There is more to the macro/micro world than minerals. I also do some botanical images
Just got an 1.4x III for 200$
Moderators: rjlittlefield, ChrisR, Chris S., Pau
There is more to the macro/micro world than minerals. I also do some botanical images
Sorry, I missed this question earlier.JKT wrote: ↑Wed Jan 25, 2023 1:08 pm...or is it actually f/24? Which is the accurate formula - m/(2*NA) or m*fwork?rjlittlefield wrote: ↑Tue Jan 24, 2023 8:36 pmThat will also be effective f/25 as seen by the sensor. (Feff = m/(2*NA), m=14, NA = 0.28)
Combining the latter with fwork = 1/2/tan(asin(NA)) gives the other value ... but have I gotten that formula right?
Yes I know - practical effect is minimal.
Anyway, with that formula the Mit 20 is f/21.6 without TC and f/30.3 with it. What that means in sharpness should be easy to test with any lens that stops that far down.
OK - I had a nagging suspicion I had seen something to that effect. Thanks for settling it!rjlittlefield wrote: ↑Sun Feb 05, 2023 5:49 pm[It is f/25. Both formulas for Feff are correct, but your formula for fwork is not correct. See viewtopic.php?p=196511#p196511 and following.
My understanding is that https://www.photonstophotos.net//GeneralTopics/Lenses/OpticalBench/OpticalBench.htm operates by tracing rays and then interpreting their paths to derive f#, NA, and pupil ratio. So the reported f# is automatically the effective f#, as seen by the sensor in the lens's normal orientation. NA is reported for the same place, so it should be that NA = 1/(2*f#), and that has checked OK in all the cases that I have seen. For the OpticalBench simulator all these numbers are outputs, not inputs, and they are all on the image side. The simulator could give corresponding numbers for the object side, and if it did, then I expect that all the numbers on both sides would be consistent with the standard formulas. But it does not, so I do not see any way to check that aspect.JKT wrote: ↑Sun Feb 05, 2023 11:12 pmAs a side note, is it really so that the f# reported at Photons to Photons optical test bench is effective f-number, BUT calculated with P=1? That seems to be the only way to make the results work. There was also a small remark in one of the Optics primers that could be interpreted that way. The effect becomes rather drastic when the P drops under 0.1 in the worst case...
The worst case I run into was Tamron SP 180mm f/3.5 macro. The table below is copied from the optical bench and the white cells are inputs. WD, H, H', S & L are not relevant here. The problem is with the higher magnifications. If the effective aperture (for nominal 3.5) was indeed 5.43 at 1:1 as the bench results claim and the pupil magnification 0.08, the required nominal aperture [ f.eff / (1 + m / P ) ] becomes 0.4 and that seems impossible. So there is clearly something wrong and it likely is in my end, but I just can't figure it out.rjlittlefield wrote: ↑Mon Feb 06, 2023 10:01 amCan you give me a specific example of the situation that you're wrestling with, and a specific pointer to that small remark you mention?
Thanks, I see the issue. It would take me too long to work up a detailed math explanation of what's going on. Conceptually, the key element is that at close focus this lens is working its way toward being telecentric on the object side. That means the entrance pupil is moving far back and getting very large. If you tried to shine parallel rays in the front, you could not come close to filling the pupil. The entire pupil can be seen only by an object that is close to focus. In the lens's working configuration, at close focus, all the angles make sense and it's only these mathematical entities called "pupil magnification factor" and "nominal f-number" that get weird. I expect the standard formulas would still work out OK if you're careful to apply them only to configurations where the entire pupil is in play, and if you're prepared to ignore craziness in the intermediate numbers that eventually disappears in the final result. As a matter of practice that means you could apply the formulas to compute the effect of small deltas like say adding some extension behind the lens that is already in its close-focus configuration, to compute how the f-numbers will change. But computing a nominal f-number, which corresponds to an object at infinity, when only a tiny part of the pupil could be seen from there, will produce an intermediate number that does not correspond to physical reality.
OK - that explains it ... at least enough to tell me not to touch THOSE formulas. So the values are effective and correct after all. Theoretically that should make the lens useful for stacking in some cases, but unfortunately it is not that sharp at 1:1. And Thanks again!rjlittlefield wrote: ↑Mon Feb 06, 2023 3:02 pmThanks, I see the issue. It would take me too long to work up a detailed math explanation of what's going on. Conceptually, the key element is that at close focus this lens is working its way toward being telecentric on the object side. That means the entrance pupil is moving far back and getting very large. If you tried to shine parallel rays in the front, you could not come close to filling the pupil. The entire pupil can be seen only by an object that is close to focus. In the lens's working configuration, at close focus, all the angles make sense and it's only these mathematical entities called "pupil magnification factor" and "nominal f-number" that get weird. I expect the standard formulas would still work out OK if you're careful to apply them only to configurations where the entire pupil is in play, and if you're prepared to ignore craziness in the intermediate numbers that eventually disappears in the final result. As a matter of practice that means you could apply the formulas to compute the effect of small deltas like say adding some extension behind the lens that is already in its close-focus configuration, to compute how the f-numbers will change. But computing a nominal f-number, which corresponds to an object at infinity, when only a tiny part of the pupil could be seen from there, will produce an intermediate number that does not correspond to physical reality.
For a lens that is perfectly telecentric on the object side, the entrance pupil is infinitely large and located at infinity, so p=0, and that wreaks total havoc with the standard formulas even though the ray paths don't look strange at all. There are some counterintuitive behaviors such as object-side f# not changing if you focus the lens closer by adding extension. (See viewtopic.php?p=249989#p249989 for some discussion of that.) The formula about f_image = m*f_object still applies, so this means that focusing a telecentric by extension, f_image varies as m instead of (m+1). Surely there must be some way to rewrite the equations so that plugging in p=0 gives that same result, but I've never spent time figuring that out.
The hard way. Copy values, change focus and repeat ... ad nauseam.Question: how did you construct the table? I don't know how to get the photonstophotos OpticalBench to do that.
Kurt, I think this is not necessarily true, though maybe your numbers could be close to correct for the particular combinations you gave. Unlike digital zooming (which does just give empty magnification), increasing the magnification with a teleconverter can increase the actual resolution on the subject, if the aerial image of the original lens out-resolves the sensor.With teleconverter the resolution of the object is not higher (empty magnification).
You may be right that there is empty magnification in this case, but maybe the reason for that is not that TC is not doing the job, but probably because of the diffraction limit there is nothing to magnify ;-)