Objective magnification

Somewhere here I read that adjusting the length of the objective tube will allow for modifying the magnification of the objective. After an arduous redesign process I was able to print 4 different tubes with 5mm length increment using the inverted RPI lens approach. However, I did not see much difference. Does anyone have a model to share to imitate a 10x and 20x objective?

Hi @dgrosen, are you using the “camera platform/lens spacer” version, or the long optics module? If the latter, a 5mm shift will only produce a slight change in magnification, as it will be roughly linearly proportional to the lens-camera distance.

Getting the field of view you obtain with a 10x or 20x objective is quite hard: with a 10x objective, the RMS optics module gets a sample-to-sensor magnification of only about 2.5, i.e. the field of view is about 1.5mm across (this is off the top of my head, but I think those numbers are within about 50%). To do that with the Raspberry Pi lens (focal length about f=3 mm), you would need the lens-to-sensor distance (call it a) to be m= 2.5 times the lens-to-sample distance (call it b). So:

a = mb

From the thin lens equation
1/a + 1/b = 1/f


1/a + m/a = (1+m)/a = 1/f
a = (1+m)f = (1 + 2.5)\times 3 = 10.5 mm

So you’d want the lens to be about 10mm away from the sensor, give or take a few millimetres, because the lens has a finite thickenss, etc. etc.

The current “lens spacer” is about as short as I thought I could sensibly make it, and so it represents the lowest magnification I could get that works nicely. Figuring out a way to get the lens closer to the sensor would be useful, but not trivial - if you can work out how to do it it would definitely be worth including as an option.

Hi Richard, thanks for the detailed info…I get it, is hard. :slight_smile:
I am using the basic optics module approach. But I think I may have printed to wrong one. I will keep on trying. thanks

ah ok - just in case it helps clear things up, if you have followed the instructions for the “basic optics module” and you used lens_spacer_***.stl and camera_platform_***.stl (where the *** probably includes LS65 and picamera_2) that’s the best one. If you used a single piece, called optics_***.stl, this will have a much longer tube than you want - I tend not to use that any more for the inverted pi lens.

All this years I used the wrong one :slight_smile:!!!
I am using the one piece camera platform with the long tube. That is why it is sooooo close. :slight_smile:
I will print the this models tonight and try it out. How silly I am :wink:

ah, glad I checked! I only added the two-piece option relatively recently, so it’s possible you started before it existed, and I guess I didn’t make a lot of fanfare about the change. Once the new STL selector goes live, hopefully it will be easier to pick the recommended options - but that’s proven harder than we thought to get right. The new module might not be exactly what you’re aiming at, but it should be much, much closer than the one-piece version. Do let us know how you get on with the zoomed-out version.

Success!!!. Printed both the RPi camera platform and short lens spacer. This is perfect!

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Awesome, glad we made that change!