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Monday, October 1, 2007

ISO & Digital Image Quality


Lowest ISOs • Blowin' In The Wind • Stacking Tele-Extenders • Performance In Cold Environments

An ISO of 50 allowed a shutter speed of 1‚ÅÑ8 sec. at ƒ/32 to give the desired effect on these waterfalls in the surf on Molokai, Hawaii. A Canon EOS-1Ds Mark II was used with a 100-400mm lens set to 180mm
An ISO of 50 allowed a shutter speed of 1/8 sec. at ƒ/32 to give the desired effect on these waterfalls in the surf on Molokai, Hawaii. A Canon EOS-1Ds Mark II was used with a 100-400mm lens set to 180mm.

Lowest ISOs

I’ve noticed that many of the Nikon cameras, such as the D100, have a starting ISO of 200, while Canon digital cameras start at ISO 100. Is there a difference?
Sal
Via the Internet


A number of the early Nikon digital cameras had a lowest ISO reading of 200, but the current cameras start at ISO 100. The ISO of 200 was a function of the interface between the image sensor and the camera's processor. The camera was optimized to this ISO and attained excellent quality, even though it had an increased sensitivity of one stop over the Canon cameras. This tended to be problematic for photographers wanting to use slow shutter speeds in midday to render blurred water, however. It was then, and still is, necessary to use neutral-density filters to achieve the equivalent of lower ISOs.

Canon has standardized from the beginning to an ISO of 100 minimum, with the possibility of setting an ISO of 50 on some camera bodies. This ISO of 50 doesn’t increase quality; it simply lowers the sensitivity of the image sensor. Again, it’s necessary to use neutral-density filters to accomplish special techniques where long exposures are needed under bright lighting conditions.

Blowin’ In The Wind

How do you get sharp images when photographing in a stiff wind? I get so much vibration through the tripod that few images are sharp.
G. Hansen
Baltimore, Maryland


There are a few conditions that keep me out of the field, but photographing in a driving wind is nearly impossible for the very reason you mention: The tripod transmits the wind to the image. But there are some techniques that can work to minimize the problem. Collapse the tripod so that it’s as low to the ground as possible. Hang extra weight from the center of the tripod. It helps to have a hook at the base of the center column where you can attach your camera bag, a bag of sand or water, or even a rope that you step on to increase the tripod’s rigidity. When shooting close to the road near my vehicle, I’ve often positioned myself so that the van creates a wind break for me. The answer sometimes is to photograph early in the morning or later in the day when it’s usually less windy.

Stacking Tele-Extenders

I remember seeing details on how to stack the 1.4x and 2x Canon extenders, but I can’t find a copy of the procedure. I have both converters and a Canon 12mm extension tube. In what order should they be stacked?
Bill
Via the Internet


You’re referring to the older Canon 1.4x and 2x extenders. The order for these to be stacked and still focus to infinity is to place the 1.4x on the camera body, the 12mm extension tube and then the 2x extender. With the newer EF II extenders, a 12mm extension tube isn’t necessary for the two extenders to mate and to focus to infinity. Again, place the 1.4x on the camera body first. The result will be an increase of 3.4 times the original focal length of the lens and a loss of three stops of light.

To fully achieve the potential of this combination, use excellent technique, including a quality tripod and ballhead, an if needed, a cable release and mirror lockup. There’s no room for error at these focal lengths. Increase ISOs to compensate for the three stops of light loss.

The extender combination should be used only on high-quality lenses that are extremely sharp because everything you put between the camera and the lens degrades the image. I wouldn’t try this with any of the telephoto zooms, such as the 70-200mm ƒ/2.8 and 70-200mm ƒ/4. The dual extender method can be best handled by the prime single-focal-length telephoto "L" lenses.

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