JML 20x vs Nikon CFI E Plan Objective 100x

[brant]

Hi Everyone,

I realize that this is probably an easy question for most of you, but why am I getting a much higher magnification on the JML 20x vs the Nikon CFI E Plan Objective 100x? I realize I don't really understand everything on how these objectives work yet, but it seems like the Nikon stays in focus all the way through the zoom, but doesn't nearly magnify as high as the 20x, which does not stay focused through the zoom, but only when within millimeters of the subject??

The JML 20x doesn't have any marking or information other than it's a JML Optical 20x. What is the next step up from this type of objective in magnification? I was thinking it would be these Nikons, but it looks like I was wrong.

I should note that I am using these on the end of a MP-E 65mm.

Thanks for your help!

[rjlittlefield]

I should note that I am using these on the end of a MP-E 65mm.

Yep, that's a key point. What it says is that you've set up a situation so far from normal that it's almost impossible to say what's going to happen.

To recap, infinity objectives are designed to be used in conjunction with a secondary "tube lens" that is focused at infinity. In that configuration, the objective acts like a sort of extreme closeup lens. It makes an object that is actually small and close appear instead to be big and far away ("at infinity"). The tube lens then converges the light from the apparently big and far away subject to form an image on the sensor of the camera.

When used in that configuration, objectives have behavior that is simple to understand. The overall magnification is just the ratio of focal lengths -- tube lens FL divided by objective FL -- and the focus point of the objective stays at a fixed position in front of the objective no matter what focal length the tube lens has or what the separation is between objective and tube lens.

Unfortunately, your configuration is quite different from that. The MP-E 65 does not focus at infinity. In fact at 1:1 its farthest focus point is only about 100 mm in front of the lens. As you turn the ring on the MP-E 65 toward higher magnifications, its focus point moves even closer to the lens. At 5:1, the focus point is only about 45 mm in front of the lens, and further, the focal length of the MP-E 65 has shortened from its original 65 mm to someplace around 40 mm.

Because your secondary lens is focused so very far from infinity, it's no longer simple to understand what happens when you stick an objective on front. In fact, the behavior depends on details of the lenses that there is no simple way to measure. Even worse, you've dragged the objectives so far away from their design point that whatever image you do get from them is likely to be far lower quality than what they could do when used properly.

Short summary: take out the MP-E 65 and replace it with a roughly 200 mm lens that focuses at infinity.

Nikon CFI E Plan Objective 100x

I'm hoping you mean 10X. If you really mean 100X, then be aware that 100X objectives are the devil's own time to work with. They have vanishingly short working distances (fractions of a mm for inexpensive ones), the depth of field is something like 1 micron, the slightest vibration looks like an earthquake, and even when everything is working properly, optical wave effects often make the subject look like it's squirming around in strange ways as you change focus.

But if you do mean 10X, then I'm puzzled about why you would think that the Nikon would be a step up from the JML 20X. So I'm confused. Can you clarify?

--Rik

[brant]

Thanks Rik, yes, I do indeed mean 100x. I am returning it, as I think I made a big mistake. In your opinion, what is the next step up from the JML 20x? I have been looking at Nikon M Plan 40x, which seem to fit what I am looking for. In the meantime, I will try using the 100x on a roughly 200mm to see if I can get any kind of results.

Thanks again for all of your help.

-Brant

[rjlittlefield]

I have two objectives that work well in the range you want. One of them is a finite Nikon BD Plan 40X NA 0.5 ELWD 210/0 with working distance about 10 mm, sample application HERE and at the links therein. The other is an infinite CF Plan 50X/0.55 inf/0 EPI ELWD with 8.7 mm working distance, sample application by morfa HERE.

The ELWD spec is important. That's what gives you enough working distance to be comfortable.

The CF Plan 50X/0.55 also works well on a shorter tube lens, for example at 25X/0.55 on a Canon 100mm f/2.8L IS USM macro as HERE. In this application it will be quite a bit sharper than the JML 20X (which has NA 0.30), but at 25X and NA 0.55 it will have significantly less DOF than the JML.

--Rik

[brant]

What is the difference between the Nikon BD Plans and M Plans? I'm looking at a Nikon BD Plan 40x/0.5 210/0 ELWD vs Nikon M Plan 40x/0.5 210/0 ELWD. The only difference is the BD vs M.

[ChrisR]

Optically we think they're the same,
The BD has a less convenient larger mounting thread ( see the FAQ on threads),
The BD has an annular gap for lighting to go down which you'd have to fill,
Sometimes you can remove the outer part of a BD to give more subject access.

[brant]

Thank you for the answer. I really appreciate it!

[rjlittlefield]

The BD has a less convenient larger mounting thread

Yes, I bought a BD-to-RMS adapter. This works well with the rest of my equipment. The adapter adds about 6 mm to the flange distance, so if you're going to use the lens in a microscope frame, be sure you have enough extra stage travel to allow focusing.

The BD has an annular gap for lighting to go down which you'd have to fill,
Sometimes you can remove the outer part of a BD to give more subject access.

The outer collar is often glued in place and may be impossible to remove by just twisting. Mine and others yielded quickly to ethyl acetate, as described HERE and later in the same thread.

--Rik