Tech Tips
September 2008

by Chuck Westfall

I currently have an EOS 20D DSLR and recently purchased a "refurbished by Canon" EF400mm f/5.6L USM lens. At first I thought it was a soft copy, but after inspecting a few images and doing some testing, I discovered it was a front focus issue (about 1.5 inches at 15 feet and 3 inches at 30 feet). My question, is this within the range correctable by a micro focus adjustment? If so, is it likely that any Canon crop factor DSLR will have this feature any time in the near future, or should I go ahead and have the lens calibrated at this time?

A focus adjustment of 1.5 inches at a subject distance of 15 feet is definitely within the range that could be handled with AF Microadjustment. The EOS 50D that was announced on Aug. 26, 2008 is the first Canon SLR below the 1D series to offer an AF Microadjustment feature. This new camera is scheduled to be available in early October.

Why does Canon no longer use Cyan/Yellow/Magenta color filter arrays in its digital cameras? As far as I know the Canon PowerShot S10 among others used such a filter. A CYM filter should yield better low-light sensitivity. Am I wrong?

It is true that CYGM filters of the type used on the PowerShot S10 back in 1999 are more efficient than RGB filters in terms of light transmission. The relatively low sensitivity of the CCD that was used for the S10 camera was one of the main reasons why Canon used CYGM on that camera and its predecessors, like the A5 and A50. Another reason was the relatively slow image processors in use at the time. However, image sensors and image processors have come a very long way in terms of technological advancement since then. For those reasons, together with the fact that RGB color filters provide better overall color reproduction accuracy than CYGM, Canon switched to RGB filtration for all of its digital cameras as far back as the year 2001.

Quick question: I have an EOS-1D Mark II and have purchased the Sekonic C-500 color temperature meter that recently came out. In the camera manual, it states the mired values for the Amber/Blue adjustment, and I am able to figure that out. But it does not mention any details on the values for the Green/Magenta other than 1-9. Do you know what CC filter numbers would correspond to the 1-9 steps of adjustment on the Green/Magenta settings? Does Canon have any in-depth info on this that would give equivalent CC filter numbers?

There's no official information from Canon on this topic. My suggestion would be to perform your own tests to determine this data. You could do that by photographing a neutral test target that's been custom white balanced in the camera, then taking a series of JPEG images at each of the Green and Magenta WB Shift settings. The results could then be measured in an image editing program and compared to the results from images shot with actual CC filters.

I am curious as to why the manual AF point selection process was changed so dramatically on the Mark III cameras. I would love to be able to select all 45 AF points like I can with the Mark II cameras. I can understand that perhaps there were complaints about too many points, but I liked the options to limit the number of points 11 or 9 while retaining the option of selecting from all 45 points when desired. Also with the EOS-1V camera, for film advance you have a high- and low-speed continuous and an ultra-high speed. This was the coolest feature. Perhaps there is a way to have three motor drive speeds on the Mark III cameras easily? For example: I like how I can change the frame rate for L/H continuous, but I find myself needing 3-4 fps for portraits and the like, 6-7 fps for action and 8+ for fast action. Otherwise I am really happy with Canon products.

The designers of the Mark III cameras haven't answered your first question directly, but they have made it clear that they were trying to improve the functionality of the cameras with the new AF sensor. Setting aside the discussions on focusing accuracy, one obvious difference between new and old is the presence of 19 high-precision cross-type AF points spread throughout the AF Area Ellipse (versus seven such points in the central area for the Mark I and Mark II cameras). This change was intended to improve the performance of the peripheral AF points not only in terms of precision with high-speed lenses but also in terms of subject recognition due to the cross-type design.

By limiting manual selection to those 19 points instead of all 45, they accomplished the objective of speeding up manual focusing point selection, especially after the firmware update that made it possible to use the multicontroller for that purpose. But I suspect (personal opinion) that they were also thinking about the difference in performance between the high-precision cross-type AF points and the single-axis normal precision "assist" AF points. They may have wanted to ensure that any manually selected focusing point would be consistent in terms of performance with any other manually selected focusing point. In any case, we have received several requests to restore the full selection of all 45 focusing points from several users, and your request will be added to the queue.

The Mark III cameras already support multiple framing rates to some extent through Custom Function III-16, which allows users to specify alternate framing rates for both high-speed and low-speed continuous. You can move that C.Fn to the "My Menu" tab for quick access, and then turn the function off or on relatively quickly to get the drive speed you prefer. So for instance, you could set 1 or 2 fps in low-speed continuous, and 6 or 7 fps in high-speed, which could be alternated with the camera's default settings of 3 fps and 10 fps respectively.

A friend of mine took a "bee on flower" macro with a Canon ring flash using a Canon 100mm macro at f/22 on a 40D set to ISO 200 in M mode with 1/200 second dialed-in. For all the world it looks like there's motion blur in some of the bee's legs. I can only surmise his flash was in High-Speed Sync mode, which caused the light to be on the whole of the 1/200 second shutter open time. I'm not near him, so I can't look over his flash settings nor do I own a Canon Ring Flash. Anyone familiar with how one determines/enables/disables High-Speed Sync mode on the Canon Ring Flash?

The controls for high-speed sync on the Macro Ring Lite MR-14EX are on the back of the power supply's control panel. However, high-speed sync is not the best solution for this problem. High-speed sync works by firing the flash at an extremely rapid rate for the entire duration of the exposure. It effectively turns the flash into a constant light source when used at shutter speeds higher than maximum X-sync speed, which is 1/250 for the EOS 40D and the 1D series digital SLRs. It has to be that way in order to provide even illumination across the frame as the moving slit of the focal plane shutter executes the exposure. That's also why the exposure level drops as the shutter speed increases, because you are in effect reducing the amount of time that the light is reaching the sensor. You can see a diagram that shows the difference between standard flash exposure and high-speed sync in our Flash Work Web site here:

Bottom line, the 'high speed' in high-speed sync refers to shutter speed, not flash duration. One additional item of information may be helpful: Even in normal flash mode (i.e., not high-speed sync), flash duration at full power with an MR-14EX can get as long as 1/750 sec. That might not be fast enough to freeze the motion of a bee's wing. One way to shorten the flash duration is to raise the camera's ISO speed setting. Another method for doing that is to open the aperture. Making the same shot at f/16 and ISO 400 instead of f/22 and ISO 200 would have cut the flash duration significantly with very little difference in noise levels or depth of field. Every little bit helps!

Thank you, Chuck. It always helps to know how things work to get the most out of using them. By the way, my friend made changes per your suggestion and retried his bee shots. The results were much better; the bees were all sharp and the 40D didn't deliver any distracting noise to speak of. I'm curious: Is flash duration information for various Canon Flashes available anywhere?

Canon does not publish detailed specifications on flash duration for its Speedlites, but the information I quoted for full-power flash duration on the MR-14EX is also applicable, generally speaking, for the MT-24EX and the 540EZ, 550EX, 580EX and 580EX II flash units. There are several reasons why full-power flash duration is designed to be approximately 1/750 second. Here are the top two:

1. Maximum X-sync shutter speed on high-end EOS SLRs like the 1D/1Ds series and the 40D/30D/20D cameras is either 1/250 or 1/300 second depending on the camera model, but the actual amount of time that the shutter blades are completely retracted during that time is only about 1/750 second. Makes sense if you think about it; it takes a certain amount of time for the first shutter curtain to move out of the way so that the entire image sensor is fully exposed, and it takes an equal amount of time for the second shutter curtain to cover the sensor and end the exposure. Keep in mind that except during high-speed sync mode, the flash can only fire when the shutter is completely open.

2. For any given Speedlite, the longer the flash duration, the greater the light output. Considering the relatively low-capacity flashtubes and capacitors used in these Speedlites, a 1/750 second flash duration provides the greatest possible output at all shutter speeds up to maximum X-sync. That's usually a good thing, except in the case of a very rapidly moving subject like the wings of a hummingbird or a bee.

The MR-14EX and MT-24EX each have two flashtubes, whereas the 540/550/580 series has only one. If both flashtubes are fired at full power simultaneously, their individual durations will be half that of the single tube Speedlites. However, it is also possible to fire only one flashtube at a time with the Macro Speedlites, which will bring the full power duration back to approximately 1/750 second. I could go on, but hopefully this information provides some additional useful perspective on the topic.

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

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.)

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© 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.