Very unsharp corners, adapter issue? And q about infinity

shoenberg3 · Joined: 04 Aug 2010 Posts: 22

Hello,
My question is two fold.
I've been noticed with my two lenses (SMC Takumar 50 1.4 and S-M-C Takumar 28mm 3.5) that I adapt to my canon XTI, that taking pictures at infinity did not yield the best results. For both lenses, I have to pull back slightly to achieve their best. And holy crap, they are sharp when I do that, especially the 50!
I've always thought that with these older manual lenses, they did NOT have focusing beyond infinity and to achieve optimal infinity focus, you could safely it turn it all the way?

The second part of question is that I don't think my 28mm is achieving its best performance. At centers, it is quite sharp but the quality declines very sharply at the sides (sort of blocky rendition), esp. when it is pushed all the way to infinity. It sort of reminds of field of curvature problem, but I am not sure. I also feel that it should be still sharper at the center.

So from these two issues (the first one may not be an unusual issue, persay), I am wondering if I may have an adapter problem. I use a cheap pixco m42-eos adapter I bought from ebay.

Any help would be appreciated.

revers · Posted: Wed Aug 18, 2010 11:56 pm Post subject:

I find that my adapted lenses on my G1 have to be backed off from infinity.

gcogger · Joined: 23 May 2010 Posts: 18 Location: Southampton, UK

The adapter is probably slightly too thin. It's safer to make it too thin than too thick (as that would prevent infinity focus completely). I don't know if an expensive adpater would be better in this regard...

As to the edge/corner softness, this is unlikely to be the adapter. I'd guess it's due to the nature of digital sensors - they work best with light arriving perpendicular to the sensor rather than at an angle. Film was much more tolerant of this so a lens that worked well on film may be poorer on a digital body. Newer lenses tend to be made with this in mind and therefore work better with digital sensors.

woodrim · Joined: 14 Jan 2010 Posts: 4058 Location: Charleston

Your questions were actually statements, so I'll respond in agreement. I have infinity issues more times than not. I test it with every lens I get by taking a picture at infinity, then several others after backing off tiny amounts. That gives me some idea of where I need to be on each lens... if I could only remember. The 28mm I have no idea about.

The good news with the infinity issue is that if you dedicate an adapter to it, you might be able to correct the register distance by adding tape layer(s) between the adapter and lens.

shoenberg3 · Joined: 04 Aug 2010 Posts: 22

Thanks for the response so far.
To add, the 28mm is already at F8. The corner sharpness barely improves from F3.5 to F8. Also the softness is more evident on one side (left) than the other three (top right bottom). It just seems like a completely different beast than the typical weakness in lens' corners.

the_Suede · Joined: 30 Oct 2008 Posts: 67 Location: Sweden

It is quite unlikely to be an adapter problem, neither lens contains any floating elements.

Unit focusing lenses are in no way affected by adapters (unless the adapter is seriously tilted/shifted, ie really wedge-shaped or decentered. Both are highly unlikely...) IF the corner issue is adapter related, then you should quite easily see the tilt of the focus plane when you focus F/2.8 at say 10m distance.

The hard-stop infinity is quite useless anyway, unless you're shooting starscapes - in ANY other situation than this you get a sharper horizon by focusing at something about a mile away - which is far from infinity, both in an optical and real sense of the word... Smile

With 85/1.4 or 135/2.0 you can easily see a difference between "1mile focus" and "infinity focus", but with a 28mm F/2.8 lens the difference is bordering on impossible to even measure accurately. And by focusing on infinity you loose at least 2/3 of your DoF in landscaping.

So the unsharp corner is far more likely to be a decentering defect in the lens, something more common than uncommon with WA lenses.

shoenberg3 · Joined: 04 Aug 2010 Posts: 22

Thanks for the answers.
I suppose I will have to live with the infinity focus "issue" but the second problem is still a concern.

I am trying to find the "sweet" infinity spot for each of these lenses. If I were to upgrade to a 5D with the same lenses and adapter, would this "spot" remain the same or be different?
I ask this, because I will be upgrading and soon after travelling to a photo trip.