What's all that noise about High-ISO?

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nixda
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What's all that noise about High-ISO?
9 months ago

A couple recent and revived threads, one year of playing with my X-E1 and more than two years with my D7000 prompt me to make some comments about High-ISO.

High-ISO comparisons
There is a lot of emphasis on High-ISO comparisons in the camera world. Unfortunately, most of the time, the discussions on forums seem to be based on flawed approaches and are misleading and/or inconclusive. Rarely does one see a thorough analysis; probably rightly so, because it's not to so easy to carry out one.

ISOless sensors
Fuji's APS-C X-Trans sensors are based on a SONY sensor that can also be found in the Pentax K5, Nikon D7000, and a few others. These sensors are ISOless, meaning that above a certain ISO number, the signal amplification to achieve the final image brightness is entirely digital. For the Fuji sensors, that threshold is at ISO 1600. Intensities recorded with settings of ISO 3200, 6400, etc., are really taken with ISO 1600, then simply multiplied by 2, 4, etc., i.e., they are underexposed by one stop, two stops, etc.

High-ISO comparisons again
With ISOless sensors, it seems to me that the only way to make a meaningful comparison is to shoot at the highest ISO number that does not yet rely exclusively on digital amplification, do careful raw processing and then compare the results. If images are taken straight out of the camera, the JPEG engine and its treatment of the raw data become more important than the underlying sensor characteristics one wishes to assess.

For example, there is a lot of hubbub about High-ISO images from the X-E2 having very little noise, but also being overly soft and devoid of details. These are typical consequences of aggressive noise reduction. One would be left to believe that the sensor is awesome, because it has so little noise, yet there is something wrong with the images, because they lack detail and have reduced dynamic range. The only way to really know for sure is to analyze the raw data to find the right balance between detail, noise, etc.

Does it make sense at all?
With the Fuji sensors, under High-ISO conditions, one could simply set the ISO to 1600, shoot according to one's artistic vision for shutter speed and aperture and then worry about getting the exposure right in post processing. It seems that there is no High-ISO setting at all anyway, because one can't shoot at any true ISO higher than 1600! That is unless one considers 1600 to be 'High-ISO', but most such comparisons focus on 3200, and 6400.

The SilkyPix developers in fact make the recommendation (with different wording) to use low ISO numbers throughout, and to me it seems as if Fuji supported that notion outright: initially the X-E1 and X-Pro1 did not have a WYSIWYG display; rather the display would always brighten so much that one can see the scene for focusing and composition purposes, no matter how dim it is. In hindsight, now that we do have WYSIWYG and I thought about it, that makes a lot of sense to me, when considering the ISOless nature of the sensor. Because it can always be adjusted in post-processing, there is no need to see the true brightness of the final image. In fact, it is more dangerous to the unsuspecting user to actually see the true brightness, because the unsuspecting user then tends to set shutter speeds and apertures such that there is a danger of clipping highlights. Preserving highlights is a good thing, though, so many would be quite happy to be unknowingly 'cheated', thinking they are taking an image at ISO 6400, not realizing that the camera actually uses ISO 1600, i.e., underexposes by 2 stops.

Sometimes, being cheated is a good thing

 nixda's gear list:nixda's gear list
Fujifilm X-E1 Fujifilm XF 14mm F2.8 R Fujifilm XF 18-55mm F2.8-4 R LM OIS Fujifilm XF 55-200mm F3.5-4.8 R LM OIS Fujifilm XF 27mm F2.8
Fujifilm X-E1 Fujifilm X-E2 Fujifilm X-Pro1 Nikon D7000 Pentax K-5
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