I had a long, very in depth and very interesting web meeting with several members of the FX9 engineering team in Japan last week. They have been looking into the green fringing that some FX9 users have been reporting in various user groups. I have to say that this hasn’t been something that I have found to be a problem, but if you keep on reading you’ll find out why that is!

The first thing that became very clear in the discussion was that they take issues like this extremely seriously. The FX9 is their baby and they want end users to be happy with it. I was shown a lot of examples of very carefully executed tests using calibrated light sources, test charts, and various objects with different colours or reflectivity placed in extreme contrast situations.

And yes, in some cases the images would show a green edge around the high contrast edge of an extremely backlit object. Not just with the FX9 but also with the FS7 they were comparing it against.

The cause is Chromatic Aberration in the lens.

In every case the cause of the coloured edge is chromatic aberration. Chromatic aberration is an optical phenomenon cause by the way the glass in a lens will inevitably refract different wavelengths of light (and thus different colours) by different amounts. So when one colour is in precise focus, other colours may be slightly out of focus and this causes blurring of one colour and this typically manifests itself as a blue, green or red fringe – chromatic aberration.

So the root cause of this issue is not the camera, it is the lens and it will vary from lens to lens. But do keep reading – there is more you need to know…….

Other Factors.

However there are other factors at play. One is the way a bayer sensor works. There is a huge resolution difference between the green channel and the red and blue channels. A lot of complex processing is used to compensate for this and that processing is optimised for what you might call “normal” or “typical” scenes. So when presented with a shot that has extreme contrast and a lot of optical chromatic aberration sometimes this processing isn’t going to deliver a fully optimised image and it may reveal the differences in resolution between the 3 colour channels. This is particularly the case with green CA as the green channel has much greater resolution than the others. 

But the FS7 doesn’t do it….

But this bayer/processing issue will be broadly similar between the FX9 and FS7, so why are more FX9 owners reporting a problem than FS7 owners?

The answer it seems is white balance!

All the  reports of this issue have occured when the camera has been set to a daylight white balance, most of the time it has been when the camera has been set to it’s default 5500K daylight preset.

When comparing the FX9 with the FS7 with both set to 5500K the FX9 has a small but noticeable bias towards green. I and many others have been aware of this from the launch of the camera, especially when you use the camera in the CineEI mode and use the standard s709 LUT from Sony. The images are distinctly more green than any other Sony camera and a fair bit more green than the FS7, FS5 and Alphas.

It’s not that white objects won’t be white, it’s just that overall the image looks a bit green. This is easily dialed out by adding a +20 offset to the cameras tint control and that is how I have been using my camera pretty much from day one.

When you think about it – or when you analyse the situation carefully as Sony have, if the camera has more green, what will happen if you have green CA? Well it’s going to be much more pronounced.  And that is what we are seeing. The way the colours in the FX9 are tuned has an unfortunate side effect of making green CA much more obvious.

This explains why I have never really had an issue with green CA. I’m normally running a +20 tint offset which reduces the green in the camera plus I tend to use my own less green versions of the s709 LUT or I use ACES which is also less green than Sony’s own s709 LUT. So overall I have already dialed out the green and unknowingly fixed an issue I didn’t know I had.

So the engineers in Japan are currently recommending adding a +20 tint to the camera when shooting daylight. In particular instead of using a preset of 5500K to use 5000K and +20 tint to achieve the same white balance but a less green image. They are also assessing whether the camera needs to have it’s processing adjusted so that it is less green when using the 5500K preset. At warmer colour temperatures this issue does not seem to ever be a problem, it’s only with daylight.

Push Auto White Balance.

Further to this it’s also worth noting that if you use the cameras Push Auto white balance to set the white balance with a white or grey card then this will normally deliver the optimum white balance for the scene you are shooting and it won’t be biased towards green. And we should remember that Push Auto white balance works correctly in both the CineEI mode and Custom mode. If you use Push Auto WB there is no need to add a +20 Tint and the pictures should look natural and well balanced. Having become used to using preset white balance values when shooting S-Log with Sony cameras for so many years I keep forgetting that you can now do this. 

Another benefit of Push Auto WB is that as it balances the camera correctly it also helps bring the FS5, FS7, F55 etc and FX9 closer together if you are using a mix of different cameras. I was recently shooting with a Venice and FX9 side by side and by using the Push Auto WB on both Venice and the FX9 it became extremely hard to tell one from the other. Previously when using the presets the FX9 always looked a touch green.

Also – if you are using daylight balanced LED or fluorescent lights using Push Auto white balance will help correct out any tendency towards green that the lights may have. So really Push Auto White Balance is a win-win situation and let’s face it a lot of you have been asking for it in CineEI for a long time – so now we have it, let’s use it.

What about Aperture Correction?

In my own experiments I found that in Custom Mode turning off the cameras Aperture correction (https://www.xdcam-user.com/2020/07/reducing-ca-artefacts-in-the-sony-fx9/) can help reduce the visibility of the CA. As CA is an optical edge effect, anything that enhances edges will also enhance the CA and turning off aperture correction prevents the CA from being boosted by the aperture correction. Overall I personally prefer the images you get from the FX9 with aperture correction off anyway, they are a little bit less sharp and more rounded. 

And slightly off topic, what about image quality at FF and S35 2K scan? This was discussed as well, the engineers want people to be happy with the camera. The issue is that this is a camera with a 6K sensor and an optical system designed around that 6K sensor. So if you then reduce the resolution of the sensor readout the optics are no longer optimised (mainly the Optical Low Pass Filter). As a result there is an inevitable increase in aliasing and moire. Unfortunately this is just the way the physics works and there is not much that can be done about it. But the engineers know that the FX9 will be compared with the FS7 – which also has similar issues in it’s S35 2K scan modes. The goal of the engineers is that the FX9 should not be worse than the FS7 and there are some tweaks in the pipelines to the image processing that will bring some improvements to the FX9 when using 2K scan. But let’s be realistic, this will always be a camera with a 6K sensor and a 6K OLPF, so the Full Frame and Super 35mm 2K scan modes will never match the quality of the 6K and 4K scan modes, it just isn’t possible so don’t expect miracles. These tweaks may take a bit of time to be finalised, so I’m not sure when we will see them. I think what we should see however is the 2K scan from an FX9 being indistinguishable from the 2K scan of an FS7.