Raynox (150 & 250 - DCR) apperture.

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All Ex
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Location: Greece Thessaloniki

Raynox (150 & 250 - DCR) apperture.

Post by All Ex »

As we all know, these macro, let's say, adapters are wide open by default.
I use them to form tube lenses, as Rik suggests.
My problem is that due to the very wide aperture (I use 1.4 for f-number ) the estimation equations give me a very large number of stacks (600-700 or even greater). I asked to place an iris in the front part of the tube lens, but Rik told me that it will affect my image quality.
what are your suggestions, if any, on lowering the number of the stacks?
All--Ex
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rjlittlefield
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Post by rjlittlefield »

It seems clear that you are using either the wrong equations or the wrong numbers, but at this moment I have no idea how to correct your misconceptions.

Let's start with this: what equation are you using, and where did you get the number 1.4 to plug in as an f-number?

--Rik

All Ex
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Location: Greece Thessaloniki

Post by All Ex »

2cn(m+1)/(m*m)
Where c=circle of confusion, I use a D800E
n=f number
m=magnification
f=1.4 is an approximation I make based on a common lens (the number most common lenses use when they are wide open)
I use (for image quality) a bit of a smaller step.
All--Ex
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rjlittlefield
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Post by rjlittlefield »

That formula only applies to a single lens that is focused entirely by extension.

When you say that you're using the Raynox as a "tube lens", I assume you have the Raynox focused at infinity, and there's another lens in front of it.

What is the other lens in front? If the front lens is adjustable, then how do you have it set?

--Rik

All Ex
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Location: Greece Thessaloniki

Post by All Ex »

Sorry for my absence, I have an afoul situation with my health, what do you mean I should have another lens in front of the tube, other than my objective?
I use your Raynox "adjustment" attaching my objective in front of it. Just as you sow in that thread.
All--Ex
My YouTube initial video

rjlittlefield
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Post by rjlittlefield »

Aha, "an objective". That is the other lens that I was asking about.

In that case it makes no difference whether you are using a Raynox or any other tube lens, and it makes no difference what the aperture of the tube lens is.

The DOF is determined entirely by the working aperture of the objective. If you want to use a formula, the simplest one is DOF (in mm) = 0.00055/(NA*NA). For example if your objective is NA 0.25, then the calculated DOF is 0.00055/(0.25*0.25) = 0.0088 mm, equal 8.8 microns.

However, I think I now understand that your basic problem is just the shallow DOF of a microscope objective. It really doesn't make any difference whether you use the right formula or the wrong formula, the actual DOF is just whatever it is.

If you want more DOF per image when using a microscope objective, then the only thing you can do is to reduce the working NA.

You can do that by adding an iris just behind the objective. For example if your objective is NA 0.25 and the hole in the back of the objective is 10 mm diameter, then adding an iris that is only 4 mm diameter will reduce the working NA of the objective to only NA 0.10 (=0.25*4/10)

However, that will reduce the resolution and sharpness -- not because I say so, but because that's just the way light works. Smaller NA implies more DOF but less resolution.

To reduce the number of frames in your stack, without focus banding, you will have to give up some sharpness & resolution.

--Rik

All Ex
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Joined: Mon Jul 20, 2015 3:54 am
Location: Greece Thessaloniki

Post by All Ex »

As always Rik I thank you.
In a previous thread of mine, I`ve got, more or less, the same answer of you.
The information though you gave me is valuable. I think it worth the thread.
Have a nice day.

:smt039

P.S.
in your formula, I noticed, that the better is the objective (bigger NA) more stacks are required.
All--Ex
My YouTube initial video

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