Timeline for Why doesn't the focus distance at which my lens has magnification 1 match the formula?
Current License: CC BY-SA 4.0
9 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jun 16, 2020 at 11:21 | history | edited | CommunityBot |
Commonmark migration
|
|
| May 30, 2018 at 18:09 | history | edited | Edgar Bonet | CC BY-SA 4.0 |
Clarify: thick lens model → description in terms of cardinal points.
|
| May 30, 2018 at 17:48 | comment | added | Edgar Bonet | @PhotoScientist: My “thick lens model” is only as a way to parametrize a compound lens in terms of its cardinal points. Its surfaces are not considered. Maybe I should clarify this in my answer. The cardinal points can be determined computationally if one knows the optical formula, or experimentally on an optical bench. Both methods are difficult, although the experimental one may be more accessible to a non expert, at least if high precision is not required. | |
| May 30, 2018 at 17:06 | comment | added | PhotoScientist | I assume that, unlike Alan Marcus, we agree that the location of the front and rear nodal points is the key to accurately assessing the conjugate distances OP is inquiring about. | |
| May 30, 2018 at 17:06 | comment | added | PhotoScientist | @EdgarBonet Granted that is correct; but for a complex optical system like the 100mm macro in question your model would require that appropriate front an rear surfaces be virtualized whose indices correspond to the idealized thick lens but have no bearing on the actual optical system. Only then can the location of the nodal points be predicted. What I was implying is that the actual elements of the lens can be used to make this determination only if there is no negative element. I worry that your approach, while computationally correct, is difficult to implement in the field. | |
| May 30, 2018 at 16:47 | comment | added | Edgar Bonet | @PhotoScientist: The thick lens model is applicable to any non-afocal, axially-symmetric optical system within the paraxial approximation. Wether the system is made of positive elements, negative elements, or a mixture of both makes no difference. The model can obviously not predict the performance of a lens, as the paraxial approximation essentially ignores all aberrations. It can, however, exactly predict the position of the paraxial image. | |
| May 30, 2018 at 16:17 | comment | added | PhotoScientist | The thick lens equation is slightly better than the thin but neither would materially predict the performance of the lens OP is inquiring about. The thick lens equation can only be used for an optical system with no net negative elements. Given that the aperture (presumably the entrance pupil) of this lens is less than 200mm from the sensor, we know that negative elements are in the lens. Rather than attempting to provide OP with an equation (perhaps lensmakers?), it may be better to help him to discover the characteristics of the assembly empirically. I may revise my answer. | |
| May 30, 2018 at 15:41 | comment | added | Alan Marcus | @ Edgar Bonet --- Nodal Points: The camera lens has several principal points. The two in this discussion are the forward and rear nodal points. While named front and rear nodal. It may be the case that they are reversed as to their actual locations. The essence of their significance – a ray coming in to this system aimed at the forward nodal, exits the system aimed away from the rear nodal. The object distance is subject to front nodal. The image distance (back focus) is focused image to rear nodal. | |
| May 30, 2018 at 14:38 | history | answered | Edgar Bonet | CC BY-SA 4.0 |