Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-13-2012, 07:08 PM   #1
Pentaxian
PPPPPP42's Avatar

Join Date: Jan 2012
Location: Wisconsin
Photos: Albums
Posts: 847
How does a DSLR "fake" ISO changes?

With film its obviously chemically more or less light absorbent, but you can't really make a sensor more or less light absorbent, just process the light that hits it differently.
As I recall the "native" ISO on my K20D is 100, anything else is done with processing but exactly how baffles me and I never really thought to ask until now. After seeing 51200 ISO shots cleaned up from a K-5 I'm doubly baffled.

I think a K5 on max ISO with an F1.2 lens would actually become a black hole on pressing of the shutter button.

04-13-2012, 07:10 PM   #2
Pentaxian
JinDesu's Avatar

Join Date: Jun 2011
Location: New York City
Photos: Gallery
Posts: 5,626
Very good guesswork
04-13-2012, 07:18 PM   #3
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 42,395
I think a lot of math does the trick

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

04-13-2012, 07:36 PM   #4
Senior Member




Join Date: Mar 2011
Posts: 140
I think that ISO on digital cameras comes from an amplification of the current to the sensor. That's only a guess, but if you ever shoot moving subjects in poor light with a high ISO (especially on a K7), you may notice the "waves" of light, as opposed to the smoother motion you get from film. Once again, it's only a guess.

04-13-2012, 07:41 PM   #5
Pentaxian
JinDesu's Avatar

Join Date: Jun 2011
Location: New York City
Photos: Gallery
Posts: 5,626
The true ISO on the camera is a function of the amplification of the sensitivity of the sensor. However, expanded ISO is a pure function of algorithms to "guess" at what is in the picture. I believe ISO 51200 is an expanded ISO.

Actually, I think a good portion of the k-5 high ISO including 25600 and 11something is "fake ISO"
04-13-2012, 07:49 PM   #6
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
A sensor has a base ISO; all higher ISOs result from positive analog amplification. Alas, these aren't rigged as op-amps, with negative feedback for negative amplification. I'd love to have a grainless ISO 10 sensor. Somebody should work on this.
04-13-2012, 07:55 PM   #7
Veteran Member
Anvh's Avatar

Join Date: Sep 2011
Posts: 4,616
QuoteOriginally posted by JinDesu Quote
The true ISO on the camera is a function of the amplification of the sensitivity of the sensor. However, expanded ISO is a pure function of algorithms to "guess" at what is in the picture. I believe ISO 51200 is an expanded ISO.

Actually, I think a good portion of the k-5 high ISO including 25600 and 11something is "fake ISO"
It's suggested that above ISO1600 the algorithm kicks in

with the K5
04-13-2012, 07:56 PM   #8
Site Supporter
Ex Finn.'s Avatar

Join Date: Nov 2007
Location: Southern Maryland.
Photos: Gallery | Albums
Posts: 6,196
QuoteOriginally posted by RioRico Quote
these aren't rigged as op-amps,
I thought they were, in a sense. Less negative feedback = more gain.

04-13-2012, 10:08 PM   #9
Loyal Site Supporter




Join Date: Jun 2009
Location: Tumbleweed, Arizona
Photos: Gallery | Albums
Posts: 5,091
QuoteOriginally posted by RioRico Quote
A sensor has a base ISO; all higher ISOs result from positive analog amplification. Alas, these aren't rigged as op-amps, with negative feedback for negative amplification. I'd love to have a grainless ISO 10 sensor. Somebody should work on this.
Analog engineering is an absolute black art (and also a dying art since everything has gone digital).

04-14-2012, 01:47 PM   #10
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
To answer directly: the sensor records voltage levels, always at base ISO. For the first few jumps in ISO, an analog amplifier kicks in to boost the voltage level before the analog to digital converter does its bit (sorry) to convert the voltage levels into numeric values. At some point - anywhere from around 800 to around 3200 depending on the camera - the analog amplifier maxes out, and the numeric values output by the analog to digital converter are simply multiplied by 2 for each stop of increased ISO. On some cameras, I understand you can actually see that happen if you examine the file with a hex editor, as all pixel values will have zeroes in their bottom bits. But this is also the point at which many cameras kick in some amount of automatic noise reduction, even for RAW files, and that might disguise this fact.
04-14-2012, 03:45 PM   #11
Veteran Member
Anvh's Avatar

Join Date: Sep 2011
Posts: 4,616
QuoteOriginally posted by RioRico Quote
A sensor has a base ISO; all higher ISOs result from positive analog amplification. Alas, these aren't rigged as op-amps, with negative feedback for negative amplification. I'd love to have a grainless ISO 10 sensor. Somebody should work on this.
Now that you mention it the ISO80 of the K5 is also extended mode and the D7000 that would have the same sensor doesn't have ISO80 so quite curious how that work because DXO shows increase in quality when using ISO80...
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, iso, light, pentax help, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Don't say Pentax "Q" in French ... "Q" = "cul" = "A--" Jean Poitiers Pentax Q 52 11-10-2013 06:25 AM
Has anyone used h2testw to check for "fake" flash drives? Alliecat General Talk 8 03-27-2012 10:16 AM
Photos: Where does a "fake" picture start? beholder3 Photographic Industry and Professionals 4 03-10-2012 06:02 PM
Wierd "pentax lens" (probably fake) summonbaka Pentax SLR Lens Discussion 4 01-01-2010 07:59 AM
At attempt at fake HDR and emulating "The Dave Hill Effect" lastdodobird Post Your Photos! 2 03-05-2008 06:51 PM



All times are GMT -7. The time now is 04:05 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top