Just a brief note before I begin that this edition of Tech Tips marks the second anniversary of the column. A sincere thanks to our readers for your continued support and interest!

Maximum ISO settings always grow higher and higher: 800, 1600, 3200 and now 6400 (whow !) with Canon EOS-1D Mark III. As far as I understand it is done by amplifying the nominal signal provided by the sensor. But is there a technical reason why Canon DSLRs have a minimum in-camera ISO setting of 100? If not, could in-camera ISO settings like 12, 25, and 50 ISO be added as additional choices? In other words, is it possible to "downplifye" the sensor signal (using something like a virtual neutral density filter)? This feature would be very helpful to replace the use of various neutral density filters in order to obtain long exposure time in bright light situations and, if still possible, to increase noise performance.

There is a lot more to high ISO ratings on digital cameras than signal amplification alone, but the drive towards image quality improvement has led to some remarkable accomplishments recently. The EOS-1D Mark III is a good case in point, with greater maximum sensitivity and better tonal gradation than any previous Canon digital camera.

The same degree of sensitivity that makes it possible to achieve amazingly good image quality at high ISO ratings on EOS Digital SLRs makes it difficult if not impossible to maintain adequate levels of image quality with these cameras when ISO settings are reduced past a certain point. That's because, unlike traditional film emulsions, which can be made with smaller silver halide particles to reduce sensitivity, today's digital image sensors feature individual photodiodes whose size is fixed and therefore cannot be freely enlarged or reduced. Even if the size of an individual photodiode were somehow adjustable, smaller photodiodes would not necessarily be desirable because of issues with degraded signal to noise ratios. Under the circumstances, the use of ND filters on your lens is still the best available solution to achieve lower ISO speed settings on your digital SLR while maintaining maximum tonal range. By the way, it is technically possible to provide ISO settings lower than 100 with modern digital SLRs; in fact, Canon offers ISO 50 as an optional setting with all current EOS models above the Digital Rebel series. The extended dynamic range of current Canon CMOS sensors makes it possible to simulate ISO 50 by changing the processing of the sensor data, but exposure latitude in highlight areas of the image is less at this setting than at higher ISOs.

I have a Canon EOS 20D with two 580EX and one 430EX flashes. Parts of the wireless flash control system are confusing to me. If I use a 580EX as the master on camera, does it become part of one of the slave groups? I have read in a forum that it is part of slave group A, but don't remember seeing this in any Canon manual. Or another way of asking this question: If I have the 580EX on camera as the master, can I control the automatic flash exposure (E-TTL) to make it behave as a "fill" flash by adjusting its exposure compensation without affecting all the other (slave) flashes? Or, can I reduce the master's output (in E-TTL) along with just one of the slave groups? I know I can turn the master off, but can I simply reduce its output and maintain high output from the slaves in E-TTL mode?

Any time the master unit in an E-TTL wireless configuration is set up to fire during the exposure, it is considered to be part of Slave Group A. Adjusting the output of Group A versus Group B is done with the ratio settings on the back of the master unit, rather than adjusting flash exposure compensation. A photographer by the name of Chuck Gardner has posted an excellent Web tutorial showing the correct method of setting up a 2-flash E-TTL wireless system such that the master flash on camera fires during the exposure as a fill light and the off-camera flash is used as the main, or key light:

http://super.nova.org/DPR/Canon/

You should read the whole article, but be sure to scroll down to the diagram marked: "My Recommended A:B Fill:Key Configuration" to learn more about this particular set-up. Next, check your 580EX manual on page 36 to see how to set the master flash to fire during the exposure. Finally, review pages 38 and 39 to see how to use the controls on the master unit to adjust lighting ratios between Group A and Group B. (Ignore the part about setting the master unit not to fire during the exposure.) In this case, you would set ratios such as 1:2, 1:4 or 1:8 if you want the slave unit to put out more light than the master.

I've been using a wonderful, 25-year-old, 500 mm f/8 fixed aperture, Tamron mirror lens on my EOS-1Ds Mark II and 5D bodies for some amazing shots. The lens mates to the EOS mount via a Tamron to M42 adapter and then an M42 to EOS adapter. With the camera set to Aperture Priority and the aperture control set to f/1, the meter works perfectly. Of course, the viewfinder is a tad dark as the lens's largest (and only aperture) is f8, and focusing is sometimes challenging. But the lens, which I've had since around 1981, is a champ and is capable of remarkable images. My problem is this: When I add a 1.4 tele-converter to this rig, theoretically turning the lens into a 700 mm f/11 optic, the EOS cameras refuse to work properly. When I trip the shutter release, the shutter seems to open -- but it stays open for several seconds even in bright sunlight, and when it closes, no image has been captured! The CF card simply has no data for that shot. Even though the lens, the two adapters, and the camera function absolutely perfectly together when it's just a 500 mm optic, the addition of the 1.4 tele-converter seems to stop the camera from functioning correctly. I tried this with my Canon 1.4 TC and also with a Tamron SP TC -- neither worked. What's causing this failure? And is there any fix, such as perhaps taping over some of the contacts on the tele-converter -- and if so, which ones? Back in the Middle Ages, when I originally bought the lens, I sometimes used it with a 2x tele-converter on the Olympus OM series cameras I was shooting back then -- it worked great with a tripod -- and, if possible, I'd like to do something like that with this lens and the brilliant Canon digital cameras. I'd be grateful for any advice about this.

The phenomenon you are reporting is normal and to be expected. It arises from the fact that the extender you are using "expects" to see a lens with fully functional electronic coupling. When it does not, the camera is programmed to lock up, just as you've reported. You may notice that it does the same thing when you remove the mirror lens, leaving the extender mounted to the camera by itself.

There is more to the story. The reason your mirror lens is able to function properly when mounted directly to the camera is that its lens mount is slightly modified compared to a regular EF lens. Specifically, one of the claws on the bayonet is shorter than the corresponding part of a regular lens. There is a mechanical switch on the camera's lens mount to inform the camera when a coupled lens has been mounted, thus causing the camera to send an ID request through the electronic contacts. When the mechanical switch is not engaged, as is the case with your mirror lens, the shutter releases normally. When it is engaged, as is the case with a regular lens or extender, the shutter locks up unless the camera determines that the lens or extender is responding correctly.

It sounds like a "catch-22" situation, but there is a workaround. You can fool the camera into releasing the shutter by pressing in the camera's lens mount release button and twisting the extender just enough to disengage the mechanical switch. This is not 100% secure, but in the case of mounting the lens on a tripod, assuming the lens has a tripod collar, you should be able to get away with it as long as the camera isn't bumped while the extender is unlocked.

I currently use Windows XP x64 and I would really like to try tethered shooting. This is currently not possible due to there being no drivers. Are there any plans for x64 drivers?

According to Canon Inc., there are no plans to offer XP-compatible x64 drivers for EOS Digital SLRs. However, tethered shooting via EOS Utility 2.0 is supported with 64-bit and 32-bit versions of Windows Vista for the following models:

EOS-1D Mark III
EOS 30D
EOS Digital Rebel XTi/400D

When these cameras are connected to a PC running any version of Windows Vista, no additional drivers need to be installed. Incidentally, 32-bit versions of Windows Vista also support tethered shooting with the following additional models:

EOS-1Ds Mark II
EOS-1D Mark II N
EOS-1D Mark II
EOS 5D
EOS 20D, 20Da
EOS Digital Rebel XT/350D

For these cameras, Canon supplies Vista-compatible drivers that must be installed along with EOS Utility 2.0 for tethered shooting with PCs running 32-bit versions of Windows Vista.

From the April '07 question/answer on AF adjustment settings:

"To delete the registered lens settings, select 1 or 2, then press the Erase button. All the registered AF micro-adjustment settings will be cleared."

Why the all-or-nothing (if I understand correctly)? Isn't it likely that a user will obtain a different example of a given lens, and wish to adjust for the new example without having to reset all the other individual settings?

The easiest way to handle the situation you describe is to overwrite an existing AF Microadjustment setting for an individual lens model. This method preserves registered microadjustments for other lens types. Let me know if this answers your question.

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

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.)