more pixels are better!

Started Dec 14, 2008 | Discussions thread
bobn2
bobn2 Forum Pro • Posts: 70,320
Re: SNR vs exposure

Les Olson wrote:

I apologise if I have misunderstood what folk meant or quoted
anything out of context.

Here is a link to the relationship between SNR and exposure:

http://www.microscopyu.com/tutorials/java/digitalimaging/signaltonoise/index.html
Nikon allergic folk can find the same information on the Olympus site
or on the original Florida State U site.

The key point the graphs make is that you need a lot of light to make
sensors photon-noise limited, ie, to make it true that SNR is related
to total sensor area, rather than to the size of individual
photosites.

Improvements in sensor design are a factor, but DxO's data show that
they have not been sufficient to outweigh the effects of increased
pixel density: SNRs have been falling steadily over the last few
years. From their graphs "normalised" to constant pixel pitch it is
clear that SNRs have gone up 3-6 dB because of improved sensor
design. That is nice - but it means that if the manufacturers had
not increased pixel numbers we could have had 1-2 stops more ISO
head room, instead of a nett effect of nothing which is what we got!

It should, however, be noted that DxO's graphs of SNR vs date are all
at 18% reflectance, which is quite a lot of light - roughly, the
centre of the histogram. Noise is not such a big deal at the centre
of the histogram, and the question is what has happened down the left
hand end. If the weather here stays miserable I will re-plot the SNR
vs date graphs using SNRs for low light and post the results.

I think the point is that in photography, we always are dealing with situations where there's a 'lot of light'. You set the xposure so that the highlights pretty much fill the pixel well at base ISO, and even with a typicl 16x exposure push for high ISO, we're working with wells 1/16 full. Many of the CCD tutorials that are cited are primarily aimed at astronomy/science users, where very often the aim is to catch some phenomenon in a single or very few pixels - that's a completely different regime. In photography the design aim is to get as many pixels as possible over any individual piece of detail, in which case 'image level' noise is the thing we're interested in.

-- hide signature --

Bob

Post (hide subjects) Posted by
thw
cpw
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow