Voigtlander 125 @ f2.5

Technically, a mirror lens is APO

So, I hope it's OK to intrude a bit w/ a converted FD 500/8 ... taken today from my rooftop in Tokyo:

Paul Yi wrote:
CV 90/3.5....
I think this is one of the sharpest lens for the money.


+1. I'm not a fan of the bokeh (this is where I think the CV 180 has it over it's shorter APO siblings) but I am a fan of everything else this lens does.

AlexTokyo wrote:
Technically, a mirror lens is APO
It's not that I care for the weird bokeh, but this lens is fun to play with and manual focus is sooo smoooth. Glad I have LiveView though.

Those are beautiful, Alex. What is it about the mirror design that makes it technically APO?

Those are beautiful, Alex. What is it about the mirror design that makes it technically APO?

Brad

Mirror lenses, as long as they have no lenses elements - just mirrors, do not refract light and therefore do not creat chromatic error (different colors of light focused at different points). APO lenses are corrected for three wavelengths (colors) of the light spectrum (generally red, green, and blue). Meaning that these three colors will be brought to focus at the same point. Lens designers use glass with differing refractive indicies to bend the light (remember the sciince experiments in school using a prism to split white light into the different components of color?). This is where all those terms like UD, LD, AD, ED come from describing, the types of glass used in the optical design. Wikapedia has a pretty do explanation here: http://en.wikipedia.org/wiki/Apochromat


Back to your question: As long as a mirror lens uses no lenses they do not refract the light and introduce color error (chromatic abberation). Actually most photography mirror optics do include refractive lenses elements to correct other optical problems in the overall design and can actually have chromatic aberations. Its just easier for the optical designers to keep the chromatic errors small and unnoticable because the lenses in mirror optics do not need to do a lot of light bending and can also use expensive glass since they are small.

Sorry for the long winded answer, but optical terms are often poorly understood and erroniously used by folks that should know better like on popular lens testing site that equates chromatic aberations to anti-reflective lens coatings rather than the true source which is the designers choice of glass and the optical formulation to correct aberations.

Ed

freaklikeme wrote:
Those are beautiful, Alex. What is it about the mirror design that makes it technically APO?

Thx for the kind words Freaklikeme.
From the little I understand, in Mirror lenses, light paths of different colors are the same when reflected from a surface. An extension of this property is that mirror lenses have no IR focus shift (real APO). Obviously, in a dioptric design (normal lens) light paths of different colors are refracted and focus at different points, creating chromatic aberration. So, Mirror lenses are kind of the poor man's APOs. It comes at a cost though: vignette, fixed and small aperture and for the worst ones poor sharpness. FDs were among the best (w/ Zeiss), and I find sharpness of my copy excellent. I suspect the dark image which made these lenses hard to focus before LiveView might be a contributor to their (undeserved IMO) bad rep on sharpness.

[Edit]: I was typing this while Ed sent his reply. Thx Ed for the more complete explanation. Actually, in typical Mirror lenses there is a corrective element (for spherical aberrations) so one could argue they do have CA. But then, is there a truly APO lens? OK, I'll stop hijacking this great thread. These lenses & pics are fantastic.

Alex - Greeting to your beautiful country

I suppose by the strick definition (focusing three wavelengths and correcting spherical aberation) there are "true" apo designs but in optics, like life, everything is a compromise and the marketing guys love to throw terms around to confuse the consumer. I'm waiting for:

Are you shooting more and enjoying less? Then try the 100% Natural Organic APO Lens - with reduced arsenic!

Optical perfection is a goal not a reality but there is alway - Good Enough.

Best Regards and Happy New Year

Ed

Edward Gill wrote:
Lens designers use glass with differing refractive indicies to bend the light (remember the sciince experiments in school using a prism to split white light into the different components of color?). This is where all those terms like UD, LD, AD, ED come from

You are mixing up refractive index and dispersion here. Dispersion means that every color of light sees a different refractive index and is refracted under a slightly different angle. All those acronyms UD, LD, AD, ED (which are really marketing terms) have the same meaning, namely low dispersion. Different colors of light are dispersed less by this glass.

High refractive index glass is also used but that is not the same thing. Sometimes they are used i.c.w. low dispersion glass to correct CA.

Thank you very much for the explanation, Ed. I like long-winded answers that lead to better understanding. I appreciate it.

Alex- thanks for sharing the experience. I've been curious about mirror lenses, but have never come up with an use for one that required the longer focal lengths. But your examples have a wonderful crispness to them. And great color.

AhamB wrote:
Edward Gill wrote:
Lens designers use glass with differing refractive indicies to bend the light (remember the sciince experiments in school using a prism to split white light into the different components of color?). This is where all those terms like UD, LD, AD, ED come from

You are mixing up refractive index and dispersion here. Dispersion means that every color of light sees a different refractive index and is refracted under a slightly different angle. All those acronyms UD, LD, AD, ED (which are really marketing terms) have the same meaning, namely low dispersion. Different colors of light are dispersed less by this glass.

High refractive index glass is also used but that is not the same thing. Sometimes they are used i.c.w. low dispersion glass to correct CA.

Yeah, there are different glasses like fluorite crystal and etc. used to combat achromatism (CA) but there is also achromatic designs (doublets, triplets, etc., basically, two or more lens elements glued together) and aspherical shapes used for the same reasons.

At the risk of being pedantic, dispersion and refractive index are related--generally, the higher a material's RI, the more likely it is to have greater dispersion as well. But dispersion is indeed a distinct phenomenon, best described as variable RI as a function of wavelength.

What makes correction of CA possible is the combination of two materials with different dispersive qualities in such a way that there is still a net refractive power. This was traditionally accomplished using a crown/flint glass doublet and is called an "achromat" design. The correction occurs near the extremes of the visible spectrum, so that red and blue wavelengths converge at the image plane. But green wavelengths are not made to converge at the same point with this design. This explains why we see the green/magenta "bokeh fringing" in out of focus areas of wide-aperture shots. The achromat design puts red+blue = magenta, versus green on opposite sides of the focal plane.

This is where a material like fluorite comes into play. Fluorite exhibits what is called extraordinary partial dispersion. It is a low-dispersion material, but more than this, its dispersion as a function of visible wavelength is different than for typical glasses. If you plot the dispersion as a function of wavelength, the curve is much flatter whereas the curve for a typical glass is more sloped. As a result of the different shape of these curves, it is possible to construct an optical group that corrects the secondary spectrum to a greater extent than possible with the flint/crown doublet. In essence, one calculates the necessary curvatures to focus three wavelengths together. This is called an "apochromat" design.

With increasing optical complexity and variety of materials, it is possible to construct a "superachromat" design that corrects secondary spectrum at four points in the spectrum. But these are extremely costly and the benefit in most photographic applications is small in comparison. One application of such highly correct lenses, however, is in infrared photography, where correction for the visible spectrum is often done at the expense of the IR spectrum, making many lenses unsuitable for IR imaging. Greater correction in this portion of the spectrum would make a lens suitable for both visible and IR applications.

Although Canon is the only manufacturer to use crystalline fluorite elements to control CA, I presume other optics manufacturers have used fluorite-doped glasses to great effect. Sadly many of Canon's lenses, although employing fluorite, don't seem to correct very well for longitudinal chromatic aberration, and I believe this is in part due to other design considerations. A true "APO" lens should display almost no noticeable transverse CA and very minimal longitudinal CA (green/magenta bokeh fringes in high-contrast, out of focus areas).

wickerprints ...

Thanks ... nice explanation, I've got a better clue now as to why the magenta / green oof's show up. I had figured out that it was one or the other, but it didn't quite make sense to me why ... now it does.

Thanks.

Benjamain

Yes you are correct that my reference to refractive indices (note I used the plural form of index intentionally) really should have been tied to dispersive power (low or high) which is the technical term to describe how differing transmissive materials effect the velocity of differing wavelengths of light passing through the material (refractive index at a specific wavelength).

I had not intended my answer for those like yourself who are apparently highly knowledgeable of optics and optical theory but for someone who wanted to why a reflective optical system (more correctly a catadioptric system) would be an Aprochromat. I assumed the person asking the question might remember light refraction and prisms from early science classes and so kept the terms on a basic level.

All said, you are absolutely correct concerning UD, SLD, ED, LD glass being marketing terms for low dispersive power material (glass, crystal, plastic, etc), meaning the refractive indices vary within a narrow band as a function of the light wavelength.

Thanks for the catch

Ed

Benjamin

Apologize for the typo in your name. I need to proof read before posting.

Ed

wickerprints

There are so many variables to deal with in a complex optical systems, I am truly amazed at the price/performance ratio we see in modern camera lenses. The new high density sensors are also really the pushing limits. I find it no surprise that manufacturers have apparently chosen to allow processor correctable aberrations such as light falloff and distortion to focus (no pun intended) on resolution and lens compactness and let the computer chips correct aberrations in camera. We live in amazing times and really are blessed with fantastic imagining tools.

Ed

Bifurcator

Now you're taking! Give me a good clean hand ground triplet and a starry night to enjoy photons on the retina.
We don't need no stinking 17 element UD AD SLD (sounds like a mental condition or a drug add) magic pipes.

Old school and fading.

Ed

P.S. You related to Euler or just hit a fork in the road?

Again, at the risk of being pedantic....

Reducing the problem of chromatic aberration *requires* the use of at least two different materials of different dispersion. That is the basic principle of an achromat. It's not simply that a given design might have one or more low-dispersion elements, UD or fluorite. For example, the traditional achromatic doublet uses crown and flint glass, neither of which is particularly "low dispersion." But to achieve even better correction of secondary spectrum we need to use another material and this is where fluorite or fluorite-doped glass comes into play.

That is to say, low dispersion glass in itself doesn't entirely fix CA. If you made the entire lens out of it, you would certainly see less CA than if you made it out of normal glass (assuming refractive index is the same but it won't be), but it still won't be an achromat or apochromat. The key to correction is different materials, not necessarily a material with low dispersion. You take one element with one dispersion characteristic and positive refractive power, cement it to another element with another dispersion characteristic and negative refractive power, and if calculated correctly, you get a doublet with net refractive power (positive or negative) and some degree of correction where red/blue wavelengths are made to focus.

Personally, I believe there is still much reason to continue R&D in optical glass to reduce CA of all kinds. First, the amount of CA depends strongly on the design of the lens, with retrofocus and telephoto designs being particularly prone to CA. You can see this especially in UWA zooms, in which the corners are almost inevitably marred by strong transverse CA. We would see a lot more CA in the supertelephotos were it not for the use of low-dispersion or fluorite glass. What is not as obvious is that CA is the primary reason for differences in sagittal and meridional MTF in superteles, not astigmatism which is inherently small for long focal length (due to small angle of view -> low field curvature).

So when we talk about wanting to keep things simple and not use fancy constructions, that's well and good for a 50mm prime, but it just isn't going to fly with a 16/2.8 or 500/4. You can't rely on software to correct this because CA manifests in a variety and combination of forms, not just the familiar transverse CA in which the image is sharp but magnification varies by wavelength. It is undesirable to have to software correct for longitudinal CA and overall haze/loss of contrast due to CA. Distortion is probably the easiest aberration to correct in post--even the "mustache" distortion can be corrected if one collects the relevant data. But spherical, chromatic, astigmatic, and comatic aberrations are best corrected in the lens.

Pedantics ROCK!

wickerprints wrote:
What is not as obvious is that CA is the primary reason for differences in sagittal and meridional MTF in superteles, not astigmatism which is inherently small for long focal length (due to small angle of view -> low field curvature).


This is interesting, I've seen it mentioned before on the forum. So can we tell by looking at the MTF curve of a lens just how much CA it's likely to have and whereabouts in the field it will occur?

Please keep being pedantic, this is great stuff.

Wickerprints

Some of the tests and anacdotal information I have seen on imaging star fields with camera lenses seems to indicate camera lens designers are not fully correcting for monochromatic aberations such as coma in addition to astigmatism which would also probably give measurable differences in differences in the sagittal and meridional MTF results. I would suspect that the introduction of aspheric elements in optic mix would be the primary contributor but I am quickly getting out of my depth in optical design.

Here is an interseting blog on using camera lenses to image star field, some pretty high end APO designs are used and evaluated that indicate astigmatic and coma aberations.

http://www.cloudynights.com/ubbthreads/showflat.php/Number/3534557

Ed