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FF Zoom to Medium Format Prime Conversion
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PostPosted: Wed Mar 29, 2023 3:25 pm    Post subject: FF Zoom to Medium Format Prime Conversion Reply with quote
No, i don't have a recipe but i know some of you study and understand lens designs thus asking:
would it be possible? Which elements to remove?


PostPosted: Wed Mar 29, 2023 5:28 pm    Post subject: Reply with quote
don't remove elements! leave your lenses alone Smile
someone tried to sell a nikon 35/.9 - but the rear element was sawn off. miraculously, the lens would now cover much more. but it's no longer an f/.9. and it's no longer worth anything, either.
have a look at which lenses cover more than FF already:
docs.google.com/spreadsheets/d/1uxvvpxJ9QVFFyh0pW2rs9KBmUW9vlh-d-VnbcLDCTn8
buy those, and leave them alone Smile


PostPosted: Wed Mar 29, 2023 5:52 pm    Post subject: Reply with quote
kathala wrote:
don't remove elements! leave your lenses alone Smile
someone tried to sell a nikon 35/.9 - but the rear element was sawn off. miraculously, the lens would now cover much more. but it's no longer an f/.9. and it's no longer worth anything, either.
have a look at which lenses cover more than FF already:
docs.google.com/spreadsheets/d/1uxvvpxJ9QVFFyh0pW2rs9KBmUW9vlh-d-VnbcLDCTn8
buy those, and leave them alone Smile

I share the love for unaltered primes. But cheap zooms? With 12 elements and always hazy in the middle? Come on, lets experiment Smile.


PostPosted: Wed Mar 29, 2023 7:01 pm    Post subject: Reply with quote
Well, the front focus objective of a telephoto zoom can be used as a decently powered (250mm FL and stronger) diopter, often in filter thread.

See (my) post https://www.dpreview.com/forums/thread/4605882#forum-post-65577935 which goes into details. But I've since learnt keeping the front focus group together gave better performance.

Nevertheless, as the thread shows performance of a doublet removed from the focus group on it's own was still usable (I used it like this for a long time) and offered lower power.


PostPosted: Wed Mar 29, 2023 7:06 pm    Post subject: Re: FF Zoom to Medium Format Prime Conversion Reply with quote
Pancolart wrote:
No, i don't have a recipe but i know some of you study and understand lens designs thus asking:
would it be possible? Which elements to remove?

I would be better to dismantle the lens entirely & construct a new one from the elements.
You're not just changing which elements that are present but also their relative spacing, and probably messing up the correction of aberrations even so.


PostPosted: Wed Mar 29, 2023 7:33 pm    Post subject: Reply with quote
Yes, just 'removing' elements until you get something else would end up in problems, unless under some circumstances with certain styles of (telephoto-type) zoom which have a fixed rear 'relay' group, which imply (right choice of word here) correction on their own. But in my experience, not always the case. I neglect to go into any further detail to give the wrong impression. But this is not inaccurate - just needing further elaboration.

Anyway, they aren't going to do medium format that way regardless.


PostPosted: Wed Mar 29, 2023 7:43 pm    Post subject: Reply with quote
eggplant wrote:
Well, the front focus objective of a telephoto zoom can be used as a decently powered (250mm FL and stronger) diopter, often in filter thread.

See (my) post https://www.dpreview.com/forums/thread/4605882#forum-post-65577935 which goes into details. But I've since learnt keeping the front focus group together gave better performance.

Nevertheless, as the thread shows performance of a doublet removed from the focus group on it's own was still usable (I used it like this for a long time) and offered lower power.


I have found that the whole front-focus group of telephoto zooms, can make for a very well corrected low-power hand-held magnifier (clean the helicoid first Wink ).

Incidentally, I assume you know that dpreview post may only be accessible for another then days or so... Sad


PostPosted: Wed Mar 29, 2023 7:46 pm    Post subject: Reply with quote
RokkorDoctor wrote:
eggplant wrote:
Well, the front focus objective of a telephoto zoom can be used as a decently powered (250mm FL and stronger) diopter, often in filter thread.

See (my) post https://www.dpreview.com/forums/thread/4605882#forum-post-65577935 which goes into details. But I've since learnt keeping the front focus group together gave better performance.

Nevertheless, as the thread shows performance of a doublet removed from the focus group on it's own was still usable (I used it like this for a long time) and offered lower power.


I have found that the whole front-focus group of telephoto zooms, can make for a very well corrected low-power hand-held magnifier (clean the helicoid first Wink ).

Incidentally, I assume you know that dpreview post may only be accessible for another then days or so... Sad


Yes... I need to get on and archive it.

And yes.... they make for excellent low power handheld magnifiers! I actually reach for those quite often first.


PostPosted: Sat Apr 01, 2023 2:07 pm    Post subject: Reply with quote
Dumb idea, to be honest, just forget it.

If it has to be explained to you why it is a dumb idea then you don't have anything approaching the level of understanding to take on such a project.


PostPosted: Sat Apr 01, 2023 3:47 pm    Post subject: Reply with quote
As Ian said, it is a silly idea.

But, if you really wanted to experiment just for fun (with almost certainly disappointing results), then as DConvert suggested you should remove all the lenses and build something new from those. Be prepared to be wasting all of your investment though.

I think your best bet would be to have two identical zoom lenses, and use the two sets of front elements / focus sections in a symmetrical setup, aperture in the middle. That would at least give you some hope of controlling distortion levels at least whilst also giving a large enough image circle.

The result would likely not be fast, useful or great though, and possibly complete rubbish.


PostPosted: Sat Apr 22, 2023 12:03 pm    Post subject: Reply with quote
eggplant wrote:
I'll make sure to let you know and compare it to my copy of a S1 70-210mm f3.5 - on speedbooster ultra.

I was aware of that image, but didn't look to the end version and noted 'apochromat'.

It is interesting the element count remains unchanged. I wonder if it's the same case as how the Canon EF 50-200mm f3.5-4.5 for the L version had one element of it's focus group become low dispersion.

L version


Non L version




Wouldn't rear system make an excellent prime lens?


PostPosted: Sat Apr 22, 2023 2:14 pm    Post subject: Reply with quote
Pancolart wrote:

Wouldn't rear system make an excellent prime lens?


Highly unlikely. It might work as an objective lens and form an image, but it will be very poorly corrected


PostPosted: Sat Apr 22, 2023 4:19 pm    Post subject: Reply with quote
I remembered I got some test shots of the front half of a Auto Flex 55mm f1.7.


#1


#2


#3


PostPosted: Sat Apr 22, 2023 6:16 pm    Post subject: Re: FF Zoom to Medium Format Prime Conversion Reply with quote
Pancolart wrote:
FF Zoom to Medium Format Prime Conversion

No, i don't have a recipe but i know some of you study and understand lens designs thus asking:
would it be possible? Which elements to remove?



When a lens is said to be "corrected," that means the lens designer tried to mutually cancel the aberrations of the individual elements so that the final resultant aberration is zero. In practice, however, some residual aberration always remains.

The figure below shows in a real example the final aberration (Sum) and the contributions of each of the 13 surfaces of a Double-Gauss 50mm F1.4 lens. The Seidel coefficients are presented for each of the 5 monochromatic aberrations: Spherical, Coma, Astigmatism, Petzval Sum (field curvature) and Distortion.



From the figure above, you can see how the aberrations introduced by each optical surface can be much larger than the overall lens aberrations. Thus, it is easy to understand that it is enough to remove a single optical element to destroy the delicate balance of aberrations. Often, simply changing the spacing between elements by just a few tenths of a millimeter can make the final aberration unacceptable.
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