Author Topic: Anyone tried f 0.7 lens?  (Read 554 times)

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Offline cejoba

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Anyone tried f 0.7 lens?
« on: September 08, 2022, 01:10:20 pm »
Hi.
I saw most old tube based thermal cameras use f 0.7 lens, that is 2 1 stops (4 2x) brighter than f 1.0, and 3 2 stops (8 4x) brighter than f 1.4.
So does that mean If I replace the f 1.0 lens on 100mk NETD BST cameras, it transform to 25 50mk?
Or replace the f 1.4 lens on a 50mk VOx camera, it becomes 6 12.5mk?!
Have anyone tried that?
« Last Edit: September 08, 2022, 06:17:18 pm by cejoba »
 

Offline Bill W

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Re: Anyone tried f 0.7 lens?
« Reply #1 on: September 08, 2022, 05:49:42 pm »
f/0.7 is only 1 stop from f/1.0

It does however change the system noise:signal ratio as you would expect.

Remember though that NETD is quoted in an f/1.0 system regardless of lens fitted, so that '50mK  f/1.4" is actually a 100mK system due to the lens fitted, so the f/0.7 lens only brings things down to looking like a 25mK f/1.0 system

As there's a lot more going on in the system, it is not that simple.  the camera might not be running in the same state as gives the quoted NETD, then add in ADC noise, quantisations and display stretches which are more likely to limit overall system MDTD / sig/noise.

As for the experiment, yes (as I have all those pevicon lenses to hand).  I have documented a thread (and now on my website) where I change a FlirOne lens for anything I could lay my hands on.

A complication is that Pevicon lenses are designed for what it essentially a 100um pixel 200x200 sensor.  That means several things are rather different when on a 12um sensor.
The common focal lengths will be quite a narrow field of view
The resolution and depth of field will be rather poor
Poor edge performance will not matter - it is off the edge of the sensor

https://www.eevblog.com/forum/thermal-imaging/flir1-g2-lens-experiments/

http://www.fire-tics.co.uk/project4/index.htm


Bill
 
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Offline Lambda

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Re: Anyone tried f 0.7 lens?
« Reply #2 on: September 08, 2022, 06:13:57 pm »
Hello. :)

I made a little spotter from a core micro A10 "Indigo" (160x120, 51 um) originally equipped with a ZnSe lens  8 mm/1.2.

I replaced quite quickly this lens by a Ge lens 15 mm/1.0 provided by Bill W, and after a lot of comparison, i noticed indeed a significant improvement in contrast, definitively.

But i guess it would also depend on the couple AR coating/nature of the material for the lens.....
For two lenses having  the same optical geometrical features, (especially same aperture (F/#)), but different material and/or AR coating, the result would be different in term of contrast.

Regards.
Lambda
 
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Offline cejoba

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Re: Anyone tried f 0.7 lens?
« Reply #3 on: September 08, 2022, 06:18:02 pm »
Thank you guys for replying.

f/0.7 is only 1 stop from f/1.0
I corrected OP.
 

Offline Vipitis

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Re: Anyone tried f 0.7 lens?
« Reply #4 on: September 09, 2022, 07:32:47 am »
a f/0.7 lens will not be T0.7 and T stops is also just measured in the center. There will be significant falloff towards the edges.

I dropped and broke one element of my f/0.7 lens before really getting a chance to compare it to my f/1.0 lens. But without having a correction profile for the lens made and applied you will notice some non uniformities.


If there are still spares for sale ask Bill or try and find something on eBay and test it out.
 

Offline Bill W

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Re: Anyone tried f 0.7 lens?
« Reply #5 on: September 09, 2022, 02:01:40 pm »
If there are still spares for sale ask Bill or try and find something on eBay and test it out.

Yes, I still have 15mm f/1.0 and 18mm f/0/7 available.
http://www.fire-tics.co.uk/spares.htm

For experiments you may be OK without the shutter motor / iris motor - if so can reduce prices GBP5.

PM or use the website email link for payment methods etc

Bill
 
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