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Author Topic: Lens/Adapter Combo Focus Problem  (Read 843 times)

Jens Peermann

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Lens/Adapter Combo Focus Problem
« on: February 15, 2016, 10:35:35 am »

Last weekend I got confirmation for something a suspected for a long time: my Metabones IV + Sony A7  combination throws the calibration of the lens out of the tolerance range. It became very obvious when using Zeiss ZE lenses.

Those lenses behaved like expected when using them on my 5D2 Camera, i.e. they had some extra travel past the infinity focus point before they come to a hard stop, to compensate for small expansions and contractions due to temperature changes. When I received the Sony A7 with the Metabones IV that extra travel was gone on the 21mm and 28mm lenses and reduced to less than a quarter of what it used to be on the 50mm and 100mm. The Sigma 70mm and Canon 135mm, that I also use on that camera, are AF lenses that don't have a hard stop when used in MF mode.

When I received the Sony A7II I initially used it exclusively with my only Sony E-mount lens, the Loxia 35/2, to make sure that the A7II is indeed the solution to an unusual flare problem that I experienced with the Loxia/A7 combination. This lens works normal on both Sonys, focusing on infinity with some extra travel possible before hitting the hard stop. I used most of my other lenses for the first time on the A7II last weekend and found this (all lenses focused at the infinity indicator or the hard stop):

On the 21/2.8 ZE infinite focus is impossible to achieve. At the hard stop the image is still out of focus from about 7 yards from the focal plane on. The area between the camera and 7 yards away is tack sharp. This lens has recently been serviced by Zeiss and should be the last one in my bag to have problems.

The 28/2 ZE shows similar results, with the oof area starting at about 8 yards and being not quite as blurred (my impression).

The 100/2 ZE is tack sharp at infinity and beyond ( I know, sounds funny; but you know what I mean) when turned to the hard stop, without extra travel left. Infinity focus is also possible with the EF 135/2.

Interesting is the EF 400/5.6. Focus on a point about a quarter mile away was not possible. It comes close, but continuing to turn the focus ring increases blur again. Staying at the closest-to-focus setting and exposing resulted in an image that is tack sharp behind the point focused on and completely oof up to and including that point.

From all this I conclude:

   
  • Both cameras have slightly different flange distances but are within tolerances, as the problem free use of the E-Mount Loxia shows.
  • The Metabones adapter is slightly too long. It barely stays within tolerances with the A7 but exceeds those with the A7II.
  • With increased focal length the problem shifts from focus before the intended point to focus behind it.

Of course, not being an expert I cannot be sure about my conclusions being right or what action to take to correct this. I have ordered a Photodiox adapter, not the least because it is guaranteed allow infinity focus with any lens. A possible hint at the problem with the Metabones being a known issue?
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E.J. Peiker

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Re: Lens/Adapter Combo Focus Problem
« Reply #1 on: February 15, 2016, 11:04:58 am »

What that means is that the lens is sitting slightly farther away from the sensor plane than it should.  This is probably a bad break on adding up of tolerances - the mount may be a bit on the long end on the camera of the in-spec tolerance window and the adapter may be on the high end of it's total thickness spec window as well and when you add them up they can no longer achieve infinity focus - basically like adding a really tiny extension tube.  Add to that warmer temperatures which cause some expansion and you can have a real problem.  The wider the lens, the more sensitive to this it would be as that difference becomes a bigger and bigger part of the total focal length.  My original a7R would no longer focus perfectly to infinity with the Zeiss 25mm f/2 (ZF2) lens after I put the stronger Fotodiox mount on the camera rather than the plastic backed one that Sony used on the original (the one much maligned for being a bit wobbly).  Of course stopping down a tiny bit will usually be enough to make up for it but there are scenarios (like astro photography) where this could be a real problem.

Now, how to remedy it...  The first thing I would do is to make sure that the all of the mounts - camera, both sides of the adapter, and the lens, are all screwed in very tightly and not loose in any way.  It's likely that if you can find just a half millimeter someplace you will be fine.  If that doesn't work, you would have to get a little more drastic and actually slightly grind down one of the mount surfaces - or you could get another adapter to see if that helps as it may be on the narrow end of spec.  If you have a store that sells them, you could try several until you find one that allows infinity focus.

« Last Edit: February 15, 2016, 11:08:22 am by E.J. Peiker »
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