Z9 burst shooting buffer depth

Just some basic tests with the few cards I have.

 Lexar 2933x 128 GiBProGrade Gold 256 GiBPergear 512 GiBAngelbird AV PRO 1 TiB
TypeXQDCFExpressCFExpressCFExpress
20 FPS (lossless)26 (11 - 37)40 (34 - 43)36 (36 - 37)37 (37 - 37)
20 FPS (HE*)60 (57 - 61)60 (49 - 77)60 (59 - 61)62 (60 - 64)
20 FPS (HE)75 (34 - 95)85 (45 - 101)100 (98 - 103)104 (98 - 112)
30 FPS196 (187-198)183 (52 - 198)192 (137 - 258)192 (142 - 198)
120 FPS706 (667 - 736)706 (558 - 739)737 (734 - 739)736 (734 - 739)
Cost per GiB (Feb 2022)$2.54$1.13$0.62$0.57
Amazon Listing

Values shown are the average over all trials with worst & best individual results shown in parenthesis.

Commentary

Surprisingly little performance difference

None of the cards tested are among the “known fastest” CFExpress cards, like the Delkin Blacks or ProGrade Cobalts. Nonetheless, I’m surprised at how minor the performance difference is between all of them, especially given there’s an XQD card in the mix.

CFExpress cards are not necessarily fast.

Angelbird AV PROs do not meet their promised performance

The Angelbird card claims a 1,000 MB/s minimum, sustained write speed. The XQD format is incapable of speeds above 500 MB/s. Yet the Angelbird is at best just 40% faster than the XQD Lexar. This suggests either the camera is the limiting factor – unlikely given that others have demonstrated much deeper bursts with other, apparently faster cards – or that the Angelbird doesn’t live up to its claims.

Blackmagic Disk Speed Test with a Pergear USB-C reader indicates the Angelbird almost hits 1,000 MB/s at the start of a sequential read or write, but within a second or two falls down to a sustained speed of only about 700 MB/s. And there’s that 40% difference again, vs XQD.

Average performance correlates with consistent performance

e.g. the Pergear 512 GiB is nominally about the same performance on average as the ProGrade 256 GiB, but the Pergear was much more consistent. The Angelbird was a tad faster & more consistent again.

This also highlights why many trials are important, in order to determine the variance. I’d rather have an on-average slower card that’s very consistent than a “bursty” card that might crap out in a critical moment and cause me to miss the moment completely.

30 & 120 FPS modes are camera limited

There was practically no difference in performance between the cards in 30 FPS & 120 FPS modes.

The bandwidth demonstrated is well below the demonstrated capabilities of all these cards, at just a few hundred MB/s.

All this seems quite conclusive that in these extra-fast burst modes the Z9 is the bottleneck, not the memory card.

Sidenote: The ProGrade card showed occasional glitches (three in total across twenty trials) – where the Z9 would suddenly stop shooting mid-burst, where a split second prior it had still shown a significant amount left in the “buffer” (the rXXX counter). I’m not sure what to make of that – perhaps the Z9 relies on some basic level of performance and the ProGrade can’t consistently meet it, or perhaps something is glitching between the Z9 & the ProGrade card that causes the Z9 to error out and stop working.

Methodology

1/250, 24-70/4 @ f4, ISO 5000.

Z9 firmware 1.11.

I enabled the shutter sound at maximum volume, and held down the shutter until I heard a stutter.

For 20 FPS mode:

  • I counted any extra frames after the stutter and subtracted those from the numbers.
  • I also tested 1/2500 and saw no meaningful difference in results, and ISO 64 & 25,600 which improved and decreased (respectively) buffer depth by about 10% each (very likely corresponding to the file size differences, though I didn’t check).
  • Five trials, each testing each format in turn: lossless, HE*, HE.

For 30 & 120 FPS modes:

  • I never heard an extra frame after the first stutter – I don’t know if that means the camera ground to a complete halt or merely that it doesn’t reliably play the fake shutter sound in these modes. The consistency of the results in those modes leads me to believe it’s the former.
  • Ten trials, sequentially.

Cards were formatted in camera and empty at the start of each class of testing (20, 30, 120). Images were not erased between trials (empty cards are not representative of real-world conditions).

Autofocus was not engaged during shooting. I haven’t tested it comprehensively, but so far I’ve seen no impact on burst performance from using autofocus (including subject recognition).

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