Why custom fuction for iso 100?

[carshooter]

I've heard/read that the normal ISO only goes down to 200 on the 1D and that you can get down to 100 with a custom function. What I want to know is that what makes that ISO so unacceptable that Canon decided to do that? I can't imagine that it would be noise like when going to high ISO's. Would it?

I'm a car photographer and usually shoot with 50 - 100 ISO film so that I can get the slow shutter speeds during a day of shooting action shots. I generally shoot action shots of cars in the 1/15 to 1/30 second range and that is just not possible with higher ISO's during the day because you are at apertures higher than 22-32.

Any ideas other than ND filters which ruin the excellent focusing abilities of Canons?

Thanks,

Dean Siracusa
Siracusa Productions, Inc.
Manhattan Beach, CA

[Chris Livingston]

I too am interested in this but would like to know if this affects image quality? Does the 100 look better, same or worse than at 200? With the D30 the 100 is outstanding and is my prefered setting but in the 1D will it improve image quality over the 200?

[Al_Goldis]

I presume because the base sensitivity of the sensor is ISO 200 and that 100 is kind of like an "electronic pull". I believe I read somewhere that the optimum image quality is achieved at 200. But I have no doubt the custom function for 100 will produce much better results than overexposing at 200 and attempting to massage the images in Photoshop.

[ThomasSapiano]

It's possible that running at ISO 100 will provide too much light for the sensor - each pixel in a CCD is a potential well which charge builds up in. If too much light hits the sensor, that could theoretically overflow and saturate at a certain charge. Modern imagers generally have electronics to collect the overflowed charge and disipate it before it smears the image. However, all of that overflowed charge is lost - so any light captured above a certain threshold will not be integrated into the image. Kind of like slowly pouring a water into a bowl to record its mass - if you pour so much that it flows over, that additional water won't add to the total mass recorded by the scale.

Theoretically, if that happened and your amplification was too low the imager would saturate at a level lower than the maximum sampled level resulting in a reduced dynamic range (ie any brightness past, say 240 would be recorded as 240 - easy to fix, but you loose 6% of your dynamic range in doing so). With that, your optimal image quality would likely be at the level of amplification (ISO) that just placed the top sampling level at the saturation level of the imager (in this case, seemingly 200iso).

Although, given that, the lower ammount of amplification would still generate less noise than the higher optimal ISO level (not that noise should be much of a problem at 200iso) - you'd just pay a penalty of loosing highlight details near the top of the scale. Likely, the effect isn't that bad, as images usually get downsampled from 12-bits to 8-bits/channel anyway - providing headroom not to loose any tonal detail. What you would loose is a portion of the camera's ability to record a wide range of lighting in the same scene.

Canon likely added it as a custom function so that the user knew that ISO 100 wouldn't necessarilly give them a better image than 200. Once the user understands that, they can enable the function and use that ISO if 200 is problematic for their purposes. For users who don't want to deal with the details, ISO 100 won't be available and the standard procedure of setting the ISO for the lowest usable level will provide them with the best possible image.

Just a guess though [img]images/icons/smile.gif[/img]

[ October 18, 2001: Message edited by: tsapiano ]

[carshooter]

That's the detailed and intelligent response that I was looking for. Thanks!

Dean Siracusa