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norm911 Mar 29, 2003 8:13 AM

ISO Settings on the D60
I'll admit to being slightly confused here. I've had my D60 for several months and absolutely love it. I'm an long-time medium format guy and continue to learn every day with my D60. One of the things that confuses me slightly is this notion of ISO settings on a digital camera. I understand very well how ISO values in film photography describe sensitivity and drive exposure, but what is the relevance in digital photography? The CMOS sensor in my D60 only has one actual level of sensitivity, unlike the various film emulsions. I can definately understand if the notion of ISO for the CMOS sensor just "slides" the range of acceptable exposures to the left or right on the scale but anything else confuses me a little.

Can anyone shed some photons on this for me?


eric s Mar 29, 2003 1:15 PM

Other might be able to say this better. I'll take a stab at it.

The way I imagine it is that a higher iso means that the CCD/CMOS is sampling the light longer. So more light physically hits the sensor and therefor it gets a better reading (or enough light to get any reading!) The amount of time isn't long by human/shutter standards (we're not talking 1/25 of a second or something.) But slightly longer than normal.

The downside of this is that the longer the CCD/CMOS stays on, the more it heats up. Heating either of those sensor types creates more noise in the image.

sjms Mar 29, 2003 2:33 PM

"The CMOS sensor in my D60 only has one actual level of sensitivity"

is that a factual statement? or is the iso knob a gain control?

this is just feed the conversation.

if i took it down to iso 80 and shot would the noise level be lower if the same shot were taken at iso 400 or iso 800

eric s Mar 29, 2003 7:10 PM


From all I've heard, yes. Not all noise comes from the ISO setting, but some does. So it is my belief that the same picture taken an ISO 80 would show less noise than taken with ISO 400 or 800.

The other thing I wonder (which I hope someone can comment on) is if heat is the only/major cause of noise when changing ISO. I wondered if there wasn't another cause (along with heat) which made the higher ISO settings have more noise. Humm... I might just turn that into a question on the general forum.

sjms Mar 29, 2003 7:44 PM

if i were to shoot an image at iso 80 it would take considerably longer to expose thus build up more heat for the equivelent shot. in the heat thought more exposure time more heat more noise? yet the 400 image will display greater noise. which leads me to believe that there is gain(sensetivity) involved in that little adjustment.

eric s Mar 29, 2003 8:02 PM


Oh, I see what you are saying. To make up for the lower ISO, you'd have to do a longer exposure. Since *that* mean sampling longer, why doesn't it add more heat? I believe for some cameras it does. Some just don't do well with longer exposure.

But it does blow a huge hole in my theory. If a 1 second exposure doesn't generate enough heat to cause problems, then changing the iso from 80 to 400 shouldn't either. Viewing it more like gain/sensitivity is probably better.

sjms Mar 29, 2003 8:33 PM

higher gain/higher noise

norm911 Mar 29, 2003 9:01 PM

From what I've been reading today, it seems that the noise is caused by the amplifers built-in to each pixel of the CMOS sensor and not by heat. The "gain" idea mentioned by sjms above might have some validity now that amplifiers are involved. Do any Canon engineers read and contribute to these forums? That might be helpful.

Anyway, if noise is the only issue, then it would seem that Canon wouldn't even allow you to "push" the ISO of the sensor into the ranges that result in noisy images. There must be more to this.


eric s Mar 30, 2003 8:23 AM

Well, I agree that higher gain or less signal makes more sense than my answer.

But they wouldn't stop us using it "if noise is the only issue" as you say. Neat Image can do amazing things when removing noise from pictures. I've seen it save pictures which I though were unusable otherwise. Higher ISO is also a marketing issue. It helps the product when doing compairsons to the competition.

norm911 Mar 30, 2003 2:41 PM

Ok, I've been talking with a photographer friend about this offline, and something just came up that changed my thinking on this issue.

What if the ISO notion for CCD’s and CMOS sensors is just a way to set the sensitivity of the sensor in a range that a photographer can relate to. We already know that these sensors are *very* sensitive - much more so than film in some cases. For instance, a digital video camera can capture images in near darkness, as well as on the brightest sunny snow days. So if the sensitivity of the sensor in our digital SLR's was set too high – as in our analogy of the video camera in near darkness – then correct exposures in the camera for a given situation would be something like 1/2000th at f11 in lighting levels that the photographer would be expecting something more like 1/60th at f4. Just an example, but you know what I mean.

The vendors, by figuring out the CCD and CMOS sensitivity levels that closely match well-known film speeds and using THOSE values as settings in the camera rather than “CMOS sensitivity level: choose from 1-255” might just be trying to make it easier for photographers to use the cameras. This approach keeps the exposure ranges more in line with what the photographer has been used to, and puts the new control knobs of the new technology (the CCD or CMOS sensor) into terms that they can easily understand.

What does everyone think about this?


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