Tech Tips
February/March 2010

by Chuck Westfall

Q. I'm using an EOS 5D Mark II DSLR, but my thought is in general about future DSLRs. I read earlier in your Tech Tips that using video mode does not impact sensor longevity and that video recording uses the whole sensor surface. Considering those two facts, we can think a sensor won't be damaged if continuously exposed to light. I was wondering why camera manufacturers continue to use conventional focal plane shutters in HD-compatible digital SLR cameras. Can we imagine future generations of DSLRs without a mechanical shutter, replaced by an "electronic virtual shutter," only recording sensor data for the defined exposure time? It may enhance DSLR longevity by leaving the mirror the only moving mechanical part and permitting exposure times not available with mechanical shutters.

A. If the only usage of a camera like the EOS-1D Mark IV, EOS 5D Mark II or EOS 7D was video capture, it would stand to reason that focal plane shutters could be eliminated. However, the CMOS sensors used in these cameras still require focal plane shutters for conventional still captures. The reason for it has to do with the way the sensor data is "flushed" prior to exposure. I can't speculate on Canon's future plans or products, but it wouldn't surprise me if it turns out that the company's R&D engineers are already working on ways to overcome the limitations of the current technology. In the meantime, various independent videographers are already modifying HD-compatible EOS Digital SLRs like the 5D Mark II and 7D by removing the reflex mirror completely. This makes it much more practical to use those cameras with professional PL-mount video lenses, which protrude deeply into the camera body. Granted, this is not the same thing as removing the focal plane shutter, but it's a step in that direction.

Q. Is it my imagination, or has the new screen/finder system in the EOS 7D improved the compensation issue when using stopped-down metering below f/5.6 with manual diaphragm lenses?

A. I have not yet had an opportunity to test the 7D with non-coupled lenses, so I can't confirm that there's any difference in metering accuracy in that configuration compared to earlier EOS SLRs. However, it should be noted that this issue is essentially eliminated by using Live View with any EOS model that supports the feature, because the exposure measurement is taken directly from the CMOS sensor instead of passing through the focusing screen.

Q. I am using the EF24-105mm zoom lens on my EOS 5D camera. When I zoom to 24mm and AF on some very distant object, the camera sets the AF ring to something between the 5m mark and infinity, quite close to the 5m mark, actually. Is this normal?

A. On my samples of the original 5D and 24-105, I typically get a distance reading between 5 meters and the beginning of the infinity mark when I aim at a distant subject with the focal length set to 24mm and the AF mode set to One-Shot with a manually selected center focusing point. It's closer to infinity than 5 meters, but it's not all the way to infinity. If I zoom the lens to 105mm and refocus on the same distant subject, the focus index might move fractionally closer to infinity, but it never goes all the way to the end of the scale.

Q. When I then zoom to 105mm without re-focusing, the image in the viewfinder and the picture taken turn out clearly unsharp.

A. That's normal, because there's more depth of field at 24mm than there is at 105mm when the camera is positioned at a fixed distance from the subject.

Q. When I AF zoomed to 105mm the focusing ring will be set to infinity and the picture will turn out sharp, as it should be. Zooming to 24mm again and taking a picture without refocusing (i.e., still focused to infinity) will produce a picture virtually indistinguishable from the one focused to ~5m.

A. And that is as it should be, for two reasons:

  1. There's a cam inside the 24-105mm lens that is designed to maintain an accurate focus when the lens is zoomed from tele towards wide;
  2. You're starting from 105mm, a focal length with relatively shallow depth of field, and zooming to 24mm, which has greater depth of field as mentioned above.

Q. My question is this: Does the AF algorithm take the hyperfocal distance into account and does it set the focusing ring to the hyperfocal distance in order to achieve maximum DOF?

A. No, it does not. The focusing algorithm analyzes contrast in the subject and attempts to set the most accurate focus based on what it "sees."

Q. The AF behavior is interesting, though (at least for me). Does it also happen with other bodies and lenses or just with this particular combination?

A. It's a common characteristic for every EF zoom lens, regardless of the EOS camera model.

Q. I am about to buy a new Canon 70-200mm lens (either f/2.8 or f/4) and am trying to decide if I should buy the lens with IS. I have taken thousands of alpine ski race pictures with the 70-200mm f/4 without IS (using 20D and now 50D cameras). I am usually shooting with shutter speeds of 1/250 to 1/1000 depending on how much light I have. I also often pan the racers at two or three gates and with this high shutter speed I am wondering if I should even use the IS feature. Can you comment on whether using IS would be beneficial for this high-speed sports photography?

A. You would be better off with the IS version for skiing photos, for a reason that might not be obvious at first glance. Although chances are good that you'll be using a shutter speed so fast that the IS system has no effect, use of IS will present a steadier image to the camera's AF detection system. Thus, if you're using predictive focus, the IS system may provide better data for the AF system to base its calculations on. However, if you're just zone focusing manually and waiting for the subject to pass through the area you've preselected, then you might as well shut off the IS function and save some battery power.

Q. I have two unrelated questions. First, there is a very well known bird photographer who has written that the IS on lenses like the 500mm f/4L IS should always be left on and in Mode two. He claims that mode two does everything that mode one does and has the benefit of the panning feature. I would like your opinion on this. My other question relates to the AI Focus option. I shoot mostly outdoors with a 5D Mark II. I like the AI Focus option, especially in unpredictable situations with wildlife where they may be stationary one minute and moving the next. Some people on forums have claimed that this is the least accurate AF mode. It seems to me that the camera is operating in either One Shot or AI Servo and that the only accuracy issue would be whether the camera detects motion accurately and makes the switch to AI Servo appropriately. So the question is whether there is any advantage in shooting AI Servo as opposed to AI Focus.

A. The bird photographer you're talking about is Arthur Morris, whose opinions I respect on the use of Image Stabilization for bird photography. In the particular case of using a long lens like the 500/4L IS on a tripod, I would agree that Mode 2 makes more sense than Mode 1. For further comments from me on this topic, please check the December 2006 edition of Tech Tips:

My answer to your second question is that you should try both AI Focus and AI Servo for bird photography to see which AF mode you prefer. If I were photographing birds at rest, I would probably pick AI Focus, but if I were photographing birds already in flight, I would pick AI Servo.

Q. I have the 550EX and 580EX Speedlites. I would like to know the Kelvin color temperature rating of these flashes. J. Dennis Thomas in the Canon Speedlight Field Guide gives the Kelvin as 5500 for flash and daylight as 5500. Should I set camera Kelvin at 5500 to match the daylight setting? The default setting is 5200.

A. The Kelvin rating for Canon Speedlites such as the 550EX and 580EX varies from about 5200K to 5800K depending on the duration of the flash burst as well as the charge level of the battery pack. That's the main reason why Canon developed automatic color temperature compensation for the 580EX and all subsequent EX-series Speedlites. With this feature, the Speedlite uses a look-up table to apply color temperature modification for every shot based on real-time data, thus achieving better shot-to-shot consistency. As a result, you don't have to try to outguess the system. Instead, you can leave the camera's white balance setting at Auto or Flash to get the benefit of the automation. Because it's an older model, Speedlite 550EX does not have automatic color temperature compensation. 5500K is a good average WB setting when using this flash, but you can tweak the camera's WB setting higher or lower based on ambient lighting conditions as well as personal taste.

Q. I have a question about the sensor cleaning in the 5D Mark II. Every once in a while I see the "cleaning sensor" screen come up on the back of the camera when the camera has been on for a while, without me having touched anything. Does the sensor cleaning kick in if the camera goes to sleep?

A. I have not seen that behavior on my sample of the 5D Mark II. And according to Canon's Service Department, sensor cleaning does not kick in automatically when the camera goes to sleep.

Thanks for reading this edition of Tech Tips! I sincerely appreciate all the comments I've received from readers of this column, and hope that the information I've provided here has been helpful.

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.)

P.S.: The purpose of the Comments section is to allow readers to respond to the content of each month's edition of Tech Tips. New topics or questions should be submitted by e-mail (using the link at the end of each column) in order to support the development of future monthly editions. I appreciate your kind support and cooperation. Thanks!

© Chuck Westfall

After earning a degree in Professional Photography from the Rochester Institute of Technology and accumulating some valuable on-the-job experience during a 10-year stint in commercial photography and photo retail, Chuck Westfall began his corporate career with Canon U.S.A. in 1982 as a Technical Representative. He has steadily advanced through the ranks to achieve his present position as Technical Advisor for the company's Consumer Imaging Group, working out of Canon U.S.A.'s headquarters office in Lake Success, NY. Among his many assignments, Chuck Westfall is currently Canon USA’s main media spokesman for new camera products. He also provides a unique insider’s perspective to financial analysts who follow the company’s CIG sales and marketing activities.

Chuck’s involvement with digital cameras began in 1994, when he assisted Canon and Kodak engineers in developing the EOS-DCS series of professional SLRs. Since then, his responsibilities have expanded to include participation in the development and launching of many other Consumer Imaging Group products including Canon's professional and consumer-oriented digital cameras. Most recently, he has been developing content for online and on-site consumer education projects in Canon USA’s Professional Products Marketing Division.

On the personal side, Chuck enjoys sightseeing, photography, reading, music, and family life with his wife Ying and their beautiful daughter Anna.

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