Comparative focus test of 2 versions of C Sonnars

[mfogiel]

As promised, I am back to show you my findings about the 2 different versions of C Sonnar 50/1.5 - the first batch "optimized for focus at f2.8" and the second batch, "optimized or corrected (as in this case) for focus at f 1.5".

Since this is a user test and far from being scientific in precision, I'd like to say right away, that these results can be prone to some absolute error - what I was trying to understand foremost was the RELATIVE behaviour of these lenses across apertures.

My test was done on a tripod, with my M7 0.85x placed at slightly beyond 1 meter (measured with a ruler) from the "focus here" line, and at a fairly accurate angle of 45°. I wear glasses, and this is how I normally focus my rf cameras, so I kept them on for this test as well.

In order to have a reference point, I took one shot with the Planar 50/2 at f2.0, in order to see if my eye was calibrated decently, and I think the result is acceptable as a "neutral" starting point.

The shots are scanned without any post processing, with the exception of a contrast boost in PS - equal for all the scans, in order to improve the reading of the numbers.

Let's first see the results from the Planar shot, and the f1.5 and f2.0 shots. In the second post I will add the rest and discuss the results.

 

[mfogiel]

Here I include the shots at f2.8, f 4.0 and f5,6 - I did not include the test at f8,0, because I detected no further focus shift, and the dof took pretty much care of all focusing issues.

 

[mfogiel]

I've made little brackets 4cm wide, to mark the best focus zone, and put an arrow where to my eyes, the image looked sharpest.
First of all, the reference Planar shot is really quite impressive, because it really is spot on the target, i.e. the black line with the "focus here" text.

The f1.5 shots show you right away what's happening - the "new" C Sonnar looks like it's missing the precise point by 1cm, but the dof covers the best focus quite well, while the "old" version is plainly off, front focusing by about 5 cm, which is not so easy to manage in real life, making this version little usable wide open.

The f2.0 shots display a similar story, and here indeed one can see the phenomenon described by Roger Hicks and Dr Nasse - in the "new" version, the usable dof sits right behind the point of best focus. Since f2.0 is probably the most interesting aperture for low dof portraits, because it gains quite a bit of contrast with respect to wide open, yet the bokeh remains exceptional, obviously this version should perform well for the "dreamy and creamy" assignements. The "old" version is a bit like a fish out of the water here.

At f2.8 we see a capsized situation: the "new" version is disappearing behind the horizon, while the "old" starts pulling out the bite, and is entering the best overall performance zone. I suspect this lens is at it's best around f3.5, and this is where Zeiss has actually calibrated it in the first place.

At f4.0 the "new" version has become almost unuseable, while the "old" version focuses like the "new" one wide open, and, believe me, at this aperture delivers the goods in large amounts...

Finally, at f5,6 the relative best focus does not move by a detectable amount, but the dof increases, making the "new" version just useable - by f8,0 the focus stays the same and both lenses cover easily the "focus here" sign.

It is difficult to draw a definitive conclusion just on the basis of some ruler shots, and in fact I'd like to make some real portraits with both lenses, to see how it works out in practice.
However, if you are attracted to the C Sonnar for its absolute bokeh wide open, or for using it exclusively in low light and between f1.5 and f2.0, then the "new" version is for you, but you better keep another 50mm lens handy for more general shooting. On the other hand, if you like the soft but contrasty drawing with good detail, which this lens delivers between f2.8 and f4.0, plus you do not disdain great sharpnesss beyond f5.6, then the "old" version should be for you, and you might want to keep one of the older f1.4 or f1.5 lenses for the wide open soft images.

 

[mfogiel]

Just to leave you with something from real life, the attached photo has been made with the "new" version at f2.0 from a distance of about 1.3m, and I focused on the eyes of the girl to the left.

 

[thomasw_]

I enjoyed examining your findings mfogiel. thanks for posting this, as it makes it clear that the version optimised for f1,5 would be the one for me.

 

[willie_901]

This is a very useful test. Thanks for taking the time to share the results.

However, I must say that I never use my rangerfinder camera at a subject distance of 1 m. After living with a 2.8 optimized version of the C Sonnar for a year, I have not experienced focus issues in my work. Whether it's the C-Sonnar, or my AI'd Nikkor 50/1.4 on a SLR, (or the Cannon 50/1.2 LTM when I owned it) I only use f-stops wider than 2.0 - 2.8 for low light scenes where I am at least 4 (and usually more) meters from my subject(s). The increased DOF at this distance protects me from RF calibration errors, poor eyesight (in the case of SLR/DSLR), focus shifts and unconsciously moving the camera after obtaining focus.

I do enjoy photographing subjects in extreme low-light conditions compatible with a wide-angle view (zooming with my feet) using fast 50 mm lenses. I must say that I have never missed focus with the C-Sonnar due to the shift. When I miss focus, it's due to a lack of skill or just plain sloppy work.

When I do use a 50mm lens for close-up focusing, I use the inexpensive, plastic, but sharp, Nikkor 50/1.8 AF on my D200. This lens auto focuses well on the D200 and camera's noise and weight are irrelevant to the close up photos I like to take.

My ZI-M is with me wherever I go... it's my favorite camera, but when my subject will be closer than 3 M, I use a SLR or DSLR.

willie

 

[jsuominen]

Thank you Marek for your excellent test! It shows clearly that Zeiss should keep both version of C-Sonnar 50/1.5 lenses on their sales list. It would be difficult to decide, which version I would personally prefer more, if I'll buy that lens someday...

By the way, I did similar comparison - but not as clear as Marek's test - few monts ago with vintage Opton Sonnar 50/1.5 and Nikkor-S 50/1.4 lenses from 1950's. It's here if you are interested http://www.rangefinderforum.com/forums/showpost.php?p=651295&postcount=14

 

[nzeeman]

i think this shows that sonnar is better with old 2.8 adjustment
since it dont misses that much on 1.5.

 

[FanMan]

Thanks a lot for your comparison and especially for your discussion of the results!

 

[mfogiel]

Having thought a little bit about the results of this test myself, I've actually decided to keep both lenses... However, if anyone from Zeiss will stumble on this thread, I'd like to suggest, to make a new version of the C Sonnar - with an adjustable switch between the F1.5-f2.0 range and the f 2,8-f16 range. I think this is perfectly doable with little extra engineering. This way, you could switch the best focus position as a function of your aperture, just like you adjust a FLE in one of the Hasselblad Distagons as a function of distance. This lens is a really unique tool, and I think it deserves to be used in the optimal way. An alternative, would be to make a digital rf with an electronic live view finder, where you could establish your best focus with an electronic confirmation tool ;-).

 

[john_s]

......... However, if anyone from Zeiss will stumble on this thread, I'd like to suggest, to make a new version of the C Sonnar - with an adjustable switch between the F1.5-f2.0 range and the f 2,8-f16 range. I think this is perfectly doable with little extra engineering. This way, you could switch the best focus position as a function of your aperture, just like you adjust a FLE in one of the Hasselblad Distagons as a function of distance......

Good idea. Or even better, have the focus ring and aperture ring linked so that the focus will be correct without the user having to switch. It would be easier than having floating elements such as are in the Summilux 50mm ASPH because the whole optical set would move as a single unit.

 

[40oz]

seems like only a fool would want the lens adjusted for 1.5. And pretty much anyone could figure out that at 1.5 or 2, adjust focus a bit for close distances. It's not an aperture/distance combo that most of us use a 50mm for, so it's not like it's a hassle.

I would think a switch to change focus depending on aperture would be unbelievably clunky compared to exercising a slight bit of care when focus is already tricky.

I sure as hell hope I don't end up buying a 1.5-optimized lens used.

 

[ferider]

Fully agree with you, 40oz. Dealing with the shift is particularly easy
with M2 or M3 (DOF marks), but then I heard somewhere those cameras are obsolete 🙂

Also, what do you do at f2.8 and infinity ? 🙂 @ f1.5 and infinity, the Sonnar behaves like
a lens-baby 🙂 well, almost.

 

[Doug]

I like John's elegant idea! But in the meantime I think I can do well with my f2.8 optimized C-Sonnar as-is. So far I have been keeping in mind the focus shift issue and just leaning forward a couple inches after focusing close-in at f2 and wider. But this is not a common situation for me, so having to compensate for error here is acceptable.

The above test is very useful in clarifying the issue, thanks! At greater focusing distances, I expect the error will stay proportional to the distance. It seems to be about 5% short at f1.5... So one solution is to focus and then turn the focus ring to a distance 5% beyond. It appears this can be done conveniently by resetting the focused distance to the left-side f4 depth-of-field mark.

In my use of the lens, I'd rather deal with this special-case in this way than be way off at f2.8 and up, which is mostly "where I live". Lovely lens. 🙂

 

[SDK]

mfogiel, I have a question. Were your cm marks scaled for the 45˚ angle of the board (so that 1cm = 1cm closer to the camera) or were they standard cm along the plane of the target board? If they are standard cm, then the errors would be about 0.7 X what is apparent because of the board angle. This would make the focus shift for the old version about 35mm at 1m focus and f/1.5, or 1.4" error.

It seems like Doug's idea about refocusing the old version wide open is good, but one might need to go less far than the left f/4 DOF mark, probably to where you might imagine the f/2.8 mark would be, about 70% of the way between the index mark and the left 4. I have the old version and will have to try that out.

 

[Doug]

Good point, SDK!

 

[peterleyenaar]

Received my Zeiss 1.5 50mm today

Received my Zeiss 1.5 50mm today

Happy Holidays Folks,
I received my 1.5 50mm Sonnar today , it appears that mine is calibrated at f2.8,
the lens front focusses at f1.5.
The lens portrays considerable ca at f1.5 focussed at a distance of 20 feet, is this normal and nothing to worry about ?, I guess at some distance one would not use the lens wide open.
I'll try to figure out how to upload images and show my (primitive ) results.
O.K. the images are uploaded, first image at f1.5 focussed on the center of the bubble, second image f2.8 focussed on the center of the bubble, third image at f1.5 aproximately 20 feet.
I am unsure what to make of this , am I better off with the 50mm Planar ?
Best Regards
Peter

 

[mfogiel]

SDK
The cm marks around the best focus line are symmetrical, however I don't think it makes a big difference at 1m distance - if we assume conventionally that 4cm depth is the best focus zone, the "best focus sign" should probably be 1.5cm before and 2.5 cm after that point. Anyway, the comparison was intended to show what happens on a relative basis - as far as practical application is concerned, I've found that front focusing at close distances can actually be of help if you make a portrait and you recompose the shot after setting the sharpness on the eye - I'v made many photos where the sharpness began at the ear level and behind... I am just waiting for a patient model to go through this "live" test... My opinion is, that in 80% of cases this lens works better in the "f2.8" version, however, the wide open images are a particular treat, so it is difficult to decide - as I said, I believe Zeiss could engineer this lens a bit better( and sell it for a higher price quite easily too...)

 

[Mazurka]

Having thought a little bit about the results of this test myself, I've actually decided to keep both lenses... However, if anyone from Zeiss will stumble on this thread, I'd like to suggest, to make a new version of the C Sonnar - with an adjustable switch between the F1.5-f2.0 range and the f 2,8-f16 range. I think this is perfectly doable with little extra engineering. This way, you could switch the best focus position as a function of your aperture, just like you adjust a FLE in one of the Hasselblad Distagons as a function of distance.

Then it wouldn't be a Sonnar design anymore -- I don't believe the classic construction can incorporate a floating element.

 

[larmarv916]

My complements to "Mfogiel" for his diligent efforts on creating the illustrations on the differences between the two Sonnar C's of the Zeiss ZM 50mm lenses. Very nice and effective illustrations and also helpful to see the "thumbprints" in a phyiscal and visual reference. Also I like the photos of the 2 girls as another more complex illustration of the Sonnars behavior.

Here is an example of F2.8 that is a compaion to a similar shot that was taken at F1.5 ( the F1.5 shot is posted in the thread for the Review of that lens) The Shot here is at F2.8 and still shows the Sonnars better ability at definition of 3D.

I took my son with me for the first time to Milano in March 07. We walked and shot photos with both my 50 and the 35 ZM lenses. To say he enjoyed Milano, was an understatement. He has waited to visit the Armory musuem at the Fortress was worth every dollar.

This was taken with the Zeiss 25mm...at F4 on Neopan 400