Tech Tips
December 2008

by Chuck Westfall

I noticed that landscape photographers are often forced to use a gradient filter to compensate for a bright sky and relatively dark foreground. Why can't this be done digitally in camera? It seems to me that if the camera were told to look for a horizontally placed light/dark line that it could apply some exposure compensation with no need for a filter. In fact, the latest Canon 50D has some exposure compensation so perhaps this could be developed (no pun intended) to simulate a gradient filter.

To the best of my knowledge, no currently made digital camera has the ability to vary its sensitivity in different areas of an image while it is being captured, although that is a very intriguing idea. Such technology may show up in future digital cameras, but for now, there are three basic approaches to controlling dynamic range in the contrasty lighting situations you describe. The first and simplest method is to use an optical gradient filter on the lens, just as landscape photographers have done for decades. The second method involves manipulation of the image data's tone curve. This can be done to greatest effect by capturing raw image data and working on it during post-processing in your computer. Most if not all of the currently available raw conversion software applications provide various sliders or other onscreen tools to adjust highlights and shadows by modifying tone curves. Similarly, many camera manufacturers offer various in-camera settings that accommodate scenes with extended dynamic range: For instance, Canon offers both Highlight Tone Priority and Automatic Lighting Optimizer settings with cameras like the EOS 50D and the EOS 5D Mark II. Similarly, Nikon offers Active D-Lighting and Sony has Dynamic Range Optimization. These methods do as much as possible in-camera in terms of modifying the available image data from a single capture, but typically, the more the shadows are brightened, the noisier they get. The third approach, which works well in many landscape applications, is high dynamic range imaging, or HDR for short. This method involves capturing several images of the same scene at varying exposure levels, then combining the image data in a personal computer to get maximum tonal range with minimum noise. Here again, many independent software applications simplify this procedure and help create beautiful files. One of the most popular HDR programs is Photomatix Pro by HDRSoft.

Let me know if this answers your question.

What is the best way to use the Micro Focus Adjustment on the Mark III?

The question is simple enough, but the answer really depends on the lenses you're using and the way you use them. To begin with, it should be clarified that there are two types of in-camera AF microadjustment for the EOS-1Ds Mark III and EOS-1D Mark III cameras: the first adjusts the point of focus equally for all Canon EF lenses, whereas the second adjusts the point of focus for up to 20 individual lens types, at one adjustment per lens. In both cases, the point of focus can be adjusted up to +/- 20 steps in 1-step increments. Also in both cases, any adjustments you make apply only to the specific camera body in question; lenses themselves are never modified by the camera's AF microadjustment settings. The amount of focus adjustment per step is proportional to the maximum aperture of the lens, with the goal being to increase the precision of the adjustment with large aperture lenses since they have a smaller depth of focus. With all that as a preamble, here is an unofficial procedure for selecting and using an AF microadjustment setting:

  1. Mount the camera to a sturdy tripod.
  2. Position a reference target for the camera to focus on. The reference target should have sufficient contrast for the AF system to read, should be flat and parallel to the camera's focal plane, and should be centered with respect to the picture area.
  3. Lighting should be bright and even.
  4. Camera-to-subject distance should be no less than 50 times the focal length of the lens. For a 50mm lens, that would be at least 2.5 meters, or approximately 8.2 feet.
  5. Set the lens for AF and the camera for One-Shot AF, and manually select the center focusing point.
  6. Shoot at the maximum aperture of the lens via manual mode or aperture-priority AE, and adjust the exposure level if necessary to achieve an accurate exposure of the reference target. Use a low ISO setting to reduce noise.
  7. If the lens has an image stabilizer, shut it off.
  8. Use a remote switch and/or the camera's self-timer to release the shutter. Use mirror lock as well.
  9. Take three sets of images at microadjustment settings of -5, 0 and +5, i.e, three consecutive images at -5, three consecutive images at 0, and three consecutive images at +5.
  10. Examine the resulting images on your computer monitor at 100% pixel magnification.
  11. Take additional sets of test images at different microadjustment settings if necessary until the sharpest image is achieved.
  12. Register the corresponding microadjustment settings in the camera.

Here are a few additional precautions to observe:

•  Do not attempt to autofocus on an angled chart, because doing so will degrade the consistency of the camera's focusing measurement. Keep in mind that the camera's AF sensor is comprised of multiple pairs of linear pixel arrays. If you attempt to autofocus on a single line in an angled focusing chart, only a few pixels from each active pixel array will "see" the target. Ideally, the contrast in the reference target should cover the entire area of the camera's center focusing point, and the reference target should be perfectly parallel to the camera's focal plane.

•  For best results, manually set the focus on the lens to infinity for every exposure before allowing the camera to autofocus the reference target.

•  Expect some minor variations in focusing accuracy within each set of three test images, even though they were all taken at the same microadjustment setting. This is completely normal, and is due to the tolerances of the camera's AF system.

•  Expect smaller microadjustment settings to have a greater effect with telephoto lenses, and vice versa for wide-angle lenses.

•  If you are attempting to set microadjustments for a zoom lens, it is important to realize that the camera's setting may only be accurate for the focal length setting you test. The instruction book suggests testing at the longest focal length of the lens, but you may find it more efficient to choose the focal length you use most often.

•  Some EOS cameras and some EF zoom lenses may require more sophisticated calibration than the in-camera AF microadjustment settings can provide. In such cases, it may be necessary to have calibrations performed at a Canon Factory Service Center.

•  Last but not least, there is no "official" Canon method for setting AF microadjustments, so this procedure is unofficial. If you think you can do better, then by all means, go for it. Towards that end, be advised that some independently made tools are designed to help you set AF microadjustments accurately. One of these is the LensAlign kit, due out soon from

About the 1/focal [length] rule of thumb [for full-frame sensors] vs. the 1.6x crop factor: As the crop-factor sensor picks only the center part of the full image and the real focal length of the lens remains the same, how does shooting with a crop factor body differ from shooting with a full-frame and cropping the center part in post-processing? With a 50mm lens, wouldn't exposing at 1/50 s on both bodies yield the same results after cropping the biggest image? It's not as if the focal length of the lens has been directly modified, like, for instance, with an extender...

For the sake of readers who may be unfamiliar with the concept, I assume you're referring to the often repeated recommendations for minimum shutter speed to achieve blur-free hand-held photography. If so, you're correct that the degree of camera shake at the focal plane is the same for any given focal length regardless of the imaging format when all else is equal. However, you may not have taken into account the magnification factor for the final output, for example, a print. If the full-frame image is cropped to match the smaller imaging format as you suggest, then the advantage of the additional imaging area which could have been used to reduce the magnification factor of the resulting print is lost. On the other hand, if output size is equalized and the full frame is used, then lower magnification will reduce the visibility of blur in the print. That's why the 1.6x conversion factor should be applied to the 1/focal length shutter speed rule if you're using an APS-C sensor camera and you want to play it safe. If you want to play it even safer, use a camera or lens with an image stabilization system.

I think I will be a Canon customer for the EOS 5D Mark II but I feel a little scared about 21 MP because of the file size in RAW (I always shoot in RAW). The solution proposed by Canon with the 5D Mark II seems to be sRAW when the full resolution is not necessary. But can you provide some information on sRAW? There are a lot of discussions on the Web but not so many reliable conclusions. My questions are:

•  Is the 10MP sRAW obtained from a kind of interpolation from the full 21MP? Is it true that in this way the sRAW will have a less per pixel noise than a RAW?
•  Would it be possible with some software (like DPP) to get sRAW from a RAW file? This function would be very useful to me to keep outstanding shots in 21MP and reduce the size to 10MP for all the others. I would prefer to archive 10MP RAW than 21MP JPEG.

The main purpose of sRAW1 and sRAW2 reduced resolution raw capture settings on the EOS 5D Mark II is to retain the image editing benefits of RAW data in terms of shadow/highlight control and white balance, etc., at a variety of resolutions and file sizes to fit the needs of professional and advanced amateur photographers. Canon is making no claims about improvements in noise levels in sRAW1 or sRAW2 compared to RAW, and the company has not published the methods it uses to produce lower resolutions. According to my tests in DPP, there is no noticeable difference in noise levels for any of these data recording modes. Since there is no significant difference in terms of noise, the main advantage of sRAW1 and sRAW2 is more images per CF card for shooting conditions where the reduced resolution is adequate for the job at hand. Nominal file sizes for RAW, sRAW1 and sRAW2 on the EOS 5D Mark II are 25.8MB, 14MB and 10MB respectively. Thanks for the feature suggestion on producing sRAW files from RAW files in DPP. There have been several requests for this functionality, which have been forwarded to Canon Inc. for consideration.

Can you perhaps help me with a 'how-to' for an EOS 400D/Digital Rebel Xti? I am trying to hook a non-Canon flash to the hot shoe on the 400D. If the camera 'sees' enough light through the lens to believe it is OK to allow the shutter to fire, a picture is taken and the flash fires. If there is too little light, as would be the case where flash would be needed, the metering mode symbol flashes in the display, and the shutter won't fire. I have set the camera to manual mode, and can alter the aperture and speed satisfactorily. What do I have to do to the camera to get it to fire the flash? I might add that I use the flash with my two A-1s and one T90, both of which do have TTL capability, but which also work quite well with this gun in manual mode from their hot shoe connections.

The Canon A-1 did not have TTL flash metering, but that's beside the point. If it is so dark that the EOS 400D's AF sensor can't read the subject, you will need to set the lens for manual focusing (and focus the image yourself) in order to release the shutter and fire your non-dedicated flash. Canon's EX-series Speedlites circumvent this problem by incorporating an AF Assist beam that illuminates the subject for autofocusing prior to shutter release.

Back in the film days, film was rated in mired values so you could get an idea of which filters, (e.g., 81A or 81B), to use to compensate for color shifts. Do digital sensors have an equivalent?

Unlike color films, the image sensors of digital cameras are not rated in terms of specific color temperature settings or mired values. Instead, most modern digital cameras have various white balance settings including presets that correspond to daylight, tungsten, and fluorescent light sources among others. Additionally, advanced digital cameras offer manual color temperature settings and even custom white balance functions that let you match the camera's color reproduction to the specific lighting conditions at hand, using a gray card or white sheet of paper as a neutral reference target. Generally speaking, you can use the camera's LCD monitor to get a reasonably good idea of the color accuracy for the white balance setting you've selected, although it's best to use a calibrated photo-quality computer monitor for precise measurements and evaluations.

Thanks for reading Tech Tips! That's it for now. See you in 2009!

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 all inquiries, 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.)

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 in order to support the development of future monthly editions. I appreciate your kind support and cooperation. Thanks!

blog comments powered by Disqus

© 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.