FN and Sensor Coverage

[pwnell]

Can someone just confirm my understanding of the relationship between the field number of my system vs. the sensor's size and field of view coverage?

My system uses FN26.5 objectives / trinoc tube, which I understand to mean that the field of view that the system will project on the sensor is a circle 26.5mm in diameter.

My camera is a 7D II - therefore the sensor is 22.5mm x 15.0mm. Pythagoras yields a diagonal of 27mm. Therefore the image circle projected by the objectives will exactly cover the sensor, bar the 0.5mm shortfall.

This is assuming no projection optics between the camera and the objective other than the lens I presume that must be in the trinoc head to converge the parallel light rays to a focal plane.

Is this accurate?

[rjlittlefield]

This all sounds correct to me.

--Rik

[Pau]

Yes, it is if the camera sensor is at a parfocal position with the eyepieces like I assume is in your system.

[Charles Krebs]

Waldo,
Yes you are correct. Using the 26.5mm FN objectives and the 26.5mm FN eyepieces along with a Canon APS body is exactly what I do with the MM-11/Olympus setup. The only problem I had was vignetting when I initially used the Olympus U-TV1X and T-MAD components to attach the camera to the tinocular head. As I mentioned in the linked thread:

The U-TMAD has an (easily removed) thin flare-cut diaphragm inside at the top near the T-mount. The opening in this diaphragm is pretty small ~20mm, while the metal "through-hole is" about 34mm. With this diaphragm in place I would get vignetting in the corners of a Canon APS body. Removing it allowed full exposure to the corners. (After removing it, it could be seen that the inner surface machining was quite "shiny", so it was lined with the Protostar flocking material).

When I finished the "project" I made up a different (larger inside diameter) tube between trinocular head and camera primarily because I wanted to have a smooth 360 degree rotation of the camera. In my opinion a 360 degree rotating camera mount is an extremely desirable feature.

[pwnell]

The only problem I had was vignetting when I initially used the Olympus U-TV1X and T-MAD components to attach the camera to the tinocular head. As I mentioned in the linked thread:

I think it was that exact article I followed back when I got my BX53, and I promptly removed the diaphragm.

[Ichthyophthirius]

My system uses FN26.5 objectives / trinoc tube, which I understand to mean that the field of view that the system will project on the sensor is a circle 26.5mm in diameter.

Normally, this means that the diameter of the high-quality intermediate image (used by the eyepieces) is at least 26.5 mm. Usually, the actual diameter of the intermediate image is even larger unless there are any apertures or light traps in the way. It should cover the entire APS-C sensor before things can get difficult on the edges.

Regards, Ichty