A Few Projects

Show and tell for your homebrew nv projects. DIY 'Stickies' here too.
phoenix
BRUCE ALMIGHTY
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Joined: 09 May 2012, 14:40
Location: Aberdeen

Re: A Few Projects

Post by phoenix » 19 Jan 2018, 22:18

Remember that the mechanics of holding the lens in place means that there is usually a rim around the edge of the lens which doesn't receive any light.
It's quite probable that the unsupported lens could have an edge to edge diameter which would give it the claimed f number, but when fitted has a lower effective diameter and therefore a higher f number.

Cheers

Bruce
LAND ROVER - THE WORLD'S WORST 4X4 BY FAR

summer-blue
Posts: 47
Joined: 07 May 2015, 23:06
Location: ca

Re: A Few Projects

Post by summer-blue » 19 Jan 2018, 23:54

Trackerman,

The method I mentioned is to estimate small lens entrance pupil. The difference between your ruler and entrance pupil is caused by the location of the entrance pupil. Because usually the entrance pupil is located behind the object lens somewhere.

For a M12 lens, lets assume your eye is 400mm away from the lens, the M12 lens entrance pupil is 5mm and located 10mm behind the object lens, your ruler will read 4.88mm. That's 2.5% smaller.

For a C mount lens, lets assume your eye is still 400mm away from the lens, the entrance pupil is 10mm and located 20mm behind the object lens, your ruler will read 9.5mm. That's 5.3% smaller.

Now for a prime lens, lets assume your eye is still 400mm away from the lens, the entrance pupil is 40mm and located 50mm behind the object lens, your ruler will read 35mm. That's 14.3% smaller.

I don't know what L lens you have. If it's those long focal one. It will have an much bigger difference than the prime lens example I used above.

If you really want to better estimate your L lens' entrance pupil. Use a binocular, so you can check the ruler at 4 meter away. ;)

summer-blue
Posts: 47
Joined: 07 May 2015, 23:06
Location: ca

Re: A Few Projects

Post by summer-blue » 20 Jan 2018, 02:16

Forgot to mention, to verify your big canon lens. There is a simple way to eliminate the perspective effect when you measure the entrance pupil.
For example, you have a canon 200mm f2.8L. Now you know the entrance pupil should be about 71.4mm. Looking into the object lens with a ruler. This time you don't need to move your eye as far as possible, just don't move your eye. Align your eye with one edge of the "light tunnel" and your ruler's zero. Then move the lens by about 71mm to the other edge of the "light tunnel". read the ruler. Now you have a true entrance pupil diameter.
The hard part is to move the lens 71mm accurately. you can first put your lens outside edge 71mm away from a wall, align your eye to the left edge of the "light tunnel". Then move the lens to touch the wall and measure the right edge.

There is more accurate way to measure and calculate the entrance pupil. But that involve measuring camera and sliding table.

Trackerman
Posts: 134
Joined: 30 Oct 2017, 19:22
Location: ireland

Re: A Few Projects

Post by Trackerman » 20 Jan 2018, 20:37

Thanks summerblue,
Yes our have confirmed my suspecision that it's the measurement merhod which is leading to incorrect recults.
The perspective error is the culprit.

Now, consider the small 16mm CCTV whigh has an apature similar to the human eye, won't the perspective error be very small?
And won't the perspective error be greater with a canon L lens due to the ratio ( eye to lens) becoming greater?

Tracker

phoenix
BRUCE ALMIGHTY
Posts: 4752
Joined: 09 May 2012, 14:40
Location: Aberdeen

Re: A Few Projects

Post by phoenix » 20 Jan 2018, 21:15

summer-blue wrote:
There is more accurate way to measure and calculate the entrance pupil. But that involve measuring camera and sliding table.
Or use the inside jaws on a digital/vernier/dial caliper

Cheers

bruce
LAND ROVER - THE WORLD'S WORST 4X4 BY FAR

summer-blue
Posts: 47
Joined: 07 May 2015, 23:06
Location: ca

Re: A Few Projects

Post by summer-blue » 23 Jan 2018, 01:41

phoenix wrote:
summer-blue wrote:
There is more accurate way to measure and calculate the entrance pupil. But that involve measuring camera and sliding table.
Or use the inside jaws on a digital/vernier/dial caliper

Cheers

bruce
Yes, I used digital caliper. But that perspective error still exist. When I mention measuring camera, I was referring to accurately measure the distance from front of the lens to the entrance pupil position for those lens with aperture diaphragm.
Anyway, as for our M12 lens or C-CS mount lens, ruler or caliper is good enough.

summer-blue
Posts: 47
Joined: 07 May 2015, 23:06
Location: ca

Re: A Few Projects

Post by summer-blue » 23 Jan 2018, 01:45

Trackerman wrote:Thanks summerblue,
Yes our have confirmed my suspecision that it's the measurement merhod which is leading to incorrect recults.
The perspective error is the culprit.

Now, consider the small 16mm CCTV whigh has an apature similar to the human eye, won't the perspective error be very small?
And won't the perspective error be greater with a canon L lens due to the ratio ( eye to lens) becoming greater?

Tracker
I think human eye can only be treated as pin point. Try looking into a small diameter tube, you can still see the perspective error.

Trackerman
Posts: 134
Joined: 30 Oct 2017, 19:22
Location: ireland

Re: A Few Projects

Post by Trackerman » 23 Jan 2018, 13:43

summer-blue wrote:
Trackerman wrote:Thanks summerblue,
Yes our have confirmed my suspecision that it's the measurement merhod which is leading to incorrect recults.
The perspective error is the culprit.

Now, consider the small 16mm CCTV whigh has an apature similar to the human eye, won't the perspective error be very small?
And won't the perspective error be greater with a canon L lens due to the ratio ( eye to lens) becoming greater?

Tracker
I think human eye can only be treated as pin point. Try looking into a small diameter tube, you can still see the perspective error.

Got ya... Indeed that's correct .

Trackerman
Posts: 134
Joined: 30 Oct 2017, 19:22
Location: ireland

Re: A Few Projects

Post by Trackerman » 23 Jan 2018, 13:59

Just bringing topic back to an earlier point.. my foxeer night wolf V2 arrived today.

The lens seems glued in place ? Did anyone else have this problem ?
Should I just force it ?

And any special tips on removing the IR filter, it seems just glued directly to the glass (smaller than the glass actually)...
Heat with soldering iron and force with blade ?

Thanks

User avatar
MrNewbie
Posts: 298
Joined: 22 Dec 2012, 21:28
Location: Grafton, Ohio USA

Re: A Few Projects

Post by MrNewbie » 23 Jan 2018, 15:22

Test it first...it may already have an ir sens lens on it

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