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Jupiter-8 odd loss of focus in center of frame f/4 f/5.8 f/8
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PostPosted: Mon Jan 03, 2022 9:22 pm    Post subject: Jupiter-8 odd loss of focus in center of frame f/4 f/5.8 f/8 Reply with quote
I have tested my Jupiter-8 lens that i use daily and found and odd problem there is loss of focus in the center of the frame f/4 f/5.8 f/8 while corners gradually improve from f2 to f16.

Did this test three times with same results, anyone know what causes this?


Shot at 1.5meters distance.


PostPosted: Wed Jan 05, 2022 2:19 pm    Post subject: Reply with quote
What camera are you using this lens on, and how did you focus the image?

At F4, the focus shift due to spherical aberration will move the point of best focus back about 1" to 2" moving from F2 to F4.

If using a mirrorless camera, refocus the lens. If using an RF, move the focus manually.

The J-8 has a lot of field curvature.


PostPosted: Wed Jan 05, 2022 4:59 pm    Post subject: Reply with quote
I have noticed this effect with other lenses. One of my 35s has a peculiar out of focus 'ring'. At certain apertures its sharp in the centre, falls off then sharp again in the corners.


PostPosted: Thu Jan 06, 2022 10:37 am    Post subject: Re: Jupiter-8 odd loss of focus in center of frame f/4 f/5.8 Reply with quote
NiceDays wrote:
I have tested my Jupiter-8 lens that i use daily and found and odd problem there is loss of focus in the center of the frame f/4 f/5.8 f/8 while corners gradually improve from f2 to f16.

Did this test three times with same results, anyone know what causes this?


Shot at 1.5meters distance.


Probably has to do with lack of flatness of field.


PostPosted: Thu Jan 06, 2022 7:27 pm    Post subject: Reply with quote
kymarto wrote:
Probably has to do with lack of flatness of field.

Another term i learned, thanks.

fiftyonepointsix wrote:
What camera are you using this lens on, and how did you focus the image?

At F4, the focus shift due to spherical aberration will move the point of best focus back about 1" to 2" moving from F2 to F4.

If using a mirrorless camera, refocus the lens. If using an RF, move the focus manually.

The J-8 has a lot of field curvature.


I use Sony NEX-5T, focus on wide open aperture then i never touch focus ring anymore.


Indeed its a focus shift, thankfully just before i discovered this effect i learned that its best focus at aperture that will be used for final shot.

Crop of center of frame at minimal focus distance.


PostPosted: Fri Jan 07, 2022 12:52 am    Post subject: Reply with quote
https://www.35mmc.com/29/04/2020/zeiss-jena-5cm-sonnars-the-magic-of-the-prewar-uncoated-sonnar-by-brian-sweeney/

I've used Sonnar formula lenses for a long time, have "shimmed" many of them for use on Rangefinder cameras. Sonnar formula lenses, like the J-8, have more focus shift due to spherical aberration than most lenses.

I put a description of it here-

https://www.35mmc.com/29/04/2020/zeiss-jena-5cm-sonnars-the-magic-of-the-prewar-uncoated-sonnar-by-brian-sweeney/
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