Sony A7R3 zebras are truly RAW

Started 9 months ago | Discussions thread
ForumParentFirstPrevious
Flat view
Ciriaco Garcia Regular Member • Posts: 335
Sony A7R3 zebras are truly RAW
43

DPreview documented a great method to generate RAW zebras on the A7R3 in their review. After some playing, I realized that zebras can be configured to report the RAW data clipping not only in HLG gamma, but also in any camera profile (or without any profile). The key is the zebras level to setup: 107+/108+ when no picture profile is enabled (or for Picture Profile 2 with its default gamma); 95+ for picture profiles using HLG2 gamma and 100+ for HLG/HLG3.

In order to test how accurate they are, I have done a test to compare the actual RAW data clipping with the camera zebras. Note that RAW converters don't truly show the RAW data clipping; one needs to resort to more specialized applications. I ended using my own tool to get the pictures I post on this thread, which generates a black and gray photo, using colors to represent the channels with RAW data clipped (bright white means all channels clipped).

The conclusion is that camera zebras are very accurate following the RAW data clipping, except for the image extreme borders and corners (why?). It is also important to be aware that camera zebras are based only on the green channel, which still is useful enough in most cases. If the red or blue is clipped, they won't alert you.

For this concrete test I enabled the Picture Profile 10, which setups the HLG2 gamma, and configured a custom zebra lower limit of 95+. Since I shall only inline as much as 10 pictures, I omit the overall photo of the test setup (you can see it here and here).

In all of the following crops: at the left the photo of the back of the camera near the moment I shot, showing the zebras. At the right, the shot actual RAW data to compare (remember: anything in color or bright white represents true clipping).

I started from 1/80 sec. By then it there was mostly one clipped area in the whole image. The camera zebras show it well, despite this area is very close to the image borders and corners:

Increasing the exposure, some building names clip:

In the next crop, the "KPMG" characters are not totally clipped, despite the zebras suggest that. I assume that zebras are a "projection" of the actual exposure selected, which will be actually performed when shooting (so maybe there is certain margin of error):

Now zooming even more. We have less resolution here in the RAW crop due to the "cheap" binning applied during demosaicing (the camera LCD at full size and 200% is much better!):

Another extra 1/3 EV of exposure and part of the sky suddenly clips. The camera zebras accurately show that, except for the extreme borders:

Another +1/3EV and the sky continues clipping. The camera zebras struggle in the left and right sides, but in "center weighted" terms are very good (one could always recompose for metering ;-D). At least at this hour, the sky clips first in the green channel, which is good, because otherwise the zebras wouldn't have reported it (the cyan color represents the mix of green and blue):

In the next picture, more sky is clipped but the zebras are not just there in the corners/extreme top:

Nevertheless, if we zoom at 1/5 the height of the frame in this photo, the zebras are accurate. Note that the blown red area has no zebra, as we expected (remember that zebras follow only the green). The yellow (which is the addition of green and red) does has zebra thanks to the burned green:

The previous example is truly amazing. We are [b]much[/b] under 1/3 EV of accuracy. I'd give it without hesitation (what I don't want to give is a stop and a half!). Note that the zebras are in motion, and shooting the back of my camera freezes them reducing their full "meaning" in these pictures.

Dialing another full stop, let's check if the zebras continue being accurate in the not yet burned areas:

However, too much close to the image border they may start to fail following the RAW data:

In summary: I'm really happy knowing that I have in the field a tool capable of setting the best exposure. And zebras are much more useful than a histogram, because many times a photo requires some clipping.

You can generate clipping reference images (similar to the crops at the right above) to learn more about your RAW files actual exposure. There is more information on this thread.

As said, the critical point for this technique is the zebra level configured. DPreview used 100+ in their article, but I have found that the camera 100+ default setting is more like a "99+" (maybe for this reason they said that you must start seeing the zebras while the image is still not clipped): if you create a custom 100+ zebra level it will clip slightly less. In my tests, if you see the zebras, the clipping is there!. The HLG gamma also has the minor downside of skewing the reported ISO levels by 1/3 EV (ISO 100 becomes 125, ISO 640 becomes 800, and so on... but they are actually the original ISOs, simply renamed).

Alternatively HLG2 gamma doesn't renames the ISO levels and requires 95+, so I have settled on it. A mode you must avoid like the plague however is HLG1, because doesn't uses the sensor full well at any ISO setting.

As previously commented, the Still gamma zebras (no Picture Profile enabled, or Picture Profile 2) requires a zebra level of 107+ or 108+. But I find it somewhat less recommendable. The reason is that sometimes, the camera shows "traces" of zebras in the full image. Using HLG2 they may dissapear or be definitely confirmed if you zoom in in that area. Using the Stills gamma, however, the zoomed image may still have traces of zebras, leaving you in doubt. That has nothing to do with the zebras level (even set to 109+ the traces are still there, while in HLG2 setting 94+ or 96+ would have a clear and noticeable impact). Other than that, they are also as reliable and "raw" as the HLG zebras.

This is currently my setup:

  • Enable the picture profile PP10 (which is already configured with HLG2 gamma by default and other proper settings). Of course, this doesn't affects at all the captured RAW image. ISO continues starting at the normal value of 100.
  • Set the zebras level to Custom1 or Custom2 configuring it for "lower limit 95+" (experimentally discovered, but not an arbitrary value: it is just the video output documented by Sony when the gamma input maxes out in that mode).
  • Setup the Gamma Disp. Assist to Auto (I think it is Off by default). Otherwise the viewfinder is awful in many gamma modes, including those based on HLG. This has absolutely no impact on the zebras.
  • By the moment I HAVE NOT changed the black level on the picture profile (lowering it enhances the contrast, but does affects the zebras threshold). The saturation however has no impact on the zebras (rising it a notch could be useful).
  • If you are in Av mode and have enabled the Highlight metering, you should select the Entire screen Avg instead. I assume the same is recommended for M mode with auto ISO. This is to make it more unlikely the exposure compensation to go beyond +3EV (too easy on overcast days while using the zebras for ETTR in Highlight metering).
  • Do not setup a freaky WB like UniWB (the camera presets seem fine, including AutoWB, as well as a real life custom WB).

Another advantage of this setup is that the histogram in the picture review is nearly RAW in the green channel (the blinkies don't work, though).

VERY IMPORTANT:

In Av mode, just at +3EV of exposure compensation, the zebras ALWAYS stop updating! (they don't grow more, or may not appear at all, even if you continue pushing up to +5EV). That has nothing to do with the effective exposure value. And is a compeling reason to not to use Highlight metering.

In manual exposure mode (M) the zebras also stop updating under 0EV (e.g. for exposures longer than 8sec at F2.8 and ISO 100) but likely you'll not be looking at them by then.

 Ciriaco Garcia's gear list:Ciriaco Garcia's gear list
Sony a7R III Canon TS-E 17mm f/4L Sigma 35mm F1.4 DG HSM Art Canon EF 24-70mm F4L IS USM +1 more
ForumParentFirstPrevious
Flat view
Post (hide subjects) Posted by
fnf
(unknown member)
ForumParentFirstPrevious
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow