Tech Tips
February 2009

by Chuck Westfall

I have a question about the way exposure works with the newer EOS cameras in the Live View mode. I understand that for Live View to work, the mirror must flip up and the shutter must open and remain open. My question is, what happens when you make the exposure? I suspect that the exposure is completely electronic and that once it's complete, the (mechanical) shutter closes. Or does closing the shutter end the exposure? And what happens at higher speeds where the mechanical shutter would normally be only a traveling slit?

Thanks for the excellent questions! There are actually several different camera settings in Live View that determine how exposures are made. Let's take a closer look:

  • When Silent Shooting is disabled in Live View, the camera's mechanical focal plane shutter controls the beginning and the end of every exposure. When the shutter button is fully pressed, the focal plane shutter closes to suspend Live View. Then it operates normally to capture one or more images, depending on whether the camera is set for single-frame or continuous shooting. At the end of the last exposure, the shutter reopens to resume Live View.
  • In Silent Mode 1, the CMOS image sensor begins the exposure by electronically simulating the first curtain of the mechanical shutter. The mechanical shutter closes to end the exposure. This is quieter because the mechanical shutter has less work to do. (There is only one shutter click instead of two.) Silent Mode 1 works for continuous shooting as well as single-frame exposures. At the end of the last exposure, the shutter opens again to resume Live View.
  • Silent Mode 2 only works with single-frame shooting. In this mode, the exposure is controlled in the same way as Silent Mode 1. However, the mechanical shutter is not reopened to resume Live View until the photographer lifts his or her index finger off the shutter button or remote switch. This is by far the quietest way to capture a still image during Live View. It's ideal for situations when silence is essential, because it allows the photographer to control when to resume Live View.

The EOS 5D Mark II camera supports video capture during Live View. In this mode, the CMOS image sensor controls the beginning and end of each individual frame, and the framing rate is 30 fps. The mechanical shutter remains open at all times, so operational noises are kept to an absolute minimum.

Because still image captures in Live View are at least partially if not fully controlled by the camera's mechanical shutter, the entire range of the camera's shutter speeds remains available, typically from Bulb up to 1/4000 or 1/8000 depending on the camera model. However, during movie mode with the EOS 5D Mark II, shutter speeds are limited to 1/30 on the slow end, and the Bulb setting is unavailable.

Recent Canon EOS cameras have both Highlight Tone Priority and Auto Lighting Optimizer settings. What is the difference between them and when would I use each? It seems to me that they both address high dynamic range situations.

Auto Lighting Optimizer (ALO) analyzes contrast in captured images and modifies both shadows and highlights via tone curve adjustments to minimize loss of detail in contrasty lighting conditions. Current EOS models including the EOS 50D and 5D Mark II provide four settings for ALO: Off (Disable), Low, Standard and Strong. ALO can be used at any ISO speed setting. Sample images showing the effect of ALO can be seen here:

Highlight Tone Priority (HTP) is available with all current EOS models excluding the Rebel XS/1000D. HTP has no effect on the actual dynamic range of the image sensor. It's just an alternative method of image processing that preserves more highlight detail than Canon's standard processing, without significantly altering midtones or shadows. The effect of HTP is enhanced by Canon's 14-bit A/D converter, which provides finer tonal gradations than the previous 12-bit system. HTP is a Custom Function with a simple on/off setting, and the available range of ISO speed settings is slightly limited when it is on. Take a look at the following Web page for some sample images that show the capabilities of Highlight Tone Priority:

ALO does not affect RAW image data. It is at its best for in-camera JPEGs shot in extremely contrasty lighting conditions. Examples would include backlit portraits and urban landscapes on sunny days, where the tops of buildings are brightly illuminated by the sun but subject matter at street level is in heavy shadow. HTP affects RAW data as well as in-camera JPEGs. It is very useful in high-key shooting conditions such as wedding photography and certain kinds of sunsets. ALO can be combined with HTP with cameras that have both features.

I saw your comment on sRAW1 with the EOS 5D Mark II, and I understand that the smaller file size would be good for storing more photos on the memory card. I was also wondering if the speed of the camera could become faster than 3.9 fps when using the sRAW1 setting. After all, the files are smaller.

The framing rate of the EOS 5D Mark II cannot be increased by setting the camera to produce smaller file sizes. Keep in mind that all captured images start out as full-frame data, which is then downsampled and compressed according to the user-selected image quality setting. Therefore, the amount of data coming in to the processor is always the same; it's only the size of the output file that changes.

A lot of people are maligning the low-light AF capability of the EOS 5D Mark II, particularly relative to the EOS-1Ds Mark III. I do not find AF on the 5D Mark II to be a problem in low light with static objects. In fact, properly used...or at least used the way I use it...I find that it will AF in light lower than I can expect exposures with reasonable image quality using high ISO speed settings. I know the 1Ds Mark III has a lot of AF functionality, particularly with moving objects and off-center points. However, I believe I remember (please correct me if I am wrong) a past discussion of yours where the wider 5D (or xxD) AF point(s) were better in low light than the narrower 1D/1Ds points. Could you comment on how each of these (5D Mark II, 1Ds Mark III) will do in low light using center point only and/or exterior points?

Thanks for the question. I wonder how many of the people who are maligning the low-light AF capability of the EOS 5D Mark II camera have actually tried it! I've received a large number of comments from 5D Mark II owners stating that the all-around AF performance is much better than that of the original EOS 5D, which is interesting since the Mark II uses the same hardware components for autofocus as the 5D. That being the case, it's clear that at least some of the AF performance improvements on the 5D Mark II can be attributed to a faster signal processing circuit. Also, I think the availability of AF microadjustment on the 5D Mark II is a big improvement over the original 5D because it allows users to maximize the performance of their Canon EF lenses.

When comparing the low-light AF performance of the 5D Mark II to the 1Ds Mark III, which is a much more expensive camera, I'd like to make several points. First, my statement comparing the low-light sensitivity of the 1D-class cameras to the original 5D, 20D, etc., models was made before the 1Ds Mark III and 1D Mark III models were introduced. Although the statement was accurate at the time, it should be noted that one of the significant improvements of the Mark III AF system vs. the 1D/1Ds Mark II series is its low-light sensitivity, together with a significant increase in the quantity of high-precision cross-type AF points throughout the cameras' AF pattern. So, my old statement no longer applies when comparing the 1Ds Mark III to the 5D Mark II. As it happens, the low-light sensitivity of each of these cameras is actually quite similar. In fact, Canon rates the low-light sensitivity threshold for the 1Ds Mark III at EV -1 versus EV -0.5 for the 5D Mark II, i.e., a half stop better. Performance at the center point is also quite similar, although the 1Ds Mark III again has the edge because it maintains its high-precision cross-type detection capability with maximum apertures as small as f/4, compared to f/2.8 for the 5D Mark II. AF performance with the off-center AF points is also going to be better with the 1Ds Mark III, partially because there are 18 such points versus 8, and partially because all 18 of the 1Ds Mark III's off-center AF points are high-precision cross-types versus standard precision single-axis for the 5D Mark II. Let me know if this answers your question.

I have a question about my new EOS-1Ds Mark III. I am told you can enter your name into the camera's data via the supplied USB cable, also that it's possible to access some camera functions via the cable and computer. I did not see any info in the User's Manual or software on how to do this.

Some of the features you're describing are accessible through EOS Utility, a software program that is supplied on the EOS Solutions Disk CD that came with your EOS Digital SLR. Detailed instructions for EOS Utility are supplied in PDF format on another CD that came in the box. Look for the "EOS Digital Software Instruction Manual" disc, or you can download the manuals from our Web site here:

Locate the Drivers and Downloads tab, then scroll down that page until you find the instruction manual for EOS Utility 2.5 for Windows or Mac, according to your computer's operating system. The instructions for uploading Owner's Name, Copyright Notice and Date/Time information from your PC start on page 12 in the Windows edition. Older cameras like the EOS-1Ds and EOS-1Ds Mark II also had Personal Functions that could be uploaded to those cameras via EOS Utility, but all available Personal Functions were added to the Custom Function menu on the Mark III cameras, so they're already in the camera and do not need to be uploaded.

I'm using an EOS-1Ds Mark II with an EF16-35mm f/2.8L lens. In many situations I feel that the standard focusing screen is not satisfactory for two reasons:

  1. It's impossible for me to judge accurate focusing of the lens.
  2. It's very hard to keep horizon lines straight (without mounting the camera on a tripod and using a bubble level).

The obvious solution for both problems would be a focusing screen with a split-image prism in the center AND grid lines. However, Canon offers either one or another and not both in the same screen. Moreover, the only screen I've seen with both was a custom-made Beattie screen, which is no longer in production. (They still make such screens for Nikon, but not for Canon.) Is there any logical reason why Canon does not make such a nice focusing screen? Is there any chance to be able to get one someday?

I cannot rule out the possibility that Canon might eventually offer a focusing screen that combines grid lines with a split-image focusing aid, but in my personal opinion the chances of this product being developed are virtually nil. The best workaround is to use one of our current models like the EOS 5D Mark II or the EOS-1Ds Mark III, because these cameras are equipped with a Live View function that enables the use of grid lines on demand, together with the option of on-screen magnification up to 10X for critical manual focusing. In addition, the EOS 5D Mark II provides contrast-based autofocus during Live View, using a movable focusing frame for greater flexibility. Both of these cameras have the added benefit of better image quality than earlier models like the EOS-1Ds Mark II.

I have a Canon EOS 50D. I understand that picture quality is somewhat dependent on pixel pitch and stray light/light interaction between sensor sites. If I dial down the resolution of the camera from 15MP to 8 MP, does that mean that only every other pixel is "hot"? Does that mean that there is less light interaction between pixels, and that my picture is cleaner? Please provide some impressions.

Canon has not disclosed the exact methods it uses to reduce resolution for sRAW (small RAW) images and Medium or Small in-camera JPEGs, but each of these recording formats involves downsampling from the original full-resolution raw image data. In tests I've performed at various EOS 50D image quality settings, I have come to the conclusion that there is no significant change in noise at pixel level caused by downsampling alone. However, at any given print size, images captured by the EOS 50D will look their best (cleanest) when working from full-resolution files.

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

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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.