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View Full Version : Flash Question, 550EX and 1Ds MKII


Paul Caldwell
January 29th, 2008, 09:10 PM
I have a question, in regards to how a 1ds MKII and 550EX should work together. Over the years, my use of flash has been very limited and when I do use one, itís always been in M mode on the Camera.

I have recently started event shooting and wanted to expand my flash range, by using either P mode or Av mode on the camera. When I started shooting like this, I ran into some strange issues on the camera.

Here is a situation I have run across and wanted to ask the forum about.

In a room dim room, ISO 125, evaluate metering, in P mode on the camera, I get the following 1/60 at F2.8. If I switch to Av mode, and set the aperture to F2.8, my shutter speed drops to 0.8, which is way to slow to hand hold, but more importantly, I donít understand by it wouldnít stay the same as the 1/60th I had while in P mode. If you expose the frame, you get the 0.8 shutter speed. It also seems to me that the P mode of 1/60 and F2.8 is way to low an aperture.

I have tried 2 different bodies and both seem to get the same type of reading with this flash. I am wondering if there is something wrong with the flash in that itís not sending the correct information to the camera.

I can switch to M mode and in the same room I can get a good exposure at 1/100 and F5.6.

My question is does it seem possible that my flash is not working correctly when the camera is in P or Av mode? I donít have another E-TTL flash that I can put on the camera to test.



Thanks
Paul Caldwell

Peter Galbavy
January 30th, 2008, 12:50 AM
What you describe is normal for Canon cameras. In Av and Tv mode they meter for ambient light and the flash is used for fill.

You must read this set of articles before trying to seriously use a flash with any Canon EOS cameras: http://photonotes.org/articles/eos-flash/

Paul Caldwell
January 30th, 2008, 09:46 AM
Thanks Peter,

I did read the photonotes.org article, but it just seems strange to me that in Av mode the camera would attempt to take a picture at 0.8 of a sec, which is way to slow to hand hold. And yet in P mode it's at 1/60 at F 2.8.

Paul Caldwell

Doug Kerr
January 30th, 2008, 10:24 AM
Hi, Paul,

I have a question, in regards to how a 1ds MKII and 550EX should work together. Over the years, my use of flash has been very limited and when I do use one, itís always been in M mode on the Camera.

I have recently started event shooting and wanted to expand my flash range, by using either P mode or Av mode on the camera. When I started shooting like this, I ran into some strange issues on the camera.

Here is a situation I have run across and wanted to ask the forum about.

In a room dim room, ISO 125, evaluate metering, in P mode on the camera, I get the following 1/60 at F2.8. If I switch to Av mode, and set the aperture to F2.8, my shutter speed drops to 0.8, which is way to slow to hand hold, but more importantly, I donít understand by it wouldnít stay the same as the 1/60th I had while in P mode. If you expose the frame, you get the 0.8 shutter speed. It also seems to me that the P mode of 1/60 and F2.8 is way to low an aperture.

With a flash unit active, the Canon exposure metering system separately treats the "ambient" aspect of exposure (pertaining to the parts of the scene not substantially benefiting from the flash illumination, which we often speak of as the "background") and the "flash" aspect of the exposure (pertaining to the parts of the scene substantially benefiting from the flash illumination, which we often speak of as the "main subject") The way it does this may seem curious when first encountered.

In Av or Tv mode, with a flash active, the camera basically sets the exposure (by setting shutter speed or aperture, respectively), based on the measured luminance of the scene under "ambient" illumination) to seek proper exposure the "background" (as defined above).

The flash part of the metering system than chooses a luminous output for the flash that, taking into account the aperture already set or determined, should produce proper exposure of the "main subject".

In P mode, the same is true except that the camera will not set a shutter speed less than 1/60 sec or an aperture less than f/4. The rationale is that, by choosing P mode, the photographer is letting the camera make many decisions, and one decision it is programmed to make is to avoid either motion blur (from two slow a shutter speed) or too small depth of field (from too large an aperture).

If the "background" has a low ambient luminance, the result of this will be underexposure of the background. The philosophy here is that the "main subject" (defined above) is of highest priority, and with respect to the background, "underexposed is better than blurred". And to the photographer, by implication: "if that isn't what you would have decided, then you do it."

When the photographer selects AV or Tv mode, he is in effect saying to the camera, "I'll make the decisions myself." The camera then follows its basic rule: "expose to produce proper exposure of the background under its ambient illumination." Then, as before, the flash portion of the metering system then sets the flash output to attain proper exposure of the "main subject".

But in fact the photographer, as the camera does in P mode, may in fact want to avoid motion blur, even at the expense of underexposure of the background and/or shallow depth of field.

To do that, we can set the mode to M, set the shutter speed to an appropriate value in consideration of the motion blur issue, set the aperture to an appropriate value considering the depth of field issue, and proceed. As always, the flash part of the metering system then sets the flash output to attain proper exposure of the "main subject". As in P mode, the result may be underexposure of the background - perhaps substantial. If that is not acceptable, there is no choice but to make some compromise with the motion blur or depth of field considerations.

In brief, your flash is probably working just as intended.

Paul Caldwell
January 30th, 2008, 11:39 AM
Thanks Peter,

I guess that I was better to just stay on M mode, as in most cases Av will not give me a hand held shutter speed and P mode picks a aperture that IMO is way too shallow. Thought I was missing something here, but I guess not.

Thanks
Paul Caldwell

Doug Kerr
January 30th, 2008, 11:59 AM
Hi, Paul,

I guess that I was better to just stay on M mode

That's what many people do much of the time. I usually do when using flash in a poorly-lit environment.

We only really have two choices: let the camera make the decision for us (selecting from a limited number of preordained "strategies") or make the decision ourselves!

Perhaps in the future we will be able to describe to the camera what our objectives are in a particular situation, and what our priorities are in resolving conflicts, and it will make choices that serve us. Until then . . .

Best regards,

Doug

Steve Saunders
February 14th, 2008, 05:07 AM
That's one niggle I have with Canon as well and one that Nikon tackles better. The allow you to set the flash sync speed in A mode. It's just a bit more fiddly with the Canon.

Doug Kerr
February 14th, 2008, 07:11 AM
Hi, Steve,

That's one niggle I have with Canon as well and one that Nikon tackles better. The allow you to set the flash sync speed in A mode.

What does that mean? Do you mean that on Nikon cameras you can set the maximum shutter speed that the camera will adopt in A mode with a flash active?

Thanks.

Best regards,

Doug

Tim Dolan (Longwatcher)
February 14th, 2008, 01:47 PM
Thanks Doug for the great explaination,

I just knew I either ran both the flash and camera in full auto or I ran them both in full manual mode, anything else usually produced crappy results.

I have noted that for whatever reason (and it may have been circumstances ), that my 550EX seems to work better with the 1DsMkIII then it ever did with the MkII. But the experience is still limited compared to the MkII.