Dark Field illumination improvements

[nlamendo]

Hello folks,

I'm using an old AO Spencer microscope to take pictures of snow flakes using a 4x Lomo objective. The scope has a condenser but I wanted to improve my dark field shots. I finally figured out the best way to do this. The shots below are the best I've managed to date (shots taken last weekend). I made an obstruction that is in place below the condenser. I've also discovered that I need to remove the top element of the condenser.

I only get an image in the central part of the camera frame. Do I need to just shorten the distance between the objective and the camera to get the image to fill the frame? Should I be able to fill the frame of a Nikon D300s with a 4x (finite)objective?

Thanks for any help.
-Nick

[Tom Jones]

I'm not going to be at all helpful with your question, except to suggest you might want to describe which AO Spencer scope you're using, and the camera setup. It would probably make it easier for some of our experts here to help.

What I will say is that those two shots are very, very nice. . If I actually had snow this winter , your shots would make me want to try shooting snowflakes in darkfield. I'd be pretty happy if I got close! How are you keeping it cold enough to get the shots before they melt?

Tom

[rjlittlefield]

I only get an image in the central part of the camera frame. Do I need to just shorten the distance between the objective and the camera to get the image to fill the frame? Should I be able to fill the frame of a Nikon D300s with a 4x (finite)objective?

What problem are you running into: vignetting or too much loss of image quality?

In either case, the usual cure is to lengthen the distance, not shorten it. You want the light spreading out farther so as to fill the sensor.

These images look great, by the way.

--Rik

[discomorphella]

Even without filling the sensor, these are spectacular. You have inspired me to try snowflake photomicrography, so I left some slides in the freezer to prechill them. I tried to capture some flakes in our latest snowstorm, but true to the coast range in Oregon, it wasn't quite cold enough to generate such symmetrical specimens. Just cold enough to generate 1.5 ft of snow...

David

[nlamendo]

Thanks all for the replies,

David - Sorry your weather isn't better, maybe if you are ever up in the mountains in a snow storm, for a few days, with your camera and microscope and...... Never mind just enjoy the weather you have.


Rik- I'm OK with the image quality, it must be vignetting. I would like to be able to have the image fill the cropped sensor of my D300s, right now I just get a circle in the middle of the image frame. That means I can only shoot small snow crystals. I'm not sure if it's means I need less magnification or what, That's why I thought I might need to shorten the distance between the sensor and the camera. I'm just using a microscope to camera adapter w/ a T-thread adapter for my DSLR. I remove the eyepiece from the old black AO Spencer microscope (1942 vintage) and shoot through the scope that way.

Tom - I hope my explanation above to Rik describes my setup well enough. It's just a simple old scope with my camera couple to the eyepiece tube with the eyepiece removed. I'd like to fill the camera sensor with the image. The scope and slides are left in an unheated barn, temps in the teens (deg F) work best.

Thanks all, for the positive comments on the images, mother nature is incredible.

[nlamendo]

Hi Rik,

The link below shows a sample image right out of the camera, I'd like to be able to use more of the camera sensor. Is this considered vignetting? Any suggestions on what I could try?

Thanks
-Nick

http://www.flickr.com/photos/46796256@N05/6728596613/

This shows the microscope I'm using.
http://www.flickr.com/photos/46796256@N05/6728595983/

[rjlittlefield]

Yes, this is vignetting.

That black border is basically a shadow. It's caused by something mechanical that intrudes between the objective and the sensor.

You can probably figure out what that is by just removing the camera and looking down at the objective along the same light of sight that the sensor would have. When you're looking straight down the center of the tube, you'll see the objective. Slide your head sideways and at some point the objective will be cut off by something. That's your problem.

If the problem is the camera adapter, then perhaps you can modify or replace the adapter to reduce or eliminate the problem. If it's the tube, you're stuck unless you want to do away with the microscope entirely and just stick the objective on the end of bellows or extension tubes. That works great from an optical standpoint (see the FAQ), but then you'd have to rework the illumination.

Moving the camera upward, farther from the objective, will let the image expand to fill more of the sensor. But that comes along with higher magnification, so it won't let you photograph larger crystals.

Sorry I don't have a great solution here -- hard problem!

--Rik

[nlamendo]

Yes, this is vignetting.

That black border is basically a shadow. It's caused by something mechanical that intrudes between the objective and the sensor.

You can probably figure out what that is by just removing the camera and looking down at the objective along the same light of sight that the sensor would have. When you're looking straight down the center of the tube, you'll see the objective. Slide your head sideways and at some point the objective will be cut off by something. That's your problem.

If the problem is the camera adapter, then perhaps you can modify or replace the adapter to reduce or eliminate the problem. If it's the tube, you're stuck unless you want to do away with the microscope entirely and just stick the objective on the end of bellows or extension tubes. That works great from an optical standpoint (see the FAQ), but then you'd have to rework the illumination.

Moving the camera upward, farther from the objective, will let the image expand to fill more of the sensor. But that comes along with higher magnification, so it won't let you photograph larger crystals.

Sorry I don't have a great solution here -- hard problem!

--Rik

Thank You Rik,
This really helps. I'll see if I can figure it out now. I recently purchased an RMS threads to M42 cone adapter and now I'm waiting for a set of M42 macro extension tubes. So I may be switching to what you suggest, but it will mean starting over with the illumination.
Thanks again.
-Nick

[g4lab]

This thread reminded me of something that I don't think got said, in a not too long ago thread, on darkfield for larger sized objects.

The condensor you are probably using is likely a variant of the Abbe condensor. Even if it isn't, better ones work the same way. That is, they all have very short focal length (on the order of 2mm ) for the top element so that they can properly match the oil immersion objective.

They also can usually match the 40x range objective and even a 10x. But 10x is usually the lowest magnification they will go. If you want to do a lot of work (like take central obstruction, darkfield pictures) you do need to take that top element off. This makes the condensor longer focal length, and a larger diameter image of the illuminant.

On the other thread this thought was lurking just below consciousness, but close enough that it was annoying me. But I did not think about it hard enough to bring it to the surface. I guess I'm gettin old.

Edit: It was the recent Rheinberg Illumination thread but the same principle applies.

[nlamendo]

This thread reminded me of something that I don't think got said, in a not too long ago thread, on darkfield for larger sized objects.

The condensor you are probably using is likely a variant of the Abbe condensor. Even if it isn't, better ones work the same way. That is, they all have very short focal length (on the order of 2mm ) for the top element so that they can prope-*/rly match the oil immersion objective.

They also can usually match the 40x range objective and even a 10x. But 10x is usually the lowest magnification they will go. If you want to do a lot of work (like take central obstruction, darkfield pictures) you do need to take that top element off. This makes the condensor longer focal length, and a larger diameter image of the illuminant.

On the other thread this thought was lurking just below consciousness, but close enough that it was annoying me. But I did not think about it hard enough to bring it to the surface. I guess I'm gettin old.

Edit: It was the recent Rheinberg Illumination thread but the same principle applies.

Thanks for your comments. It's a good reminder for me to always first search previous threads and or the FAQ's when I have a question or can't get something to work. This would have saved me a lot of time figuring out by trial and error to remove the condenser top element. Yes. it is an Abbe condenser.

-Nick