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new Samyang 135mm f/2
#21
http://lcap.tistory.com/entry/Samyang-13...-135mm-f18

 

Doesn't the first comparison (of the city shot) look like it is more a depth-of-field issue rather than resolution? The Sony ZA looks sharper at f/1.8 on the distant corner than the center air conditioner unit, which isn't how (centered) lenses work.

#22
I'm trying to figure out. I frankly prefer a brick wall to start with  Rolleyes . Let's say that the Samyang looks as an excellent performer again, but probably the test isn't good for a real comparison with the Zeiss.

stoppingdown.net

 

Sony a6300, Sony a6000, Sony NEX-6, Sony E 10-18mm F4 OSS, Sony Zeiss Vario-Tessar T* E 16-70mm F4 ZA OSS, Sony FE 70-200mm F4 G OSS, Sigma 150-600mm Æ’/5-6.3 DG OS HSM Contemporary, Samyang 12mm Æ’/2, Sigma 30mm F2.8 DN | A, Meyer Gorlitz Trioplan 100mm Æ’/2.8, Samyang 8mm Æ’/3.5 fish-eye II | Zenit Helios 44-2 58mm Æ’/2 
Plus some legacy Nikkor lenses.
#23
A couple comments:

 

* "proper" lenses can perform better in the corner than in the center, there is no rule against this.  If the edge of the image field is weighted higher than the center of the field any CAD software be it Zemax or Code V or HEXAGON or OSLO or some other will produce a better corner image than center one. 

 

In a theoretical context, if you perfectly correct coma astigmatism and polychromatic aberrations but leave spherical aberration, a lens with any amount of vignetting will perform better in the corner than the center, since the vignetting will eliminate the most poorly behaved rays.  Not that they are poorly corrected, but this is the case with a couple of nikon's supertelephotos.

 

The ZA 135/1.8 does have quite a bit of axial color, I would imagine based on the results that one of two things is the case:

 

* The review website has a bad copy which is relatively unlikely to have happened twice and the lens is out of alignment

 

* Or the lens is focused better in the corner than the center indicating that there is some field curvature and the center is backfocused.

#24
Quote:A couple comments:

 

* "proper" lenses can perform better in the corner than in the center, there is no rule against this.  If the edge of the image field is weighted higher than the center of the field any CAD software be it Zemax or Code V or HEXAGON or OSLO or some other will produce a better corner image than center one. 

 

In a theoretical context, if you perfectly correct coma astigmatism and polychromatic aberrations but leave spherical aberration, a lens with any amount of vignetting will perform better in the corner than the center, since the vignetting will eliminate the most poorly behaved rays.  Not that they are poorly corrected, but this is the case with a couple of nikon's supertelephotos.

 

The ZA 135/1.8 does have quite a bit of axial color, I would imagine based on the results that one of two things is the case:

 

* The review website has a bad copy which is relatively unlikely to have happened twice and the lens is out of alignment

 

* Or the lens is focused better in the corner than the center indicating that there is some field curvature and the center is backfocused.
 

Hey, maybe that will be a new trend in lens design - sharp in the corners and mush in the center. You can call it the 'Anti-Holga'.

 

As for the ZA 135/1.8, I'm just basing it on the PZ review:

http://www.opticallimits.com/sonyalphaff...ff?start=1

 

(Even though PZ doesn't test at infinity, I still say that in the comparison it looks like it is back focused.)
#25
There are many aspects to lens design which would baffle most photographers.  In general the optics industry really does not care what photographers want for lenses on a "quality" note.  For example, photographers would love a lens that is completely transparent and has no meaningful aberrations of any kind and such a lens can be designed, but no one could pay for it if it were fast.

 

Designing lenses is a balancing act and a compromise between the various aberrations and it takes a great deal of skill to get to a good result and often things are rounded.  For example, nikon's 50mm f/1.8G is really closer to being f/2 wide open.  The first commercial lens to ever feature an asphere was "f/1.2" but the manufacturing specs would only support f/1.4 -- the rays for f/1.2 are mechanically cut off.  I would state with a great deal of confidence that if the transmission of the 135/1.8 is under t/2.2 the lens is f/2 at best and not actually f/1.8 as well.

 

Anyway, in simplest terms distortion results from an uneven distribution of power around the aperture stop.  The double gauss design form corrects distortion through symmetry which is why it is uncommon to see high distortion fast 50s.  The sonnar form, which most faster lenses over 85mm use, solves for distortion by placing a negative element before the aperture stop to reduce the power of the lens before the stop and meet it with mild power behind the stop.  The result is lowish distortion and is fairly brilliant.  The radii of curvature in front of the stop on a couple surfaces are very low, so there are massive aberrations there.  By making the lens longer and splitting elements the designs are improved to a usable level.

 

If you fix distortion by moving the stop, you entirely redefine the corner performance of the lens - the chief ray which defines the image circle is pointed at the aperture stop... if you move it the chief ray moves with it.  Aspherical elements will also add "wiggle" to the distortion which there is no mechanism for correcting.

 

Field curvature (hence forth referred to as petzval) is the source swirly bokeh.  Add a little vignetting and you have the complete formula for it.  If you consider a single lens, it will be free of lower order petzval and reasonably free of higher order petzval if the curvature of the first surface is equal and opposite to the curvature of the second element.  If these elements are split and compounded several times you get symmetric design forms like the biotar or double gauss or landscape lens, etc.  These designs have a flat field through symmetry as well - if the "net refractive index" of the front half does not equal the back half behind the stop, you get an uneven distribution of power about the stop even if the curvatures match.  This produces distortion.

 

Weighting the optimizer and guiding it is also a balancing act.  Typical color weighting is red+blue=1 each, green=2 to mimic the eye.  Weighting center vs corner depends what you are trying to accomplish.  But for example, "no coma" would mean weight the corner strongly.  I believe samyang does this and in general due to their size I think they "have a thumb" on the photographic community's wants more than bigger designers. 

 

But bigger 'design centers' attend industry events, read JOSA and SPIE publications, talk with industry members, and so on and so forth.  Never do the desires of photographers re: aberration balance come into the question.

 

https://dl.dropboxusercontent.com/u/3928...Design.pdf

 

Here is an article that predates CAD that is somewhat relevant- it is about the implementation of computers into the design of lenses and how the process works (or worked, but things have not changed too much in that regard).

 

Many commonplace things today are referenced in their infancy in this one.  'frequency response' is the thought or idea that grew into MTF, and the diagrams of methods of interferometric testing have become commonplace within optics.  Particularly figure 4 has seen almost no change but is now known as a Fizeau interferometer.

 

Note that the target for the hypothetical lens is not even in terms that photographers would use.  Part of that is the historical nature of the article, but things are still done that way.  Depending on the background of the lead designer you design for a certain amount of wavefront error in waves, or you design for a certain MTF value (usually > 50% across the field at the nyquist of the sensor is the goal) or you design for a certain spot size (usually on the order of a pixel in rms diameter) or you design for the RIM plot to nicely fit in a certain scale, for example.

#26
I like the look of this, specially that it has 9 aperture blades, an odd number of blades makes for the pretties starts IMO. I have the Pentax K 135mm f/2.5 which is my most favorite lens, and it would be interesting to compare these. Specially now that I have the NX1 on the way, 28 megapixles on APS-C would really show how sharp both are.

  


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