Long Post - Synopsis of a Fascinating Fuji Board Thread on EV

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Greg7579
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Long Post - Synopsis of a Fascinating Fuji Board Thread on EV
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There was a fascinating thread on the Fuji Board that has since locked from max responses, and another one immediately started on the same subject. It had a lot of arguing and messiness in it and some statements that were probably wrong, but there was some great stuff in that thread.

The thread was mostly about maximizing exposure, but like all DPR threads it varied wildly off-track and had some amazing variation in it.

  • What ETTR is and how to maximize it using ISO
  • What ISO is and how it works
  • ETTR – what does the E mean? Exposure, histogram, or brightening with ISO?
  • The in-camera histo being off the jpeg data and not the raw, but really not even the jpeg but the EVF video.
  • What ISO-Less (or ISO Invariance) is or is not
  • High DR scene metering using ISO
  • Use Raw Digger to really know what is happening on your camera and learning how to max EV decisions
  • The Raw Digger creator Borg came on with a long string of technical responses that were fascinating

Here are some lifts from that thread that are interesting:

Borg, the Raw Digger Creator, said some interesting things:

- Borg: The thing is that one starts editing knowing what data is available, and adjusts the image based on that knowledge. It really matters.

- Borg: Signal is the same, it is determined at capture time, together with photon shot noise. The idea that the signal and data numbers are the same thing leads to misconceptions. What you see in the raw data is data numbers. Those are produced through measurement of the light hitting the sensor during exposure and signal processing. Raising ISO may result in adding less noise to the signal during the signal processing, but it doesn't affect the signal, nor does it reduce noise in the signal.

- Someone said that one must understand how the sensor renders a scene in raw. Borg: “It doesn't, a sensor is a bunch of exposure meters behind color filters and protective glass layers - and raw data is the record of exposure metering, factored by ISO.

- Someone said that he suspects that there is some color and luminance noise suppression baked into Fujis RAFs. Borg: Data analysis doesn't support this, at least with firmware 1.34. Sometimes excessive noise is added in signal processing chain if ISO is set to lower values, sometimes raw converters have insufficient precision, sometimes colour transforms are not accurate enough and/or applied too early in the workflow. True noisiness is better be judged on raw data itself.

- Borg: The term "ETTR" leaves a lot of room for interpretation. Different people understand ETTR differently. On a side note, given that most cameras don't have raw clipping warnings, histograms are skewed by white balance and generally are presented for JPEG shooters while ETTR for JPEGs has very limited utility, practical methods of achieving ETTR and pseudo-ETTR also vary between ETTR practitioners.

One guy said he saw an article that said "The camera is actually capturing every shot at base ISO and increasing the brightness after the exposure for whatever ISO was selected. You are completely unaware, and it is automatically done, even to RAW files. That’s why they call it ISO-less."

Finger Painter said this was wrong on a number of points. The article suggests that Fujifilm's Sony sensors do not use variable analog gain to implement an ISO increase. They do. It suggests that post-capture lightening is done only in software. It isn't. Variable analog gain is used. It suggests that the sensors are completely ISO-less. They are not. It is also wrong where they suggest that older sensors were made more sensitive to light by increasing the power to the sensor. In fact, the sensitivity of the sensor is constant with both current and older sensors. And they are wrong to say that these sensors are called ISO-less because the effect of the ISO setting is applied automatically without the user being aware of it.

Someone said: Like the vast majority of digital camera sensors, the Fujifilm sensors capture light at the single, constant sensitivity of the sensor, and then apply an amount of analog gain to the signal coming from the sensor before that signal reaches the Analog to Digital Converter. The amount of gain applied is proportional to the ISO setting.

Someone said: Certain sensors are mis-labelled "ISO-less". A better label is "ISO-invariant", and that label applies only above a certain threshold ISO setting. On a X1000F that threshold is ISO 2000. "ISO-invariant" means that there is no effective change in the effect of read noise between different ISO settings for a given exposure level. This is in contrast to older sensors where the effect of read noise was diminished as ISO setting was increased for a given exposure.

Someone said: In the case of a low DR scene or an easier to meter scene in general. I agree, the ETTR principle still mostly applies at higher ISOs. The sensor isn't perfectly invariant, so if you can set a higher ISO in-camera without risking any highlight information you will probably best off doing that. When I tend to intentionally "under-ISO" a bit is when I'm shooting in very unpredictable, difficult to meter and/or high DR situations which will provide a significant highlight headroom buffer with pretty much the same level of apparent noise that I would have gotten with a higher in-camera ISO setting. A dark/high DR scene with a spotlight on a white dress/spotlight in the face kind of stuff...

Someone said in response to "the ETTR principle still mostly applies at higher ISOs." Depends on how you interpret ETTR. If "E" stands for "exposure", lightening with ISO (moving the in-camera histogram to the right) is not a part of ETTR. HTTR (histogram-to-the-right) is not the same as ETTR, and may contradict ETTR by ignoring the priority of exposure - one can set exposure much lower than possible and promote it to the right by raising ISO. That's not ETTR, but that's a common mistake.

Someone said: Maximizing exposure is a valid technique to counter noisiness and artifacts for any ISO.

Ysarex said: Most camera functions aimed at controlling the JPEG output have no effect on the raw file. Fuji DR is an exception and does effect your raw file. Note that DR 200/400 require that you raise the ISO to 320 and 640 respectively. Normally when using ISO values above base ISO the signal from the sensor is boosted before or during the creation of the raw file and so effects the raw file reflects this signal boost. If DR 200% is engaged then 1 stop of ISO brightening is withheld during the creation of the raw file. If DR 400% is engaged then 2 stops of ISO brightening is withheld during the creation of the raw file.

Truman said: As far as raw DR200 is the same as dialing in -1 Exposure Compensation. DR400 is the same as dialing in -2 EC. Simply put your target on a tripod pointing to the same target in the same lighting and note what happens when you take a shot at DR100 and then one at DR200. The camera will increase the ISO by "one stop" which is equivalent to -1 EC in the exposure. That's the reason if you use DR200 you have to have the ISO set so camera can shoot at base two times base ISO.

Then someone shot at Rico, who said in his book that if you shoot RAW and set DR to 200 or 400% the image brightness will be amplified in-camera to one or two stops less than what the ISO number says. So the picture I took at ISO 800 and DR400 is actually an ISO 200 RAW image.

But then Ysarex said: It is not actually an ISO 200 raw image. The ISO on the camera was set to 800 and you took an ISO 800 image. This is a strained explanation used by people who do not understand ISO and that would apparently include Rico.

When the DR setting on a Fuji camera is increased to DR200/400 one or two stops of ISO lightening is implemented in the camera EXR processor as opposed to the more common implementation of either analog sensor gain or digital scaling in the ADC. If you open the raw file that was taken at ISO 800 and DR400 the metadata in the file will identify the ISO value that was used for that image as 800. During import, Lightroom recognizes the DR setting and pushes the brightness to the ISO number dialed in the camera. Which it is able to do because that ISO value that was dialed in on the camera is also recorded as the ISO value in the metadata for the raw file.

Someone said: By implementing ISO in the image processor rather than through analog sensor gain or digital scaling potential ISO clipping in the raw file is avoided. And yes that could be apparent when trying to process a raw file. That doesn't mean that the raw file is actually recorded at ISO 200. It's not necessary to twist thinking around like that to explain what is happening. The ISO standard that Fuji complies with explains what Fuji is doing just fine. If the ISO was set to 800 then it's 800.

When the DR200/400 modes are used in a Fuji camera the raw files are effected such that one or two stops of ISO lightening that we would otherwise expect to see implemented in the raw file by either or both analog gain/digital scaling is not applied. There's no need to twist it beyond that.

Someone said that the twisted thinking explanation results because people incorrectly define ISO as not what ISO is defined as in the ISO standard which Fuji helped write and which Fuji complies with. A lot of folks, Rico apparently included, define ISO by how it's implemented. The most common way to implement ISO is through analog amplification of the sensor signal. And so these folks define ISO as such. Then they get tripped up trying to explain what's happening when a camera in full compliance with the standard implements ISO in a different way. That's what's going on here.

Erik B said:

Increasing the ISO isn’t actually increasing exposure, it is brightness gain that is applied after the sensor has already been exposed, it can be applied in-camera when shooting or later on in post. If your sensor is ISO invariant, there is no significant downside to applying it in post, but there are some significant upsides. Once you’re above base ISO, and especially ISO 800 in the case of the newer dual-gain sensors in the Fujis, for the best image quality it makes the most sense to get as much actual “exposure” as you can manage by setting your aperture as wide as possible while still meeting your DOF needs, setting your SS to the lowest value that will prevent any kind of motion blur, and setting your ISO at ISO 800 or some higher value that is still well below where you might encounter any highlight clipping (the highlight warning blinkies are great for this). The newer Fuji sensors (Sony)are almost completely invariant at ISO 800 and above, so you’ll get pretty much the same image quality at ISO 800 with 3 stops of exposure push in post as you would at ISO 6400 in camera, except that with ISO 6400 in camera you have thrown away 3 stops of headroom and have possibly irretrievably lost some important highlight detail. If you “underISO” at ISO 800 you can push the brightness back up in post with no significant noise penalty compared to “correctly” exposing at ISO 6400, but you will now potentially have 3 stops of highlight information to work with in post that might have been lost otherwise.

Erik B said:

If you are a RAW shooter, “Exposing to the right” (ETTR) is indeed the way to go for maximum dynamic range and minimum shadow noise, but it is really only relevant at base ISO where it is good practice to record the important highlights as far to the right (of the histogram) as possible without clipping to ensure the maximum signal to noise ratio and cleanest shadow detail to work with later. I find the highlight warning blinkies to be the most reliable method for judging sensor saturation, far better than the histogram which can easily fail to register small areas of overexposure. With my usual jpeg settings (Provia, -1 Shadows, -1 Highlights, settings which do affect the EVF view and how the blinkies work), at base ISO and DR100, there are two things to consider for the RAW shooter - “just” blinking highlights are always fully recoverable in post (it’s generally OK to let specular highlights and direct light sources to “hard blink”), but if you have any blinking skin, back off the exposure compensation until it isn’t. This results in consistently fully exposed RAW files with maximum signal to noise ratio with no irretrievably blown highlights - easily accomplished with just your eyeball and exposure compensation assigned to the front command dial in “C” mode usually in Multi metering mode: Set your ISO to base value 160 or 200 depending on which camera you have, set the appropriate SS to eliminate potential motion blur (yours or uour subject’s, set the aperture for the desired DOF, compose you image normally, focus in the right spot, adjust the exposure compensation until the important highlights “just” blink (but not skin) and press the shutter button. A dull, flat scene will appear very bright in the EVF, and very high DR scene will often appear dark - it’s all about how the brightest highlights are recorded, not how “correct” the image in the EVF looks - a fundamental difference to optimal jpeg exposure.

Erik B said:

It is important to note that neither the histogram or the blinkies accurately represent the RAW file (which is why some blinking is OK) Would it be better if we had metering and blinkies that accurately reflected the RAW file? Sure, but this actually works just fine. I know that some folks will make radical adjustments to their jpeg settings to get the histogram/blinkies to better represent the RAW file (Google UniWB), but I find this to be a monumental waste of time, the above method works just fine - it’s easy, quick, simple and your EFV looks normal while using it. If I’m ultimately being a bit conservative and leaving a tiny bit of highlight headroom on the table, I can live with that, the tonal response gets non-linear and unattractive right at the edge anyway.

Erik B said:

In low light things are different. Considering the invariant nature of the more recent Fuji sensors at ISO 800 and above (depending on the model), you don’t necessarily want to “expose to the right” for the best results. If you’ve maxed out your aperture and set the SS as low as possible to prevent motion blur, you’ve already exposed the sensor with the maximum amount of light possible - ISO is applied afterwards (either in-camera or in post in the form of brightness amplification and/or a tone curve) and it doesn’t make your sensor more sensitive or add any actual exposure, it is only adding additional image brightness to make up for the exposure shortfall. If you shot at ISO 800 or above you can boost the brightness (“exposure”) in post with very little to no noise penalty relative to initially using a higher ISO in camera (and risking blowing the highlights). So.... if you’ve already got your exposure maxed out using SS and aperture and your minimum ISO is set to 800 where the sensor becomes essentially ISO-less, you can simply adjust your exposure compensation (effectively the ISO in this case) so that the image looks “ok” in the EVF with no blinking and that’s it, you’re done. At ISO 800 minimum and no blinking, it matters very little where you set the ISO in-camera if the image looks reasonably decent (not ridiculously dark) in the EVF because you can adjust it later in post with very little noise penalty, but a big potential advantage in highlight protection headroom in different to meter situations. Setting the ISO higher in-camera to push the “exposure” all the way to the far right may limit the amount of highlight information you will have to work with later in post, particularly in high dynamic range/difficult to meter scenes.

Erik B said: ISO invariance trickery really only comes into play at higher ISOs (which will always be noisier in general.

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