01-07-2015, 06:05 PM
@Joju, you will lose approximately one stop of light with a 1.4x teleconverter. f/4 will become f/5.6.
@Brightcolors
That's not true at all.
A fresnel lens allows you to focus light with less material than a spherical lens. It's based on the theory that light is bent when it hits the material and does not get further bent inside the material. A fresnel lens has many small prism-like structures on it, to focus light you just increase the angle of inclination continuously as you move away from the center of the lens. You can reduce the necessary amount of material in this way by producing a nearly perfectly flat lens that still focuses light (and thus can be very thin), or you may apply it to an already curved surface to simulate a much higher refractive index and thus focusing the light much more strongly. The fresnel element will not contribute third order aberrations to the system, but higher order aberrations must be calculated as they will be present in sufficient quantities so as to impact the system. This is also already a strongly curved surface, so it imparts rather large aberrations already.
To drive the telephoto ratio further from one you must increase the power of the positive and negative elements. To first order (that is, arrangement of thin lenses with positive and negative power simulating the net result of the system) this allows you to place the positive and negative lenses closer together to achieve the same result (and thus, shorten the overall length of the lens). The trouble with this is that spherical aberration can become quite immense when you bring thick lenses close together, astigmatism will rise sharply the closer they get together, and it becomes more and more sensitive to assembly tolerance as well as the presense of local minima within the optimization of the lens, as spherical will quickly invert from being over to under corrected.
The asymmetry of power about the aperture stop will also drive distortion up rather quickly.
Nikon has used a fresnel lens to massively increase the power of the "P" group in the telephoto arrangement and met it with a large increase in the power of the "N" group. The fresnel's contribution to the power of the "P" group is free, diffractive effects aside, but the "N" group's power absolutely is not. Many extra elements were clearly needed to correct the aberrations induced, and either coma or astigmatism has still been corrected imperfectly.
@Brightcolors
Quote:
Scynthels, you seem to not be aware that the reason for the short length is due to the special nature of the PF element, and that such an element does its work without increased aberrations (and so without the need for extra elements)? same as with the DO elements from Canon.
That's not true at all.
A fresnel lens allows you to focus light with less material than a spherical lens. It's based on the theory that light is bent when it hits the material and does not get further bent inside the material. A fresnel lens has many small prism-like structures on it, to focus light you just increase the angle of inclination continuously as you move away from the center of the lens. You can reduce the necessary amount of material in this way by producing a nearly perfectly flat lens that still focuses light (and thus can be very thin), or you may apply it to an already curved surface to simulate a much higher refractive index and thus focusing the light much more strongly. The fresnel element will not contribute third order aberrations to the system, but higher order aberrations must be calculated as they will be present in sufficient quantities so as to impact the system. This is also already a strongly curved surface, so it imparts rather large aberrations already.
To drive the telephoto ratio further from one you must increase the power of the positive and negative elements. To first order (that is, arrangement of thin lenses with positive and negative power simulating the net result of the system) this allows you to place the positive and negative lenses closer together to achieve the same result (and thus, shorten the overall length of the lens). The trouble with this is that spherical aberration can become quite immense when you bring thick lenses close together, astigmatism will rise sharply the closer they get together, and it becomes more and more sensitive to assembly tolerance as well as the presense of local minima within the optimization of the lens, as spherical will quickly invert from being over to under corrected.
The asymmetry of power about the aperture stop will also drive distortion up rather quickly.
Nikon has used a fresnel lens to massively increase the power of the "P" group in the telephoto arrangement and met it with a large increase in the power of the "N" group. The fresnel's contribution to the power of the "P" group is free, diffractive effects aside, but the "N" group's power absolutely is not. Many extra elements were clearly needed to correct the aberrations induced, and either coma or astigmatism has still been corrected imperfectly.