75mm APO depth of field

[Peter Branch]

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........ The whole concept of DOF is based on physical properties of point images. ..........

 

Unfortunately the whole concept is actually based on the determination under some very specific conditions of how far apart two image points have to be before they are no longer perceived as being a single point.

 

The way this figure is measured makes various assumptions including that a 35mm frame will be enlarged to 7.2 X 10.8 cm and then viewed from 25cm. Under these specific, and somewhat atypical conditions, it is asserted that the human eye cannot distinguish two points if they are less than 0.1 mm apart on the resulting print. Hence, given the 3X magnification, the largest circle of light on the negative that will be indistinguishable from a point, and thus be accepted as being in focus, is 0.1mm/3, that is 1/30mm.

 

On this figure all the calculations and engraving on lenses relating to DoF tend to be based.

 

Many factors which are known to influence the perception of sharpness in the real world are simply ignored.

 

Hence DoF is actually based on an empirical formula which relies on average properties of the human eye - it's not physics.

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.... relies on average properties of the human eye - it's not physics.

 

Horticulture, then?

[mmradman]

Unfortunately the whole concept is actually based on the determination under some very specific conditions of how far apart two image points have to be before they are no longer perceived as being a single point.

 

The way this figure is measured makes various assumptions including that a 35mm frame will be enlarged to 7.2 X 10.8 cm and then viewed from 25cm. Under these specific, and somewhat atypical conditions, it is asserted that the human eye cannot distinguish two points if they are less than 0.1 mm apart on the resulting print. Hence, given the 3X magnification, the largest circle of light on the negative that will be indistinguishable from a point, and thus be accepted as being in focus, is 0.1mm/3, that is 1/30mm.

 

On this figure all the calculations and engraving on lenses relating to DoF tend to be based.

 

Many factors which are known to influence the perception of sharpness in the real world are simply ignored.

 

Hence DoF is actually based on an empirical formula which relies on average properties of the human eye - it's not physics.

 

It is either physics and averages based on one predictable model or, the way you describe it, lens DOF scales would be individually engraved to take into account individual preferences and eyesight, so which one is it?

[pico]

We may promote hard metrics for DOF that satisfies charts, however as mentioned earlier in this thread, there is the profound issue of perception - our brain. Humans have not been able to KNOW what they do not or cannot know.

[CheshireCat]

Humans have not been able to KNOW what they do not or cannot know.

 

...and what they really want

[CheshireCat]

I happen to have a couple old DNGs comparing 75 Lux @ F2 and 75 APO @ F2. I'm know I'm going to regret this somehow, but here they are... (dropbox).

 

John, thanks for sharing. You will not regret it

 

I have processed the files with Lr 5.6.

 

I set the WB to D50 for both files. First thing I noticed is how much the Summilux shot has much colder colors. It takes +500K for the Summilux shot to match the Summicron shot colors. This may be caused by changing ambient light conditions or difference in lens color rendering.

The Summicron has better colors overall.

 

At that focal distance, the Summilux is a little wider.

 

The other very interesting thing is that the Summicron is about half a T-stop slower than the Summicron.

 

The Summicron holds highlights better.

 

The Summicron is better corrected for longitudinal CA, which is to be expected from an APO lens. However, it is not corrected as well as other APO lenses.

 

Checking the "EST 1887" mark at the top of the bottle and the red logo in the neck, my subjective feeling is that the Summicron has more DOF. This feeling is increased if I boost the sharpening to about 50.

This is probably due to the much better color correction of the Summicron.

On the Summilux shot, the red logo in the neck casts a red halo that is really annoying and does not help with the perception of DOF.

 

I am attaching a comparison detail in color and then the red channel.

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[John Black]

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I use that bottle for LOTS of test shots with different lenses, and even with Live View putting focus on the exact same spot is really difficult. The shooting distance is around 2 meters, the slightest move in the focus ring can moved focus fore or aft by a couple millimeters. I use these test images to look at bokeh and CA. These are not ideal images by any measure to compare DOF. Before the M-240, these type of test shots were torture... LV is still tricky, but worlds easier than the M9 days.

 

All that said, your conclusions are what I would say about the 75 APO vs Lux. The APO is can be wickedly sharp at F2 and the CA can be non-existent in the plane of focus. And the 75 Lux is about the opposite For the Lux, F2 is sharpish and the magenta / green CA is usually quite evident in the bokeh. There can be some CA in the plane of focus too - the purplish, blooming type. And that 75 Lux had been to Leica NJ for calibration. Maybe somebody somewhere has a better calibrated 75 Lux, but when Leica sends it back with their certification card, I have to assume the lens is as good as it going to get from Leica's point of view. (This was a serial number 36xxxxx-something w/ the slide out hood, still Canadian made.)

 

Coming back to the DOF debate, there have been so many times with the 75 Lux that I've wondered where the exact the plane of focus actually was in F1.4 and F2 images. There is a region that is sharpish, so it's obvious where the plane of focus was placed, but critical focus is a semi blurred mess. Saying where DOF starts and ends could be a very subjective interpretation. Whereas the 75 APO leaves little to question. If a 75 APO is well calibrated, I trust it more than the 75 Lux. The 75 Lux can be tricky with its focus shift.

 

What was the original question in this thread? LOL

[CheshireCat]

Your Summilux is sharper than the one I had tried (and did not buy - now I know I did the right thing).

The big problem of focusing the Summilux (and many other non-APO fast lenses) is that focus is only possible on some light frequencies. E.g. if you focus for green, then red is off.

Andy posted a nice example here:

http://www.l-camera-forum.com/leica-forum/digital-forum/145063-what-chromatic-aberrations-really-look-like.html

[pgk]

Saying where DOF starts and ends could be a very subjective interpretation.

 

What was the original question in this thread?

I think that you just answered it;).

[Peter Branch]

It is either physics and averages based on one predictable model or, the way you describe it, lens DOF scales would be individually engraved to take into account individual preferences and eyesight, so which one is it?

 

I was careful to point out that the data was for an "average" eye.

 

It does not and makes no attempt to apply to individuals. There is, of course, a possibility that in practice no individual has the average value.

 

Individual perception is just one of the many factors that are not taken into account in determining the totemic 0.0333 mm.

 

As the Zeiss paper on this subject makes clear there is precious little physics involved and lots of geometry.

 

Once the 0.0333 mm "datum" is assumed then a whole edifice of calculations can be, and has been, constructed.

 

It all depends on whether or not the conditions which gave rise to the 0.0333 mm figure apply.

 

It is my contention that the 0.0333 mm figure is obsolescent in the context of high pixel digital sensors, highly corrected lenses and the totally different viewing conditions currently employed.

[jpk]

The other very interesting thing is that the Summicron is about half a T-stop slower than the Summicron.

 

Thanks for posting the comparison! Please clarify: which lens is slower and at what F-stop or T-stop or lens marking?

[lct]

Individual perception is just one of the many factors that are not taken into account in determining the totemic 0.333 mm.

We would need big sensors to get such a "totemic" value. 8x10 at least. But you made a typo i guess.

[pgk]

It is my contention that the 0.333 mm figure is obsolescent in the context of high pixel digital sensors, highly corrected lenses and the totally different viewing conditions currently employed.

Over 30 years ago my undergaduate project was a comparison of Leitz and Minolta lenses by MTF and practical means. I am pretty sure that I made prints designed to fulfill those standard viewing requirements you refer to. And whilst the MTF data showed differences, the prints, when viewed under these conditions did not. Depth of field scales have always IMO been indicators, nothing more. Their use has been as guidance only.

 

[And FWIW I specialise in underwater photography, much of it carried out with wide-angle lenses behind large spherical domes through which the lens 'sees' a spherically curved image far closer than the subject matter, except of course at close focus distances. Usually the aperture used and precise point of focus are an utter compromise with good corner performance remaining the holy grail. Some lenses work better than others of/at identical focal length. DoF characteristics do vary, believe me.]

[Peter Branch]

We would need big sensors to get such a "totemic" value. 8x10 at least. But you made a typo i guess.

 

Thank you - now corrected.

[lct]

We may discuss at lengths about the validity of such or such CoC standard value, and we did it many times in this very forum already, but whatever value we choose, it won't change anything when comparing the DoFs of two different lenses for the simple reason that the same CoC value will be retained for both. Unless one wants to compare apples to oranges of course.

[CheshireCat]

Thanks for posting the comparison! Please clarify: which lens is slower and at what F-stop or T-stop or lens marking?

 

Oops.. I actually meant that the Summicron seems slower than the Summilux.

 

This is what I can tell by John's raw files, though someone who has both lenses should confirm this hypothesis.

[CheshireCat]

We may discuss at lengths about the validity of such or such CoC standard value, and we did it many times in this very forum already, but whatever value we choose, it won't change anything when comparing the DoFs of two different lenses for the simple reason that the same CoC value will be retained for both.

 

This is false, and we should be discussing the validity of the simplistic CoC concept altogether.

 

Given an APO and a non-APO lens, such as the 75 Summilux and Summicron, the average CoC for all the frequencies of the visible spectrum near the "intended focus point" will be smaller for the APO lens.

It is interesting to note that, for some color frequencies, a lens may have a CoC greater than the pixel size even at the "intended focus point".

Please check Andy's example (cited in one of my previous posts).

[John Black]

Both files have full EXIF data with custom lens names embedded - specifically spelling out 75mm F2 Summicron-M APO and 75mm F1.4 Summilux-M Version II.

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[CheshireCat]

I noticed the "Version II" tag.

What's the difference from "Version I" ? Do they have different 6bit codes ?

[pico]

I noticed the "Version II" tag.

What's the difference from "Version I" ? Do they have different 6bit codes ?

 

 

No optical difference.