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The D200 has the ability to take images at ISO100 while the D70 range starts at ISO200. In similar vein other digital cameras have had slightly different ISO ranges.
Is there a simple explanation of why this difference occurs, I could hazard a guess at why for high ISO but what limitations? or other reasons are there for a digital camera having a lower bound in ISO?
This question is prompted by a recent post that noted ISO200 with the D70 was better in the owners initial opinion for noise than his D70.

That was a pretty inconclusive thought on my part, and there was some postproduction involved with the shot. Just sort of vague impressions. I'm going to do a more thorough test comparing the two at ISO 200, and then one at ISO 100, long exposures, without long exp NR and i'll bring you my results. It won't be really soon though, my car blew-up last night.

Edit, I also shot in Raw compressed, I'll shoot uncompressed next time. That may have been the difference.

Thanks for the reply Dooda. Those personal tests would be very interesting for those in 'transition' to the D200. I am still wondering what technical reasons there are for a sensor to be limited by an upper and lower ISO boundary by the manufacturer. I will have to do a few searches but assumed someone on the forum would have investigated the topic well before me.

I have VERY little understanding of this,

However I have read that each sensor has it's own 'native' sensitivity, at which is has no additional signal amplification or modification to achieve a certain ISO rating (eg ISO200 for the D70)

Any other ISO rating will need amplification (positive or negative) to increase it's sensitivity. I read this in a discusiion about why the D70 didn't have ISO100... the theory was that the sensor has a native ISO of 200 and to get ISO100 would require negative amplification and wouldn't have any image quality benifits.

As I said... I have no understanding of this concept and am just regurgitating stuff I've read elsewhere. I have no idea if this is actually correct or even if it's what you're asking

Paul

My thoughts as i am aware of it:

Every CCD and CMOS, being an electronic circuit, suffers from noise. All electronic components create noise. When a signal is amplified in a digital circuit, noise is also amplified. As the amplification is increased so too does the noise increase. At some stage, the noise will overpower the original signal and be the dominant signal in the circuit. (This is called sensitivity)

The key to this original signal is the sensitivity of the receiving equipment, in this case, the CCD/CMOS. There is a point where the sensitivity of the CCD/CMOS is not enough to gather enough data(light) to create information for amplification. This would be the point where the lowest ISO is acheivable to provide consistent results.

It may be possible to get enough light data out of the D70 CCD if the sensitivity were reduced to, let's say 100 ISO. However the shutter speed required may be far too long. It's possible that the sensitivity drops off sharply lower than 200 ISO. So for the CCD to provide an accurate and consistent image, 200 ISO is the lowest ISO the CCD is capable of.

Bear in mind that there is much more to it than just that.

It seems that the D200 suffers far less amp glow than the D70. Does anyone else notice this? I get a tiny bit in the lower right, and sometimes the upper right, but for the most part it takes some really sloow shutters (4+minutes) to get any ampglow from the D200, and even then, only towards the end of the evening of shots. So I've been leaving long Exp NR off, leaving a little bit of room for cropping (still way more resolution than on the D70) if needs be, and letting the cloning tool take care of the rest of the amp glow. This is a real relief because:

A) With the inferior battery length of the D200, I was going to need three batteries per night.

B) At ISO 100, my shots were going to take 2x longer on the D200 than on the D70 (which also meant 2x longer noise reduction). So I'm pretty relieved, the pictures from the D200 are way smoother, and I'm not waiting for NR to do it's thing for infinity amounts of time.

I still haven't done a test comparing ISO 200 on D200 and D70 though, and I probably won't be able to unfortunately. Sorry.

NikonUser wrote:... the theory was that the sensor has a native ISO of 200 and to get ISO100 would require negative amplification and wouldn't have any image quality benifits.

Assuming there wouldn't be any negative effects by doing it, if the D70 did allow negative amplification (ISO100), that would negate the need fror a 2x ND filter in some situations... There have been times when I would have liked to slow the D70 down but I don't (yet) own ND filters.

there are some noise comparisons on a DPReview thread that would interest those comparing the D70 and D200 as well as others ...
http://forums.dpreview.com/forums/readf ... d=18919912