Lenses with flat rear elements ?

[taffer]

Hi there,

so far I can recall having two lenses on which the most rear element is just that, flat on the side facing film. They are the Yashinon 55/1.2 and the Nikkor 105/2.5, both for SLRs.

A while ago I saw a review thread at pnet talking about the (most sought after) Konica Hexanon UC 35/2 in LTM, and also mentions that flat rear element.

Is that just a plain characteristic of some optical designs, or is it made that way for a special purpose, and if it's the later, what's the thing going on behind it, are they know to render light rays in a perfectly perpendicular way to the film plane, or what ?

😕

TIA

Oscar

 

[Pherdinand]

55/1.2?? never heard of such a yashinon.
I would think it's the "plain characteristic of some optical designs". The direction of the rays hitting the film plane as well as the planarity of the focal plane(??) is influenced by all the lens elements, not only the last surface. But someone more in the field might correct me on this.
If it would help for what you think, the lenses designed for digital sensors would all be like this, since there it is of high importance that the angle of incidence is close to zero (that is, to perpendicular to the sensor).

 

[bmattock]

55/1.2?? never heard of such a yashinon.

A very famous lens. It was also made in LTM, reviewed in July 1955 Modern Photography. I have the article, but have never seen the lens. Also a 35mm f2 in LTM. But I digress...

I would think it's the "plain characteristic of some optical designs". The direction of the rays hitting the film plane as well as the planarity of the focal plane(??) is influenced by all the lens elements, not only the last surface. But someone more in the field might correct me on this.
If it would help for what you think, the lenses designed for digital sensors would all be like this, since there it is of high importance that the angle of incidence is close to zero (that is, to perpendicular to the sensor).

I don't know the optical physics necessary to figure out things like this. But I suspect that the lens design in this case is common to very large aperture lenses. I have a Canon 55mm f1.2 FL and a Canon 58mm f1.2 FL, and both have large, flat, rear elements. I also have some non-camera lenses of large aperture, like a Kowa 58mm f1.13 and a Rodenstock XR-Heligon 86mm f1.2 and both have flat rear elements. My 178mm f2.5 Kodak Aero Ektar (WWII vintage) has a flat rear element as well.

That's the sum of my knowledge on the subject, sorry!

Best Regards,

Bill Mattocks

 

[Sonnar Brian]

The designer often split either the front element or rear element of a classic planar/xenon or Sonnar lens into two elements of lesser power. At the larger aperture, the first/last element would require more "power" and cause more aberrations. The Nikkor 5.8cm F1.4 lens split the front element, Summarit split the rear element. The 5cm F1.1 Nikkor split Both front and rear element. The lesser power lens will be flatter.

 

[taffer]

Thanks for your answers guys !!!

@Pherdi, yep, that issue related to digital sensors stroke me as well. Haven't seen any of the new Zeiss RF lenses therefore can't state that but I've read nothing about them having those kind of rear elements, so yes, I guess it's particular of some optical designs when applied to somewhat fast lenses.

@Bill, thanks for the info ! It made me recall that I think the RF Canon 50/1.2 has one of those as well (not 100% sure though). If that would be the case then maybe we would have a similarity between the super fast 50s and short teles as well.

Just thought about this, we have lens diagrams in dantestella's site and also at the Canon museum... http://www.dantestella.com/technical/block.html

Btw, here I'm attaching a shot with the 55/1.2 wide open, shares the same oof lights behaviour as the RF Canon. It was also marketed under another name and all them have also the Tomioka brand. This particular lens was a present from a friend, it's fungused and the auto diaphragm is not working, but nonetheless is a huge nice piece of glass, makes the plastic Zenit 122 feel like a toy.

Thanks,

Oscar

 

[bmattock]

Doing some Google searches for "flat rear element" and "camera lens" I did come across some interesting comments from a few folks who think that the flat rear element may be contributing to their problems with their old film lenses on their digital SLR bodies - spots and such as the light bounces back off the digital sensor onto the flat rear element and then back to the sensor again. Hmmm.

Well, would not bother me in any case - my 1.2 lenses are Canon FL mount, no digital SLR will take them.

Best Regards,

Bill Mattocks

 

[taffer]

The designer often split either the front element or rear element of a classic planar/xenon or Sonnar lens into two elements of lesser power. At the larger aperture, the first/last element would require more "power" and cause more aberrations. The Nikkor 5.8cm F1.4 lens split the front element, Summarit split the rear element. The 5cm F1.1 Nikkor split Both front and rear element. The lesser power lens will be flatter.

Thanks for the info Brian !

If I understood correctly, at fast apertures both front and rear element play a major role regarding image aberrations, right ?

Therefore by splitting the 'force' of them into two, and giving the front/rear one a flat end, they will reduce their 'power', and reduce the amount of aberration in the final image, yet still rendering it correctly as the two extra splitted elements made part of the work, right ?

I think you already did, but if you give me the title of that book I'll add it to the 'post your WTB' thread 🙂

 

[Kin Lau]

I did come across some interesting comments from a few folks who think that the flat rear element may be contributing to their problems with their old film lenses on their digital SLR bodies - spots and such as the light bounces back off the digital sensor onto the flat rear element and then back to the sensor again. Hmmm.

I have a Russian 35/2 m42 mount lense with a very large and flat rear element that gives my Drebs fits with metering and what looks like internal flare. I don't have this problem with any other m42 or t-mount lenses.

 

[VinceC]

My W-Nikkor 35mm f.1.8 (RF mount) has a large flat rear element. It's an optically outstanding lens, one of the best I've ever used.

 

[taffer]

Interesting, never thought of the reflection issues. With the glass in front of the sensors that's another new thing to have in mind...

 

[jlw]

Interesting, never thought of the reflection issues. With the glass in front of the sensors that's another new thing to have in mind...

I've been noticing what I think are reflections when using some old LTM lenses on my R-D 1; in fact, I started a thread long about about it under the name "Sensor Bounce".

Example pic from that thread

I drew the red line in the pic to show where the reflection is most prominent. When seen, it seems to take the form of a fairly-well-defined "second edge" around edges of a bright subject against a dark background. (If the subject isn't bright, the reflection doesn't amount to much, and if the background isn't dark, the reflection gets lost in it.)

The example pic isn't a perfect example, as some have suggested it is actually lens flare or highlight "blooming" caused by overexposure of the sensor. However, I've seen it in other pictures (which I'm too lazy to dig out and post at the moment) where it's well-defined enough to make it evident that it's a reflection.

Normally I just don't worry about it much, since it only appears under specific conditions and with specific lenses (unfortunately, my fave Canon 50/1.4 seems to be one of them.)

But now I can see why manufacturers of "digitally optimized" lenses take special care in coating the rear elements to deter reflections (I read in 'Pop' that this is one of the characteristics of "digitally optimized" lenses.) I'd love to get hold of an RF lens that claims to have been designed with digital use in mind, such as one of the new Carl Zeiss models, and test it under similar conditions against my Canon, to see if I could pin down the effects.

 

[VinceC]

Interesting. My above-mentioned w-Nikkor 35mm shows a nearly identical characteristic with highlights on film when shot wide open at f/1.8. Disappears completely when stopped down slightly to f/2. I've always taken this to be a design trade-off for pushing the high-speed envelope half a century ago. I just shoot it at f/2 unless I'm struggling for every photon in available darkness.

 

[taffer]

Thanks for posting that example jlw, I can imagine that even if only happening in certain light conditions, that can be annoying. As you say, would be interesting to know how 'digital optimized' lenses behave in those conditions.

Btw, speaking of the Yashinon, here's the Revuenon version, identical except for the rubber grip on the Revue vs scalloped metal in the Yashinon.

http://cgi.ebay.com/Tomioka-55-1-2-...oryZ4688QQssPageNameZWD1VQQrdZ1QQcmdZViewItem

Maybe I should consider having that lens cleaned and serviced in the end...