Archived from groups: rec.photo.digital (More info?
Roger N. Clark (change username to rnclark) wrote:
> This is a good question. I think the answer is partly that
> ISO was originally defined as a density on film at a given
> exposure time for a given light source intensity. If one takes
> a percentage of full well capacity in electronic sensors,
> as a substitute for density on film, then we see ISO definition
> has to do with how many photons are collected as a percentage
> of the total possible. That says nothing about noise, just as
> with film the specification says nothing about noise
> (or film grain).
> Side note:
> Because film has a much lower quantum efficiency than CCDs or
> CMOS sensors, it collects many fewer photons, about 6 times
> lower (at least). Scaling the numbers here,
> we could compute the number of photons incident per
> square micron at a given ISO, and f/ratio. It appears to me
> that film too may be close to photon noise limited, or at
> least a large part of the noise we see in film, which we call
> grain is due to photon statistics, added to the random
> distribution of film grains, which induces its own noise.
> But I believe the photon noise is the larger of the two
I would welcome a more sensible definition of ISO as applied to digital
cameras. Percentage well filling without specifying a noise level is a
plain nonsense. Perhaps then we could move on from the DLSR versus Point
& Shoot argument and simply say if you can be satisfied with a low ISO
camera go P&S and if you need high-ISO go DLSR?
>> 2 - Number of pixels for a given sensor size.
>> I know I've raised this before but I'll do so again. If you have a
>> given sensor (let's think small sensor here - e.g. 8.8 x 6.6mm),
>> then you could, for example, have either 2MPixels at one size or
>> 8MPixels at half the linear size or a quarter of the area. Neglecting
>> packing efficiency, you could have a higher-resolution
>> image with poorer SNR in each pixel, or a lower resolution image
>> with higher SNR per pixel. Which does the eye prefer in typical
>> viewing conditions? The image with the smaller pixels will appear
>> crisper. How is the different noise spectrum perceived by the eye?
> This is a very good question, and I have been researching this
> subject. It appears that to first order there is an even trade for
> signal-to-noise and spatial resolution. So in your example,
> the images would appear similar. But because it is
> subjective, some people will prefer one, others the other,
> but they would be judged close, especially compared to the
> same images with equal signal-to-noise ratios. I've created
> images like this and that is what I see and what others have
> indicated. But it is a subjective test. I plan on creating a
> series of images, trading spatial resolution and signal-to-noise
> ratio and presenting that on a web page for people to judge for
> themselves. But it will be a few months before I get this done.
When I was involved with this professionally some years ago, we used a
weighted SNR based on the sensitivity of the eye & brain at various
spatial frequencies. Rather as in audio where there are various weighting
curves you can use to make the measured SNR tie up better with the
subjective SNR than an unweighted flat frequency response figure. This
attempts to remove at least some of the subjective element, but you are
right that different observers will have different preferences making a
weighted SNR only representative of a median of observers. Viewing
conditions would need to be standardised as well!