Digital primer

[jeffreybehr]

[font color=\'#000000\']Well, I'll bite.  My answers apply to digital single lens reflex cameras (DSLRs), not fixed-lens digicams.

'What exactly is meant by exposure time in digital?'  This has exactly the same meaning as in film SLRs.  It's the amount of time any part of the film or sensor is exposed to the projection of lite from the lens.

'Does the camera "scan" the image from top to bottom to create a stream of data?'  Don't know.  Doesn't matter to me.

'Does this reading the data stream occur after the sensor is exposed?'  Yes.  There's no info (voltage) in each sensor element until it's exposed to lite.

'If the camera can achieve a "speed" of 1/1000 second, How does the camera gather more light by longer exposer time?'  It's just like film.  The sensor requires a certain level of lite to create a voltage or create a chemical reaction in the film's coating.  If the incoming lite level is low, one must expose the sensor a longer time to end up with a useful voltage.

'If a camera is labled maximum ASA/ISO of 100, what is the camera doing when you shoot at 400?  What are the trade-offs?'  If the camera's maximum ISO rating is 100, you can't shoot at 400.  Did you mean 'minimum'?  If so, shooting at 400 v. 100 exposes the sensor for 1/4 of the time, generating a lower voltage in the sensor's elements.  That lower voltage is amplified more by the camera's operating system to create a useful voltage.  Unfortunately, the sensor's random electronic noise is also amplified more, creating a higher proportion of noise to useful signal at higher ISOs.  In other words, the image's signal-to-noise ratio worsens continuously the higher the ISO is set.

'(D)oes it have a physical shutter?'  Yes, all DSLRs I'm aware of have shutters just like film SLRs.

Your last question would be good for a doctoral thesis.  Perhaps others will contribute something on it.

Jeffrey Behr
Behr Photography, Phoenix, AZ  USA[/font]

[Randi]

[font color=\'#000000\']I just purchased my first digital camera (Olympus C-2100). I literally did weeks of intense research about cameras and digital photography. Here are some of the sites I saved regarding digital learning.

http://mavicausers.org/NYIphotoCover.html
http://www.dpreview.com/learn/
http://www.kodak.com/global/en/consumer/pi...ing/index.shtml
http://www.schoolofphotography.com/digital.html
http://internetbrothers.com/phototips.htm

(Sorry, I don't know how to add the actual links for you).

There are tons on sites out there. I must have 30 or more saved. I learned so much and there is so much more to learn.

Good luck.[/font]

[dfourer]

[font color=\'#000000\']I'm a little behind the curve on learning about digital.  Below I'm listing some basic questions that are on my mind.  Do you have a favorite on-line primer on the subject?  Something that's not out of date already?  

What exactly is meant by exposure time in digital?  Does the camera "scan" the image from top to bottom to create a stream of data?  Does this reading the data stream occur after the sensor is exposed?  If the camera can achieve a "speed" of 1/1000 second, How does the camera gather more light by longer exposer time?  

If a camera is labled maximum ASA/ISO of 100, what is the camera doing when you shoot at 400?  What are the trade-offs?  

I imagine there are lots of subtle differences between digital and film that I would anticipate if I knew the basics of how the digital camera works.  By the way, does it have a physical shutter?

What are the most important specs - besides # of pixals and sensor size - that are likely to vary from one brand to another or from consumer to professional camera?

----David[/font]

[jwarthman]

[font color=\'#000000\']David,
Put simply, the digital SLR (DSLR) has a sensor (CMOS or CCD) that is exposed much like film. These cameras have "normal" shutters and lenses with variable apertures, just like a typical film SLR.

When the shutter is open, the sensor "accumulates" light energy, much as film does. The longer the shutter is open, the more energy is accumulated by the sensor.

When the camera saves the image (after the shutter closes), it "reads" the sensor, which includes amplification of the signal from each pixel. DSLRs have a sensitivity to light expressed in ISO terms - again just like film.

But unlike film, one can adjust the sensitivity of the sensor "on the fly". Increasing the ISO has the effect of increasing the amplification of the energy that was accumulated by each pixel.

As with film, increased sensitivity comes with a price. In the case of DSLRs, the higher ISO one uses, the "noisier" the image gets. This is because more amplification causes both the "wanted" signals and "noise" to be amplified.

As for specifications, I don't know what specific measurements are available other than those you mentioned. Obviously important is how the images look! It turns out that resolution only tells part of the story. One should look at test images and reviews, and try to gather a sense for the "quality" of the pixels, not just the quantity. For example, Canon's CMOS-based D30 and D60 are said to produce very "film-like" images, but many (cheaper) CCD-based digicams present a more, well, digital-looking image. This has to do, at least in part, with the signal-to-noise ratio of the pixels. You can read descriptions of how the various cameras perform in this regard, but the best evaluators are your own eyes!

I think this site is an excellent place to learn more! Beyond that, following are some helpful links:


A very good overview of digital sensors:
   http://www.extremetech.com/article2/0,3973,15465,00.asp

A good overview of how digital cameras work:
   http://www.extremetech.com/article2/0,3973,15466,00.asp

Focus is astrophotography, but has good info about sensors:
   http://users.erols.com/szykman/Astro/AstroDigiCamFAQ.html

Good tutorial from Kodak:

   http://www.kodak.com/global....ing.pdf

Interesting catch-all site with many links:
   http://www.dcviews.com/tutors.htm

Hope This Helps!

-- Jim[/font]

[dfourer]

[font color=\'#000000\']Thanks very much for your replies.  The links are great.[/font]