a7III PDAF striping in practice, part two

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JimKasson
JimKasson Forum Pro • Posts: 17,316
a7III PDAF striping in practice, part two
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This is a continuation of the now-full previous thread on the same subject .

Before we get started, I'd like to say something the brand advocacy that, in my opinion, marred the previous thread. So far, most of the digital ink about PDAF striping has been passionate advocation of one of the following positions.

  • The striping on the a7III is a fatal flaw. You can never know when and where it will occur, and all the fixes are a PITA and impair quality. Don’t buy this camera, and everybody who says something different has a hidden agenda.
  • The striping on the a7III occurs only on a specific set of lenses under an extremely rare set of clearly-identifiable circumstances. In the incredibly-unlikely circumstance that it should occur, there are fixes that will remove it perfectly. It’s a nearly-perfect camera, and everybody who says something different has a hidden agenda.

I’ve seen this movie before. Two cases come to mind, both involving Sony cameras.

When the Sony a7R came out, there was a lot of noise about “shutter shock” (which was kind of a misnomer, since the root cause of most of the vibration was from the motor that wound the shutter). Mike Collette, Joe Holmes, Huntington Witherill, and I, along with others, spent a lot of time and effort analyzing the cause of the problem, quantifying its effects, and seeking mediation strategies. For our troubles, we were attacked by two groups of people who said:

  • The a7R shutter shock is a fatal flaw. You can never know when and where it will occur, and all the fixes are a PITA and impair quality. Don’t buy this camera, and everybody who says something different has a hidden agenda.
  • The shutter shock on the a7R occurs only under an extremely rare set of clearly-identifiable circumstances. It’s a nearly-perfect camera, and everybody who says something different has a hidden agenda.

Eventually, we came up with a set of circumstances where shutter shock was significant, another where it was inconsequential, and – unfortunately – a set where the behavior was not well defined. A few people seemed to care about that. Finally, Sony released a replacement for the a7R which didn’t have the problem, and the hoopla died down.

When the NEX line of cameras came out, and especially when the first alpha 7 cameras were released, there was a big fuss about Sony’s lossy raw compression algorithm, which was always operating in those cameras. There were two schools of thought:

  • The a7x lossy compression is a fatal flaw. You can never know when and where it will affect the final image, and all the fixes are a PITA and impair quality. Don’t buy this camera, and everybody who says something different has a hidden agenda.
  • The lossy compression on the a7x affects images only under an extremely rare set of clearly-identifiable circumstances. They are nearly-perfect cameras, and everybody who says something different has a hidden agenda.

A few people reverse-engineered the Sony compression algorithm. I wrote software to simulate its effect on images. Eventually, we came up with a set of circumstances where raw compression artifacts were significant, another where it was inconsequential, and – fortunately – a very small set where the behavior was not well defined. A few people seemed to care about that. Finally, Sony released a firmware update and new cameras which – with what I’ve called malicious obedience – allowed uncompressed raw but not losslessly compressed raw, and the hoopla died down.

I’ve emphasized the similarities with some cutting and pasting above; I apologize if that made it a boring read.

I find this tribalism distinctly unhelpful. The arguments for the extreme positions obscure complexities that are important to photographers, and the filling of fora with extreme comments makes it hard on people who just want to get information about what camera to buy and how to use it to best effect. Not only is it difficult to separate the wheat from the chaff, it is unpleasant to wade through all the name-calling, ad-hominin arguments, and outrageously false statements.

Having said all that, I expect the cycle to repeat with the a7III PDAF striping. Undaunted, I will continue to work on it.

Here’s what we know:

  • It occurs with the a7III with some lenses in backlit situations with lens flare present.
  • The a9 had similar issues, but people don’t seem unhappy with the a9.
  • It is related to off-axis light impinging on the sensor.
  • It is related to the sensor rows with PDAF pixels.
  • It is not strongly dependent on f-stop.

Here’s what we don’t know:

  • If there are any lenses that can’t generate these artifacts on the a7III.
  • What lenses are the worst, and what ones are the best
  • If you can get objectionable artifacts without visible flare (we do know you can get measurable and visible artifacts in parts of the picture with no visible flare, and measurable artifacts with hardly any visible flare present at all).
  • The exact mechanism by which the artifacts are generated.
  • If we can make a short list of situations to avoid if you are worried about PDAF striping.
  • How big you have to print to see the artifacts.

My first follow-on post is going to be about the last question.

Jim

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