Lens adapters: flange-to-flange distance

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Today I received a “K&F Canon EOS-M4/3 PRO” adapter (bought from https://www.kentfaith.com/). This is a new design advertised as being of high precision. I was specially interested as this is one of the very few or maybe the only series of relatively low cost adapters with a non-reflective black interior finish. This is is important to prevent reflections that otherwise degrade image contrast or create flare and ghost images. Please see Black anodised aluminium in IR and Commlite products: how I wasted my money.

In my previous post Lens adapter with filter drawer I described the problem of rear mounted filters modifying the effective flange distance. I described how this “shortening of the flange distance” made it impossible to focus a very wide angle lens at infinity. I solved this by adding 0.3 mm of shim to an Canon EF to Nikon G adapter.

Photograph of the adapter
K&F Concept lens adpater

When I tried the same wide angle lens on the “K&F Canon EOS-M4/3 PRO” I noticed an even worse focusing problem than with the rear mounted filter. I decided to measure the flange to flange distance of this adapter. The flange focal distance of Micro 4/3 cameras is 19.25 mm while that of Canon EF and EF-S cameras is 44.00 mm (please see Wikipedia). The difference is 24.75 mm, and this is the expected flange-to-flange distance that an adapter for mounting Canon EF lenses on M4/3 camera bodies should have. The K&F adapter has a flange-to-flange distance of 23.85 mm, that is 0.9 mm too short. In the case of the 4.5 mm objective this is 1/5 of the total focal length (i.e. equivalent to a 10 mm shift for a 50 mm objective or 40 mm for a 200 mm objective). For optimal lens performance a precision of 0.01 mm in the flange focal distance is considered to be required (see Roger Cicala’s post “The Great Flange-to-Sensor Distance Article: Part 1“). A 0.9 mm difference is enough to affect the focusing scale of 24 mm lens so drastically that focus at infinity is achieved when the scale is at 0.7 m and even a 100 mm lens focuses at infinity with the scale scale showing about 4 m. For computed values see the post “Still more on adapter tolerance” by Jim Kasson.

I also measured the flange-to-flange distance of the “DEO-TECH OWL EF/MFT” adapter I described recently: exactly 24.75 mm. This adapter measures correct to the 0.01 mm resolution of the digital calipers I used. With no filter in the adapter, when focusing the same lenses as tested with the K&F adapter, the focusing scales show the correct distances.

If the flange-to-flange distance is too long it prevents infinity focus, which would be more of a problem than an increase in the nearest possible focusing distance. From the post “Adapter flange focal distance errors” by Jim Kasson it turns out that even expensive adapters can be significantly shorter than expected, although not as short as this “K&F Canon EOS-M4/3 PRO”. Contrary to Jim Kasson’s report the Novoflex MFT/CO (M42) and MFT/OM adapters I own are both within 0.01 mm of the expected flange-to-flange distance while a “K&F OM-M4/3 (not Pro)” is only 0.19 mm too short. With respect to image quality, a short adapter is not much of a problem with an old-fashioned prime lens focused by displacing all the optic elements and groups together. In contrast in many modern primes or zooms with focusing mechanisms that work by moving individual optical groups the design assumes that the rear optical element is always located exactly at the designed distance from the sensor. Altering this distance degrades image quality. A nice explanation is given in the post “Lens adapters — FUD to enlightenment” by Jim Kasson.

The “K&F Canon EOS-M4/3 PRO” adapter seems well built, strong without doubt but on the other hand weighing 111 g is one of the heaviest glass-less adapters I have ever encountered (unnecessary weight is a disadvantage). The inside of the adapter as per the description has a black non-reflecting finish. The fit onto the camera is smooth and with minimal rotational play. The lens-side mount is also strongly built and provides a good fit. On the other hand the too short by 0.9 mm flange-to-flange distance is such a major flaw as to make this adapter of little practical use with most wide-angle and normal lenses. With the odd out-of-specifications lens, a shorter flange-to-flange distance can be an advantage. I do have one such lens in M42 mount, but the error is less than 0.5 mm. Delivery was very fast and the parcel was posted to me from within EU, i.e., the carrier used by K&F cleared the parcel through customs at no cost to me. It is also remarkable that prices are significantly lower at https://www.kentfaith.com/ than those asked by Aliexpress sellers.

I have bought earlier some other K&F Concept adapters and they have been o.k. in design although they have in several cases required some tuning of the tension of springs and once of them required the fine-adjustment of the position of the tab that engaged a lever in the adapted lens. They have also been in the past very good when I have complained and solved problems smoothly. [2020-07-01] I wrote to them some minutes ago and I will update this post when I receive an answer. [2020-07-04] Three days have passed and I haven’t received an answer.


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