Cameras, Magnification and Field of View, Part 2

I had the pleasure last week of demoing a Yokogawa CSU-W1 spinning disk confocal – this is the new large field-of-view Yokogawa scanhead. Andor and Technical Instruments arranged a demo for us and paired the scanhead with an Andor Zyla camera. I’ll have more to say about the demo later – it’s a pretty cool confocal – but for now I want to focus on some field of view (FOV) issues it raised.

Side port photo.

Nikon Ti side port, with projected image.

The CSU-W1 FOV is 17 x 16 mm, and the Zyla chip size is 16.6 x 14 mm. If you refer back to my first post, you will remember that the side port FOV is 18 mm in diameter, and the largest square you can inscribe in it is 12.7 mm.  If this is correct, we should have seen vignetting with the CSU-W1 / Zyla combo, because they’re larger than 12.7 mm across. But we didn’t.

It seems that the FOV is bigger than 18 mm.  But how big is it? The first, simplest experiment I did was to put a kimwipe over the side port as a screen, near the image plane, and measure the size of the image projected there. If you measure the size of that projected image, it’s about 28 mm in diameter.  That’s the second clue that the FOV is larger than 18 mm.

Camera_translator

The camera mounted on its translation stage.

To get a better sense of what the exact FOV is, and to eliminate possible errors from the fact that that kimwipe is not quite in focus, I next mounted a cheap CMOS camera on a
translation stage, and rastered it across the field of view. I took a set of tiling photos
as I translated the camera, and then stitched them back together in ImageJ. It turns out you can translate the camera almost to the edge of that side port tube and still get an image. You’ll notice that we don’t have the C-mount adapter attached either; we did the experiment both with and without the adapter. Then just by measuring the size of the stitched image we get the size of the field of view.

For a 40x /1.3 NA oil lens, we get a field of view of 21.2 mm with the C-mount adapter attached and 29 mm with it removed. With a 20x / 0.75 lens and no C-mount adapter, we get a field of view of 31 mm.  We haven’t yet checked other objectives. Here are the images. They’re shown at the same scale and link to the full-size images.

40x_cmount_stitched-1

40x / 1.3 objective, with C-mount. 21 mm FOV.

40x_stitched-1

40x / 1.3 objective, no C-mount. FOV 29mm.

20x_stitched-1

20x / 0.75 objective, no C-mount. FOV 31 mm.

It seems that you can get a larger field of view than the 18 mm that Nikon claims – at least 21 mm and even larger if you get rid of the C-mount. So why does Nikon claim 18mm? I can think of a couple possibilities. One is that all objectives may not give this large a field of view. Another is that the optical performance may suffer at the edge of the field of view. It’s hard to rule that out with the low quality brightfield images acquired here. In the future I intend to repeat these experiments with a higher quality camera acquiring point spread functions across the field of view. But for now it’s encouraging that we may be able to access a larger field of view than the 18mm Nikon guarantees.