First, thank you for all the excellent information you provide. It is great to have a one-to-one about tech questions. Now my question: I replaced the focusing screen in my EOS 5D with the "Ee crop lines focusing screen set." This screen is not listed in the manual. Do I need to make any changes to C.Fn-00? Does changing the screen affect the auto focus?

Using the optional focusing screen with crop lines (as opposed to cropping mask) does not require any change to C.Fn 00 on your 5D, and neither AF nor AE will be affected. If you use one of the screens with the cropping mask, your AF will still be good but you will need to restrict yourself to Spot metering for accurate auto exposure.

I have questions about how quickly Image Stabilization (IS) kicks in:

1: If I'm shooting street photos rapidly and I autofocus with the shutter button (half-pressing then shooting very quickly) am I losing the IS (does it take a half second or so to kick in? And does the camera fire before it has kicked in?

2: What if I focus with the thumb exposure button: once the focus has been set with that, does IS remain functional until I shoot?

3: What about in manual focus: when does IS activate then? Or does it?

4: What's the best way to street shoot and have IS working, assuming some of the shots are going to require quick shooting? (I'm aware that IS is less necessary at 35mm or less, corrected for crop factor, but I'd still like it there, as I'm more likely not to hold the camera steadily under such circumstances.)

When using Canon EF or EF-S lenses equipped with an Image Stabilizer function, it does in fact take about half a second after it has been activated for the function to become fully effective. Therefore, if you want to take full advantage of this feature, it may be necessary to modify your shooting technique to allow enough time for the system to work. With EOS Digital SLRs, Image Stabilization works in both autofocus and manual focus, so you're free to choose any focusing method you prefer. If you set up your Custom Functions with an EOS Digital SLR so that AF is operated with the AE lock button on the back of the camera, you can activate IS with either the AE lock button or the shutter release, and the choice between the two becomes a matter of personal taste. If you need to shoot so spontaneously that there's not enough time for IS to become fully effective, you may want to consider helping yourself out by using faster shutter speeds or flash to minimize or eliminate motion blur. This may involve using higher ISO speed settings than you're used to, but with the excellent low-noise characteristics of current EOS DSLRs, that's not much of a concern any more.

I own a Canon EOS 10D and am shooting weddings with the 550EX flash, CP-E3 battery pack, and an EF24-70mm f/2.8L lens. I come from over 20 years of shooting weddings with MF and manual flash. I shoot with the camera on manual, typically ISO 400, and typically at f/5.6 with an Omnibounce. I can't seem to get consistent exposures even though I am not focus-locking and recomposing. Short of moving to a new camera, is there anything I can do to achieve greater consistency?

The EOS 10D, which was replaced in 2004 by the EOS 20D and subsequently in 2006 by the EOS 30D, used the original version of E-TTL, which has a flash metering pattern that is very sensitive to variations in the reflectance of subject matter. Because of this sensitivity, wedding photography can be difficult with this camera unless you essentially "trick" the flash metering system into an averaged flashmeter reading that looks at the entire picture area rather than the small zone around the active focusing point. There are two ways to do this when using Canon Speedlites with an EOS 10D:

1) Set the focus mode switch on the lens to manual. This setting forces the flash metering to be averaged across the entire picture area.

2) Leave the focus mode switch on the lens set to AF, but use Custom Function 4-1 on the 10D and refrain from autofocusing during the exposure. C.Fn 4-1 initiates AF from the AE lock button on the back of the camera, but it averages the flash metering pattern whenever the AE lock button is not being pressed. Using this approach, you would autofocus the subject first by pressing the AE lock button with your thumb, and then make sure to lift your thumb off the AE lock button before taking the picture.

Either of these techniques should improve the consistency of your flash exposures during wedding photography with the EOS 10D and a compatible Canon Speedlite, but if you're looking for something better than that, I would recommend upgrading to a current EOS model with E-TTL II.

I have an EOS 30D and a Speedlite 580EX. I was thinking about buying a Speedlite Transmitter ST-E2 and also a Speedlite 430EX so I can use both flashes off-camera. Will it work? Will it work on TTL? Or do I have to buy another 580EX so I can work with both flashes off-camera in TTL mode?

Speedlite Transmitter ST-E2 can control any number of off-camera 430EX, 550EX, 580EX or 580EX II Speedlites in E-TTL/E-TTL II or Manual flash exposure mode as long as they are set to slave mode and they are in range. The 550EX, 580EX and 580EX II can also be used as master units to fire off-camera 430EX, 550EX, 580EX and 580EX II Speedlites set to slave mode.

I've just bought an ST-E2 and I can't make my Speedlite 580EX fire in manual mode when it's not on camera. When I try to change the mode in the strobe head to manual flash it just doesn't change. I didn't find any reference in the instructions about using a flash in manual mode off camera with the ST-E2. How can I do it?

In this case, switching to manual flash exposure mode is done on the Speedlite when it is set to Slave mode, by holding the Mode button down for several seconds.

I just purchased an EOS-1D Mark III and thoroughly enjoy just about every aspect of it. I upgraded from the 20D so you can understand it was a large upgrade. My question to you is, I am having problems with AI Servo tracking of birds in flight. Correct me if I am wrong, but my understanding is that once you lock into the bird with the center focus spot the tracking should follow the bird even when it goes outside that area. My camera seems to lose the focus the moment the bird leaves that center spot. My question then is, assuming this is a problem, how can I address it? I did notice that on the forums that some people have different firmware versions. Will I need to send my camera to Canon Repair (not my 1st choice) or will there be a firmware upgrade to address this? Or perhaps there is some kind of custom function I need to adjust. Any help in regards to this will be greatly appreciated.

If you have selected the center focusing point manually and all other settings are at their defaults, then the center focusing point is the only one that's active. If you want the camera to track the bird using all 45 focusing points, then you must set the camera for automatic focusing point selection. When AI Servo AF is combined with automatic focusing point selection, focus tracking starts from the center point, as mentioned on page 83 of the EOS-1D Mark III owner's manual.

"When the AF point selection is automatic (p. 84), the camera first uses the center AF point to focus. During autofocusing, if the subject moves away from the center AF point, focus tracking continues as long as the subject is covered by the Area AF."

Page 84 shows how to set the camera for automatic focusing point selection. Hope this helps! By the way, I'll have more on EOS-1D Mark III autofocusing in the next edition of Tech Tips.

Thank you very much for taking the time for responding to my question concerning the 420EX flash not firing on my 1D Mark II in your June Tech Tips. Your insights are greatly appreciated. I wanted to follow up to clarify the nature of the issue. As mentioned, my 420EX -- very rarely, without apparent reason, but sometimes as many as 10 times in a row -- fails to fire on my 1D Mark II in single shot mode, even with high ISO settings and large apertures, where flash output should be very minimal. In these instances, all indications are that the flash is ready and more than amply charged for the light called for by the metering. Likewise, the green indicator light illuminates after the shot, incorrectly suggesting that the flash did fire. The Exif data even shows that the flash fired, although the shot is underexposed as if the flash did not fire at all. I attempted to reproduce the problem by using the 420EX on a 20D body, and by using a different flash, but of course, when you want the problem to repeat so that you can analyze it, it won't! Likewise, even the 420EX / 1D II combo will go thousands of shots without exhibiting the problem at all. It seems to be a truly mysterious phenomenon. Any further insights you have now or in the future would of course be appreciated. Likewise, if I ever figure it out, I will plan to drop you a line to give the story an ending. Thanks again.

Thanks for clarifying the problem. It may very well be that the combination of lighting conditions and camera settings you've selected in terms of ISO, aperture value, flash-to-subject distance is such that the 420EX cannot provide a proper exposure. One way you could troubleshoot the issue would be to run through a series of test exposures at various apertures, starting with a relatively small one like f/8 and then progressively working your way towards the maximum aperture of your lens. This would enable you to determine when you're getting close to the limit of what the 420EX can handle in terms of accurate exposure with minimum output. If it turns out that you need more control, you may want to consider using a bounce card or diffuser to force the Speedlite to produce a longer burst than it ordinarily would in a direct flash situation.

Thanks for reading Tech Tips! That's it for now. See you in August!

You are invited to submit questions about photo equipment, imaging technology, or photo industry trends that may have a bearing on your work or interests. I cannot promise to answer everything, but I pledge to do my best to address the issues that concern you. (Please use the e-mail link provided at the end of this article.)