Focusing long lens camera

[beardown911]

Dear Guru,


First of all, my question is how to test focus of long focal length camera indoor?
I am really new in photography and I may use wrong terminology.
Please correct me, if my question doesn't sound right.
 
Our department has been building an aerial multi-spectral camera with 150 mm lens attached.
Lens model is sigma 150mm f2.8 macro lens.
Depth of field will be fixed at infinity and f-stop will also be fixed at f2.8.
The reason that we would like f2.8 is because very short integration time, 0.2 milliseconds will be needed.
I've calculated hyperfocal distance using CoC =d/1500 and a number that I've got is about 1070m.
CCD detector pixel size is 6.45 micron.

One thing I am planning is to do focus (or spatial performance) test indoor or similar environment before flying the camera.
I am not going to use such a sophisticated optical bench system.
As said, lens will be fixed at infinity with f2.8. This is a real big limitation.

Based on hyperfocal distance, minimum distance at which an object is within focus would be 535m (1/2 hyperfocal), right?

Would it be any method that the camera is able to take a picture of MTF chart(or similar target) indoor?

Or outdoor test would be the best choice.

Any comment will be very appreciated.

Thank you,

Hojin

 

[KeithB]

You can do what my eye doctor does.  He uses mirrors to bounce the image around in the exam room.  You could get a buch of (good!) mirrors and arrange them so that you have 500 m of path length.  I do not know how the mirrors will affect the resolution, you may need to test them.

[Bob Atkins]

While in theory I soppose you cound use multiple in practice that wouldn't be the best approach. For resolution testing you'd still need a long path, plus you'd need front surface mirrors of optical telescope quality (flatness) and they aren't cheap. Plus no matter how long the path and how many mirrors you use, you'll still not get to infinity!

On an optical bench you can use a device called an autocollimator to adjust focus for long distances. It's a device which has both a high quality lens and mirror in it. A web search will turn up a lot of descriptions of it and how it works.

The best simple test of focus and resolution at infinity can be made by imaging objects such as the Moon and stars, though it's rather difficult to quantify resolution in terms of cycles/mm that way. However you can see how shifting focus from infinity to the HFD affects the sharpness at infinity.

To actually measure resolution (or MTF) at various very large focus distances would need some specialized optics which could effectively project a virtual image of a resolution test target at various distances. If you know the MTF of the lens when it's in focus, you could probably calculate the MTF with various amounts of defocus. Ther are formulae available for the calculation of the defocused MTF of a lens which you can probably find in advanced optics textbooks.

[beardown911]

Dear Bob and Keith,

Thank you so much for your comments and advices.
Bob's comments sound a bit technical to me, but I will definitely research on that.

Thanks and have a nice weekend,
Hojin