The Mystery of the Lost Parfocality

[Jay60p]

Here is a Nikkor AF 28-85mm f/3.5-4.5 SLR zoom lens mounted on my X-T3,
with the basic Fotodiox adapter for Nikon F.
The lens focus is parfocal from 28mm to 85mm, with a smooth zoom ring, great for video zooms:

 

So I was surprised to find weeks later that it suddenly was not
holding its focus, it was focusing farther at wide angle (??)

At 85mm, focused manually (a screen grab cropped into the video):

 

Then zoomed out to 28mm, the focus shifted behind the subject:

 

Eventually I realized that I was using a different Fotodiox Nikon F adapter this time,
and the only difference was an aperture control ring built in for G lenses:

 

I compared the thickness of the two adapters with a small C clamp.
The G adapter was a hair shorter than the basic adapter,
and the G adapter had the lens focus going too far past infinity.
So I decided to extend the G adapter’s length a slight bit.
I removed the back of the adapter by removing four screws,
and I added a thin layer of tape to extend the adapter’s overall length:

Finding the right thickness of tape was trial and error.
Thin cardboard, too thick. Duct tape, too thick. Black masking tape worked.

Now after zooming out it is back in focus:

 

So a mounting error of the thickness of masking tape threw off the zoom’s parfocality!
How thick was this tape? I have an old caliper from my grandfather, and it measures
in 1/1,000 inch increments. (But not wide enough to measure the Nikon adapters):

The masking tape is seven thousandths of an inch thick. That's about a sixth of a millimeter.

The moral of the story: if your vintage zoom lens adapted to your mirrorless camera
appears not parfocal for video, try another brand of adapter, especially if it has infinity focus
adjustments. Apparently all it takes is a few thousandths of an inch to throw it off.

 

[BrunoCH]

I have the same problem with my nikkor zoom 25-50. Back focus need to be very precise and it can move just with a change in temperature. This is why there is a back focus adjustment ring on the zoom fujinon MK.

This Ebay seller

https://www.ebay.fr/usr/ramir73?_trksid=p2047675.l2559

sells M42 adapter to Fuji X with a back focus adjustment. I have exchanged messages with him and he will see if he can do something for a Nikon to Fuji X adapter. I’m waiting !

[heart0less]

Same story here:

I recently managed to acquire Olympus OM Zuiko 75-150 / 4, which is considered to be parfocal. 

The issue was I couldn't achieve the so-called parfocality, no matter what adapter I used (OM - EF and then EF - FX was a no-no; using EF - NEX or OM - NEX didn't help, at all). 

I felt quite disheartened, haha. 

In the end, I was able to correct the backfocus with my speedbooster (rotating the rear glass element did it), so at least it keeps its focus when I use it with Lens Turbo II. 

I'll have to take some measurements and try to somehow alter the 'dumb' adapters to match the speedbooster.

[Jay60p]

10 hours ago, heart0less said:

In the end, I was able to correct the backfocus with my speedbooster (rotating the rear glass element did it), so at least it keeps its focus when I use it with Lens Turbo II. 

 

I've been considering the Lens Turbo II, Nikon F to Fuji X.

Which one do you use? And how did you rotate the rear element? I assume that is not an "official" feature.

 

[heart0less]

I opted for the most versatile option - Canon EF to Fuji X, since you can adapt almost everything to Canon EF and that's exactly what I do (I have M42 lenses, Nikon F, Pentax K, Olympus OM - all of them work okay).

There is some little screw in the back, which you can loosen up and then rotate the element as you wish.
Just like on this video:

https://www.youtube.com/watch?v=CQ_PFMLonpU

Unfortunately, I heard that some copies of LT II doesn't feature the above mentioned screw and have to be taken apart in order to make any adjustments.
What's more, it's quite difficult to mark your default setting before you try to even do anything, because there are no retaining rings (like in Metabones) or anything like that - just bare glass.

[Jay60p]

Thanks, that is good info. I also have M42 and Pentax and Nikon.

Too bad it doesn't take Canon FD.

The only thing is, that means having two adapters on a lens instead of one,

so I would have twice the chance of back focus error.

I suppose I could buy both and see which maintain best parfocality with the Nikon zooms I have,

and return the other. B&H in New York is very good about returns within 30 days.

Will keep this in mind.

[Jay60p]

P.S.   If my X-T3 shots don't look so hot, they were blow-ups from the full frame's view:

and shot in available light in UHD at 60p but only 100mb/s, at ISO 10,000 since it was a quick and dirty lens test.

Didn't know I'd be putting this online.

[Jay60p]

I found the same problem with my SMC Pentax-FA 100-300 and the Fotodiox PK(A)-FX adapter.

Symptoms are focusing past infinity, and shifting focus distance farther when zoomed back to wide angle.

I fixed that adapter the same way, but that needed more than twice as much extension (18 thousands of an inch).

The lens is now parfocal. I can see why Fotodiox would want to err on the short side with these adapters since

prime lenses aren't much affected as long as they reach infinity. Zooms need more accuracy for parfocality.

Maybe the more expensive "pro" Fotodiox are more accurate?

[heart0less]

1 hour ago, Jay60p said:

Maybe the more expensive "pro" Fotodiox are more accurate?

That's the question I was asking myself. You can't really know that unless you buy it.

And when you do and it turns out that it isn't any better, then you are screwed, since in many cases it's impossible to return it - I get all my adapters from eBay or AliExpress, because we have no B&H in Poland (nor any close equivalents) and those kind of things are really tricky to get.

I guess I prefer to correct them using DIY methods rather than order some different copy from other brands and have no guarantee they would work.

[Jay60p]

 

5 hours ago, heart0less said:

That's the question I was asking myself. You can't really know that unless you buy it.

And when you do and it turns out that it isn't any better, then you are screwed, since in many cases it's impossible to return it - I get all my adapters from eBay or AliExpress, because we have no B&H in Poland (nor any close equivalents) and those kind of things are really tricky to get.

I guess I prefer to correct them using DIY methods rather than order some different copy from other brands and have no guarantee they would work.

The good news is that these DIY methods are easy, I'm set now and don't need to try the Pro adapters.

If the adapters were too long instead of short, then we'd really be screwed!

Here's my final results with my 35mm SLR zoom lenses & adapters I use for video (your results may differ)...

Nikkor AF 28-85mm f/3.5-4.5 on Fotodiox NIK-FX - parfocal, no adjustment needed.

Nikkor AF 28-85mm f/3.5-4.5 on Fotodiox NIK(G)-FX - needs extension for parfocality.

Canon FD 35-70 f4, and Canon FD 70-210 f4 on Fotasy FD-FX - parfocal, no adjustment needed.

SMC Pentax-AF 100-300 f/4.5-5.6 on Fotodiox PK(A)-FX - needs extension for parfocality.

 

[Jay60p]

Update:

The Mitakon Zhongyi Lens Turbo II for Nikon to Fuji X also needs back focus adjustment on my X-T3.

It has the aperture ring in the center, so you can remove the back flange with four screws the same way as shown above

with the Fotodiox NIK(G)-FX dumb adapter.

It needed the black masking tape plus an extra layer of thinner tape (about 11 thousandths of an inch total) to keep my

Nikon AF 28-85 in focus, as well as my newer Nikon 24-85 f2.8-4.