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Field curvature? Or something else?


Jon Warwick

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I have a 50mm Summicron v5 (latest non APO). Imagine a mountain range, say 5km away. I've shot it at f5.6-f8.

 

The middle 1/3rd of the file is razor sharp.

The left- and right-hand 1/3rds are both (equally similarly) quite a bit softer than the middle.

This is visible even on 24x16" prints.

 

Same things occurs on scenes, say, 100 metres away etc.

 

This doesn't occur with my 35mm f2 ASPH.

 

Is this field curvature causing this with the 50mm .... or something else?

 

If the latter, any idea what it could be?

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That is interesting. I had the same lens and found that it was hard to focus on mid-range subjects, but close and distant was fine. I traded the lens in towards a Summilux and I could focus it fine.

 

Just yesterday, I learned that the Noctilux ASPH and 35mm FLE lenses have field curvature, but there was not mention of any other lenses. The f/1 Noctilux and AA Summicron were mentioned as being flat in field.

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Hello,

 

Film or digital ?

 

Maybe the filter (if used one) is too thick.

If digital, you may see quite easily when on site.

 

Arnaud

Hi, thanks for your reply.

I see it on both film and digital.

I do have a Leica UVa filter permanently attached on that lens.

Edited by Jon Warwick
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Take the filter off and try it again.

 

I had the same with M Apo 90 and Apo 3.4/180 and UV filters. especially when the upper part of the image is à bright sky.

Probably due to reflections between the UV filter and the front lens the image is considerably less sharp.

Forget UV filters.  

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I believe the Summicron 50 does have some focus shift when stopping down. I'd expect to see the behavior your describe -- at least, if by "visible" in 16x24" prints you mean "upon close inspection" -- for someone who is focusing with with live view in the center of the frame when stopped down. If that's the way you're using it, one method to see if this is the culprit would be to focus the same scene with the lens wide open before stopping down, then take another photo of the same scene that is focused while stopped down; when focused wide-open, the edges will likely be in better focus with mild to negligible deterioration on center starting around f/8. If you're focusing with a rangefinder, then these comments probably won't help much. :-)

 

Cheers,

Jon

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I agree with Jon. The cron has field curvature, and normally Leica adjusts the focusing calibration to counter balance the curvature. If one focuses with LV, the curvature becomes more apparent. Filters have nothing to do with this issue.

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All lenses have more or less field curvature besides exceptional ones like 50/2 apo but it is more visible at wide apertures than smaller ones like f/5.6 or f/8. At f/5.6 you might see some focus shift that does not exist on the 35/2 asph but if the effect is still visible at f/8 your lens might need some CLA. 

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I agree with Jon. The cron has field curvature, and normally Leica adjusts the focusing calibration to counter balance the curvature.

How does Leica adjust for field curvature? In the focusing helix?

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Hi Pico,

 

Leica knows darn well what the field curvature of each lens is.  They cut the helicoid to optimize the theoretical flat focus plane within the focus footprint of the curvature of field for the distance chosen and the particular lens.  This may place the center of the field forward or rear in the DOF in order to get the largest amount of the rest of the field within acceptable sharpness.  Also, field curvature can and does change with different focus distances and can be taken into account when designing the helicoid.

 

This is why Edward stated that the EVF will not do the same job as the RF because it has no direct link to the helicoid.  The RF is designed to work as an integral part of the system and as such is designed to maximize the image across the field at different distances.  The EVF can not be expected to do this and as such is often an inferior way to focus.

 

Also, the EVF DOF peeking becomes wider as the lens is stopped down.  Making it more difficult to place the focus: 1) in the center of the DOF, and 2) in the correct portion of the DOF to correct for the particular field curvature characteristic of a lens.

 

Rick

Edited by Rick
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Leica knows darn well what the field curvature of each lens is.  They cut the helicoid to optimize the theoretical flat focus plane within the focus footprint of the curvature of field for the distance chosen and the particular lens. [...]

 

I wondered about that. If I can put it together, I might use my dial indicator to measure the variation of the throw of various lens helix. Leica lenses perform a straight thrust against the RF roller, but whether it is linear is something I would have to measure myself, or have another authoritative source.

 

I once though that the surface of the focus ring turned and was formed to accommodate lens focus characteristics, at least for lenses that do a full focus range in one turn. Looking at, for example, version 1 of the 75mm Summilux it is clear it does not. Others? Dunno yet.

 

75mm-summilux-differences.jpg

Edited by pico
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In LV ,  focus 1/3 from center to corner.

 

I have never seen a Leica lens without field curvature .  Well a few exceptions,  black 65 Elmar and 100 APO.  It may be there, but very reduced.  It is one reason I bought the F2 35 ASPH instead of the FLE.

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In LV ,  focus 1/3 from center to corner.

 

I have never seen a Leica lens without field curvature .  Well a few exceptions,  black 65 Elmar and 100 APO.  It may be there, but very reduced.  It is one reason I bought the F2 35 ASPH instead of the FLE.

 

 

Yes.  The 35 FLE has wicked field curvature and to my eye busy background OOF rendition.  My 35 cron seems to be pretty flat at all apertures.  But, the color and contrast trump the 35 cron by a margin.  And, for many subjects FC is not important.  Folks have to make their own choices.

 

Rick

Edited by Rick
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I wondered about that. If I can put it together, I might use my dial indicator to measure the variation of the throw of various lens helix. Leica lenses perform a straight thrust against the RF roller, but whether it is linear is something I would have to measure myself, or have another authoritative source.

 

I once though that the surface of the focus ring turned and was formed to accommodate lens focus characteristics, at least for lenses that do a full focus range in one turn. Looking at, for example, version 1 of the 75mm Summilux it is clear it does not. Others? Dunno yet.

 

75mm-summilux-differences.jpg

This construction is seen on other longer lenses as well, the helicoid is there, but internal, transferred to the roller by a push bar.

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I wondered about that. If I can put it together, I might use my dial indicator to measure the variation of the throw of various lens helix. Leica lenses perform a straight thrust against the RF roller, but whether it is linear is something I would have to measure myself, or have another authoritative source.

 

I once though that the surface of the focus ring turned and was formed to accommodate lens focus characteristics, at least for lenses that do a full focus range in one turn. Looking at, for example, version 1 of the 75mm Summilux it is clear it does not. Others? Dunno yet.

 

<pic>

Highlight mine.

 

Whether it is linear or not, it has to be the *same* for every lens since the remaining part of the RF mechanism is in the camera (which is same for every lens). For simplicity I would assume that it is linear.

 

To deal with field curvature, Leica adjusts the focal plane (as measured by lens cam+RF mechanism) somewhere appropriately within DOF space and not in dead center (rather behind or in front). This way objects to the side also get covered (kind of) in the complex focus curvature space. It actually depends on the shape of the curvature. This is why LV focusing is inferior to RF focusing in lenses with strong curvature. In LV focusing we tend to maximize the center sharpness (meaning putting the object in dead center of DOF space) which is not ideal for the objects to the side.

 

Complicated? Tim Ashley had a nice write up about this using 35FLE. He explains it really well in his article compared to my four sentences above.

 

Tim's response in the prior discussion:

http://www.l-camera-forum.com/topic/202466-tim-ashley-35mm-fle-review/page-2#entry2298913 

 

His article:

http://tashley1.zenfolio.com/blog/2013/4/leica-m-240-with-35mm-f1-4-fle---some-observations

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[...] This is why LV focusing is inferior to RF focusing in lenses with strong curvature. In LV focusing we tend to maximize the center sharpness (meaning putting the object in dead center of DOF space) which is not ideal for the objects to the side. [...]

 

Current EVFs allow to move the focus patch anywhere into the frame hence are more accurate than any rangefinder thanks to focus magnification. Only problem is speed where the RF is unbeatable as far as manual focussing is concerned at least.

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Current EVFs allow to move the focus patch anywhere into the frame hence are more accurate than any rangefinder thanks to focus magnification. Only problem is speed where the RF is unbeatable as far as manual focussing is concerned at least.

You are right that movable focus patch will allow you to focus anywhere. But if you want to have good focus all over the plane then field curvature makes it tricky. Then one needs to sacrifice a bit of sharpness in the center to get sharpness in the corner (or vice versa). Tim's articles concludes that RF is superior in this aspect. Or if you do want to use EVF then you need to know where to focus. In 35FLE he recommends approaching focus from behind to first focus point (and not going for sharpest center) to have adequate sharpness in corners. The curvature shape is quite complex for FLE and it changes shape with different focus points and aperture.

Edited by jmahto
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I know only Mr Ashley as the discoverer of the "boricron" 28/2 sorry but in case of field curvature and focussing in the center of the frame the subject matter won't be sharp if it is not in the center itself unless DoF is wide enough to compensate the effect. Easy to check at fast aperture with everyone's body. In such circumstances the RF's accuracy is always inferior to a modern EVF with moving focus patch and focus magnification in my modest experience but i'm not a reviewer.

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