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UltravioletPhotography

Learning About Your Digital Camera's Light Meter


Andrea B.

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In another thread I had complained that UV exposures seemed to change slightly when I changed white balance (in aperture mode with matrix metering). This puzzled me. But I might have an explanation for it now?

 

In the Raw Digger tutorial Digital Camera Light Meter Calibration I learned that changing white balance changes the amount of "headroom" in highlights. Headroom = the number of EV stops between metered middle gray and the highlight saturation point (hitting the wall on the right side of the histogram). This of course makes sense if you stop to think about colour of light and so forth. But it was nice to use the Raw Digger raw histograms to confirm the changes.

 

And so I'm thinking that maybe this changing headroom-in-highlights thing helps explain the exposure changes I saw. The camera, in aperture mode with matrix metering, lowers the exposure if highlights are blowing out too far after a white balance change even though underlying raw data is the same regardless of what white balance is applied. Matrix metering must be paying attention to the JPG and not the raw NEF. As I understand it, matrix metering relies on some canned data about the scene. That data could go all to heck in a UV or an IR scenario.

 

Maybe all that is obvious to everyone, but I've been curious about it all and thought I'd pass along what I discovered. :)

 

 

I also learned that:

The spectral sensitivity of the exposure meter can differ from the spectral sensitivity of the sensor.

 

That is interesting. Maybe the sensor can record more, less or the same (choose one, ) in UV than the exposure meter suggests. I think my D600 is an underexposer in UV relative to the metering.

 

Of course, in UV we learn not to pay much attention to the metering in the first place. But if I can figure out how much "headroom" I have it could be possible to come up with some rule-of-thumb about how much EV to apply over or under the meter reading. This could be useful.

 

CONTINUED in POST #5

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I find I much prefer center weighted over matrix or spot (point) metering. It seems more intuitive for me at this point. However my UV metering always seems low, just a starting point.

 

If I understand correctly what you are observing is that the matrix selects a different midtone level depending on WB used. So is it when the WB does not actually match the scene the exposure is off? Please excuse my naïveté, but is that not expected?

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Don't know, John. It is one of those things which I suppose I never paid all that much attention to.

So I'm investigating until I fully understand it.

 

Added: I just put the Nikon 1 V3 into Aperture Mode with Matrix Metering and rotated through all the WB settings. The exposure did NOT change on that cam.

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CONTINUED from POST #1

Last Update: 23 April 2015

 

In this post I'm going to calculate how headroom there is in the UV highlights for the following set-up. To do this you need to make two shots of a middle grey target. The first is as fully saturated as you can get - that is, shoot a few stops over the suggested metered exposure so that the R, G and B channels hit the right wall of the histogram. The second shot made at suggested metering of the grey target.

 

Equipment: Nikon D600-broadband + Nikon 105/4.5 UV-Nikkor + BaaderU UV-Pass Filter + Labsphere 50% Reflective Standard + Sunlight

Added Settings: Matrix Metering + Neutral [0] Picture Control (except for Sharpen = +4) + ISO400 + f/8 + ADL=Low.

 

Fully-Saturated Example

By full saturation, we mean pushing the exposure until all 3 channels 'hit the wall' on the right side of the raw histogram. Nikon D600 has 14-bit colour, so full saturation is reached would theoretically be reached in bin 16383.

214 -1 = 16383.

Added: Shane Elen has added a note to this thread that for some cameras the saturation point(s) may be set lower then the theoretical maximum bin value. That seems to be true for the Green channel in this experiment as you'll see.

 

I needed to go into bulb mode to produce this almost fully saturated example. At bulb for almost a minute, I decided enough already. Raw Digger (RD) recommends shooting slightly soft so that minor imperfections (dust, etc.) play less of a role. Of course hand-held for a minute guarantees softness. :D

 

f/8 for 58.9" @ ISO-400, Matrix Metering, Raw Composite, Almost Full Saturation

PLEASE NOTE: The raw composite you see here does not have any white balance applied. But you are seeing it as an RD TIFF export which is then converted to an sRGB JPG and also through the smokescreen of whatever your monitor/browser adds to the mix. This is why we like seeing the raw data histograms.

More UV lands in the Red channel than in the green or blue channels. No surprise, of course.

fullSat_rawComp_gamma16.jpg

 

 

This is the sRGB JPG extract from the NEF showing the in-camera white balance. (And the subsequent loss of any image when the data is stuffed into a smaller space.)

fullSat_jpgExtractResize.jpg

 

 

Here is the corresponding histogram of the raw. The two greens have been averaged. Both the linear and log versions are shown. Full saturation for this UV setup was reached around 3EV over metering for the given settings. (RD not really needed to determine this, but we like 'proof', yes?)

 

f/8 for 58.9" @ ISO-400, Matrix Metering, Linear Histogram, Raw Composite, Almost Full Sat

histoSat_linear.jpg

 

 

f/8 for 58.9" @ ISO-400, Matrix Metering, Log Histogram, Raw Composite, Almost Full Sat

histoSat_log.jpg

 

 

Metered Example

RD asks you to shoot a typical grey target for the metered example. I'm using a 50% reflective grey standard. Given that we are in UV, my D600 does not exactly see middle grey, but I'm going to pursue this anyway.

 

f/8 for 1/4" @ ISO-400, Matrix Metering, Raw Composite, No EV

AGAIN, PLEASE NOTE: The raw composite you see here does not have any white balance applied. But you are seeing it as an RD TIFF export which is then converted to an sRGB JPG and also through the smokescreen of whatever your monitor/browser adds to the mix. This is why we like seeing the raw data histograms.

meter_rawComp_gamma16.jpg

 

 

This is the sRGB JPG extract from the NEF showing the in-camera white balance. My weird D600 white balance comes fairly close to capturing the "real" raw apparently.

meter.jpg

 

 

Here is the corresponding histogram of the raw. The two greens have been averaged. Both the linear and log versions are shown. We are interested in the right-most peaks of the histogram corresponding to the grey target.

 

f/8 for 1/4" @ ISO-400, Matrix Metering, Raw Composite, No EV

Two big peaks, of course. One for the dark background and one for the 50% reflective grey standard.

histoMeter_rawComp_gamma16_linear.jpg

 

 

f/8 for 1/4" @ ISO-400, Matrix Metering, Raw Composite, No EV

histoMeter_rawComp_gamma16_log.jpg

 

 

Headroom for R, G, B Channels

So comparing the histograms of these two shots we can see how much headroom (on average) there is in the highlights for each channel for the target subject. This should give an indication of how far exposure can be pushed to retain details in highlights while opening up the shadows with less noise. Exposing to the right can be useful in UV.

 

Sample of the grey target

The sampled area is indicated by the grey overlay.

meter_rawComp_targetSample_samp.jpg

 

 

Bin statistics for the sampled target area

meter_rawComp_targetSample_stats.jpg

 

 

Histogram (linear) for the sampled target area

histoMeter_rawComp_targetSample.jpg

 

 

Headroom calculation in EV

The bin where the average number of sampled pixels lands is used in this calculation. See the green column in the bin statistics above. In this case the average corresponds to the histogram peaks.

From these calculations I conclude that I can easily shoot in UV about X stops over the matrix metering using the BaaderU and in-camera WB and still recover the shot.

 

Calculation: saturation_bin/ave_grey_bin

 

Red: 16383/5968.6 = 2.74 --> just under 1.5 stops over metered value. log22.74 = 1.4567...

The red saturation value is about 2.7 times greater more than the average metered red value from the 50% grey target.

 

Green1: 16383/1985.8 = 8.25 --> 3 stops over metered value.

Green2: 16383/1987.7 = 8.24

The green saturation value is about 8 times greater more than the average metered green value from the 50% grey target.

 

Blue: 16383/1446 = 11.33 --> 3.5 stops over metered value.

The blue saturation value is about 11 times greater more than the average metered blue value from the 50% grey target.

 

Conclusion

The difference between the metered example and the (almost) fully saturated example is about 8 stops. The Red channel blows soonest. At f/8 with in-camera WB and ISO-400 using the BaaderU, the headroom calculation shows that you can shoot about 1.5 stops over metered value before blowing that Red channel.

 

Look below for shots which are 1.33 and 3.33 stops over metered value to see what happens when I try to recover them.

 

*********

 

I have to stop and think whether I've used the correct target.

OK, yes. We are aiming for "middle grey" on this experiment.

I don't want to get too hung up on reflectivity vs. brightness vs. lightness.

See Middle Grey.

 

*********

 

So I am wondering, just out of curiosity, what would happen if you exposed separately for the R, G and B channels, extracted the relevant channel and then re-combined them. We would never do this in the Visible case, but I'm curious about the UV case.

 

******

 

CONTINUED in POST #7

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I use to overexpose several stops in UV based on the internal meter to get it right. The photodiode seems to be several stops less sensitive in UV (as expected).

Once figured that out and looking at the histogram and not exposing over say 80%, things works quite well (with my Panasonic GH1 and GH2)

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CONTINUED from POST #1

Last Update: 23 April 2015

 

Now that the headroom in the highlights has been determined, I want to look at two examples of photos made at exposures above metering for the given setup.

 

Example of Exposure 1.33 stops above Metering

f/8 for 1/1.6" @ ISO-400, Matrix Metering, Raw Composite

This exposure was pushed until the Red channel was just short of maximum saturation.

With another 1/3 stop exposure, the Red would have blown.

max_rawComp_gamma16.jpg

 

 

This is the sRGB JPG extract from the NEF showing the in-camera white balance.

(The D600 has major dust bunny splatter problems.)

max_jpgExtractResize.jpg

 

 

And the histogram (linear) for this example showing the Red channel bin maximum at 15945 with Green and Blue maximums considerably below that at bins 5456 and 3937, respectively. Again, we are interested in the right-most peaks of the histogram corresponding to the grey target.

f/8 for 1/1.6" @ ISO-400, Matrix Metering, Raw Composite

Raw Digger is not correctly reporting the exposure time in the histogram labeling.

max_rawComp_gamma16_histoLinear.jpg

 

 

Can we recover this shot in a converter/editor? Yes, easily. I've calculated that there is a minimum of between 1.33 - 1.5 stops of headroom in the Red channel and even more in the Blue and Green channels. So this shot is about as close as I could come with the D600 set to 1/3 stop increments.

 

 

Here is the Photo Ninja conversion. It has no Exposure slider tweaks !!! White balance was easily made on the target because the Red channel was not blown. I would say it could perhaps have handled a weensy little +1/6 increment on the Exposure slider if I had been feeling picky about it. :D But the point is that our headroom calculation above is quite accurate - at least for this particular camera/lens/filter combo.

maxpnExpOnly.jpg

 

 

 

Example of Exposure 3.33 stops above Metering

 

Here is a shot made 3.33 stops over metered value. The Red channel has blown and the Green channel is very close to maximum saturation.

 

f/8 for 2.5" @ ISO-400, Matrix Metering, Raw Composite

redGreen_rawComp_gamma16.jpg

 

 

This is the sRGB JPG extract from the NEF showing the in-camera white balance.

This cyan tint is an indication of a blown channel for my D600.

redGreen_jpgExtractResize.jpg

 

 

And this is the histogram (linear) for a square sample of the round grey target showing that the Red channel has hit the wall at bin 16383, the Green channel(s) are close to the wall at bin15781 and the Blue channel is not far behind at bin 13966.

Raw Digger is not placing the correct exposure speed of 2.5" on the labeling.

redGreen_rawComp_targetSample_histoLinear.jpg

 

 

The log version of the histogram.

Raw Digger is not placing the correct exposure speed of 2.5" on the labeling.

redGreen_rawComp_targetSample_histoLog.jpg

 

 

Can we recover this shot in a converter/editor? Yes, but there is a side effect.

 

Here is the photo converted in Photo Ninja with the Exposure slider decreased by -.75. The Highlights slider was not used. (Although the highlights slider is very good and would ordinarily be applied by default, I wanted to be pure in this recovery effort.) So the photo is quite easily recovered except for white balance. Read on.

IMPORTANT: I could not white balance on the target in Photo Ninja because of the blown channel. PN gave me the stop sign. So I had to white balance on non-blown areas of the photo. I have not done any fine tuning of this wb.

When we shoot above metering and blow a channel, we will face this kind of WB problem. Thus it is probably better to shoot above metering only so far that you do not blow a channel. That is the purpose of calculating the headroom in the highlights for a particular shooting situation. :D

redGreen_pnExpOnly.jpg

 

***********

 

As a final comparison, here is the original metered photo which was converted in Photo Ninja with the Exposure slider increased +1.33 because the metering is underexposing in UV. White balance was made on the target.

meter_pnExpOnly.jpg

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So here are the remaining questions: What happens to the highlights headroom when I shoot UV on the D600 using a different white balance? a different filter? a different ISO? If I worked at this for awhile, could I come up with a useful reference table for UV exposures?

 

Review of the steps:

For a fixed camera + lens + filter + aperture + iso + in-camera white balance:

  • Shoot a middle grey target at metered value.
  • Shoot a middle grey target until channels begin to saturate to determine to the right-hand wall of your camera's raw histogram. (Or determine this theoretically by how many bits your camera uses to record raws - understanding that some channels may actually reach saturation short of the theoretical value.)
  • Calculate highlights headroom multiplier for each of the R, G and B channels. The two green channels can usually be averaged.
  • Determine the highlights headroom in stops (log base 2) of the channel which saturates first.
  • Shoot with that number of stops over metering for a good exposure.

*********

 

As always, I'm eager to hear of any errors, misconceptions, awkward phrasing or typos.

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Nice to see you "digging" in. I have followed a similar process for most of my DSLR over the years to determine headroom.

 

I started using UniWB in 2004 along with an uploaded linear tone curve in order to help use the histogram for evaluating saturation in UV and IR. I have found this method particularly useful for 830nmIR conversion where the Bayer dyes are almost equally transparent and, the largest component of most landscape images (being vegetation) is effectively white details or "Zone VII". By comparing RAW data and converted aRGB values along with some test exposures, it is possible to determine how much Highlight Warning blinkies (typically the R channel) can be acceptable, in conjunction with what the right hand side of the histogram is doing, to place those highlights details at optimum exposure for retaining detail in the whites.

 

Since I don't want to deal with different "methods" for different DSLRs & wavelength conversions, I use it with all my DSLRs as I also found this method useful for unmodified Nikons with respect to exposure evaluation using the DSLR histogram. Independent of the histogram and HW, for visible photography white with details I can meter the whites and know exactly how much exposure compensation to add to put the whites at a converted RGB value of around 200-220 for example. For 830IR photography I use the histogram & HW.

 

Here's what I have found for Nikon:

 

Metering an 18% grey card I have found the in-camera meters to be set between around 9% to 13% (depending on model) i.e. between 1 to 1/2 stop underexposed. Theoretically a 18% grey card should produce 2.47EV headroom but due to the above meter response this often turns out to be between 3 and 3.5EV. If I find this a problem I permanently adjust the spot & center wtd meter response using the b (varies with model e.g. D810 it is b7 Fine Tune Optimum Exposure) setting in the Custom Menu. It does not work well for matrix metering. With the newer DSLRs and the ability to recover shadows (eg D810) I find the high headroom (~3.0EV) a safety margin with little drawback, so have not implemented an exposure compensation for the D810. Implementing an exposure compensation for UV and IR did not seem to work so well. For instance, implementing an exposure compensation for 830IR was unreliable (for some of the reasons posted earlier) and guestimating was far more accurate, so I guestimate and then evaluate the histogram & HW then reshoot if required.

 

Nikon HW blinkies turn on at RGB, R , G or B values of 248 (at least on the D300S, D700, D800 and D810) not 255. However a value of 255 is at the end of the tone curve, whereas 248 gives a little "breathing room". It would be nice to be able to set the HW trip point from the menu. A pseudo adjustment can be implemented by playing with the DSLR Picture Control settings for Sharpness & Contrast. I also tried experimenting with uploaded "odd" tone curves which give the "appearance" of effectively raising or lowering the trip point but I found them difficult to work with. The same was true for an inverse "tone curve" which effectively removed gamma compensation producing a linear "RAW" histogram rather than the typical gamma "RAW" histogram..

 

HW will under-report "saturation" due to downsized jpg resolution, and the histogram can also suffer the same effects, LCD resolution (often reducing an 8 bit histogram to a 7bit histogram) and histogram bit depth (7 or 8 bit jpg histogram representing 12 or 14 bit RAW data).

The use of HW while looking at individual colour channels is important with respect to the luminance histogram for the following reason:

Example D800/810 RGB indicates Luminance value of 248 as the trip point turning on HW. As this is based on luminance rather than individual channels, it under-reports blown pixels which can have a value of 255 all the way out to the Luminance value of 240!! For example a luminance value of 240 can have R 255, G 255 and B119 for which the RGB ("luminance") HW indicator will report on as not saturated. Checking the individual R, G or B channels will clearly show the R & G to be saturated.

 

What happens to the highlights headroom when I shoot UV on the D600 using a different white balance?

Another great reason for UWB - never have to worry about it as it is constant.

 

a different ISO?

In the beginning this did confuse me for a while during ISO testing as my saturation point seemed to vary. However, most of the time I shoot as base ISO (visible and IR) so I don't worry about it. UV is more challenging with regards to ISO, so it would be important to see if the saturation values are at different ISO settings.

 

With regards to your methodology, it is important to understand the saturation point but it is probably more important to know the optimum exposure for whites (or for colour UV, red, green or blue) with detail. For a specific set of parameters, as you have outlined above, your approach will get you closer but I think you will find that UV and IR (below 830nm) doesn't play nice due to wavelength interaction i.e. the Bayer response and wavelength response of the meter with respect to the range of reflected wavelengths.

 

I am certainly interested to see what you find.

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14-bit colour, so full saturation is reached in bin 16383.

214 -1 = 16383.

 

This is the theoretical value but it is not a given. It is not uncommon for sensor saturation points to be set in-camera lower than 16833 or 4095, especially for the G channel.

In the 59sec ISO 400 exposure "Almost Saturated" - your G channel is clearly saturated at 15795 even though it is below 16383. Possibly internal black point subtraction, ISO related or, low saturation point for the G channel set in firmware.

D100 12bit R saturation was found to be 3650 rather than 4095.

D810 with black point subtraction 15782, with black point subtraction turned off 16383, D810 black point value is 610.

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It is not uncommon for sensor saturation points to be set in-camera lower than 16833 or 4095, especially for the G channel.

 

Aha!! I was wondering what was going on in those Green channels. Clearly there's a serious spike before 16383 in the Greens.

I will add a note above about theoretical versus actual values of the saturation bin.

 

**********

 

Regarding black point subtraction - this can be turned off in the D810?

 

Thank you for the insights, Shane.

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Shane, I saw your Post #10 before seeing your Post #9 which contains so much important, valuable information about exposure, in-camera histograms, saturation and so on. Hard-won, good practical knowledge !! Thank you for writing all that up.

 

 

Question about UniWB: What is the status of UniWB these days? Can files be found to load onto one's camera? I gave up trying to create them some time back. Am I correct in thinking that it is pointless to use UniWB unless you use an accompanying Flat Curve?

 

Question Nikon Picture Control Settings: You are saying that the settings in these can affect the headroom values ??

 

Comment on Luminance and Blown Channels: I learned this the hard way. I made what I thought were going to be good UV exposures during some field work by shooting UV in Monochrome and "balancing" the Luminance histogram. Naturally I had all kinds of problems later with blown channels. "-(

 

Comment on ISO: There is a point where further amplification does nothing useful. There is a Raw Digger tutorial about this. (Of course ! These guys have looked at everything !!) I want to work through it and try to find the max useful ISO for the D600-broadband when shooting UV.

 

Question about Matrix Metering: I'm thinking that we shouldn't be using Matrix Metering at all in UV or in IR (the preceding experiment being an exception). What do you think?? There is no way the D600 or any other camera can have any canned UV or IR scenes to draw upon when it is trying to "recognize" what kind of scene it is metering. But I don't like spot metering either. I guess that leaves Center-weighted. :lol:

 

Added: It is still difficult for me to believe that white balance affects headroom values (or exposure). Raw is raw, I always thought.

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It took me less than 5 mins to create a D810 UWB with decent values of R 1.002 and B 1.008. You can use it with a modified DSLR curve i.e. adjusting sharpness contrast etc or better (no hidden TC modifications) just create one using Picture Control Utility. Or I can send you one, does your cam use piccon*.ncp or piccon*np2, I think they are transferable within the file extensions noted.

 

NPC settings do not affect headroom values, they just give the illusion of shifting the HW set point. You are basically changing the tone curve shape pushing or pulling values away from the original values. Example, using an aggressive PC, the jpg representation image becomes more saturated, this pushes some colours into saturation where in fact they might not have been using a gentle PC, thereby turning more HW blinkies on. The RAW data however is still unaffected.

 

On the D810, gain switches from analog to digital above ISO 800. So I find it is better to underexpose using ISO 800 (rather than for example ISO3200) and then push the exposure post process where you can control the saturation using curves rather than straight +2EV exposure adjustment. Testing or googling will probably produce the ISO numbers you are looking for.

 

I would imagine Matrix metering internal ref. data will not work for IR or UV scenes for obvious reasons. My point was that, adding in-camera exp comp to "correct" metering issues does not work well for matrix metering even in the visible.

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It is still difficult for me to believe that white balance affects headroom values (or exposure). Raw is raw, I always thought.

 

It doesn't, it just affects the perception of how much headroom you think you have when using different WB. After all, the RAW data doesn't change when you change WB, it only changes in response to the intensity and spectral quality of light which in turn effects individual channel headroom.

 

What you are seeing on the Rawdigger site is the combination of spectral response of the R and B channel, with respect to the G channel, and the spectral distribution of sunlight and incandescent light. This clearly shows that to balance incandescent light to sunlight (daylight balance) then the R channel would need to be reduced and the B channel increased and this is accomplished by implementing Incandescent WB. On the DSLR histogram this can mislead the photographer into believing the exposure is under or over exposed when in fact it may be perfectly OK. If you use a UWB then the DSLR histogram would closely match the Rawdigger histogram under both lighting conditions.

 

The Rawdigger plots are showing you the actual RAW data collected (based on the spectral properties of light) regardless of whether the WB has been implemented or not.

 

With regards to metering in UV and IR, Illiah sums this up nicely

 

"The spectral sensitivity of the exposure meter can differ from the spectral sensitivity of the sensor."

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Raw is raw, sort of.

 

But if your only practical access to the image data is via a converter which does only logarithmic conversions based on pre-loaded white-balance metadata (the present Sony situation,) then maybe sort of not. In that case, the camera white balance is your only tool for ensuring that all the histograms fall on the same scale without pegging at one or both ends.

 

And don't even get me started about the antics of Sigma X3F files.

 

Many things can affect the sensitivity of meters, obviously. Not only do different kinds of meters (silicon, GaAs, CCD, etc.) have different native sensitivity curves, but outside the visible, one must take into account what is between the meter and the environment--not only lenses and filters, but any plastic or glass covers or diffusers which may be installed for protection. It is a real can of worms. However, for some meter/sensor/lens combinations, there can exist a "metering ISO" which will fall on the scale of the meter's adjustment range and which will work surprisingly well. I used to shoot Kodak HIE film through an 093 filter by setting the ISO to 3200--and it worked for AE, at least on that camera. With digital cameras, you have less latitude to play with, because the meter and sensor gain can only be decoupled about 3 stops. I can shoot IRG at -2 stops through a standard filter with appropriate white balance. Beyond that, it's pretty much manual mode and checking the preview screen after each shot.

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Clark: ...logarithmic conversions based on pre-loaded white-balance metadata (the present Sony situation,)

 

I'm not familiar with Sony conversions, but perhaps Raw Digger could also give some info about it?

 

****

 

Clark: ...digital cameras...the meter and sensor gain can only be decoupled about 3 stops.

 

That has been my informal observation. Now it is interesting to take a look at that (and other stuff) with some actual data.

 

****

 

Shane: It took me less than 5 mins to create a D810 UWB with decent values of R 1.002 and B 1.008. You can use it with a modified DSLR curve i.e. adjusting sharpness contrast etc or better (no hidden TC modifications) just create one using Picture Control Utility.

 

Oh man !! No kidding?? I gave up too soon I guess. Did you use the magenta monitor shot to do this?

 

I need to explain sometime how I "pushed" the D600-broadband in-cam white balance tool to get the wb setting I am currently using for UV. It was not a one-shot measurement. It was a sequence. I don't know why Nikon can't just tweak something somewhere so we can get lower temperature WBs in our cams like Pentax and Lumix provide. Sigh.

 

****

 

I'm just going out to shoot a new set: one full sat and one metered shot as well as several in between - this time using some other white balance. I'm still just experimenting to see what turns up.

Now that I have presented an example of how to analyze the headroom on the right, I think I can just report the next results without a lot of extra reading to wade through.

So stay tuned.

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