William-Optics New Adjustable Flattener P-FLAT6AII

William Optics has a new adjustable flattener with (they claim) 97% of full-frame (43mm image circle) coverage for a wide range of refractors from around 480mm focal length, up to 970mm. This is almost certainly the replacement for the old P-FLAT4.

There are several other adjustable flatteners such as the Long Perng one (which is cheaper).

WO publishes some suggested spacings on their web site, which I have summarized here:


There is what I believe to be a typo. Field curvature for refractors (whether doublets or triplets) is proportional to the focal length only (not the focal ratio). However we see in WO's suggested spacings that the Zenithstar 71 and Gran Turismo 71 which have basically equal focal lengths, have significantly different spacings.

If we do a least-squares interpolation using WO's suggested spacings, we get the following:

Notice that the R-squared is 0.9173 which is not very good. However if we drop the Zenithstar 71 data point and keep all the rest, we get this:


A much better fit (almost a straight line) with an R-squared of 0.9926.

In summary: to determine the optimal spacing for your particular refractor, use m=-0.014 and b=14.855 (remember the formula, y = mx + b).

Once I get my copy of the P-FLAT6AII, I will be able to validate if the above formula holds.

AWS M5A (AMD EPYC) Performance Comparison

This is the time-honoured Linux kernel benchmark.

Preparation was simply unzipping the 4.20-rc3 tarball from kernel.org, running "make menuconfig" and immediately saving the config (no changes) and then make -j 4.

This was on Amazon Linux 2, with a 40GB GP2 EBS block storage volume.

AMD EPYC m5a.xlarge:

time tar zxf linux-4.20-rc3.tar.gz
real 0m6.109s
user 0m5.928s
sys 0m2.838s


time make -j 4
real 18m31.421s
user 66m52.071s
sys 5m54.968s

Intel Xeon Platinum m5.xlarge:

time tar zxf linux-4.20-rc3.tar.gz 

real 0m4.693s
user 0m4.688s
sys 0m1.767s

time make -j 4
real 14m4.332s
user 49m12.569s
sys 5m44.682s

So there we have it: on a kernel compilation, one run, the Intel instance completed the kernel compilation 25% faster.

Viltrox EF-FX1 AF Adapter Brief Impressions

Overview

I have been using a Mitakon Zhongyi Lens Turbo II (Canon EF to Fuji X-mount) for almost a year and I'm pretty happy with it: you get "full frame" cropping, and the optical quality is surprisingly high. The only downside is that a lot of wide lenses (anything wider than 24mm actually) don't work, as their rear elements collide with the reducer's front element near infinity.

I'd read about various Canon EF to Fuji X AF adapters, and the cheapest one out there is the Viltrox EF-FX1, so I bought one off ebay for $140 (expressed-shipped from China) and got it a few days later.

I will dispense with all comments about build quality, etc. Build quality is adequate, equivalent to a third-party lens. That's good enough.


I originally intended to do some videos comparing the AF performance of a Fuji XT-1 with the Viltrox adapter, with my Canon 6D, but it became quickly apparent that such a comparison was useless. The long and the short of it is: the XT-1 (or XE-2, I tested both) with the Viltrox adapter and almost any Canon lens, behaves like a circa 2011-2012 mirrorless camera in terms of AF performance: it's slow, hunts a lot, and often does not find focus.

There is a review here with videos of the AF performance with a wide variety of Canon lenses, but I do not consider this very useful for several reasons:
  • high contrast subject
  • well-lighted
  • the lenses were already "close" to good focus
Basically: the video above makes the adapter look a lot more performant than it really is. I watched that video and was impressed with the AF performance, so when I actually got the Viltrox, expectations did not match reality.

My testing is much less rigorous, but I did it in a dimly-lit room with low contrast subjects. Also, I made sure to crank the lenses to their minimum focusing distance before engaging the AF. For example on the 180mm f/3.5 Macro, it takes quite a long time to motor from MFD to the correct focusing distance (i.e. this is the worst-case scenario).

Strange Bugs and Quirks

  • The adapter forces the lens EMD to act as an "auto iris" all the time - you can actually see the lens diaphragm stopping down when you point the lens at a bright light source. This is disconcerting and may not do wonders for the lens' longevity.
  • Aperture EXIF data is mis-reported for many lenses when wide open; this does not affect the actual aperture, just the reported EXIF data. For example, the Canon 50mm f/1.8 STM wide open (at f/1.8) is recorded as f/20; the Canon 35mm f/1.4L Mk I wide-open is recorded as f/16. Stopping down to f/2.0 or f/1.6 respectively records the correct aperture value in the EXIF data. However, other lenses (I tested two zooms - the 24-85mm f/3.5-4.5 USM and the 70-200mm f/2.8L USM) do record correct aperture data even wide open.
  • Only the widest focal length EXIF data for zoom lenses is recorded in the EXIF; for example the 24-85mm always reports 24mm whatever the actual focal length, and the 70-200mm always reports 70mm.
  • On STM lenses (only tested with the 50mm), the AF/MF switch is disobeyed - MF is always possible (it's manual focus-by-wire, and the adapter always enables it). This is probably a good feature to have rather than a bug.


Lens AF Performance

So to the meat of the summary: AF performance. I tested this with a variety of lenses, and as stated above, AF performance is equivalent to a circa 2011-2012 mirrorless (well, a Panasonic GF2 because that's the mirrorless I owned in that time frame). Or perhaps a sluggish modern prosumer camera like a Canon G5X, or a 10-year old entry-level Canon DSLR (like a 350D).
  • Canon 50mm f/1.8 STM - easily the worst-performing of the first-party lenses I tested. Unable to reach focus in many (somewhat dark) situations. Hunted a lot and slow. This same lens performs very well on a Canon 6D: moderately faster AF performance, but very accurate and doesn't hunt at all in low light.
  • Canon 35mm f/1.4L Mk 1 - focuses surprisingly fast, though not as fast as natively on the 6D and hunts.
  • Canon 85mm f/1.8 - same as the 35mm.
  • Canon 135mm f/2L - same as the 35mm.
  • Canon 180mm f/3.5L Macro - I take it back, this is the worst-performing lens with the Viltrox adapter. Gets lost more often than not, AF is pretty much useless on this lens. But this lens also has mediocre AF performance on the Canon 6D.
  • Canon 24-85mm f/3.5-4.5 - focuses quite fast (see the theme? "real" ring USM lenses perform well).
  • Canon 70-200mm f/2.8L non-IS - also focuses quite fast.
  • Canon 16-35mm f/4L IS - focuses fast, and IS works.  I was able to get sharp photos at 35mm and 1/2 second exposure time. There was an instance where the lens got disconnected and AF stopped working (and the display showed f/0 - as if no lens was attached) but restarting the camera fixed this.
  • Sigma 50mm f/2.8 Macro (the old one that locks up your camera with Error 99) - this does not lock up a Fuji camera! however aperture cannot be controlled, so it only operates wide-open (on any modern Canon DSLR, this lens locks up the camera if you try to set the aperture to anything other than wide-open; on old DSLR's like the 5D, it would not lock up the camera but apertures smaller than wide-open cannot be commanded).
  • Tokina 80-400mm f/4.5-5.6 AT-X AF - useless.  At 80mm it seems to reach focus, but at 400mm it hunts around, runs back and forth past the correct focus point, then indicates correct focus (double beep) even when the lens is clearly not focused.

Conclusion

I would not characterize the Viltrox EF-FX1 to be a cheap parlor trick (it is on the cheap side, admittedly): on fast ring USM lenses it is actually usable, although the user experience is sub-standard.

If you have a large pile of Canon lenses, then this adapter is useful.  I do not know if Canon lenses AF faster on say, Sony A7-class mirrorless cameras, or if the more spendy Canon-to-Fuji AF adapters focus faster. But the Viltrox is $140, which is less than the cost of the cheapest Fuji primes. So if say you want a 50mm AF prime and have the Canon lying around, it's cheaper to buy the Viltrox than the Fuji XF 50mm.

Some Canon lenses (well the cheap 50mm STM that I tested) don't work very well, and the AF performance of all the Canon lenses is nowhere close to a 6D, which is a 4-year old, mid-tier body. Granted the XT-1 and XE-2 are also of equally dated vintage. Maybe a more modern Fuji body would perform better, but the XT-1 and XE-2 AF swiftly with native Fuji lenses, so I don't think the problem is in the body.

That said, if Viltrox came out with a version of this converter with a built-in reducer like the Mitakon Zhongyi, I'd probably buy it.