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CZJ Biometar 2.8/120 on K-10D
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PostPosted: Sat Sep 13, 2014 5:33 am    Post subject: CZJ Biometar 2.8/120 on K-10D Reply with quote
Dull overcast day for the most, then a short period of sunshine.
Some at f2.8, some not.
OH














Last edited by Oldhand on Sun Sep 14, 2014 9:10 am; edited 1 time in total


PostPosted: Sat Sep 13, 2014 7:17 am    Post subject: Reply with quote
I am jealous,very nice images (was the lens found locally? ).I do like the details in the 2nd and 3rd images.Can't say I like the bokeh in #6 but its still a great image


PostPosted: Sun Sep 14, 2014 6:06 am    Post subject: Reply with quote
mo wrote:
I am jealous,very nice images (was the lens found locally? ).I do like the details in the 2nd and 3rd images.Can't say I like the bokeh in #6 but its still a great image


Thanks Mo
Yes the bokeh there is a little nervous - too many sticks too close to the subject doesn't help either.
I have had this lens for quite a while - just haven't posted any images with it.
Not local I'm afraid - Germany I think because the postage was reasonable at the time and Germany has better postage rates than some other parts of Europe.
I am still playing with this lens.
I think it has more to offer than I am getting from it ATM.
We'll see
OH


PostPosted: Sun Sep 14, 2014 8:21 am    Post subject: Reply with quote
It does take a little while to get used to a lens,I think you are doing ok so far Very Happy What styling is your lens?


PostPosted: Sun Sep 14, 2014 9:03 am    Post subject: Reply with quote
mo wrote:
It does take a little while to get used to a lens,I think you are doing ok so far Very Happy What styling is your lens?


It's the zebra version Mo.
Here is a piccie
OH



PostPosted: Sun Sep 14, 2014 12:17 pm    Post subject: Reply with quote
Wow, Oldhand. I am impressed by this set. Beautiful shots!


PostPosted: Sun Sep 14, 2014 1:19 pm    Post subject: Reply with quote
I have a copy of the CZJ 120mm F2.8 Biometar. Its design is a 5-element Double-Gauss. The lateral chromatic aberration is practically nil, but there are some longitudinal chromatic and spherical aberrations, which produce a slight smoothing effect when the lens is wide open. Nonetheless, this effect can be desirable for portrait, which was the main application of this lens in the Pentacon Six system. From F5.6 onwards, the lens is sharp as a razor.

A little known feature of Biometar 120mm F2.8 is that there is significant focus-shift from F2.8 to F5.6. Always focus at working aperture in applications that require maximum sharpness.

Due to the Double-Gauss design, the Biometar can be used as an excellent macro lens with the aid of extension rings or bellows. In contrast, the performance of the usual lens 135mm F2.8 with type design and Ernostar Sonnar degrades rapidly for focus distances less than 1 m.

The main advantage of Ernostar design compared to the Double-Gauss is the smaller size. Therefore, for normal photographic use, 120mm F2.8 Biometar does not take much advantage in relation to a good lens 135mm F2.8, except for a lesser degree of lateral chromatic aberration and coma, which results in a best image quality at the edges.


PostPosted: Sun Sep 14, 2014 10:45 pm    Post subject: Reply with quote
Gerald wrote:
I have a copy of the CZJ 120mm F2.8 Biometar. Its design is a 5-element Double-Gauss. The lateral chromatic aberration is practically nil, but there are some longitudinal chromatic and spherical aberrations, which produce a slight smoothing effect when the lens is wide open. Nonetheless, this effect can be desirable for portrait, which was the main application of this lens in the Pentacon Six system. From F5.6 onwards, the lens is sharp as a razor.

A little known feature of Biometar 120mm F2.8 is that there is significant focus-shift from F2.8 to F5.6. Always focus at working aperture in applications that require maximum sharpness.

Due to the Double-Gauss design, the Biometar can be used as an excellent macro lens with the aid of extension rings or bellows. In contrast, the performance of the usual lens 135mm F2.8 with type design and Ernostar Sonnar degrades rapidly for focus distances less than 1 m.

The main advantage of Ernostar design compared to the Double-Gauss is the smaller size. Therefore, for normal photographic use, 120mm F2.8 Biometar does not take much advantage in relation to a good lens 135mm F2.8, except for a lesser degree of lateral chromatic aberration and coma, which results in a best image quality at the edges.


Thank you Gerald for this information.
Your observation on focus shift I will keep in mind, but I am having difficulty seeing it through the viewfinder.

As a comparison, I shot some side by side images with the Biometar 2.8/120 and Mamiya 2.8/110
First at 2.8 then at f8.
Here are the results:
OH

Biometar @ 2.8 and then f8





Mamiya @ 2.8 and then f8




PostPosted: Mon Sep 15, 2014 1:09 am    Post subject: Reply with quote
Oldhand wrote:


Your observation on focus shift I will keep in mind, but I am having difficulty seeing it through the viewfinder.

Hi Oldhand,

Focus shift is a little hard to see when shooting a three-dimensional subject like a field of flowers because the act of closing the aperture simply changes the position of the plane of focus, but something will always be in focus. In contrast, when shooting a distant landscape or a flat object, the focus shift has the potential to reduce visibly the sharpness of the image.

I will try to show the focus shift with an example. I took three photos:

1) Aperture wide open, focus adjusted precisely with 11.7X live view magnification.
2) Aperture F8, without refocusing (same focus adjustment of photo no. 1)
3) Aperture F8, refocused to the working aperture.

The crop area is marked by a yellow rectangle in the full picture.

Full image:



100% crop - F2.8:



100% crop - F8 without refocusing, same focus as for F2.8:



100% crop - F8 after refocusing:



Some additional comments:
1) The resolution in the center of the field is already pretty good for wide open aperture.
2) The resolution drops significantly when closing the aperture to F8 without refocusing.
3) After refocusing, the resolution at F8 increases dramatically and, as expected, is much better than for wide open.
4) A rough estimate is that the focus shift reduces the resolution to half it could be.
4) It was not shown, but if the aperture is closed to F16 or F22, the depth of field is sufficient to mask the effect of the focus shift.


PostPosted: Mon Sep 15, 2014 1:42 am    Post subject: Reply with quote
Thanks Gerald.
The difference is quite pronounced isn't it.
Much appreciated
OH


PostPosted: Mon Sep 15, 2014 6:40 pm    Post subject: Reply with quote
Yes, the focus shift effect is pronounced when viewed at 100%. However, to be fair, the degradation in sharpness is not very noticeable in pictures viewed at normal distances. Besides, I don't know if it is easily visible in the optical viewfinder of a DSLR camera.

Maybe the pictures below give an different idea of the magnitude of the Biometar's focus shift. The pictures show the Biometar distance scale when the camera was on a tripod in a fixed position, and the lens was adjusted to best possible focus for F2.8 and F8, respectivelly.

Best focus for F2.8:



Best focus for F8:


Note that the variation in focus distance is about 5cm for a distance subject-to-camera equal to 2.5 meters. Focus errors of this magnitude are common for handheld operation, especially when the subject is not static. In fact, the focus accuracy of most phase-detection autofocus systems is not much better.


PostPosted: Mon Sep 15, 2014 10:20 pm    Post subject: Reply with quote
Gerald wrote:
Yes, the focus shift effect is pronounced when viewed at 100%. However, to be fair, the degradation in sharpness is not very noticeable in pictures viewed at normal distances. Besides, I don't know if it is easily visible in the optical viewfinder of a DSLR camera.

Maybe the pictures below give an different idea of the magnitude of the Biometar's focus shift. The pictures show the Biometar distance scale when the camera was on a tripod in a fixed position, and the lens was adjusted to best possible focus for F2.8 and F8, respectivelly.

Best focus for F2.8:



Best focus for F8:



Note that the variation in focus distance is about 5cm for a distance subject-to-camera equal to 2.5 meters. Focus errors of this magnitude are common for handheld operation, especially when the subject is not static. In fact, the focus accuracy of most phase-detection autofocus systems is not much better.


Thank you Gerald.
This is an excellent illustration of the point that you are making about the Biometar.
Much appreciated
OH
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