Quick Test: Canon 300mm f/4 L (non-IS) for astrophotography

The Canon 300mm f/4 L (non-IS) from the 1990's is one of Canon's discontinued, older and slower telephoto lenses. It does have UD glass. Because I already had the artificial star set up, I decided to see what star shapes look like off-axis on a Canon EOS 6D full-frame body.

Note that this is a contrived test using a 50 micron artificial star, 8m away (because it's cloudy).

And here it is:

It is not bad at all.

Compare to the APM Lomo 80mm f/6 Super-Apo triplet ("the best 80mm APO in the world," according to some), with the Televue TRF2008, which got the best results in my artificial star test:

Not too bad a showing for the Canon, I must say, given that the Canon is a 300mm f/4 (75mm aperture). The Lomo is the equivalent of a 384mm f/4.8 so not too far off.

Conclusion: the Canon superficially looks capable of challenging the "best 80mm APO in the world" on full frame.

William-Optics New Adjustable Flattener P-FLAT6AII on AP130GTX

I've attempted to create a general formula for spacing of the William-Optics New Adjustable Flattener P-FLAT6AII here.  I've also done some rudimentary testing of this flattener on a Lomo 80mm f/6 triplet as well. The key takeaway at the 480mm focal length is that this William Optics flattener performs better than the half-priced Orion flattener/reducer, but is marginally outperformed by the 20-year old design of the Televue TRF2008, at least in my testing with an artificial star that was quite close by, not at infinity.

What about on the Astro-Physics AP130GTX, which has an 819mm focal length?

Based on William-Optics' tables, the closest focal lengths are the 970mm Z126, with a 1.4mm spacing, and the 711mm Z103, with a 4.6mm spacing. Neither is particularly close to 819mm, but using my least-squares approximation with m=-0.014 and b=14.855 a spacing of 3.4mm is obtained.

However, based on my previous experience with the Lomo, where the best spacing was around 2mm shorter than indicated by the table (may be caused by the artificial star not being at infinity) I decided to try several spacings of the Flat6 to determine the best one.

First the AP130GTX with no flattener, on the corners of a Canon EOS 6D. Performance is decent, actually: the field curvature is much less than with the Lomo 80mm.

We do observe that there still are comet-shaped stars, although they are pretty tight.

The Televue TRF2008 did very well with the Lomo, in spite of its 20-year vintage (it was released in 1999). It does not do quite as well with the AP130GTX, however, in spite of (supposedly) being designed for 400mm - 600mm focal lengths.

The stars are not comatic, but they are quite eggy.

The Flat6 supposedly is best at 3.4mm spacing. Here's 3mm. It's not great. The stars are round (ish), but quite diffused compared to without the flattener. This could also be down to my technique or lack of it (that's a typo, the spacing is 3mm, not 3 meters).

 I decided to try an even shorter spacing of 1.5mm which would correspond to 900mm of focal length.

While the stars are tighter, they are less round than at 3mm, and show a bit of a comatic shape. I suspect that 3.4mm or 4mm is a better choice for spacing for the AP130GTX. But performance is still only tolerable.

I guess that's why AP gets away with charging $825 for their dedicated flattener.

Refractor Flattener/Reducer Comparison on Lomo 80mm f/6 Super-Apo Triplet

Some Hong Kong amateur astronomers have done their own testing of various flatteners with the Lomo 80mm f/6 Super-Apo. Their results can be summarized as such. Note that the Televue TRF2008 performs the worst in their test (24m distance to artificial star).

I recently purchased the William Optics New Adjustable Flat6A II which supposedly works with a wide variety of refractors. I've collected the recommended spacings from William Optics and generalized it to any refractor focal length.

For this test, I'm using a Lomo 80mm f/6 Super-Apo with a Russian OK4 air-spaced triplet. Due to poor weather, all testing was done indoors at about 8m distance (admittedly, quite close) and a Hubble Optics 5-star artificial star. I used the 50-micron (smallest) star for this test.

The test images were captured with a Canon EOS 6D, which is a full-frame sensor and therefore somewhat of a challenge for these flattener/reducers.  Do note that because the test was not conducted at infinity, it is not conclusive.

First, the performance of the refractor without any correction at all. We can clearly see that there is quite severe field curvature. This performance is inadequate for even casual imagers.

Compare this to the performance with the inexpensive Orion 8894 0.8X reducer:

Performance is better than without any flattener, but still not that great. Note that this is at the corners of a full-frame sensor, so on a reduced-frame camera, performance would be much better.

Now for the oldie-but-goodie Televue TRF2008 flattener/reducer for the TV85, which is designed for 400mm - 600mm focal lengths.

This is a pretty good showing, significantly better than the Orion.

Now for a bad example: the Altair Astro (Long Perng) 0.6X reducer/corrector, which was never designed for full frame:

Suffice it to say, this is barely better than no corrector at all, however there is the 0.6X focal length reduction which may offset the ugly corner stars.

According to William Optics, the GT81 with a 478mm focal length requires 7.9mm of spacing with the Flat6. Here are two attempts with an 8mm spacing:

Neither of them are very good. Definitely worse than the TRF2008. I tried spacings of 9mm and 10mm, with even worse results. With an 11mm spacing, I could not reach focus. This was almost certainly due to the artificial star not being at infinity.

Here's the same Flat6 with a 6mm spacing, which is significantly less than recommended:

Performance is significantly better than at 8mm, and is almost though not quite as good as the TRF2008.

And with 4.5mm spacing:

Correction is almost the same as at 6mm. In general, the shorter the focal length, the more correction is required. And correction is increased by increasing the spacing. Interestingly, this Lomo 80mm f/6 triplet seems to need less field flattening than would normally be indicated.

It is somewhat disappointing, however, that after all this drama, the Flat6 cannot surpass the 1999-era TRF2008.

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.

Viltrox EF-FX1 AF Adapter Brief Impressions


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.


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.

Lightweight Triplet Super-Apochromat Refractor

The Russian Lomo 80mm f/6 and f/7.5 OK4 triplet apochromats are considered by many to be among the finest 80mm refractors in the world. However, they are also known for being built like tanks. My APM Lomo 80mm f/6 in a William-Optics tube and with a Feathertouch focuser weighs 4.26 kg (9.4 lb) all in, which is heavy for an 80mm refractor.

Here we see the weight with a typical 18mm AstroTech Paradigm ED eyepiece, 2" diagonal, tube rings, and small Vixen dovetail:

On the other hand, the modular Borg refractors are well-known for being supremely portable and lightweight. However, they have been less known for being at the pinnacle of optical performance.

So I wondered, what if you could marry the best traits of the Lomo OK4 triplet and the Borg refractors?

Behold - the Lomoborg:

It is constructed from the 7803 80mm diameter x 205mm long Borg tube and the 7835 helical focuser. The tube ring is from a Takahashi FS-60. This setup was for my Borg 76ED, which has a 500mm focal length. The OTA is a little too long for the Lomo 80mm f/6 which is a 480mm focal length, so some eyepieces won't reach focus (the 18mm AstroTech Paradigm ED eyepiece barely reaches focus with a 2" diagonal, with about 2mm of in-travel left).

All-up weight is 2.9 kg (6.4 lb) which is a win!

In comparison, the Borg OTA with the 76ED objective weighs 2.46 kg (5.4 lb) so the Lomo objective adds 1 lb of weight.

The Lomo lens was adapted to the Borg tube using a 3D-printed adapter, which is secured to both the lens cell and the Borg tube using 3mm grub screws - not an ideal arrangement. For an actual production setup, the adapter would need to be made of aluminum and with threaded ends. Also, some sort of dew shield would be necessary.

3D Printer Use Case: Repairing a Dented Lens Filter Ring

I dropped this ancient Schneider-Kreuznach Retina lens on the floor, dinging the filter ring:

To repair it, I 3D-printed two plastic pieces (in HIPS with 100% infill) one matching the inner diameter of the filter ring, and the other matching the outside diameter:
Some ugly use of a C-clamp (the clamp's thread managed to put some marks on the opposite side of the lens filter ring, d'oh!):
And the result: leaves something to be desired, but still an improvement over the original damaged filter ring.

Synta/GSO Finder Bracket for Astro-Physics AP130GTX

If you've ever had a hankering to attach a cheap and cheerful Synta or GSO finder scope (or red dot finder) to your spendy Astro-Physics AP130GTX triplet refractor,  then this 3D printed design is for you.

I had forgotten (or did not want..) to buy the fancy Baader Vario finder and rings from Astro-Physics when I bought my AP130GTX. Since I had a 3D printer and some cheap no-name China red dot finders, I figured I'd design a finder bracket.

There's a bit of a complication because the AP refractor has its finder bracket screws coming out at a 20-degree angle, and the two screw holes are quite far apart. I also did not want to secure the finder bracket with a single screw. This bracket is appropriately curved to fit the AP refractor, and the screw holes are also tilted for proper alignment.

Right-Angle Finder Adapter for Takahashi Polar Scope

This is a Yashica/Contax right angle viewfinder, which can be obtained from ebay for under $20:

I decided to 3D-print an adapter allowing this finder to be attached to a Takahashi polar scope, so when using the polar scope an observer can avoid a stiff neck.  The adapter can be found here.

Here is the Contax right-angle viewfinder attached to the polar scope on a Takahashi Space Boy mount. The adapter has two holes that need to be tapped for M4 for the set screws. 3D printing technology isn't repeatable enough at a sub-0.5mm scale to allow a tight press-fit on both the Contax finder and the Takahashi polar scope, at least when using PLA (succeeding prints may be too loose or too tight, hence the need for the set screws).

Potentially if using ABS or HIPS it might be possible to exploit the material's flexibility for a press-fit on both the Contax finder and the polar scope eyepiece end.

Takahashi EM-1S Ersatz Polar Scope

I bought an old Takahashi EM-1S mount from Yahoo Japan, but it did not come with a polar scope.  Since this mount is 25+ years old, I was not even sure if a polar scope could be bought for it. Also, the Takahashi polar scope is expensive, and since I got the EM-1S fairly cheap (for a Takahashi..) I figured it would not be worth it to buy the current polar scope.

As luck would have it, I had a Celestron 6x30 "Long Eye Relief" finder scope, and I thought it might be possible to adapt this finder scope as a primitive polar scope for the EM-1S. The outer diameter of the eyepiece portion of this finder is about 24mm, and the barrel itself is 32mm in diameter. After three attempts, I came up with this design for an adapter, and 3D-printed it. The finder scope is a bit of a loose fit, so I added two masking tape shims:

There are three 3mm diameter holes in the circumference of the adapter, which must be tapped with an M4 tap and grub screws inserted to hold the finder scope in place.

Here is the ersatz polar scope in position. The adapter is a very tight fit in the Takahashi mount's RA bore. A strap wrench would be needed to remove the adapter. So it is important to place the finder scope into the adapter, and secure it with the grub screws, before inserting the adapter into the mount's RA bore. Some whacking was necessary to get the adapter to seat (although a previous version I made did not require whacking, so it's probably down to non-repeatability at sub-mm level in the 3D printer).

The polar scope manhole cover (cap) still threads on nicely:

Almost Full Frame: Fuji X Camera Lens Turbo Adapter (Ver II) from Mitakon

I very much like the form factor and UI of the Fuji X-mount cameras: they are very much the poor man's Leica.

A challenge is that wide lenses are hard to find, due to the 1.5X crop factor, since the Fuji cameras are not full-frame. The only full-frame mirrorless cameras to date are those from Sony ($$$) and Leica ($$$$$) which aren't an option for me. One could use LTM lenses such as the 15mm Voigtländer Super-Wide Heliar in LTM, but it too is rather expensive.

I've used some LTM lenses (Leica Elmar 90mm f/4 and a bunch of Russian lenses) on the Fuji XE-2, and these are fine - but still no wide-angle lens. To get say a 28mm equivalent would require an 18mm focal length. Fuji makes a very nice native X-mount 18mm lens, but it's quite expensive. All of the Fuji lenses are expensive, even the kit lens.

I discovered the Mitakon Camera Lens Turbo Adapter (Ver II) which is a 0.726X focal reducer. I've used reducers on telescopes; what these devices do is reduce the effective focal length of an attached lens, increasing the f-ratio in the process (since the focal length shortens, but the lens' physical front aperture does not change). Focal reducers are a great idea in theory, but they also decrease back focus. This means the focal reducers can only be used with SLR lenses (which have longer back focus than the X-mount). LTM lenses cannot be reduced (so no full-frame Leica lenses on a non-full frame camera).

A concern with focal reducers is that they may reduce image quality; after all, we are introducing another 2-3 lens elements between the original lens and the camera sensor. To test this theory, I took a known-good lens (the Canon 180mm f/3.5L Macro) and mounted it on both a Canon 6D full-frame camera, and the Fuji XE-2 with the Mitakon Camera Lens Turbo Adapter. And here are the results:

Here's the full-frame image with the Canon 6D and 180mm lens:

And here's the same scene with the Fuji XE-2 and Mitakon focal reducer (I literally just removed the Canon DSLR body and substituted the Fuji body, since the lens was mounted using its tripod foot to a tripod). Note the obvious vignetting in the corners, and the slightly smaller field of view. The Canon 180mm Macro with the 0.726X reduction and 1.5X APS-C crop factor is effectively a 196mm lens:

Here's the center of the Canon image (with the pants hanging out of the window). I took several shots with both auto-focus and manual focus using Live View, and this was the sharpest. The lens was wide-open and the shutter speed around 1/1000 second, with everything on a tripod.

and here's the same crop with the Fuji and Mitakon adapter. It actually looks sharper than the Canon image (and is slightly narrow in FOV). This is the same lens, although I manually focused using the Fuji's focus-peaking feature. I think the Canon image can be sharper if I had taken a RAW image. Fuji JPEG processing really does seem better than Canon's, and the X-Trans sensor probably also helps.

Here's the corner of the Canon image (top-left, but not the extreme corner):

and the same from the Fuji. Note it's a bit darker and the vignetting is obvious. Sharpness seems about the same as the Canon (note that the macro lens is wide-open, since you can't stop down the aperture on a Canon EF lens if it's not attached to an EOS body):

Long story short: at least on this sample of the Canon 180mm f/3.5L macro lens, the Mitakon focal reducer does not obviously reduce image quality, when used with a Fuji XE-2 body. I'm sure the Fuji's X-Trans sensor and JPEG processing has something to do with this. It's entirely possible that the Mitakon reduces image quality from the lens, but the Fuji's in-body processing compensates for this.

I'm not inclined to test the Canon 180mm lens "native" (with no reducer) on the Fuji body, since I don't intend to use any SLR lenses un-reduced on the Fuji body. Also, the Mitakon reducer adds a layer of protection over the Fuji sensor.

Here's the Canon lens on the Fuji body, it does look a bit ridiculous:

There's just one wrinkle with the Mitakon reducer: certain lenses don't work (they don't reach infinity with the adapter) due to projecting rear elements. Here are three Pentax screw-mount lenses that I have: the Super-Takumar 50mm f/1.4, Super-Macro-Takumar 50mm f/4, and Super-Takumar 35mm f/3.5 - and only the middle lens reaches infinity with the Mitakon reducer in place. The 50mm f/1.4 and 35mm f/3.5 have projecting rear elements that strike the Mitakon's front element (bad..) when racked to infinity:

There's a more complete list (80+ lenses) by A. Hillyard (with a Google sheet) that enumerates both compatible and incompatible lenses.

In summary, the Mitakon Camera Lens Turbo Adapter (Ver II) looks like a good choice for "almost full frame" on Fuji X-mount cameras, so long as you are willing to live without autofocus. It is also generally quite inexpensive (US$ 150).