Canon 200mm Prime as a Rail.

[jsp]

Hi,

I'm experimenting with a Canon 200mm prime lens as a rail. I wondered if you would like to see what I find as I go along?

First I tested the clarity of the Canon lens against my Olymus Zuiko manual lens that I used previously as a tube lens for my Mitutoyo objectives.

The Canon one is the Canon EF 200mm 1:2.8 L II lens
The Olympus is a 200mm prime, manual focus. It says Auto-T OM-System MC 1:4 f=200mm 324461

Attached are the two images of a ruler to show the clarity and depth of field at the focal plane.

They were taken from the same distance away outside. The canon lens seems fine.

Jen

[rjlittlefield]

Jen,

I'm guessing that "as a rail" really means "as a focusable tube lens". That is, you plan to step focus by sticking a microscope objective in front of the lens and using the lens's internal focus mechanism to step focus through your subject.

In that case, the microscope objective will be stopping down the 200mm lens to around f/20, depending on exactly which objective. Further, it will be doing that by adding a stop in front of the lens, not by using the internal aperture.

As a result of these differences, testing the lenses by themselves will not tell you much about how well they work when paired with an objective.

--Rik

[jsp]

Thanks, that's good to know. I will find out the hard way later this week when the adapter arrives. :-)

[Chris S.]

Jen,

To make sure you are aware, you'll likely want to focus the prime lens at or close to infinity when using it as a "focusable tube lens."

Rik, from offline conversations, I know that one objective Jen hopes to use this way is her Mitutoyo 50x/0.55. Her subjects are fern gametophytes, which are not terribly deep.

I've not used this approach. I'm aware that it works well at low NAs, and suspect that it becomes more limited as NA increases. Do you have a sense whether or not the "focusable tube lens" approach will work well with this objective and subject depth?

--Chris S.

[rjlittlefield]

Do you have a sense whether or not the "focusable tube lens" approach will work well with this objective and subject depth?

In her other thread, I have already pointed to viewtopic.php?p=92144#p92144 , which documents my experience with using a 50X NA 0.55 lens on my 100 mm macro lens.

One important observation there was that "So, at this magnification and NA, my stack can only be a total of 40 DOF's deep." Taking NA 0.55 DOF as 0.0017 mm, that suggests a maximum subject depth around 0.068 mm.

However, even that much assumes the rear lens has enough focus throw to reach that limit, which I am not optimistic about. I just now ran a quick test with my Mitutoyo 50X NA 0.55 lens on my 100 mm macro lens, and to shift focus at the subject by 0.060 mm I had to refocus the macro lens to less than 0.5 m. In contrast https://www.usa.canon.com/internet/port ... -8l-ii-usm lists the minimum focus for that lens as 1.5 m. So, I'm thinking that at 50X, she won't get enough focus range, regardless of image quality.

--Rik

[Chris S.]

Thanks, Rik. I hadn't noticed Jen's other thread until you moved it. Asked and answered, I see.

--Chris S.

[jsp]

Thanks for explaining. The numbers are hard for me to get to grips with, and I don't know what NA means.

I'm not really that picky about which objective I use. My subjects are all tiny plants that are all just one cell thick. I'm not trying to fill the frame with my subject. I could just use a much lower magnification objective and crop the image down. At this point, just getting the photo at all would be great, given how difficult it is. If I can't use the perfect objective and fill the frame, I'm okay with that.

I have x5, x10, x20 and x50 objectives to choose from.

The hardest photo that I want to take is a single celled fern that stands straight up like a street lamp so is very thin. When it grows up it gets to look like the attached images which were taken with a 10x objective. I have no idea what size the cells are.

I think I could do just fine using the 10x objective even on the smallest little single celled fern.

[jsp]

This is what a single shot looks like with the canon 200mm lens and the 10x mity, but I can't get the camera to remote-fire the flash with Helicon Remote, so I don't have a stack yet. Still working on it. The image seems like a good start though.

[rjlittlefield]

I don't know what NA means.

NA stands for Numerical Aperture. It describes the angular width of the cone of light that is accepted by the objective. Larger NA gives higher resolution but shallower depth of field.

On Mitutoyo M Plan Apo objectives, NA is the number that is written immediately after the magnification, either following a slash or on the next line. So, "10X / 0.28" means 10X magnification and NA 0.28 .

--Rik

[jsp]

Thank you for explaining. :-)

My investigations are bit complicated now because Helicon remote will trigger the camera, but for some reason the remote flash trigger doesn't work. Also since I set it up, the Canon EOS Utility 2 has gone on the blink, blaming a dodgy USB cable. I will keep trying to fix it.

[jsp]

Here is a stacked image from the 10x Mity and Canon 200m prime.

I had to take the images manually, turning the focussing ring of the 200mm lens. The Camera does not seem to get on well with Helicon remote. At one point the camera crashed and I couldn't even turn it off. I had to remove the battery from the camera in order to restart it.

However, it is quite easy to just take the photo by turning the lens focus ring in small increments, while firing the shutter.

The subject is an old 1p piece from 1939. The scratches are from my polishing it with a wire scourer.

[jsp]

This is a stack with the 50x objective showing just scratches. This was harder because the 50x required a lot more light, and because the vibration of my pressing the button on the camera caused noticeable movement of the image in the viewfinder. I'm not sure that 50x would work with pressing the camera shutter button by hand.

Also the depth of field that could be achieve with stacking by using the focusing motor of the 200mm lens was very shallow indeed. It was okay for this flat surface, but when I tried to photograph a single hair, which was almost perpendicular to the lens, I couldn't really get enough depth in the stack to do it.

I think this technique will not work for my subject with the 50x lens and I will probably stick to 20x or 10x.

[rjlittlefield]

The problem with the flash may be due to Helicon Remote running the camera in Live View mode, and the camera refusing to trigger flash in Live View.

If so, then you may be able to work around the problem by setting the camera to use its internal flash, blocked from illuminating the subject, and triggering the external flash as an optical slave from the in-camera flash. This is not ideal in theory because what the camera actually does is to momentarily pop out of Live View, take the flash picture, then go back into Live View, all of which introduces extra vibration from mirror and shutter movement. But with a short flash (low power) it usually works out OK.

--Rik

[Lou Jost]

Also the depth of field that could be achieve with stacking by using the focusing motor of the 200mm lens was very shallow indeed.

As we've mentioned before, it is very difficult to use tube lens focus bracketing at high m. But you could get a bit more depth of field if you used a macro lens as tube lens, because it has a longer focus throw.

[jsp]

Thanks Lou, yes I understand.

I'm still in a dilemma between trying to buy another rail, or sticking with the autofocus tube lens and using a lower magnification lens. It's a bit tricky as there are so many other problems still to solve, and I can't really predict which combination of things will solve it.

My ferns are also very hard to grow and hard to light. I will have to wait and see if I get lucky and actually have anything to photograph in a few weeks, and whether that coincides with actually having the time to photograph it.

The advantage of the autofocus lens as a tube is that it takes a huge amount of the faff out of running my system and when I'm short of time and the ferns are short of patience, then having that extra speed really helps.

I also have a 100mm autofocus macro lens, so I could try that.