[Klaus]

They underweight distortions as well I'd say.

[Guest]

The one thing that concerns me when they make a single lens with multiple mounts for digital camera; is that there has been imperical proof that the rear element design needs to match the system the camera is used with for optimal performance (lens rental showed how putting glass of different thickness between the lens and sensor improve/worsen performance for different digital cameras).

-

So.... it begs the quesiton which system the lens is optimized around....

[Rainer]

Or, alternately, I would buy a 135mm f/2 which was more compact and a lot lighter, as it would be if it was designed for the Fuji X/m43 format.

Well, it cannot be much more compact or lighter.

A 135mm f/2 lens will always have an entrance pupil of nearly

70mm ... this is unavoidable. Therefore it needs a frontlens

with at least this diameter. Even if you simplyfy the rest of

the optical construction (at the price of degraded quality)

the resulting lens will not be much lighter nor smaller.

[Scythels]

 

The one thing that concerns me when they make a single lens with multiple mounts for digital camera; is that there has been imperical proof that the rear element design needs to match the system the camera is used with for optimal performance

Much more goes into the placement of the exit pupil than the rear element.  Arbitrarily thick glass will more or less always work if the exit pupil is very far from the sensor.  The angle of incidence measured off of the normal of the rays hitting the edge of the sensor will be minimized if the exit pupil were at infinity (i.e a rear-side telecentric system).  Small AoIs mean better performance with thicker filter stacks on the sensors and more consistent performance across different stack thicknesses.  The array of microlenses on the sensor can only be optimized for one AoI which is likely a 'medium' one, but this will not especially hurt designs that have a distant exit pupil, as they should fall within the field (think collection area) for the lens they should hit. 

 

This is a telephoto lens, so the exit pupil is quite distant to begin with.  Distant enough that it wouldn't merit special consideration in the design and it likely 'just is' whatever the distance is. 

 

The lens would be optimized for canon EF mount, as this is the most popular mount and drives the most sales.  a 2mm thick coverglass would be used in the design of the lens.

 

 

Well, it cannot be much more compact or lighter.

Sure it can.  Aberrations with field dependence (coma, astigmatism, distortion, field curvature, and 5th order spherical) reach lower max values on a smaller sensor.  This would allow the pursuit of a greater telephoto ratio (=shorter) and the chief ray (touches corner of the sensor) defines the size of all elements after the peak in negative power of a lens.  For the new samyang, everything after the third element could be shrunk down.  A more tapered barrel could also reduce size.

[popo]

It seems to have some focus shift due to badly corrected spherical aberration.  All other aberrations seem to be quite well controlled.  After their new 50mm and now this I am beginning to think that samyang is designing their lenses with a workflow very dissimilar to other manufactures and strong weighting to off-axis (corner) performance.

Where do you see the focus shift? I'm not sure I can. At least for my specific use it wouldn't be a problem even if there is some, although for an AF lens it would be most undesirable.
 
As for the design, were you following them when they were designing the 14mm? A preview model was tested by a Polish site and on the basis of that performance, they went back to the drawing board to give us the 14mm they sell now... high resolution across the frame...
 

They underweight distortions as well I'd say.

...and distortion was the thing that stood out most about their current 14mm! I don't recall any significant distortions with other lenses they made since though?

[popo]

The lens would be optimized for canon EF mount, as this is the most popular mount and drives the most sales.  a 2mm thick coverglass would be used in the design of the lens.

 

As a minor variation, I'd think they'd design optically for Nikon F mount, since their mount is slightly further out than Canon. As such they could easily extend the lens body for others. Unless the optics can stick out much further than the lens mount for whatever reason.

 

Sure it can.  Aberrations with field dependence (coma, astigmatism, distortion, field curvature, and 5th order spherical) reach lower max values on a smaller sensor.  This would allow the pursuit of a greater telephoto ratio (=shorter) and the chief ray (touches corner of the sensor) defines the size of all elements after the peak in negative power of a lens.  For the new samyang, everything after the third element could be shrunk down.  A more tapered barrel could also reduce size.

I think the argument wasn't that it couldn't be made smaller/lighter, but it was to question how much smaller/lighter it could be. A 135mm f/2 lens would typically cover full frame sensor, and we would be looking at say 1.5x smaller image circle radius for APS-C sensors. Most of us aren't optical designers so don't have the experience estimate in detail, but the assumption would be the optics at the front would dominate regardless, and reducing elsewhere would lead to relatively minor gains in comparison.

 

A similar question often pops up on various forums, why aren't there longer focal length lenses (say, > 300mm) made specifically for APS-C sensors? The argument against it typically runs along the lines of the saving being insufficient to justify having two versions of the lens, so just make the one with the bigger image circle to cover both uses.

[Scythels]

 

Where do you see the focus shift? I'm not sure I can. At least for my specific use it wouldn't be a problem even if there is some, although for an AF lens it would be most undesirable.

The depth of field primarily extends backwards as the lens is stopped down, so there is minor backward focal shift, as the front depth of field doesn't expand much.

 

 

As for the design, were you following them when they were designing the 14mm?

I wasn't following the design side of things when they were designing the 14mm, at least not with photographic lenses anyway.  I certainly feel well enough educated about that lens to comment on the production design, but not the process or any previous designs. 

 

 

I'd think they'd design optically for Nikon F mount, since their mount is slightly further out than Canon. As such they could easily extend the lens body for others. Unless the optics can stick out much further than the lens mount for whatever reason.

Designing "for a mount" is fairly relative, especially with a telephoto lens such as this.  The parameter for the distance from the vertex of the rear element to the image plane in Code V (optical CAD) is "IMC" or Image Clearance.  With a telephoto lens, IMC will almost definitely be >44 naturally, so there is no need to 'design' for any one particular long flange mount as it will all work out the same anyway.  Wide-angle retrofocus lenses which want to smash themselves up against the image plane in optimization must be designed for very specific IMC constraints (i.e mount specific) but telephoto lenses generally do not.

 

 

A 135mm f/2 lens would typically cover full frame sensor

Statements like these are quite dangerous.  Here: http://i.gyazo.com/d3247be61fadc9db9268390d769d6fb0.png is a working design for a camera lens.  The focal length of a lens is arbitrary for this design form (retrofocus) since a longer focal length will just mean a bigger lens for a bigger sensor with the same angular coverage.  The red bundle is the marginal ray - it traces from the optical axis to the center of the image plane and defines the f number.  The green bundle is the chief ray - it traces from the edge of the angular field to the edge of the image plane and defines the size sensor a lens covers as well as the field of view (the two are directly linked if the focal length is held constant).  You can see that shortly after the lens begins to strongly focus the light into a smaller bundle, the chief ray extends "further out" away from the marginal ray.  If you shrink the diameter of the lens elements, you will vignette the chief ray bundle there.  Ultimately, the size of the "back" elements determines the size sensor the lens covers.  It cannot be made arbitrarily large, typically the focusing within the lens limits the rear diameter to be +/- the front diameter but some designs are larger in the rear, though that makes for an odd design.  Because long focal length lenses have.. long focal lengths and large elements in front to allow their aperture (e.g f/2) their tendency is to "want" to cover a very large sensor.  However if there is vignetting that can be observed on a particular sensor size, one may be assured the lens does not 'cover' a much larger image area.  As this lens is a retrofocus lens, it could be extended to a sensor 2-3x larger (look at the angle of the chief ray coming out of the aperture stop) but everything after the front two elements would have to be very considerably enlarged.

 

 

I think the argument wasn't that it couldn't be made smaller/lighter, but it was to question how much smaller/lighter it could be

The volume and weight can be considerably reduced, but without a radical redesign the length must remain.  A lens barrel that tapers back to the lens mount without being very wide (e.g 100mm diameter of superteles) looks funny so it is often not done. There are most likely structural issues to that as well, but I am not a MechE.  In telephoto design the would-be-called-telephoto group i.e the first several elements dominates overall size and weight due to general practices in mechanical design.

 

 

A similar question often pops up on various forums, why aren't there longer focal length lenses (say, > 300mm) made specifically for APS-C sensors? The argument against it typically runs along the lines of the saving being insufficient to justify having two versions of the lens, so just make the one with the bigger image circle to cover both uses.

In a nutshell, in very long focal length situations (mirror lenses aside) the front elements are generally very large and determine the general diameter of the barrel.  The elements that could be reduced in size to cut off the image circle contribute little to the overall weight of the lens, and there isn't much sense in making a backwards-tapering lens barrel.  I would have shown a telephoto ray trace above but my student copy of Code V hasn't been approved for this semester yet and I don't love photozone enough to travel to the lab and back to produce a lens diagram for the forum

[popo]

Thanks for some info there. I think we're talking about the same thing, but from different "perspectives". I even have wondered if there existed something like "a photographer's guide to optical theory and design" but that would be a rather niche book... I have even tried reading various book on optical theory and design, including a telescope design book, but I find my interest levels to be inversely proportional to the number of equations presented

Side note: my "day job" is as an electronic engineer, but I would love to have done physics at university. I knew even then my level of mathematics wasn't good enough to get far in that. So like most I make do with off the shelf optics.

[Scythels]

Quite, but the reasoning we use is different

 

If  you want a more... laymanable design-related book, Bentley's Field Guide To Lens Design is good.  It's still meant for engineers, but it is intended to be something you can open for a quick reference to general issues than something you open for the issues that result in a disparity between what Code V or Zemax computed and what will really happen, or when aberrations are not centered in the pupil, for example. 

[stoppingdown]

Ok, new comparison, now with a Zeiss:

 

http://lcap.tistory.com/entry/Samyang-13...-135mm-f18