CCD/film/eyes

[Guest]

I think I'm right in saying that there is a fundamental difference in the way eyes and photographic film work. An eye is sensitive to the intensity of light not to the duration of light. So if you are looking at something very dim and you can't see it, it doesn't matter how long you keep staring you still won't see it. In contrast a film acts as a kind of integrator. So if you are taking a picture of something dim you can leave the shutter open and the film adds up the light until eventually an image is formed. All within limits of course.

So my question is, what happens in a digital camera with a CCD? Is it like an eye or like a film or is it something else? I've used film cameras since I was a boy and I still use them now but I also have a DSLR. On the DLSR I can set the ISO rating but the ISO rating is a filmy kind of thing so how does that relate to the CCD? I had an idea that the CCD worked more like an eye so I would have thought that a CCD had a particular sensitivity and that was that.

Cheers

Steve

 

 

[Davey-T]

Digital sensors have wells which fill up with photons, longer exposure more photons fuller wells (simplified version)

Digital ISO is an amplifier

Dave

[Tiki]

Eyes and film emulsions only react with light of certain colours. CCD's react with light of a wider spectrum of colours (hence UV/IR cut filters) .

3 hours ago, Davey-T said:

Digital ISO is an amplifier

Or you could also say that ISO is like gain. High ISO, high gain and the signal becomes more noisy.

[acey]

Every photon detector has an integration time. In a camera it's the exposure time, in the eye it's determined by the retinal cells and is about half a second or less. The Hubble Deep Field image was constructed from exposures totalling more than 30 hours in each of four wavebands. What you see through a telescope is constructed by your brain out of half-second exposures, while your eye flicks around in involuntary movements called saccades.

[saac]

We do not bad then really for a half second exposure!

 

Jim

[vlaiv]

On 15.2.2016. at 19:28, saac said:

We do not bad then really for a half second exposure!

 

Jim

Eye is a fascinating piece of equipment (coupled with brain). It is extremely sensitive, and it has some really clever noise reduction algorithm. It can detect even a single photon, but brain stops this weak signal from being "read". I think that it takes somewhere around 10 photons or so in "single" exposure to trigger a light response. This is why we don't see any noise when looking thru the telescope (at least fine grained, coarse grained noise is present - things might pop in and out of view during observing session), and I believe that for such short exposures (order of 1/10th of a second) eye beats both ccd/cmos and classical film.

[Skipper Billy]

But what do cats see ?  Their eyes are much more sensitive than ours.  I often wonder - if I had the eyes of a cat what would the night sky look like - pretty spectacular I guess !

[vlaiv]

Cats cheat!

They use faster focal ratio (have bigger eye lenses), use binning (higher rods density at expense of angular resolution) and have enhanced coatings (tapetum lucidum) to boost their QE

[saac]

Yep Cat's do cheat. The tapetum lucidum is a marvel found in many nocturnal mammals, sadly not us.   A special layer behind the retina that acts to reflect  light that has not been absorbed by the photoreceptors on its first pass.  Reflecting it back through the retina gives a second chance for the photons to be absorbed by the retina's photoreceptors, thereby increasing the light gathering power, improving night vision.  Rather cool.

 

Jim