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01-13-2018, 01:53 AM   #1
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ISO invariance for Pentax DSLR cameras?

Are the Pentax cameras with invariant ISO or not? What are the native ISO values and what are the simulated ones?
It would be interesting to know for KP, for example, a camera with APS-C sensor that performs at high ISO values.

01-13-2018, 04:00 AM - 1 Like   #2
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The Sony Exmor sensors are ISO invariant. Here is a discussion on the topic (with a number of good links) - not necessarily with respect to the KP.
01-14-2018, 05:33 AM   #3
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Thanks for the links, useful information though I am not passionate about astrofotography (still ).

I'm still curious about KP what ISO values are "real" and what are "simulated"?
01-14-2018, 12:40 PM - 1 Like   #4
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My K01 and my K1 are certainly Iso invariant. Now almost all the time my K1 is set to base Iso (100).
Iso invariance - PentaxForums.com

01-14-2018, 12:43 PM   #5
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QuoteOriginally posted by edri Quote
I'm still curious about KP what ISO values are "real" and what are "simulated"?
I am not sure what the relevance of this is to a truly Iso invariant sensor?
01-14-2018, 02:23 PM   #6
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QuoteOriginally posted by edri Quote
I'm still curious about KP what ISO values are "real" and what are "simulated"?
Setting ISO is basically telling the camera's processor how much to mathematically amplify the digital output from the sensor's processor. The sensor itself is supplied a variable bias voltage to compensate for different light intensities and the bias voltage is determined by the sensor's processor according to instructions from the camera designers. Like any electronic device, analog performance is not perfectly constant, but the sensor's processor can compensate for that and in effect all ISO settings are artificial.

The people designing Pentax cameras know more about the performance characteristics of the both the light meter and image sensors put in their cameras than any of us do. Extending the range of allowable ISO settings involves tradeoffs, at the high end a decision is made as to how noisy acceptable images can be (and with the KP's special in-camera noise reduction processing, how much detail can be lost while still being acceptable) and at the low end, in order to offer lower ISO settings, either losing details (by "compressing" the sensor's output) or making high ISO settings noisier than they would be if the "base" ISO was higher. Either way, it's a marketing decision implemented by engineers.

The programming of the sensor and the camera combine to produce the best results at the ISO settings that the camera designers decide should produce the best results. Different cameras using the same sensor will perform differently, depending on how well the programmers compensate for varying physical characteristics of the image and light meter sensors, as well as the decisions made regarding tradeoffs of details retained versus minimum and maximum ISO settings. What are "real" and "simulated" ISO values is subjective and if you don't find images taken at certain ISO settings to be acceptable, don't take images at those ISO settings. Unless you can manipulate instantaneous light intensities, selecting preferred shutter speeds and apertures should come before selecting preferred ISO settings, because the alternative is not taking an image, period.
01-22-2018, 09:53 PM - 1 Like   #7
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I'd say Pentax sensors are ISO invariant. For night time shots I routinely set the ISO @100, the shutter speed and the aperture to suit the scene and fire away. Later in Snapseed I open the shadows, raise the exposure a bit and add contrast to taste. Try it, you will see incredible dynamic range, beautifully rendered highlights and minimal noise. You cannot chimp though as the image will be too dark.BTW: SHOOT RAW!
01-23-2018, 03:31 AM   #8
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QuoteOriginally posted by ShawnH Quote
I'd say Pentax sensors are ISO invariant. For night time shots I routinely set the ISO @100, the shutter speed and the aperture to suit the scene and fire away. Later in Snapseed I open the shadows, raise the exposure a bit and add contrast to taste. Try it, you will see incredible dynamic range, beautifully rendered highlights and minimal noise. You cannot chimp though as the image will be too dark.BTW: SHOOT RAW!
Amazing how simple it can get. Now for most shots I set the most open appropriate aperture and slowest appropriate shutter, check I am not clipping the highlights and sod the rest - just start shooting. Of course base Iso and Raw. What I am doing is giving the sensor the maximum amount of light possible and that is as good as it gets for IQ. The dim images are annoying though - it would be great if you could bracket Iso.

01-23-2018, 09:29 AM   #9
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QuoteOriginally posted by GUB Quote
Amazing how simple it can get. Now for most shots I set the most open appropriate aperture and slowest appropriate shutter, check I am not clipping the highlights and sod the rest - just start shooting. Of course base Iso and Raw. What I am doing is giving the sensor the maximum amount of light possible and that is as good as it gets for IQ. The dim images are annoying though - it would be great if you could bracket Iso.
We really need a "D" mode added to PASM. It would add a small jpg brightened for chimping , ISO bracketing and a workable live view. It is unfortunate that camera makers have pushed a phony ISO dial rather than educate us on how a digital sensor really works.
01-23-2018, 12:31 PM   #10
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QuoteOriginally posted by ShawnH Quote
It is unfortunate that camera makers have pushed a phony ISO dial rather than educate us on how a digital sensor really works.
In the past when all sensors were not Iso-ivariant then pushing the exposure triangle just like in the film days made sense. But quite agree that now it has stuffed up peoples thinking. But what worries me more is that I am not sure that Iso-invariance is seen as a major point of difference by the manufacturers. Look at the Sony a9 - it has abandoned Iso-invatriance in pursuit of some other folly. (I think it was to achieve a fast burst rate from memory).
01-23-2018, 04:11 PM   #11
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BTW; Exposure metering also becomes far less critical at night. Just set the ISO to 100 with a dynamic range of 12.5 stops (see dxomark) just about any night scene can be handled.
01-23-2018, 05:46 PM   #12
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More than "less critical" and actually unnecessary (for handheld based shooting). For instance my default night scene lenses are the A28 2.8 and the A50 17. If you accept that the 50 1.7 is better stopped down to f2 and subject movement and handholding limitations restrict you to 1/ focal length shutter speed then at Iso 100 both lenses exposed correctly for EV8 which from memory is a "brightly lit night street scene" anything darker than that is underexposed and Utilizing Iso-invariance kicks in to whatever degree needed. The only caution is to still chimp a bit and check things like the street lights themselves are not getting too fried.
01-23-2018, 06:03 PM   #13
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QuoteOriginally posted by GUB Quote
More than "less critical" and actually unnecessary (for handheld based shooting). For instance my default night scene lenses are the A28 2.8 and the A50 17. If you accept that the 50 1.7 is better stopped down to f2 and subject movement and handholding limitations restrict you to 1/ focal length shutter speed then at Iso 100 both lenses exposed correctly for EV8 which from memory is a "brightly lit night street scene" anything darker than that is underexposed and Utilizing Iso-invariance kicks in to whatever degree needed. The only caution is to still chimp a bit and check things like the street lights themselves are not getting too fried.
Exactly. I am doing a series on liquor stores with neon signs at night. Settings are M; 1l60: f2.8 ISO 100.
01-23-2018, 08:23 PM - 1 Like   #14
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This shot was with tripod so no need to underexpose except to control the highlights. A28mm 2.8 @ f5.6 1/2sec. First shot is ex camera with exposure about right for the lights. Second shot the foreground plants are brought up and laid across the first with a layer mask and gradient (Gimp). Your guess is good as mine on how much I pushed the flowers (3 stops?) - this would not be so successful without Iso invariance.


01-23-2018, 11:46 PM - 1 Like   #15
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QuoteOriginally posted by GUB Quote
This shot was with tripod so no need to underexpose except to control the highlights. A28mm 2.8 @ f5.6 1/2sec. First shot is ex camera with exposure about right for the lights. Second shot the foreground plants are brought up and laid across the first with a layer mask and gradient (Gimp). Your guess is good as mine on how much I pushed the flowers (3 stops?) - this would not be so successful without Iso invariance.

Beautiful shot. I shot the image below at ISO 100, 1/60, f4. Some simple shadow, exposure and contrast adjustment .
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PENTAX K-S2  Photo 

Last edited by ShawnH; 01-23-2018 at 11:53 PM.
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