I have a couple of older Olympus OM lenses and want to use them on my A7R2.  The 21mm f3.5 works fine and is a remarkabaly sharp quality lens.  The adapter marked OM-NEX is a low priced one acquired on eBay for about $11 (USD).  The problem I encountered is that when I use the adapter with the 300mm f4.5 Olympus OM lens, I cannot focus at infinity.  IMHO, the adapter is a bit too long for use with a 300mm lens and thus the lens set at the infinity mark, is still out of focus on a very distant object. Has anyone had this issue with long lenses?  Is there modestly priced adapter that is at precisely correct length and permits focus at infinity?

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You observations present the opposite of the facts 

of optical life. Wide angle lenses are extremely 

vulnerable to tiny differences in physical distance 

from the image receptor.  

   

Without doubting your observations, there are a 

few rational conclusions. 

  

A. Your adapter is too long, much too long, cuz it 

takes a serious does of over-length to effect such 

lenses as a 300mm .... BUT if indeed the adapter

is much too long your wide lens focuses waaaay 

past infinity, compensating that over length. This 

rationale follows the physical rules, but OTOH is  

a highly unlikely scenario in the real world. 

   

B. Your 300 is defective. Also unlikely.  

   

C. The DoF of your wide angle may be hiding a 

lack of real infinity focus. But I'd be surprised if 

it can hide the amount amount of overlength-of-

adapter that can noticably wreck infinity focus of 

a 300mm lens.  

    

Cheap adapters are intentionally blueprinted to 

anticipate slightly sloppy machining. IOW they 

are blueprinted to be decidedly SHORT of ideal 

length, so that lenses on them will always focus 

[to varying degrees] well beyond infinity position

which is a very usable result for $10 to $35, that

results in no user complaints at such low pricing.   

    

While NOT doubting your observations for sake

of "conversation" [above], given the real laws of

physics, a solution most likely does require that 

we doubt some detail of your observations :-(   

  

It's only natural to suspect the adapter cuz it's 

so cheap, but your problem, as described, just

makes no sense. 

I also own a few of these great old Zuiko/Olympus lenses and use them with the Novoflex adapter - you can find them here http://novoflex.de/de/produkte/adapter/adapterfinder.html

These adapters aren´t cheap but precisely manufactured compared to the ones from China.

I´ve never had any problems. I have a few adapters from Novoflex also for Minolta and Leica lenses.