Thursday, November 8, 2007

G9: Av and Tv Peculiarities

When the G9 is placed in Aperture Priority mode (Av), the internal logic for selecting the necessary matching shutter speed will not permit shutter speeds longer than one second. If a slower shutter speed is actually required for proper exposure, the LCD will display the “1” in red to indicate that there is a problem. However, the G9 certainly has the ability to use slow shutter speeds.

By using shutter speed priority (Tv), the shutter opening duration can be set for as long as 15 seconds. In Tv mode, if the necessary matching aperture value is not available, then the aperture value that is selected is displayed in red on the LCD.

You may not notice the red aperture or shutter speed display unless you are in the habit of pressing the shutter button halfway and holding it while you check the LCD display.

What are the limiting shutter speeds and apertures of the G9? These limits are usually as: shutter speeds from 15 seconds to 1/2500 second and apertures from F2.8 to F8.0. This is true but some combinations of shutter speed and aperture are not available at all and other combinations are only available under certain conditions.

If you have a G9, you’ve probably already noticed that the aperture range at the full telephoto zoom position is not F2.8 to F8; it is F4.8 to F8. This is a fairly typical characteristic of many zoom lenses and not only Canon products.

Imagine a hole that must be closed. A small hole can be closed more quickly than a large hole. When the G9 is set for an aperture of F2.8, the fastest shutter speed is not 1/2500 second; it is 1/1600 second. In fact, to get a shutter speed of 1/2500 second, the aperture must be nearly F8 if the zoom is at maximum telephoto.

As a means of working around these limits, a “Safety Shift” can be set in the G9 menu. With Safety Shift turned on, the aperture is automatically adjusted, if necessary, even when the G9 is in Av mode. Likewise, with Safety Shift on, the shutter speed will be automatically adjusted, if necessary, even when the G9 is in Tv mode. At first, I didn’t like the idea of the Safety Shift but, more recently, I’ve turned it on. Safety Shift does not affect Manual Exposure mode.

I don’t like the 1 second slow speed limit in Av mode. This has caused me some problems, especially when expecting to get three shots automatically exposure bracketed (AEB) for combining into a High Dynamic Range image. In these cases, I knew that the exposures would be long and had mounted the camera on a tripod. I wanted the same aperture for all three images but, instead of three, got only two different exposures – and sometimes only one! Manual mode is the answer but, unfortunately, auto exposure bracketing does not work in manual mode. This means that the camera must be touched – very carefully! -- between exposures.

These problems, quirks, bugs – whatever you choose to call them -- are not new to the G9. The G3 and, I assume, all the G Series cameras are very similar. As with all cameras, to get the most out of the G9, we have to learn its peculiarities.

1 comment:

Russell Stewart said...

My new G10 also has a 1 sec limit in Av mode.... this is crazy and I hope that it's something that can be addressed with a firmware update.