The Lens Thread

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The Lens Thread

Postby Nathan » Fri Apr 08, 2011 6:57 pm

I'm looking into purchasing my first Elphel camera and building my Apertus rig- The biggest cause for confusion for me is lenses!

I thought I would start up this thread to see what lenses people are using and what you might recommend?

At the moment, I am looking at Pentax lenses...

Pentax C60636KP 6mm, f/1.4 Manual Iris Lens
http://www.bhphotovideo.com/c/product/519631-REG/Pentax_C61232KP_C61232KP_1_2_C_Mount.html

Image

--------------------------------------------

and

Pentax C61232KP 1/2" C Mount 12mm Lens with Manual Iris
http://www.bhphotovideo.com/c/product/519631-REG/Pentax_C61232KP_C61232KP_1_2_C_Mount.html

Image

--------------------------------------------

And Maybe even this which looks like a beauty! F1.0... WOAHG!

Pentax 1/2" C Mount 8-48mm F1.0 Manual Zoom Lens
http://www.bhphotovideo.com/c/product/3 ... _48mm.html

Image


Any ideas / thoughts / suggestions?


(The lenses I have been looking at are manual Iris...
is ther currently support in elphelvision for auto iris?
Is there any benefit to getting a motorized iris?)
Last edited by Nathan on Fri Apr 08, 2011 7:14 pm, edited 4 times in total.
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Re: The Lens Thread

Postby Sebastian » Fri Apr 08, 2011 7:09 pm

There is currently no support for motorized lenses.

http://www.dvinfo.net/forum/silicon-ima ... si-2k.html has a very comprehensive list of good lenses for the SI2k which also uses a cmount.

My input is to get lenses which are easy to operate, lenses with proper focus/iris rings that have a good rotation range, the cheaper ones have these screw nobs and the focus area is very small. They are designed to be focused once then fixed and used in that position. For filmmaking you need lenses that were made for constant refocusing.
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Re: The Lens Thread

Postby Nathan » Fri Apr 08, 2011 8:21 pm

Thanks Sebastian,

that's an interesting thread- but the SI2K has a sensor size of 10.24x5.76mm (almost double the current Elphel)

the SI2K sensor is quite close to the size of 16mm film and as such the 16mm lenses discussed in that thread play nicely with the SI2K without too much cropping.

But for the elphel (with its diminutive sensor) Isnt it better to use lenses that project an image circle closer to the sensor size? That's why Ive been looking at the lenses designed for 1/2 inch chips...

Are there no C Mount lenses designed for 1/2 in chips that offer the qualities outlined in your previous post? (easy to operate, lenses with proper focus/iris rings that have a good rotation range)

------------------------------------------------------

Perhaps this one is more suitable?

Pentax C60607KP 1/2" C Mount 6mm F1.2 Manual Iris Lens

http://www.bhphotovideo.com/c/product/573470-REG/Pentax_C60607KP_C60607KP_1_2_C_Mount.html
Image
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Re: The Lens Thread

Postby arniepix » Fri Apr 08, 2011 10:04 pm

There's no downside to using a lens made for a larger format. Projecting an image circle that's too large has no ill effect. If you go the other way, using a lens made for a smaller format on a larger one, you get vignetting, meaning the corners are cut off.

BTW, I noticed that some of these lenses are labeled as TV lenses. They probably don't have the resolving power for the Elphel, so the image is going to be too soft. TV lenses are made for a sensor with only 350,000 pixels or so, the Elphel has 5 million.
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Re: The Lens Thread

Postby Nathan » Sat Apr 09, 2011 9:19 am

Okay so it's time to forget the cheapy TV lenses!!
I'm concerned about having lenses with a very narrow FOV because of drastic cropping of the image circle!?
What if I was to buy a second hand lens. For example:
Image
Kern Switar 10mm f1.6 lens H16 RX * C-mount 1.6 H16RX
What would the focal length be when used with the tiny elphel chip?

(I'm sorry I am a bit lost with all this lens stuff! It might be driving some of you crazy!! I really need a hand undrstanding this!)
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Re: The Lens Thread

Postby Sebastian » Sat Apr 09, 2011 2:32 pm

Its really simple:

Cropfactor is around 6.

f=10mm x 6

f=60mm


The term you need to look for is "megapixel lense". This means it was designed for higher resolution sensors. Sometimes lense manufactureres also supply a megapixel value: 5 megapixels, 3 megapixel but from my experience this value does not necessarily reflect the actual lenses quality.
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Re: The Lens Thread

Postby arniepix » Sat Apr 09, 2011 5:47 pm

Film lenses, like the Switar you're looking at, should be good, as well.
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Re: The Lens Thread

Postby Nathan » Sat Apr 09, 2011 6:13 pm

Cropfactor is around 6.

f=10mm x 6

f=60mm



This is the seed of my confusion...

Crop factor is the size of a sensor as compared to 35mm film.

The example lens is a 10mm lens- which is designed for 16mm film

Which means its image circle is smaller than that of 35mm film...

a
10mm lens designed for 35mm Film
is different to a
10mm lens designed for 16mm Film
is different to a
10mm lens designed for 1/2" Chip

So when using lenses which produce an image circle smaller than 35mm Film lenses... you cannot simple use a crop factor based off of 35mm standard.

true!?
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Re: The Lens Thread

Postby Sebastian » Sat Apr 09, 2011 9:20 pm

No, the focal length is independent of the camera system the lense is designed for.

And the focal length is independendet of the size of the image circle.
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Re: The Lens Thread

Postby Nathan » Sun Apr 10, 2011 1:48 pm

To quote myself later in the post:
"What I am after is a method for determining an accurate measure of a captured angle of view (with the Apertus 1/2.5" chip) Considering that different lenses are designed for different sized sensors (ie. different sized image circles)."

----------------------------------------------------------------------


Okay, So what I have gathered is that

"A Focal length is A Focal length!"
The Field Of View, not the focal length,
will change depending on the
format of the camera

http://www.rmassa.com/specsheets/Basics_of_Lenses.pdf

So using a lens on a smaller chip will produce an image with narrower FOV...
I understood that all along!

What continues to elude me is when working out the AOV for a captured image, when do image circles come into play?

Lets see if I have got this part right...
Lenses have larger / smaller image circles; Depending on what size sensor they have been designed for. For example, take three 10mm lenses: although the image circle changes in size, the angle of view remains the same? (see below)

Image

Now if the above is correct, the size of the image circle (as well as the size of the chip) affect the total captured angle of view!?

If we use a 10mm lens designed for 1/2" sensors, we are going to get a wider AOV than if we use a 10mm lens designed for a 1" sensor. (see below)

Image

Image

----------------------------------------------------------------------

What I am after is a way of determining the Captured Angle of View when using lenses with the Apertus Camera.

I have found a formula which will easily calculate the AOV- given the focal length of the lens and the size of the sensor.

Image

But this formula does not consider the size of the image circle!

My presumption is that the above formula assumes that the image circle is the exact correct size for the sensor!? (Im not sure)

What I am after is a method for determining an accurate measure of a captured angle of view Considering that different lenses have different sized image circles.

(Please bear with me... I think I am close to understanding this dilemma of mine, I hope I have articulated my issue well enough that someone can help!)


Additional notes:

(just a place for me to dump my notes on this)

Example lens

Imager Size 1/2" (5.76 x 4.29)
Focal Length 6mm
Horizontal Angle of View 56.93°

Apertus sensor size: 5.70 x 4.28

----------------------------------------------------------------------------------

Require 2 formulas

1: Calculate captured angle of view for 1/2.5" Apertus chip (5.70 x 4.28)

2: Calculate theoretical Focal Length if 1/2.5" chip was scaled up to 1/2" size.
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