DOF for ASPH

[01af]

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It's a toasty 40F today.

Forty funnygrades? How much is that in real degrees?

 

 

Does this not mean that the Asph lens has enhanced DOF?

Not necessarily. While depth-of-field indeed depends on the size of the exit pupil, not entry pupil, it also depends on the exit pupil's distance. When two lenses with equal focal lengths have different exit pupil sizes (at the same aperture, of course) then they'll also have different exit pupil distances. These two differences cancel each other out exactly—at infinity focusing. At shorter focusing distances, the larger exit pupil means less depth-of-field ... but at distances longer than, say, 0.7 m this will hardly make any perceptible difference.

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[thighslapper]

This thread really needs tying in with Lindolfis current exposition on rangefinder accuracy....

 

The two are inextricably linked in the process of achieving satisfactorily focussed sharp images....

[Mylek]

Thanks everybody!

 

I was out all day and didn't expect so much answer so i will read everything and digest it. To answer Jaapv, i was talking about depth of field only.

[pico]

You know you have really ruined the day for military resolution target fanatics. And this: "underlying reality and causality" really makes the day for philosophers and .... well, if I make a picture after drinking Guinness, should I require my viewers to have the same cause and underlying reality?

 

Ok, ok .... I'll experiment.

.

Off for a bicycle ride. It's a toasty 40F today.

 

Well, I've had a couple pints of Guinness and I must say that after looking at images made before-and-after, my perception of depth-of-field has not changed. It might have to do do with the weather rather than the Guinness. It went from 40 degrees F to 10 degrees, and my bicycle chain affair froze, the transmission failed, and I fell down. Now everything is blessedly foggy.

--

Pico - touch typing and hoping for the best

[Michael Geschlecht]

Hello Everybody,

 

Describing acceptable depth of field as: The acceptable lack of best focus on either side of the plane of best focus:

 

When popularly available floating element lenses became available in the 1970's & 1980's people realized there was sometimes a difference in the acceptable depth of field of some floating element designs as opposed to some non floating element designs of the same focal length within the inventory of a single manufacturer. That is: 1 manufacturer might make 2 different lenses of the same focal length & place the depth of field markings for a given aperture @ 2 different inscribed distances on the 2 different lens barrels.

 

During this time period what was said was: The floating element designs produced a better degree of correction in the focussed on image plane but that high level of correction w/ floating elements degraded more rapidly than conventional designs as distances on either side of that focused on image plane increased. This required a different set of depth of field markings.

 

Best Regards,

 

Michael

[adan]

I have just looked at a series 4 Summicron and an ASPh Summicron held side by side and up to a light source. Looking at the size of the exit pupil of both lenses it is obvious that the series 4 lens subtens a larger pupil than the ASPH lens.

 

Did you do this holding the lens flanges 27.8mm from your eye?

 

That's the distance from which the film or sensor will "see" the exit pupil of M lenses, and given the differences in rear-element power/construction of these lenses, viewing from some other distance will not accurately reflect the true relative exit pupils.

 

The ASPH has a thick diverging (i.e. mini-fying) rear element that the v.4 does not. That element (the largest piece of glass in the lens) will make anything on the other side of it look smaller - varying according to viewing distance.

[Michael Geschlecht]

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Hello Again Everybody,

 

As per Lars's Post # 3 in this Thread:

 

Perhaps an example of what Lars discussed as - Apparently decreased depth of field because of changes in expectations as overall lens quality improved - Might be seen as the reason the depth of field markings on the Bright Chrome Super Angulon 21mm/3.4 were recalculated & made somewhat more restrictive when its finish was later changed to black.

 

Best Regards,

 

Michael

[Lindolfi]

The symmetry of the design does influence the DOF and Depth of Focus at a given focal length. See here starting on page 19. The effect is not huge, but 30% is possible.

All usual DOF and depth of focus calculations use formula's that hold for symmetrical designs.

Now especially in the middle focal length ranges, like 50 mm on 24x36, there is no real need to make a lens asymmetrical, but looking at the designs of the various summiluxes (asph compared with non-asph at various focal lengths) there may well be differences in symmetry.

[lars_bergquist]

Some of the points taken here are a bit abstruse, but I will address a couple of the most obvious.

 

With the dear old Rodenstock Imagon, you could regulate the amount of fuzz by inserting one or other of several sieve-type aperture plates. Now if you used one that increased the minimum c.o.c. the lens could produce to, say, 1/15th of a mm, well then a d.o.f. calculation based on an acceptable c.o.c. of 1/30 would be quite irrelevant. This would be like going at that ridge in my metaphor with earth-moving machinery, removing the entire crest. The 'sharpness ridge' would have been changed into a low plateau. And the line where that plateau started to drop off, i.e. the limit of any d.o.f. consideration, would be further out, but also much lower, than before. – This also applies to pinhole 'lenses'. A pinhole does not focus the light, it just directs it. A pinhole image is unfocused and therefore equally unsharp at all distances. And you could construe that fact to mean that a pinhole 'lens' has unlimited depth of field!

 

And this whole argument tells us what's in the claim that some lenses have more d.o.f. than sharper ones.

 

Do non-symmetrical lenses (meaning lenses that have unequal dioptries fore and aft of the iris diaphragm, i.e. retro-focus and telephoto lenses) have different d.o.f. from symmetrical ones? Calculations are based on focal length, i.e. the position of the rear or image-side primary plane of the lens. This plane lies at the same distance from the image plane, 135mm, in both a symmetrical 13.5cm Hektor as in a telephoto 135mm Apo-Telyt. – Calculations are valid for the optical axis. Curvature of field may be a factor off axis, but this is a different kettle of fish. The old Novoflex and earlier telescope long lenses had so much curvature that peripheral sharpness simply did not exist, at least not anywhere near the plane of focus. But that does not invalidate the principle; it just limits its applicability.

 

We should also remember our sense of proportion. Depth of field is not part of the optical laws, it is just a (sometimes) handy quick-and-dirty rule of thumb for practical cats. Minor quibbles are simply not worth our time, unless we are in bed with a bad head cold and unable to take pictures.

 

The old man still with his head on

[jaapv]

Nice circle. My first remark was that it was " mathematically identical" . I agree. The rest of the discussion is about smoke and mirrors.

[Lindolfi]

Since the angle of the cone of rays entering the sensor depends on the symmetry of the design at a certain focal length, the design is of importance for depth of focus, and therefore for depth of field.

[jaapv]

And the cone gets asymmetrical as it moves out of the optical axis. I think we are getting to the point where we need to provide a fixing frame for the head of the observer, to ensure the exact viewing distance and planparallel location of his eyes relative to the print.

[Lindolfi]

At this point I'll take up the advise of Lars and go out to take pictures, during which visualizing DOF is not very accurate (certainly not within the difference between designs), but part of the game of working with a rangefinder.

[pgk]

Its worth thinking about this topic with a sense of proportion. As stated there can be differences in the apparent or theoretical depths of fields (assuming one can actually tie down and include all parameters in equations - Zeiss's arbitrary specifications) of various designs bu these are in all honesty very marginal. Where they may have an effect is when they result is focus shifts at different apertures and this is I would suspect, probably noticeable most when all the tolerances and adjustments lie near their extremes (which may explain why focus shifts can sometimes be minimised by lens adjustment). Unless you are pixel peeping or are running huge enlargements (rare!) then most of the time the differences in depth of field are not going to be visible. However other consequences of the lens designs which alter the depth of fields are more noticeable as has been so often stated on this forum.

[01af]

Nice circle. My first remark was that it was "mathematically identical" . I agree. The rest of the discussion is about smoke and mirrors.

Oh great ... after one full circle, you're still wrong.

 

The math is identical for different lenses because the simplifications are the same. But real-world depths-of-field are different. And no, they don't just appear to be different; they actually are different. Depth-of-field depends on more factors than just COC, distance, focal length, and aperture.

[jaapv]

Oh man - here we go again:rolleyes: Summarizing my posts - and there are dozens of them in this forum dating back to 2004 - saying the same thing I do and then presenting it as a disagreeing argument..... It is that moderators don't use ignore lists....

 

First you say that my remark that the math is identical is wrong (conveniently ignoring "all else being identical") and then you say the math is identical because...etc, which everybody above has been explaining. Do you ever actually read the posts here, including your own...?

[01af]

Do you ever actually read the posts here, including your own ...?

Do you?

 

"DOF is, all else being the same, mathematically identical between asph and non-asph lenses." Did you say this or did you not?

 

Now my point is this: The math you're using is incomplete. Simplified. It does not take all relevant factors into consideration. So if your math suggests the depths-of-field should be identical it does not mean they actually are.

[jaapv]

I am using??? I am saying the math that is being used gives the same outcome. Do you get different outcomes from the math being used? Now read the whole post. That tells you that despite the math being used the outcome may be different on differently constructed lenses...

 

DOF is, all else being the same, mathematically identical between asph and non-asph lenses. However, the newest generation M lenses is so well corrected, high-resolving and precise that the sharpness falloff that produces the illusion of DOF in our perception is better defined, so for practical purposes it is a bit more narrow. Having said that, I have learnt not to rely on DOF for focussing especially with modern lenses on a sensor. There is but one sharp area in a photograph and that is the plane of focus.

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[01af]

I am saying the math that is being used gives the same outcome. Do you get different outcomes from the math being used?

Of course I don't. I use the same simplified DOF formulas as you and everyone else, so I get the same results. But unlike most others, I understand that these results are only approximations which don't reflect the finer differences between various lens designs properly.

 

 

Now read the whole post.

I did. Did you? Let me explain your post to you. You're saying basically the same as Lars says: In the presence of higher maximum sharpness at the plane of focus, the level of what's considered "acceptable sharpness" (or "acceptable lack of sharpness", if you so prefer) will shift in the eye of the beholder. So apparent depth-of-field becomes narrower even when technical depth-of-field (as computed by the formulas) remains the same.

 

Well ... and now things are getting real complicated, so take a deep breath and read slowly. You (and Lars) are right and wrong.

 

It's true that apparent depth-of-field—i. e. what looks sharp enough to the eye of the beholder—can, and will, shift according to several factors, including not only print size, viewing distance, and vision but also the spatial frequency of the image content, the beholder's expectations, and the level of maximum sharpness. That's the general problem of the whole concept of depth-of-field right from the beginning (not just since the advent of Asph lenses). The introduction of a maximum acceptable diameter of the circles of confusion as a criterion for "acceptable sharpness" is a rather crude attempt to get a grip on all these volatile factors.

 

But—as soon as you do agree upon some maximum acceptable diameter of the circles of confusion, you defined something a formula can kick in and act upon. We call this technical depth-of-field. It's the depth of a range in the object space from where points will be depicted no larger than the agreed-upon maximum COC diameter in the image plane ... no matter if a real-world person actually accepts these COCs as sufficiently sharp in a real-world print or not. Technical depth-of-field hopefully is similar to but not necessarily the same as apparent depth-of-field.

 

And the whole point of this discussion thread is this: At the same focal length, same focusing distance, and same aperture, technical depth-of-field can be different for different lens designs—in particular for (but not limited to) Asph vs non-Asph.

[pgk]

Technical depth-of-field hopefully is similar to but not necessarily the same as apparent depth-of-field.

But in a pictorial (as opposed to scientific) photographic context there is only apparent depth of field because depth of field is a visually (not mathematically) assessed criteria. It is most likely that the OP was interested in this, unless there is some scientific application of lenses such as the 28/2.........