75mm APO depth of field

[Peter Branch]

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............It is not very probable, that the real focal length is engraved.........Jan

 

The reality is that the actual focal length is engraved on many Leica lenses.

 

For example my 50mm f/1.4 Summilux-M ASPH is engraved and it has a focal length of 51.4 mm.

[Michael Geschlecht]

Hello LCT,

 

Could you explain the math in your Post please?

 

What are the formulas that you used for determing circles of confusion of 0.02mm & 0.03mm?

 

I would think that with a smaller circle of confusion the depth of field would definitionally be somewhat less than it would be with a larger circle of confusion.

 

 

Best Regards,

 

Michael

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What are the formulas that you used for determing circles of confusion of 0.02mm & 0.03mm?

 

I would think that with a smaller circle of confusion the depth of field would definitionally be somewhat less than it would be with a larger circle of confusion.l

 

DOF for a circle of confusion of 0.02mm is 5cm, for 0.03 it is 8cm. Increase the COC by 50% and the DOF will grow by 50%, too.

[pico]

The reality is that the actual focal length is engraved on many Leica lenses.

 

 

 

For example my 50mm f/1.4 Summilux-M ASPH is engraved and it has a focal length of 51.4 mm.

 

 

I am not sure they are all marked. My 75mm Summilux is marked "00". Is that good?

 

[CheshireCat]

I am not sure they are all marked. My 75mm Summilux is marked "00". Is that good?

 

 

More than good: perfect. Your lens is exactly 75mm

[Michael Geschlecht]

Hello Jan,

 

I would think that when the 75mm Summilux set at F 16 is focussed to Infinity, that the depth of field markings show the depth of field to be 10 meters to Infinity, because it probably is.

 

I would also think that when the 75mm Summarit set at F16 is focussed to Infinity, that the depth of field makings show the depth of field to be 8 meters to Infinity, because it probably is.

 

Different lenses, of different optical designs, of the same focal length, focussed on the same point, set at the same aperture, can have different depths of field in certain circumstances.*

 

* There is, of course, the 21mm F3.4 Super Angulon (built by Schneider) which comes in at least 2 versions. Apparently with the same optical design in the earlier (chrome) & later (black) versions. These 2 versions have different depth of field scales at equivalent apertures. The reason for this difference was never officially clarified by Leitz/Leica.**

 

** the 21mm F2.8 Elmarit (a Leitz/Leica design) which succeded the above 21mm lenses continued the depth of field positioning of the later black lens which showed less depth of field at any marked aperture than its chrome predecessor.

 

When a lens is marked on the external focussing mount & internally on the metal portion of the lens assembly with a number like "18" (no "." between the 1 & the 8) on a NOMINALLY 50mm lens: This is shorthand for a focal length of 51.8mm for both the lens assembly & the focussing mount.

 

This is because during the manufacturing process Leitz/Leica has begun its "fine tuning" by first grouping components for final fitting within parameters measured in 1/10 of a millimeter.

 

If you begin with components grouped accurately to within 1/10 of a millimeter it is easier to "fine tune" to a given standard than it is if you begin with parts that are only accurate to the nearest 1 millimeter.

 

Best Regards,

 

Michael

Edited September 18, 2014 by Michael Geschlecht

[lct]

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What are the formulas that you used for determing circles of confusion of 0.02mm & 0.03mm? I would think that with a smaller circle of confusion the depth of field would definitionally be somewhat less than it would be with a larger circle of confusion.

0.03mm is the standard CoC value for 24x36 cameras and i chose 0.02mm randomly for sake of comparison. Of course a smaller CoC determines a shorter DoF as you can see in the screenshots above. You might wish to click on the links hereafter to get a couple of DoF formulas and a DoF calculator based on the latters. See also our colleague SJP's article and spreadsheet on this point.

[lct]

[...] the 21mm F2.8 Elmarit (a Leitz/Leica design) which succeded the above 21mm lenses continued the depth of field positioning of the later black lens which showed less depth of field at any marked aperture than its chrome predecessor [...]

DoF markings of lenses are just approximations. For more accurate figures, see the DoF tables of Leica's technical datas, for instance that of the Elmarit 21/2.8 asph attached below.

LeicaM_2128A_specs_p05.pdf

[jankap]

Hello Jan,

I would think that when the 75mm Summilux set at F 16 is focussed to Infinity, that the depth of field markings show the depth of field to be 10 meters to Infinity, because it probably is.

I would also think that when the 75mm Summarit set at F16 is focussed to Infinity, that the depth of field makings show the depth of field to be 8 meters to Infinity, because it probably is.

Best Regards,

Michael

 

On the homepage of Ken Rockwell I found these pictures. The two at the left side are luxes, the right one is a Voigtländer.

 

http://www.l-camera-forum.com/leica-forum/attachment.php?attachmentid=458079&stc=1&d=1411119815http://www.l-camera-forum.com/leica-forum/attachment.php?attachmentid=458080&stc=1&d=1411119815http://www.l-camera-forum.com/leica-forum/attachment.php?attachmentid=458081&stc=1&d=1411119815

 

For picture making I agree with ict, that these markings should be seen as rough approximations.

The calculator programs are on a strictly mathematical basis. If the lens manufacturers take care of more data (actual focal length, f-stop or even lens design) is possible. I don´t believe it.

But is it relevant? The result, the photo, is enlarged to wall paper or only viewed on an iPhone, giving a different impression of the sharpness. So, who cares, if the markings on the lens show 8, 9 or 10 meters?

There is no border of sharpness, the really sharp plane gives the distance marking (for the center!).

Jan

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