28 Cron Asph v2 Peculiarity

[jplomley]

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I'm really hoping to get some feedback on the attributes of this lens. Last year I purchased an SL and upgraded my 28 Cron Asph v1 to the v2 after my dealer let me do a comparison in which it was obvious there was significant improvement in corner performance. Plus the ergonomics were much improved with the design of the new lens hood. Fast forward a year and I've just now added an M10 to the arsenal after trading in an M9 and MM1. The M10 is absolutely brilliant and focusses my 35 Lux FLE, 50 Lux Asph and 90 APO perfectly (minus a bit of focus shift on the 35 Lux at f/2.8-4.0). With the 21 SEM, the DOF partitioning is perfect. Now the problem. With the 28 Cron Asph v2, at close distances and wide open, all the DOF is in front of the focus plane. At mid- to far-distances, all the DOF is behind the focus plane, regardless of the aperture. Very bizarre, and I don't see this with any of my other 4 lenses. As a result, I'm hesitant to send in the M10, because it seems exceptionally well calibrated. It just seems this particular sample of 28 Cron when coupled to the RF in this M10 does not play well. So it seems to me the lens is in need of an adjustment. Am I on the right track? Any other users of this lens have a similar issue. Note this is a late 2017 production run on the 28 Cron, which I must say has been a brilliant lens. No complaints on optical performance, just getting the DOF sorted out.....

 

Cheers in advance for your feedback,

 

Jeff

Edited February 18, 2018 by jplomley

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[Mark II]

My 28mm Summicron performs very similarly, except that focus is spot-on at near distances (< 20m) while at infinity, there is obvious field curvature and some back focus that is visible when shot near to wide open. Mostly I use the lens on film at present, where the effect is negligible.

 

This is actually the second copy of the lens that I have. The first was substantially worse, such that at focal distances of ~50m I would find the centre hugely back focused while the corners were sharp at a distance of less than 10m. This was bad enough the back focus was not covered by DOF until around f11 - which is absurd for any 28mm lens, let alone at this price level. If your lens is behaving like this, I would exchange it.

 

It would be interesting to understand what the cause of this is. I strongly suspect that the lens design has severe field curvature, and the focus is calibrated to put the frame average in sharp definition rather than at the image centre. While my first lens was unacceptably poor (and exchanged), the second shows similar behaviour - albeit less exaggerated.

[lct]

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Edited February 19, 2018 by lct

[Exodies]

Field curvature is one of the most mysterious properties of a lens. Who has ever had a scene which was equidistant from the sensor? The ideal situation would be to draw the required field with a finger on the view screen.

[Steve McGarrett]

Exodies, on 19 Feb 2018 - 14:42, said:

Field curvature is one of the most mysterious properties of a lens. Who has ever had a scene which was equidistant from the sensor? The ideal situation would be to draw the required field with a finger on the view screen.

 

And then punch the lens to the desired side to bend the mount, so the focal plane tilts accordingly?

[Mark II]

Field curvature is one of the most mysterious properties of a lens. Who has ever had a scene which was equidistant from the sensor? The ideal situation would be to draw the required field with a finger on the view screen.

 

 

There is nothing mysterious about field curvature - almost all lenses have it to some extent. But I have never seen anything as unusable as the first 28mm Summicron copy that I had - the infinity stop on the lens seemingly focussed substantially "beyond infinity" (ie it required a substantial deliberate mis-focus in the rangefinder to pull distant central subjects in to sharp definition even at f8 - even though the edges were tack sharp only a few meters away).

 

A lens performing like that is not fit for purpose - particularly a 28mm where DOF should be immense when stopped down.

[01af]

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I purchased my new Summicron-M 28 mm Asph one year ago, and it had some small yet significant front focus right out of the box. So I sent it in to Leica Customer Care in Wetzlar for calibration. Now the focus is spot-on. Field curvature is no issue.

[Exodies]

Yes, it’s an inevitable part of lenses but what I find mysterious is commenting on it.

The scenes I want to photograph have a shape in three dimensions

The lens I want to use has a field of best focus in three dimensions

These two fields are rarely flat are equally rarely ever congruent

But people say of lens X that it has field curvature as if they ever wanted to photograph a scene parallel to the sensor- that never happens.

This is the mystery.

[lct]

What may feel mysterious about field curvature is its wavy form out of some lenses including seemingly Summicron 28/2 v2 and v1.