Tube lens or 200mm lens as converging lens?

[cactuspic]

I am experienced in the macro range of magnification (1x-5x) and feel very comfortable there. But I am a relative neophyte in the micro ranges of 10x and 20x. In the past, when I have ventured into these higher magnifications, I have used my canon 70-200mm lens (at 200mm focused at infinity) as well as an older 200mm Nikon lens (focused at infinity) as my converging lenses for my 10x and 20x infinity focused objectives. I have been reading the recent tests and posts relating to tube lenses such as the Raynox with great interest. I surmise from recent comments that the tube lenses such as a reversed Raynox or possibly a Sigma lifesize diopter yield superior results. Is that a correct interpretation? I checked and could find no direct comparison between the two methods, but I may have overlooked the relevant post.

As I understand it, distance between the tube lens and the camera sensor is the extension necessary to focus the tube lens at infinity, which may be experimentally determined by using a bellows and live view to find the exact length of extension required to bring a distant subject into sharp focus. The distance between the tube lens and the objective is somewhat variable. Is this correct or am I all wet? Are there any rules of thumb regarding objective to converging tube lens distance?

As someone who lacks any scientific background past and no real experience in microscopy, I struggle with some of the technical explanations in this forum. If the answers to my questions have been answered previously or are obvious, please accept my apologies.

To the extent that it is significant, I shoot with a Canon 7D, Canon 5Dii, and Canon 5Diii. My objectives are a Nikon Pan 10x/.25 CFN22 WD 10.5 and a Nikon Plan Fluor ELWD 20x/.45 WD 7.4. I also have the Sigma diopter and a Canon 250D diopter.

Irwin

[rjlittlefield]

I have used my canon 70-200mm lens (at 200mm focused at infinity) as well as an older 200mm Nikon lens (focused at infinity) as my converging lenses for my 10x and 20x infinity focused objectives.
...
I surmise from recent comments that the tube lenses such as a reversed Raynox or possibly a Sigma lifesize diopter yield superior results. Is that a correct interpretation?

I think that's been over-generalized. There's a common problem with telephotos that they're inclined to vignette, especially on full frame. Official tube lenses don't vignette, but on full-frame the corners are inclined to go soft. The Raynox and Sigma don't have those problems, so in that sense they're superior. However, if you happen to have in hand one of the combinations of telephoto and camera that does not vignette, then I have no reason to think that you would do better by using a Raynox or Sigma instead.

As I understand it, distance between the tube lens and the camera sensor is the extension necessary to focus the tube lens at infinity, which may be experimentally determined by using a bellows and live view to find the exact length of extension required to bring a distant subject into sharp focus. The distance between the tube lens and the objective is somewhat variable. Is this correct or am I all wet?

This is correct.

Are there any rules of thumb regarding objective to converging tube lens distance?

With telephotos, I think shorter distance is usually better. Certainly it will be less inclined to vignette and will use more central areas of the glass. The one caveat is that shorter distance may also add some glare that will cut contrast, so watch for that. With official tube lenses, you should be OK anywhere within manufacturer's recommendations. With Raynox, I think it's slightly better with at least 25-50 mm of separation, but in my tests the change with varying separation seemed small.

--Rik

[cactuspic]

Thanks Rik. You're so helpful.

Irwin