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Some samples with the Experimental Optics 50mm f/0.75
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PostPosted: Sat Jun 04, 2022 7:51 am    Post subject: Some samples with the Experimental Optics 50mm f/0.75 Reply with quote
I thought some of you might be interested in how the Experimental Optics 50mm f/0.75 renders, since it is quite a rare lens and afaik still the record-holder for the fastest-ever full frame lens (might even cover GFX).
It certainly is a bit of a special purpose lens, but I very much enjoy taking it out for the right shots.
The fence detail is a 100% crop out of a 42 MPix full frame image at fully open aperture.

Enjoy your weekend!

#1
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO50_11_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO50_11_1.jpg[/img][/url]

#2
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO50_13_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO50_13_1.jpg[/img][/url]

#3
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO502_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO502_1.jpg[/img][/url]

#4
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO50_15_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO50_15_1.jpg[/img][/url]

#5
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO503_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO503_1.jpg[/img][/url]

#6
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO504_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO504_1.jpg[/img][/url]

#7
[url=http://forum.mflenses.com/userpix/20226/big_12143_EO505_1.jpg][img]http://forum.mflenses.com/userpix/20226/12143_EO505_1.jpg[/img][/url]


PostPosted: Sat Jun 04, 2022 8:07 am    Post subject: Re: Some samples with the Experimental Optics 50mm f/0.75 Reply with quote
kathala wrote:
I thought some of you might be interested in how the Experimental Optics 50mm f/0.75 renders, since it is quite a rare lens and afaik still the record-holder for the fastest-ever full frame lens (might even cover GFX).
It certainly is a bit of a special purpose lens, but I very much enjoy taking it out for the right shots.
The fence detail is a 100% crop out of a 42 MPix full frame image at fully open aperture.

Enjoy your weekend!

#1


#2


#3


#4


#5


#6


#7


Very doubtful this covers full frame- the body looks like a rebrand/reissue of a TV/XR-Heligon. In extreme macro, things can be made to look like they cover a larger format.

At any rate, nice pictures!


PostPosted: Sat Jun 04, 2022 9:54 am    Post subject: Reply with quote
Did some research. It is actually an adapted Oude Delft E50 Rayxar X-ray lens. Would be too see interesting how it performs on longer distances and on the streets at night.


PostPosted: Sat Jun 04, 2022 10:07 am    Post subject: Reply with quote
Yep thought so, just couldn't remember which it was rebranding.


PostPosted: Sat Jun 04, 2022 10:08 am    Post subject: Reply with quote
Interesting, there used to be a steady supply of x-ray lenses, mostly rodenstocks and De Oude Delfts on ebay, but not seen one for a few years, they seem to have dried up for some reason. They had immense glow so this one does look to have been significantly altered.


PostPosted: Sat Jun 04, 2022 10:25 am    Post subject: Reply with quote
That glow came from unit-focusing a superfast lens designed for infinity, to 'extreme' macro closeups - a byproduct of using a lens with an unachievably short flange distance on any modern digital camera really.

But for a while people were using these on DSLRs, which is even more extreme than mirrorless.

As I'm sure people are aware if you look historically, superfast photographic lenses that were unit-focused didn't have as great close focus ability - can't do two things at once etc.
See any phillipreeve review of close up samples for that 'glow' : https://phillipreeve.net/blog/review-syoptic-50mm-1-1/#close_M_070_m_1117_42mp_A7rII

So would be even bigger issue here.


PostPosted: Sat Jun 04, 2022 10:28 am    Post subject: Reply with quote
Some info here. The guy that adapted it took of the rear group and used a SMC Takumar 55/1.8 rear group instead (to increase the rear to sensor focus distance since that is les than a mm.)

https://u42.co/Lens-Design/Rayxar%20e50%20f0.75%20De%20Oude%20Delft/Rayxar-e50-f0.75-De-Oude-Delft.html


PostPosted: Sat Jun 04, 2022 1:01 pm    Post subject: Reply with quote
D1N0 wrote:
Some info here. The guy that adapted it took of the rear group and used a SMC Takumar 55/1.8 rear group instead (to increase the rear to sensor focus distance since that is les than a mm.)

https://u42.co/Lens-Design/Rayxar%20e50%20f0.75%20De%20Oude%20Delft/Rayxar-e50-f0.75-De-Oude-Delft.html


That‘s very interesting - thanks for the link! The CAs are so pronounced that they give some images a really unique (and quite interesting) look. I‘m not sure it works for many subjects but in some of the samples I like it.

Still gonna avoid those superfast industrial lenses for now - they‘re certainly way above my paygrade when it comes to modifying and adapting in an effective way.


PostPosted: Sat Jun 04, 2022 2:15 pm    Post subject: Reply with quote
D1N0 wrote:
Some info here. The guy that adapted it took of the rear group and used a SMC Takumar 55/1.8 rear group instead (to increase the rear to sensor focus distance since that is les than a mm.)

https://u42.co/Lens-Design/Rayxar%20e50%20f0.75%20De%20Oude%20Delft/Rayxar-e50-f0.75-De-Oude-Delft.html


Yes, you can read about swapping the rear block here https://hispan.hu/rayxar-50/

Helios 58mm f2 and Takumar 55mm f1.8 rear block tried (I am not sure why rear blocks from larger aperture lenses weren't tried).

Those are Double-Gauss lenses and the Rayxar is a heavily asymmetrical triplet. Fine you say, doesn't mean it can't work- insert 'it sort of looks like a Double-Gauss when the rear block is swapped' here...


Welllllll......

As you might expect, the system is a delicate, deliberate balance. What role does the last group play, particularly with it's strongly curved surface which features in basically all variations of this design?
Given that overlap I am going to look at the Wray f0.71. It was not the first of this design (see https://postimg.cc/WqPrkxSQ) but it has been well discussed.

The lens can be understood in two sections. Starting with the front:

Quote:
Another unusual form of wide-aperture lens (f/0.71),covering a flat field of about 20°, has been described by Wynne (1951 and B.P. 696,902) ; a front system of lenses in contact corrected for spherical aberration and coma has its field curvature and astigmatism corrected by a rear system of surfaces which are approximately in the aplanatic and normal-incidence conditions for an axial pencil, and hence free from spherical aberration and coma

[1]

Quote:
In the British patent specification 696,902 it is observed that the first portion of this objective comprising the single front lens and the two menisci, exclusive the concave rear surface of the second meniscus, can be corrected for spherical aberration and coma, but not for field curvature and astigmatism.

(exclusive meaning excluding)
[2]

Or in other words

Quote:
If field curvature and astigmatism be initially neglected, it is quite possible to design a system of lenses well corrected for spherical aberration and coma, and giving, in the glass of the last one, the convergence angle of about 45 corresponding to a relative aperture of f/0.7

[3]




So the rear section must correct for field curvature and astigmatism, by

Quote:
a rear system of surfaces which are approximately in the aplanatic and normal-incidence conditions for an axial pencil, and hence free from spherical aberration and coma; a deep converging cemented surface at the back controls higher-order aberrations

[1]

The following is technical, but I have highlighted the important stuff in italics.

Quote:
If now the convergent light emerge from the third lens C through a surface approximately at the aplanatic condition for an axial pencil, the angular convergence will be reduced by a factor equal to the refractive index of the glass, and this aplanatic surface will introduce no spherical aberration coma or astigmatism, but will have a Petzval sum of the sign required to correct the front part of the lens.

The iris aperture of the lens lies behind this aplanatic surface, and is approximately in the plane of the vertex of a fourth lens D, whose front surface is centred on the axial image point of the rest of the system; this surface has therefore no effect on the convergence, gives no spherical aberration or coma, but introduces astigmatism of the sign required to correct the front part of the lens, and some field curvature opposite in sign but smaller than that of the aplanatic surface. Finally, if the last surface be plane and close to the focal plane, the convergence angle is increased again by a factor equal to the refractive index, and the aberrations introduced by it are small. For the complete lens, therefore, the correction of the front part for spherical aberration and coma is substantially unaltered, and the astigmatism and field curvature can be reduced to the small values required to correct the whole lens by a suitable choice of the position of the aplanatic and concentric surfaces.

[3]

As said before- this rear section corrects for field curvature and astigmatism, and leaves the spherical/coma aberration correction of the front section relatively untouched.
The rear surface of lens C deals with Petzval sum, being aplanatic, and the front surface of lens D deal with astigmatism.

The gap between C and D can be described as a 'strong negative air lens' whose suitable shape corrects field curvature and astigmatism [2]. The air lens technique crops up in various lens systems. As Brian J Caldwell writes, these designs

Quote:
incorporate a meniscus doublet lens group with a very strong concave surface near the image plane. This meniscus group serves to flatten the field. These designs also have characteristics similar to Petzval lenses, particularly the minimization of negative power in large elements located far from the image plane.
[4]

Remember the obvious Petzval + negative field flattener? Smile




The next post will look at the specific composition of the rear doublet which is what was swapped. There is actually a decent amount to talk about - it's the focus of the Bulthuis patent and is central to allowing the aperture to increase beyond f0.7 or improve coverage. I will also talk about what I would do with these lenses.

Sadly they're now less common or more expensive, so difficult to justify.

Note I don't know exactly how to draw an air lens, please don't assume the entire gap is functioning as it.

References

[1] Wynne, C.G., 1956. New Lens Systems. Reports on Progress in Physics, 19(1), p.317
[2] Bulthuis, H.W., 1970. Optical lens system having a relative aperture exceeding 1/0.75 CA838967A, p.2
[3] Wynne, C.G., 1951. A new form of f/0.71 lens for 35 mm cine-radiography. Journal of Scientific Instruments, 28(6), p.172
[4] Caldwell, B.J. 2010. Large aperture imaging optical systems US20100246031A1, p.54


PostPosted: Sat Jun 04, 2022 8:10 pm    Post subject: Reply with quote
Wow - that‘s very interesting! Thank you so much for sharing that information. I’ve never heard about any of that before… certainly still got a lot to learn.


PostPosted: Sun Jun 05, 2022 10:20 am    Post subject: Reply with quote
[quote="D1N0"]Some info here. The guy that adapted it took of the rear group and used a SMC Takumar 55/1.8 rear group instead (to increase the rear to sensor focus distance since that is les than a mm.)

https://u42.co/Lens-Design/Rayxar%20e50%20f0.75%20De%20Oude%20Delft/Rayxar-e50-f0.75-De-Oude-Delft.html[/quote]

Someone may well have done this, but the Experimental Optics version uses no external components. Their project is still documented on kickstarter.


PostPosted: Mon Jun 06, 2022 3:00 pm    Post subject: Reply with quote
Certainly a distinctive look - thanks for sharing.
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