. (unless we're discussing Canon flash units; a 540EZ that doesn't support E-TTL isn't going to do anyone much good)

[bongvr]

I see...I think that's it. Thanks a lot, I cant wait to try it out, now, due to budget constraints, I have to check out both Sigma and Metz. Again, thanks!

[MatsP]

FEC should be fine with the 300D. What happens is that you tell the flash that you want some amount of compensation (+ or -) compared to what the flash would normally give out. So the flash will just do that.

When you use FEC on the camera (on cameras that have that), the camera will just report to the flash a lower (or higher) amount of flash needed for the scene, so it works out the same.

Rune explained pretty well what may be the problem. I'd just like to clarify that the AF Assist light is ONLY there for a single reason: To give the camera a luminated area to focus on. It's got nothing to do with measuring the actual light, this will be done with a preflash of some sort. However, the camera will use the AF point that you used for focusing as the preflash measuring point, so when you move the AF point from for instance the face of the bride to the dress, it will use the dress rather than the face to measure the light from the flash. If the dress is white, this will lead to an underexposed shot (since the camera wants to get about 18% "gray", and white is a bit lighter than that...).

--
Mats

[MatsP]

I just had a quick look at the Sigma web-site, and it claims that the EF 500 flash is able to support E-TTL.


--
Mats

[Rune]

Originally Posted by MatsP

(since the camera wants to get about 18% "gray", and white is a bit lighter than that...)

Minor point, but the ANSI standard says 12.5% grey (12%-13%). Which is why the most recent instructions for Kodak's 18% grey card mentions the need to open up a stop when measuring exposure using the card.

Bob Shell had an interesting article on his website that told the story of why Kodak kept the 18% grey card. Unfortunately he has since removed that article. (Basically Ansel Adams lobbied Kodak in the late 70s to keep the 18% grey card unchanged)

[bongvr]

HI!


I just got my Sigma Ef500 DG Super flash just 2 hours ago and I love it!!! I tested the FEC with my 300d and sure enough, it gave me great results...Have to practice using it coz I have a wedding to cover on sunday....again thanks!