Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-13-2014, 05:57 AM - 3 Likes   #16
Loyal Site Supporter




Join Date: Mar 2012
Location: Rochester, NY
Photos: Gallery
Posts: 1,819
QuoteOriginally posted by Hattifnatt Quote
I have no idea how film ISO works, I was only talking about digital ISO, which in signal processing is basically a scaling up.
Lecture from my old days taking photo chemistry.

Pretty easy actually. The higher the film ISO the more sensitive to light, or faster, it is. This is accomplished by using larger silver halide (AgX) crystals. Larger crystals increase the chance that an AgX will be struck by a photon of light. It takes three photon strikes to the AgX crystal to allow developer to reduce the crystal to silver. This comes at cost. Faster film is more grainy. ie. It has more artifacts in digital parlance.

By controlling the ratio of large and small crystals in a film you control the contrast of the film. High contrast graphic arts films basically have just one size crystal. It is all of nothing. Regular photographic films have a range of crystal sizes within each film type. To add a bit more to the equation you could also tweak the results while processing the film.

Other factors after exposure that can control the results include:

- Which developing agent or combination of developing agents you use.
- The length of time in the developer.
- The temperature of the developer.
- The agitation of the solution during development.
- You can also increase sensitivity by adding things like gold chloride to the mix.
- Film has a backing that absorbs light that passes all the way through the emulsion. Light bouncing back into the emulsion will affect contrast and detail. This is called the anti halation layer. Digital sensors are probably built with the same sort of backing.
- If you develop the film long enough it will reduce all crystals to silver. Photo strikes damage the crystal is such away to allow he developer faster access by the developer. They act as a catalyst to speed up the process.


Kodak developed a technology called T-Max that allowed higher film speed using smaller crystals. Silver halide crystals look like triangles with the tips cut off. Kodak found a way that made the crystals lay flat in the emulsion exposing the highest possible surface area to the light. You got he effect of higher speed with smaller crystals. Prior to this the crystals had no alignment and were at whatever angle they happened to be when the emulsion hardened. This has the effect of increasing film speed and reducing grain.

11-13-2014, 07:09 AM   #17
Loyal Site Supporter
AquaDome's Avatar

Join Date: Jan 2014
Location: New Carlisle, IN
Photos: Gallery | Albums
Posts: 1,403
QuoteOriginally posted by old4570 Quote
Iso 50
Yes. ISO 50 please. Even ISO 25 would be useful to me.
11-13-2014, 08:23 AM   #18
Pentaxian
FantasticMrFox's Avatar

Join Date: Oct 2014
Location: Scotland
Posts: 2,097
As far as I remember reviewers were very excited about the ISO 64 setting in the Nikon D810 and claimed a very appreciable difference in dynamic range.

Last edited by FantasticMrFox; 11-13-2014 at 11:30 AM.
11-13-2014, 10:57 AM   #19
Veteran Member




Join Date: Jan 2012
Location: United States
Posts: 340
That is interesting, that must be some pentax processing magic. Generally extended isos are basically taking a certain exposure and pushing exposure. In this case it would be exposing by 1/3 stop at 100 ISO to get 80 iso. There should be no image difference from a shot at +1/3 EV ISO 100 to +0 EV ISO 80. Either that or the base iso of the K-5 sensor is ISO 80

11-14-2014, 10:09 AM   #20
Veteran Member




Join Date: Dec 2010
Location: SoCal
Photos: Gallery | Albums
Posts: 494
I agree on the ability of having ISO 50 or ISO 80 - it would be great.
I'd like to be able to shoot in bright outdoor light wide open w/o having to shoot at 1/6000s at times.
I suspect that Pentax decided that with a smaller sensor (APS-C), and slower lenses (not many fast lenses that aren't primes better than f/2.8), better than ISO 100 wasn't really necessary.
11-15-2014, 06:36 AM   #21
Loyal Site Supporter
Mikesul's Avatar

Join Date: Sep 2009
Location: Phoenix, AZ
Posts: 2,479
QuoteOriginally posted by gaweidert Quote
Lecture from my old days taking photo chemistry.

Pretty easy actually. The higher the film ISO the more sensitive to light, or faster, it is. This is accomplished by using larger silver halide (AgX) crystals. Larger crystals increase the chance that an AgX will be struck by a photon of light. It takes three photon strikes to the AgX crystal to allow developer to reduce the crystal to silver. This comes at cost. Faster film is more grainy. ie. It has more artifacts in digital parlance.

By controlling the ratio of large and small crystals in a film you control the contrast of the film. High contrast graphic arts films basically have just one size crystal. It is all of nothing. Regular photographic films have a range of crystal sizes within each film type. To add a bit more to the equation you could also tweak the results while processing the film.

Other factors after exposure that can control the results include:

- Which developing agent or combination of developing agents you use.
- The length of time in the developer.
- The temperature of the developer.
- The agitation of the solution during development.
- You can also increase sensitivity by adding things like gold chloride to the mix.
- Film has a backing that absorbs light that passes all the way through the emulsion. Light bouncing back into the emulsion will affect contrast and detail. This is called the anti halation layer. Digital sensors are probably built with the same sort of backing.
- If you develop the film long enough it will reduce all crystals to silver. Photo strikes damage the crystal is such away to allow he developer faster access by the developer. They act as a catalyst to speed up the process.


Kodak developed a technology called T-Max that allowed higher film speed using smaller crystals. Silver halide crystals look like triangles with the tips cut off. Kodak found a way that made the crystals lay flat in the emulsion exposing the highest possible surface area to the light. You got he effect of higher speed with smaller crystals. Prior to this the crystals had no alignment and were at whatever angle they happened to be when the emulsion hardened. This has the effect of increasing film speed and reducing grain.
Nicely informative post and pleasant reminder of old film days.
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, dslr, iso, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
ISO 80 vs ISO 100 shaolen Pentax K-5 30 12-05-2013 11:05 AM
ISO limited to 160, not 80 in expanded. toooldtocare Pentax K-5 9 07-02-2013 12:03 PM
K-5: can't select ISO 80 (only 100) fnflying Troubleshooting and Beginner Help 6 03-22-2013 10:31 AM
Iso 80 JimD Pentax K-5 5 03-20-2012 03:31 PM
80 iso? Mark K5 Troubleshooting and Beginner Help 17 03-04-2012 01:07 PM



All times are GMT -7. The time now is 07:05 PM. | See also: NikonForums.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