Lens Adapters: Flange-to-Flange Distance

Focus Shift

lens adapters

Pedro J. Aphalo






lens adapters, focus shift

Today I received a “K&F Canon EOS-M4/3 PRO” adapter I bought at 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 0.45 mm of shim to a Canon EF to Nikon G adapter.

K&F Concept lens adpater

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 a 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 showing about 4 m. For computed values see the post Still more on adapter tolerance by Jim Kasson.

OWL adapter from DEO-TECH

OWL adapter from DEO-TECH

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 callipers 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 apparently in some cases be significantly shorter than expected, although not as short as this “K&F Canon EOS-M4/3 PRO”. Contrary to Jim Kasson’s report that Novoflex adapters are short by design, the Novoflex MFT/CO (M42) and MFT/OM adapters I own are both within 0.01 mm of the expected flange-to-flange distance. In contrast, 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. However, many modern primes and zooms have focusing mechanisms that work by moving individual optical groups and their design assumes that the rear optical element is always located exactly at the correct 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. Sale conditions are not the same, as at this website returns are accepted only up to 30 days from purchase (rather than a certain number of days after delivery) and refunds are only provided after returning items at the buyers expense (and without a feedback system, buyers lack any leverage).

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. [2020-08-08] I haven’t received an answer, yet. [2022] They never answered. I have no idea if currently produced adapters of this model are correctly sized or not.