Relay lens to increase working distance

[Lou Jost]

I have lenses with poor working distances and fat barrels, especially my reversed MFT lenses. I love these because I can use in-camera focus bracketing to capture a stack in 30 seconds instead of ten minutes. But it is difficult to light the subjects.

Instead of photographing the subject directly with one of these lenses, what if I used it to photograph the aerial image made by a different lens with a longer working distance? Then I could light them easily.

So I tried my Oly 60mm macro lens reversed, on extension, to give me 2x. I used f/4. Working distance, with my auto reversing ring in place, is less than 1cm. First I shot the subject directly. Then I used this same lens to photograph the aerial image produced by my Repro-Nikkor at f/2.8, whose working distance is about 46mm. I used a strew [Edit: "Strew" is the wrong word; probably "strew" means a loose scattering of objects, while this was made by sticking transparent tape to a butterfly wing] of butterfly scales, backlit with a Tronde light, so that lens shading wouldn't confound the comparison.

Full image (at this resolution, both images are similar so I post only one):


Below left is a 100% crop (from halfway between center and top edge) of the direct photo, and on the right is a 100% crop of the photo of the aerial image, adjusted to equalize exposure and color balance. There is a bit of CA and some loss of contrast and detail. But not huge. The difference is not noticeable at web-sized enlargements. It was worth a try. I post it here as an interesting failure.

100% crops: Left, real subject, right, aerial image through relay lens:

[ray_parkhurst]

Hah, I've always heard that the 85RN was used as a "relay lens", but until your post did not know what this meant. This is pretty cool, though I can't explain why it was a failure. The 85RN should have sufficient resolution I would think.

[Lou Jost]

I think the apertures fight each other.The 85RN has to be stopped down to 2.8 for best resolution. The -60 has to be stopped down a bit also. This might be like what happens with a tube lens, when the proper place for an aperture is between the lenses. So maybe a better approach would be to use both lenses wide open, and an aperture between them stopped down to about f/3.2. The results presented here isn't really terrible, and it might be improved by that aperture placement.

[rjlittlefield]

Considering things from the standpoint of the subject, the Repro-Nikkor at 1:1 and f/2.8 will look like effective f/5.6 at best, potentially narrower depending on the optics behind it.

Similarly the reversed Oly by itself, from the standpoint of the subject, would look something like effective f/6. (I'm assuming that the Oly manages f-numbers like Canon does, and not like Nikon, so 6=4*(1+1/2).)

So, there's not a big difference in apertures like you'd have if you were using a 10X NA 0.25 objective in place of the reversed Oly.

I'm thinking this is just a case of two sets of aberrations that play badly together, instead of somehow compensating for each other as sometimes happens.

Regarding the idea of an aperture between the two lenses, I don't see how that's going to work out in this case. That's a good approach when the two lenses are close together so there's no image inversion between the lenses. But in this case where you're forming an intermediate image, where would you put the aperture? If you put it in the plane of the intermediate image then it'll be a field mask, not an aperture. Near that plane it will be an out-of-focus field mask, no effect on most of the image. To affect the whole image, you would need to put the aperture where it would be limiting across the whole field, and the only available locations I see to do that are in or near the relay or the rear lens.

--Rik

[Lou Jost]

Thanks Rik, that makes sense. So you think this might work with a different set of lenses?

[enricosavazzi]

A couple more things to consider:

- To what magnification is the Olympus 60 mm set? This is visible on the magnification scale behind a plastic window on the lens barrel. This lens has floating elements that optimize it for the magnification it is set to. If the lens is then used at a magnification substantially different from the inverse of the one set on the lens (inverse since the lens is reversed), image quality is likely to suffer. If the lens is focused at infinity and its front element is placed a short distance from the sensor, this is obviously a mismatch.

- Focus on this lens is by wire, and with certain combinations of camera settings the lens focus might automatically rack back to infinity when the camera is switched off. The actual magnification the lens is set to should remain visible on the magnification scale of the lens even when the lens is no longer attached to the camera, since this is a mechanical scale.

For all practical purposes, the lens is not electrically attached to the camera when reversed, unless you are using a custom extension cable that joins the contacts within the lens mount of the camera body with those of the lens even when the lens is reversed.

Whether there is any way to stop down an unpowered Olympus 60 mm is very unlikely, since the lens has no aperture ring and by default the aperture is fully open.

[Lou Jost]

Enrico, as I mentioned in the post, I used my automatic reversing ring, which I built. I wrote a post explaining its construction, but the words are too common and I can't find it now....I had shown that I can even control the lens when using it on a Nikon camera, by using a dummy Oly camera to adjust the lens while it is on the Nikon.

The camera is set to return to last focus point when turned on, and it does.

I set the lens to marked 1/2x and, after reversing, adjusted the extension to get 2x.

I use a marked pin attached to the lens flange to ensure that the lens flange is 19.2mm from the subject plane when reversed.

[rjlittlefield]

Thanks Rik, that makes sense. So you think this might work with a different set of lenses?

I don't know any theoretical reason why not, except that there are a lot of possibilities and most of them probably aren't what you want so the search for a good pair might take a long time.

--Rik