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02-04-2021, 10:20 AM   #1
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645D: better at 200 or 100 ISO?

Dear forum friends

I am (recently) happy owner of a magnificent 645D that I am learning to use.

I ask the experts: the "base" sensitivity of the 645D is 200 ISO and many photos I have seen in the forum were taken with this sensitivity.

What happens when you expand it to ISO 100?

What changes in overall performance?

Do they change, improve or get worse?

Thanks for your advice.

02-04-2021, 11:37 AM - 1 Like   #2
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QuoteOriginally posted by GPMASS Quote
I am (recently) happy owner of a magnificent 645D that I am learning to use.
I ask the experts: the "base" sensitivity of the 645D is 200 ISO and many photos I have seen in the forum were taken with this sensitivity.
What happens when you expand it to ISO 100?
What changes in overall performance?
Do they change, improve or get worse?
Congratulations on your new 645D!!!

In my experience, the base sensitivity of digital sensors is similar to the base sensitivity of film emulsions. At the native ISO you get the best performance mostly in terms dynamic range and to a lesser degree color rendition. At a lower ISO than the base, highlights begin to lose details and colors become less saturated.

ISO is a quality certification set by the International Standardization Organization. ISO numbers meet or exceed ISO quality standards. Lo and Hi settings don't, and thus are generally not recommended, but Pentax and others give us this option but cannot call it 100 ISO even though it is effectively 100.

If you've had any slide film experience, it's similar to pull processing by one stop. You can do it, it will maintain your mid tone exposures, but there are artifacts due to the processing compensation.

The best way to know if it's acceptable to you is to run tests to get to know how your camera performs at all ISO and Lo/Hi settings in low light, bright light, and high contrast lighting.
02-04-2021, 04:04 PM   #3
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Usually what happens when using lower than base is that the camera will overexpose and then artificially pull the exposure down as if you had intended to shoot at a lower ISO. This will usually result in lower dynamic range.

It would be similar to using +1.0 exposure compensation at ISO 200 and then using the -1.0 slider for exposure in your RAW software. You’ll probably risk clipped highlights.

Newer cameras are very tolerant of under exposure where doing the above in the opposite direction produces an image nearly identical to using the implied ISO set in your camera.
02-05-2021, 01:39 AM   #4
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100 vs 200

Pentax 645D 100iso vs 200iso

I remember reading this when I first bought my 645D and have rightly or wrongly stuck to 100 whenever possible.
Cheers Paul

02-05-2021, 06:05 AM - 3 Likes   #5
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Hi,

Well, looking at the Kodak sensor data sheet, where they don't actually state it in terms of ISO, and doing a little ciphering as my Dad referred to it, I come out with a base ISO around 120. But, that's above 100, so I think Pentax then chose 200 as being the base. And, then gave us the option of using 100. My own testing and then really shooting at both 100 and 200 shows the unit works equally well at both. It doesn't have the usual issues with a lower than camera base ISO the way other cameras do. So, I keep mine at 100 unless I need to boost it.

Stan
02-05-2021, 11:47 AM   #6
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As suggested, I'll do some tests.

The strange thing is that somewhere I read that at 100 ISO the result is "lower" than that obtainable at 200, especially as regards the control of the "highlights".

And that the best dynamic range is just 200 ...

boh ...
02-05-2021, 12:18 PM   #7
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QuoteOriginally posted by GPMASS Quote
As suggested, I'll do some tests.
Yes and please share your results on this thread. The key will be what happens in the highlights like clouds in higher contrast lighting.
02-05-2021, 05:29 PM - 1 Like   #8
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The only potential problem with running a camera at a lower ISO than the sensor's official base ISO would be a slight pastel mottling in the over-exposed areas.

The reason is sensor physics. All CCD and CMOS sensor pixels have a linear range where the output signal is directly proportional to the input light level. That defines a range of light levels that create an accurate response. And then the pixels have a saturation point in which more input light does not create any more output signal. The exact saturation point varies for R, G, an B colors and also varies from pixel to pixel and chip to chip due to minor manufacturing variations.

Typically, sensor makers and camera makers pick a base ISO to ensure that all the pixels (except a small number of dead ones) have a nice linear response form 0% to 100% light levels at that ISO. It is possible to run a sensor at less than base ISO but then there's a risk that some pixels might get saturated at less than 100% of the signal level. The result is that no matter how much you overexpose the image, some pixels might never reach 100% brightness. Given the interactions with the color array, the result would probably be a faint pastel hue to some pixels in parts of images that have bright white objects (e.g., clouds, walls, snow, etc.). The effect would impact colored objects, too, but be much less noticeable.

02-06-2021, 04:38 AM   #9
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Very interesting.

What questioned me are the DXOMARK charts, where you see an increase in all the features (especially on the prints), going down from 200 to 100ISO.
For example, the increase in dynamic range seems to be really noticeable (and useful in landscape)
Immagine 2021-02-06 122851.png
Immagine 2021-02-06 122913.png
Immagine 2021-02-06 122932.png
Attached Images
     

Last edited by GPMASS; 02-06-2021 at 04:52 AM.
02-06-2021, 06:04 AM   #10
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Hi,

Well, that nicely illustrates it.

Stan
02-06-2021, 08:46 AM   #11
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QuoteOriginally posted by wa2kqy Quote
Hi,

Well, that nicely illustrates it.

Stan
I think that your analysis indicating an intrinsic ISO of 120 suggests that the performance would improve to 100 ISO when there is no way of testing at intermediate ISO levels to see any change between 120 and 100.
02-06-2021, 12:29 PM - 1 Like   #12
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QuoteOriginally posted by GPMASS Quote
What questioned me are the DXOMARK charts, where you see an increase in all the features (especially on the prints), going down from 200 to 100ISO.
For example, the increase in dynamic range seems to be really noticeable (and useful in landscape)
If the DXO charts are accurate, then everything I've posted on your thread is not true. However, what I've posted is not based on my own scientific tests but rather my personal experience and observations when shooting below the base ISO number into the Lo options.

The 645D processor could make a difference when shooting jpegs, but I shoot RAW 99% of the time and I assume the DXO chart is based on RAW data.

I also know that on many cameras, you can shoot extended ISO only as jpegs. To quote JM Peltier: Here’s an example of how Extended ISO works on a camera that has a Native ISO range of 200:

You set your ISO to 100. The minimum amplification that the sensor is capable of is ISO 200. So the sensor will output an overexposed image at ISO 200, then the processor will pull the exposure down one stop to an equivalent ISO of 100.


The 645D's Kodak KAF-40000 sensor has a native ISO of 200. So any extended lower ISO is processing, so I don't understand how the DXO test shows improved performance in those three categories over the base ISO.

The conflicting input you've gotten in this thread can be resolved if you run your own real world test comparison on your 645D.

---------- Post added 02-06-21 at 02:06 PM ----------

Just found this comparison on line. Click on the ISO 100 and compare it to the ISO 200.

Look carefully at the highlights. You'll see they are better controlled and less blown out at ISO 200 which is counterintuitive unless the extended 100 ISO reveals the pixels can't handle the over-exposed highlights.

Pentax 645D medium format digital camera ? The Register

Last edited by Alex645; 02-06-2021 at 01:07 PM.
02-07-2021, 07:09 AM   #13
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Immagine 2021-02-07 102434.png
Immagine 2021-02-07 102510.png
Here are two enlarged details from the site indicated by Alex645.
On the left 100ISO, on the right 200ISO.
Highlights at 200 ISO are better, more controlled
Attached Images
   

Last edited by GPMASS; 02-07-2021 at 01:25 PM.
02-07-2021, 03:43 PM   #14
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I always used to find ISO 200 on my 645D gave me better highlights than ISO 100
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