Infinity optics - position of tube lens

[stephenx_86]

Hi, I was hoping someone would be able to help me with a microscopy problem that has been puzzling me for a while.

I'm building a basic microscope from scratch using a 50x long working distance Nikon objective (LU Plan ELWD, wd = 10.1mm, NA = 0.55). The sample is illuminated from above using a white LED source. I am currently using a "Mounted Visible Achromatic Doublet" from ThorLabs (http://www.thorlabs.com/thorProduct.cfm?partNumber=AC254-150-A-ML) with a focal length of 150mm as a tube lens to focus the light onto a CCD.

As I understand it, if this were working correctly, I should get the image formed on the CCD at 150mm CCD-lens separation and this happens (Although I'm not certain if the CCD should be at the focal point or just beyond it). What confuses me is that as I vary the CCD-lens spacing I can still form a sharp image (by changing the focus of the objective). As the spacing decreases I get a larger image. However, looking at all the infinity optics pages I can find online it appears like I should get image inversion (as I pass from a real to virtual image) around the focal point, but I do not observe this. The image just gets bigger and smaller. At the maximum separation I can get with my equipment the image is still a long way off shrinking to a point.

Please could someone explain what is happening to the light as it passes out of the achromatic doublet lens. Also, is using an achromatic lens as a tube lens not appropriate ? From what I have observed, it does not appear to be behaving as expected.

Any light that could be shed on this would be greatly appreciated. Please let me know if I've not explained things clearly.

Thanks
Stephen

 

[reasonableman]

Hmm I find your description a little confusing. You seem to imply that because the focal length of the achromat is 150 mm then the image should be at 150 mm. This is not correct, you need to consider the output of the objective (which will in turn depend on the object-objective distance).

My suggestion is draw a simple ray diagram starting with the centre line and marginal ray. Newport have some decent introductory articles: http://www.newport.com/Optics-Fundamentals/604533/1033/content.aspx

 

[Andy Resnick]

<snip>
Please could someone explain what is happening to the light as it passes out of the achromatic doublet lens. Also, is using an achromatic lens as a tube lens not appropriate ? From what I have observed, it does not appear to be behaving as expected.

I agree, this is a confusing description. If the objective lens is infinity-corrected, the image is formed at infinity.

The tube lens then has an object distance of infinity, so the image is located at the rear focal plane- as you see. Now, as you make changes to the image distance of the tube lens by changing the tube lens/CCD spacing, you also need to make object distance changes for the objective lens (I don't think changing the tube lens-objective spacing will have any effect- although there may be a 'sweet spot' for aberration balancing). The magnification changes because that's the ratio of the tube lens image distance to the objective lens object distance.

there's nothing wrong with using an achromat for a tube lens- most likely that's what's used in my 'scope.

 

[stephenx_86]

Thank you both for your replies, I now understand what is happening. It seems I had a fundamental misunderstanding regarding the objective lens - I thought the distance from the objective lens to the sample was always at the "working distance". What you have said now makes sense in terms of what I've seen, because when I moved the tube lens I had to refocus the objective. Those links about optics also look very handy.

Thanks
Stephen