Going Thermal...

[Cadmium]

I would like to make some kind of telephoto attachment, which I think would be good for doing stitched pans, but maybe I am thinking wrong?

What is your focal length Andy?

[Andy Perrin]

Thermal cameras typically list angular FOV instead of focal length, and AFOV is 25 degrees. The minimum working distance is 0.6m, and the sensor is 120x120 pixels, each with a pitch of 45 microns, so that gives a focal length of 122mm. In my case at least it is already pretty "tele."

[Cadmium]

Andy, I 'think' the FLIR E4...E8 all have a 45° angle of view, not sure, but I think that is about 50mm?

I compared my D610 (full frame) using 50mm focal length and it looked pretty close to the FLIR view.

Also, I think the infinity focus is a bit off, not sure, but seems like that.

So, I don't really know.

[Andy Perrin]

Hah, no, I don’t think so. You are forgetting how tiny the sensor in these things is. FLIR seems to be using a 25 micron pixel according to this:

http://www.flir.com/cores/display/?id=60983

It could also be the 17 micron one, but the 25 looks closest to 320x240 resolution. That means the sensor is 8mm horizontally.

 

Following Edmund’s handy guide, using a 45 degree field of view and a closest working distance of 500mm (from the Ex series data sheet), I get a horizontal field of view of

HFOV = 2*500*tan(45/2) = 414mm.

 

The focal length would then be

f = (8mm * 500mm)/414mm = 10mm.

 

You can see that the reason it has to be so tiny is that the 8mm sensor requires it. I just redid my calculations above and mine is 12mm, not 122mm. I’m not sure what I did wrong earlier.

 

https://www.edmundoptics.com/resources/application-notes/imaging/understanding-focal-length-and-field-of-view/

[Cadmium]

FLIR E-4...E-8 field of view: Field of View 45° × 34°

Looks about the same as 50mm lens view on my D610.

[Andy Perrin]

Cadmium, it’s a different size sensor. A 35mm sensor would correspond to an equivalent f of 10mm*(36mm/8mm) = 45mm, which is pretty close to the 50 you are claiming. (35mm sensor is 36mm horizontally)

 

For mine it would have an equivalent f of (12mm)*(36mm/5.6mm) = 77mm

[Cadmium]

I am just telling you what the specifications say the field of view is, and it has the same field of view as my D610 using a 50mm lens.

It would be like doing stitched pans with 50mm lens shots (with much lower resolution), so to get more resolution, one would need to use a longer focal length and stitch more of those shots together.

See what I mean?

 

Thermal sensitivity = 320 x 240

Lens = 45° (45° x 34°)

 

Those are the specs, and also what the internal camera information menu shows.

[Andy Perrin]

Yes, I understood that from the beginning. But you asked me what the focal length is! So I figured it out. The true focal length for yours is 10mm and the crop factor is 4.5 so the equivalent is 45mm. About what you observed.

 

Anyway yes, of course you have to zoom in to do good panoramas. Mine was an f equivalent of 77mm, so that’s why they seem to work on mine without adjusting.

[Cadmium]

Where are you getting the 10mm focal length and 4.5 crop factor from?

 

Where are finding the sensor size?

 

OK, I found it on the link.

[Andy Perrin]

The sensor size is (25 microns)(320 pixels) = 8mm. I showed you the calculation in detail above and the formulas are just from the Edmunds link. The 4.5 crop factor is 36mm for a full frame in the horizontal direction divided by the 8mm for the actual sensor size. All calculations were done for the horizontal case.