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Suggestions for Improvement of 4x-6x Setup?

 
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STJohnson



Joined: 21 Nov 2017
Posts: 1
Location: Portland, OR

PostPosted: Tue Nov 21, 2017 12:53 pm    Post subject: Suggestions for Improvement of 4x-6x Setup? Reply with quote

I was hoping I might find some guidance here on ways to improve the image quality of one of my setups.

At the moment, I'm mostly interested in photographing Collembola (ranging from ~1mm-4mm in size), but the image quality isn't quite where I'd like it to be. I've been shooting with a Nikon D7000 (APS-C) and a Nikon SB-400 flash mounted off-camera. Lenses are often a Nikon 55mm f/2.8 AI-S and a Nikon 24mm f/2.8 AI-S (reversed) used separately (not in combination) on extension tubes (85mm of Nikon PK tubes) or bellows (Nikon Bellows III, 140mm of extension).

I am quite happy with the quality of the 55mm mounted on extension tubes, though it doesn't quite give me the magnification I'm after (only about 2x mounted on 85mm of extension tubes or about 4x with the extension tubes and bellows combined (this is one combination I still need to explore more)).

This photo was taken with the Nikon 55mm mounted on the bellows, shot at f/8 or so:



Often, I'm using the 24mm reversed on 85mm of extension which gives me the kind of magnification I'm after (~6x), but I'm often shooting at f/8 or f/11 to get some decent depth of field and so the photos are often soft from diffraction.

Here are a couple of decent images I've managed to take with the 24mm reversed:







For the most part, I find those images acceptable, however when photographing subjects that are near the 1mm range the quality drops off considerably when using the 24mm reversed.

I must also state that I am most interested in single-image photos. While I acknowledge the benefit of stacking images, I am often photographing subjects that are quite motile and rarely stay in one position long enough to get a usable set of images to stack.

I understand that I should be using a larger aperture, such as f/5.6 rather than f/8 or f/11, as this is where the images would be at their sharpest, but, again, the larger apertures don't quite afford me the depth of field I would like. Perhaps the larger aperture would be acceptable on the smaller subjects, though?

So, my main question is: what should my next step be in order to increase my image quality at a magnification at 4x-6x? Should I pursue a "tube lens" setup and if so, which combination of focal lengths will give me the magnification I'm after? Should I pursue a setup with a microscope objective or would this require me to stack my images?

Of course, there will always be a trade-off between each setup, but is there a better setup than what I have now that will help to increase my image quality without losing much depth of field and without having to resort to image stacking?
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 18246
Location: Richland, Washington State, USA

PostPosted: Tue Nov 21, 2017 7:35 pm    Post subject: Re: Suggestions for Improvement of 4x-6x Setup? Reply with quote

STJohnson wrote:
is there a better setup than what I have now that will help to increase my image quality without losing much depth of field and without having to resort to image stacking?

Short answer: no.

More precise answer: yes, but not by enough to matter, using equipment that you can buy off the shelf.

The problem is that the tradeoff between depth of field and diffraction is determined only by the angular width of the entrance cone (the light entering the lens that eventually forms the image). The wider the cone angle, the more resolution but the less depth of field.

Some lens aberrations can give increased depth of field at reduced quality, and certain types of severe lens aberrations combined with specialized post processing can give substantially increased depth of field with only modest reduction in quality. (See for example:http://optics.org/article/18342 and https://en.wikipedia.org/wiki/Wavefront_coding .) However, I am not aware of any systems that will do what you want and can be purchased off the shelf.

In my opinion, the most promising route for your application is through fast acquisition of focus stacks. A simple approach is to shoot HD video and extract a set of still frames from the video file. Alternatively, there are a few cameras that can acquire full resolution focus stacks very quickly through a combination of electronic shutter and camera control of lens focus. For your application, the technique would involve sticking a microscope objective in front of the camera's lens so that the objective would provide magnification while the camera lens would control focus. Some of our other members have experience with this latter technique and will probably chime in with more information.

--Rik
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