Focus shift

[jaeger]

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I didn't know 50 summilux ASPH have FLE?!?  Where is the source please.

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[Jeff S]

7 minutes ago, jaeger said:

I didn't know 50 summilux ASPH have FLE?!?  Where is the source please.

Well known and often discussed.  Karbe has talked about the difficulties of incorporating it and still maintaining small lens size.

https://www.shutterbug.com/content/leica-lens-saga-interview-peter-karbe

Jeff

[adan]

It is also right there in Leica's technical specs .pdf for the lens.

 

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[jaeger]

49 minutes ago, adan said:

It is also right there in Leica's technical specs .pdf for the lens.

 

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Wow!  I am so happy, it has FLE and implied APO (not official).  In addition, I have the black chrome version that give it a vintage look as well.  No wonder I get very good results every time using this lens...  I have a model requests me using it on his photoshoot too. 

Edited September 29, 2020 by jaeger

[Jeff S]

1 hour ago, adan said:

It is also right there in Leica's technical specs .pdf for the lens.

 

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Similar diagram appears in link I provided earlier, with further commentary by Karbe on how it came to fruition.

Jeff

[lct]

The 50/1.4 asph's floating element has been reported since 2004 or 2005 if memory serves. No sure if it was mentioned by E. Puts then but i knew it when i bought my copy in 2005. I was not aware of the apo correction though. 

[adan]

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The APO label was simply never applied. Leica (correctly, IMHO) figured it was a bit too much on a non-telephoto lens, since longitudinal CA is not usually a big issue with shorter lenses.

Then they changed their mind with the APO-Summicron.

The 50 ASPH design (2006) is based off the 75 APO-Summicron (2005), just as Dr. Mandler reverse-engineered the 75 Summilux from the 50 Summilux - thus has the same features of the 75 APO, in a shorter focal length.

[Jeff S]

1 hour ago, adan said:

The 50 ASPH design (2006) is based off the 75 APO-Summicron (2005), just as Dr. Mandler reverse-engineered the 75 Summilux from the 50 Summilux - thus has the same features of the 75 APO, in a shorter focal length.

As Karbe explains (above).  I guess pretty soon we will have covered the interview.

Jeff

[Harpomatic]

11 hours ago, BrianS said:

From an article that I wrote a while ago- 

Hello Brian, many thanks for the info and the article link. I had not read it and found it incredibly interesting!

[BrianS]

Thankyou, and thankyou for your article. I read it through, very interesting. I read up on this subject when shimming Sonnars and Jupiters to use with the Leica standard.

1/f = 1/u + 1/v

I used the formula for focal length to compute the change in the shim thickness required to make the lens agree with the rangefinder at 1meter. With the Sonnar, the focus shift when stopping down would allow use at infinity. If that did not work with a particular lens, it meant the focal length needed to be changed- done  by moving the rear group as per the Lens Maker's formula.

For focus shift caused by Spherical Aberration, the explanation from the formula for focal length is that the focal length of the lens is not constant across the diameter of the lens. Lens designers knew this. Additionally, the rate that the focal length changes across the diameter is responsible for "Bokeh Ball Density". Add up the Area of the lens at each focal length- get a density function for the Bokeh Ball.

[Punkrockemo]

How do people using a lens affected by focus shift deal with it? Do you avoid using Appertures that are affected or do you shift the focus to suit.

I have the 35mm Nokton, I find if i shift the focus a bit after focusing it helps but its not very accurate.

Does the distance to subject make a diffrence.

Thanks for article interesting and useful. 

 

[astrostl]

My order of preference:

- avoid lenses with significant focus shift

- shoot them at shift-safe apertures

- know the tendency of the lens to front or backfocus and lean forward or backward after focusing with the RF

- focus at the shooting aperture with LV / EVF

[BrianS]

I test the lenses for focus shift at various apertures. When using the lens in practice, focus the lens using the Rangefinder and then shift the focus ever-so-slightly to compensate. A lens that is over-corrected for spherical aberration will shift towards infinity when stopped down. SO, nudge the focus slightly closer.

[LocalHero1953]

I had the Sonnar 50/1.5 for a while, calibrated at f/2. I shot it mostly wide open, and learned to lean in a bit after focusing. This was a lot easier to verify and learn once I had an EVF for my M240.

I believe some versions of the lens were factory-calibrated at f/1.5 (apologies if this was mentioned earlier in the thread).   

Edited October 25, 2020 by LocalHero1953

[Harpomatic]

8 hours ago, Punkrockemo said:

Thanks for article interesting and useful. 

Thanks!

As for your question:

7 hours ago, astrostl said:

My order of preference:

- avoid lenses with significant focus shift

- shoot them at shift-safe apertures

- know the tendency of the lens to front or backfocus and lean forward or backward after focusing with the RF

- focus at the shooting aperture with LV / EVF

+1!!

[MrFriendly]

On 9/29/2020 at 4:02 PM, adan said:

The 50 ASPH design (2006) is based off the 75 APO-Summicron (2005), just as Dr. Mandler reverse-engineered the 75 Summilux from the 50 Summilux - thus has the same features of the 75 APO, in a shorter focal length.

And why does the 50 APO-Summicron ASPH cost twice as much as the 75 APO-Summicron ASPH?

[MrFriendly]

On 9/27/2020 at 8:14 PM, Harpomatic said:

For all the doubters and the curious out there I wrote a short article on focus shift, what it is and what causes it.

I hope you’ll find it interesting!

https://www.47-degree.com/focus-shift/what-is-focus-shift

I’m really happy to discuss it!

I did find it interesting, and very helpful. Thank you for writing it.  Questions:

1. As far as FLE, my understanding was that non-FLE lenses achieve their maximum performance at infinity (or minimum focus distance, whichever the lens designer went with).  The FLE part was added to have maximum performance across the board (i.e., from minimum focus distance to infinity).  Is that not correct?
2. Isn't the whole purpose of aspherical lens element to fix focus shift and other spherical aberrations?

 

[Harpomatic]

9 hours ago, MrFriendly said:

I did find it interesting, and very helpful. Thank you for writing it.  Questions:

1. As far as FLE, my understanding was that non-FLE lenses achieve their maximum performance at infinity (or minimum focus distance, whichever the lens designer went with).  The FLE part was added to have maximum performance across the board (i.e., from minimum focus distance to infinity).  Is that not correct?
2. Isn't the whole purpose of aspherical lens element to fix focus shift and other spherical aberrations?

 

Thanks for the feedback!

1.my understanding is that the FLE helps keeping the optical performance at a high level throughout the focusing range, especially close focus, and to reduce focus shift. I also am under the impression that all photographic lenses, except macro or special purpose ones, are optimised for infinity focus.

2. Aspherical elements are not necessarily helping at all with focus shift. Eminent examples are the two aspherical versions of the 35mm Summilux M before the FLE. Another point is that you don’t necessarily need aspheres to correct focus shift, like in the Nokton Classic 35mm 1.4 version 2: there is no aspherical element in there, they used an anomalous partial dispersion glass element to eliminate, for all practical purposes, the focus shift that plagued version 1. With a small trade-off in corner sharpness. I go in more detail in the comparison between them in my blog.

 

Anybody more knowledgeable than me can correct me if wrong please, I’m always happy to learn!

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Hallo,

Look here: Leica M

[adan]

1. I believe that "general purpose" photographic lenses are optimized for a "compromise infinity" of about 1000x the focal length. E.G. a 50mm lens is optimized for 50 meters, with a range of "pretty close to optimized" that extends from, say, 2m to true infinity. The primary purpose of an FLE is to slightly adjust the optical formula to provide a lens structure more optimized  for infinity when focused at infinity, but re-optimized for closer distances when focused closer (as a continuum, not a sharp change).

"Aero lenses" for aerial photography are more likely to be optimized for "true" infinity. One example being the Zeiss/Hasselblad 100mm Planar f/3.5, designed at NASA's request for orbital and space pictures with better long-distance performance than the standard 80mm f/2.8 Planar, and also recommended for "airborne" aerial photography from 1000s of meters altitude.

http://www.hasselbladhistorical.eu/pdf/lds/CFi100.pdf

If the lens designer, playing with the design software, can fix something else with an FLE as well, that is just icing on the cake.

2. Don't forget the use of cemented elements as a substitute for ASPH surfaces (and vice-versa). By cementing two elements of glass with different light-bending properties (refractive or dispersion indices), along a curved surface (a Merté surface) in the middle such that the edges of of the glass are mostly one glass type, while the center is mostly the other glass type, one can approximate the same effect as an ASPH surface with a non-constant radius. (Hence the idea that one ASPH surface can do the job of two spherical surfaces (and vice versa)).

See, for example, the rear group of a Tessar formula below.

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In an extreme case, ca. 1966, a Leitz designer, Prof. H. Marx, came up with a five-glass "doublet" of layers cemented and ground, and then cemented again and ground, and then cemented and ground again, so that the extra glasses phased in only at the edges. Completely unmanufacturable economically, but a thought experiment (and coincidentally, an attempt at an "improved" 50mm f/2.0). A diagram of this lens can be found on page 39 of Erwin Puts' Leica Lens Compendium.

Edited October 26, 2020 by adan

[Dennis]

On 9/27/2020 at 7:14 PM, Harpomatic said:

I hope you’ll find it interesting!

Very interesting. Thank you for sharing. Before reading it, my knowledge about the topic was like the three draws at the beginning of your article  😂 Now, I know much better. I like the "option" to shoot wide open, and at f/8, it makes sense.
If you don't mind, a question for you. Sometimes I just want to go out and shoot and don't think about technicalities. Or think about them at home before to go out and be free and focused to the moment. Sometimes, I feel would love to know deeply my camera/s and/or lens/es limitations to have the best image possible. For example, I didn't understand one thing. Is it different (about focus shift) to focus on the OVF or LV? And what about to focus wide open first then change to the given aperture? I'm asking because I use very little LV; I prefer the OVF very much; that's why I love the M system. In other words, if I want to use the OVF only, what do you recommend? Are these specific techniques recommended for the problematic lenses, or do they apply for any lenses to prevent an eventual focus shift?
Thank you in advance for your feedback