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06-28-2013, 05:13 AM   #1
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Why no low ISO settings?

Dunno if anyone here knows the answer to this, but... Is there some technical reason why they can't put low ISO settings in a DSLR? It's not just a Pentax thing.. Even Nikon's top dog, the D3x has a minimum ISO setting of 100.

Back in the film days, you could buy ISO 25 speed film. I'd think some lower settings would come in handy for stuff like time exposures in daylight, etc.

06-28-2013, 05:18 AM   #2
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Limitations to sensor technology at this stage preclude any SNR advantage to lower ISO options. I recall asking Pentax why no ISO 100 option on the K100D and their response was because it was not an acceptable result when tested. If there is no advantage to SNR and thus DR, then it wouldn't be a greatly marketable feature to have.
06-28-2013, 05:34 AM   #3
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QuoteOriginally posted by GibbyTheMole Quote
Is there some technical reason why they can't put low ISO settings in a DSLR? It's not just a Pentax thing.. Even Nikon's top dog, the D3x has a minimum ISO setting of 100.
Both my K-r and K-30 offer an ISO of 100 - you have to enable expanded sensitivity. However, when you do this, you will also disable auto highlight and shadow adjustment as it takes advantage of that reserved +/- 1 stop.
06-28-2013, 05:36 AM   #4
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Those tiny photocells can only take a certain amount of light and you want them to be able to handle 4 times as much? Don't think so.

06-28-2013, 06:05 AM   #5
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QuoteOriginally posted by SteveB Quote
Those tiny photocells can only take a certain amount of light and you want them to be able to handle 4 times as much? Don't think so.
Well... it's not just me. Quite a lot of other folks would like this capability as well. I found a plausible technical reason here.

Guess I shoulda researched it before I posted, eh?
06-28-2013, 06:07 AM   #6
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Just like a film emulsion, a sensor has a single best performance ISO. The other settings effectively "push" or "pull" processing of the image, but like with film the results are not as good as the base ISO, although they have really improved high ISO settings due to the demand for it. I doubt if they have put much effort into reducing the sensor sensitivity, as there isn't customer demand for it. Besides, to lower the effective sensitivity you can always add neutral density filters. Some people like to shoot the Leica Noctilux (f0.95) lens outdoors at full aperture, and use a 6x ND filter to do so.
06-28-2013, 06:21 AM   #7
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There have been DSLRs that sort of supported lower ISO settings. The Kodak DCS Pro comes to mind. Basically, they cheated by taking several pictures and averaging every pixel. It kind of works too, except when one of the comprising images is blown out by overexposure.

Last edited by topace; 06-28-2013 at 06:21 AM. Reason: Spelling
06-28-2013, 06:32 AM   #8
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Increasing the ISO means to increase the amplification and by this increasing the noise produced by the amplifiers. To prevent this, in a laboratory you would cool down the amplifiers to very low temperatures, not possible in a consumer camera.

Decreasing the ISO means to dampen the sensitivity of the photo cell, but by doing this you worsen the signal to noise ratio of the photo cell itself.

There is a "sweet spot" somewhere between these 2 disadvantages. With present photo cells and amplifiers, this is somewhere in the range of ISO 80-100.

New technologies may change it some time in the future, but actually at ISO 80-100 you get the best overall signal to noise ratio.

06-28-2013, 06:40 AM   #9
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Because if you lowered the nominal sensitivity of the sensor, you'd not be able to reach the higher ISO's. The IQ of a lower than 100-200 ISO is negligible from an ISO 50 or even 25 sensor. You could not see the difference in the middle of the DR and the highlights would explode all the time. So they design sensors with a baseline ISO of 100-200.

Therefore you don't want ISO lower than 100 because it would do nothing to increase IQ but would lead to substantially more highlight clipping AND worse performance at higher ISO's.
06-28-2013, 06:52 AM   #10
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Gibby, I'd often wondered the same thing...glad you started this thread.

What would be very cool is if they could achieve these low ISO's and make them variable across the sensor. It would be a built-in, shape-customizable neutral density filter!
06-28-2013, 07:13 AM   #11
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I used a Canon PowerShot S2 IS for several years, and its minimum ISO was 50. Unfortunately, its max was 400, but that wasn't out of the ordinary for a point-n-shoot in 2005. ISO 400 on that camera looked worse than ISO 6400 on the K-5.

I've been wondering about this, too, but the answers make sense. You can't have everything all the time...
06-28-2013, 02:35 PM   #12
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QuoteOriginally posted by TomB_tx Quote
Just like a film emulsion, a sensor has a single best performance ISO. The other settings effectively "push" or "pull" processing of the image, but like with film the results are not as good as the base ISO, although they have really improved high ISO settings due to the demand for it. I doubt if they have put much effort into reducing the sensor sensitivity, as there isn't customer demand for it. Besides, to lower the effective sensitivity you can always add neutral density filters. Some people like to shoot the Leica Noctilux (f0.95) lens outdoors at full aperture, and use a 6x ND filter to do so.
So what is the base ISO of a Pentax DSLR? Is it 100 and from there the higher ISO's pull/amplifier the sensor?
06-28-2013, 04:22 PM   #13
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QuoteOriginally posted by 6BQ5 Quote
So what is the base ISO of a Pentax DSLR? Is it 100 and from there the higher ISO's pull/amplifier the sensor?
On the old K20D, it's ISO200. Same w/ the D7000 so it's probably the same w/ the K-5...
06-29-2013, 03:45 AM   #14
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QuoteOriginally posted by 6BQ5 Quote
So what is the base ISO of a Pentax DSLR? Is it 100 and from there the higher ISO's pull/amplifier the sensor?
It depends on the camera. DXO Mark measures isos and they say base for the K5 is 70, for the K20 is 100, and for the kx was 170-ish.
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