Question about a Carl Zeiss lens

**FocusedWolf** · Nov5-09, 02:51 PM

Ok so this really has nothing to do with astronomy, besides the use of lens,... but it is a lens question so here goes :P (moderators may move this if their's a more appropriate place for it)

Information printed on the lens: "Carl Zeiss Tessar 2.0/3.7 2MP Autofocus".

This is from a Logitech webcam that my team might be using for our college senior project.

I'm trying to determine as much information as possible about this lens... because knowing the field of view for this lens at different distances is critical for our project. So anything and everything about "Tessar 2.0/3.7" i need to know.

from this webpage, i found a partial explanation of what those numbers sort of indicate: http://www.zeiss.com/C12567A8003B58B...25731D004AC1B7
"They feature a Tessar 3.7 mm, f2.0 autofocus lens that has been optimized for a resolution of 2 megapixels, thus enabling outstanding detail rendition and clarity."

so, i'm guessing here:

3.7 mm lens diameter?

"f2.0" so focal length = 2.0 mm?

And if you can point me to some webpage that has formulas for calculating other useful information about lens' in general, then that would be most helpful :P

**chemisttree** · Nov5-09, 03:07 PM

f2.0 is the focal ratio (unitless). Obtained by dividing the focal length by the diameter of the lens. 3.7mm is the lens diameter. At f 2.0, the focal length is equal to 7.4 mm.

**Borek** · Nov5-09, 03:41 PM

2.0 is aperture (that's more or less the same thing as focal ratio, just this name is used more often).

**chemisttree** · Nov5-09, 03:56 PM

Huh?

**Borek** · Nov5-09, 04:24 PM

From wiki:

In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.

Aperture is a term that you will find in every camera manual, I don't remember seeing focal ratio used in such places.

**chemisttree** · Nov5-09, 04:34 PM

So we are both agreed that the f ratio is not the aperture itself but is the ratio of the focal length to the lens diameter (or at least the entrance pupil)?

**Borek** · Nov5-09, 04:39 PM

I see your point, sorry - I am translating directly from Polish in my head, and there is no distinction between aperture and relative aperture (f ratio) in Polish terminology.

**S.Vasojevic** · Nov5-09, 04:45 PM

3.7 mm is a focal length of this lens. F 2.0 is maximum apperture. Field of view depends of size of film, or in this case cmos or ccd detector. Rule of thumb is that if diagonal of detector is equal to focal lenght of lens, you will get, what is considered as "normal" angle of view, which coresponds to angle humans have with their vision.
In order to calculate exact angle of view, both horizontal an vertical, you have to know both focal lenght an exact dimensions of detector you are using.

**chemisttree** · Nov5-09, 04:58 PM

Hmmm. If 3.7 mm refers to the focal length, at f2.0 the entrance pupil would be 1.85 mm diameter. It could be that an entrance pupil this small is used but it looks bigger than that. I'm not convinced.

**DaveC426913** · Nov5-09, 05:10 PM

Originally Posted by chemisttree

Hmmm. If 3.7 mm refers to the focal length, at f2.0 the entrance pupil would be 1.85 mm diameter. It could be that an entrance pupil this small is used but it looks bigger than that. I'm not convinced.

Seems about right for a webcam. They're not much more than a mm or two from what I've seen.

**S.Vasojevic** · Nov5-09, 05:14 PM

You should never consider diametar of lens. If good people from Carl Zeiss say it is 3.7 mm, F2.0 then it is so. Diametar of lens never exactly coresponds to focal lenght - maximum apperture ratio, because it also depends on losses which are different for every lens. It is not telescope mirror.
To OP- if you describe your setup more precisely maybe we will be able to help you furthermore.

**chemisttree** · Nov5-09, 05:33 PM

Originally Posted by S.Vasojevic

You should never consider diametar of lens. If good people from Carl Zeiss say it is 3.7 mm, F2.0 then it is so.

Yes, photographic lenses are described by focal length and focal ratio, not diameter. You're convincing me....

**FocusedWolf** · Nov6-09, 07:29 PM

Originally Posted by S.Vasojevic

3.7 mm is a focal length of this lens. F 2.0 is maximum apperture. Field of view depends of size of film, or in this case cmos or ccd detector. Rule of thumb is that if diagonal of detector is equal to focal lenght of lens, you will get, what is considered as "normal" angle of view, which coresponds to angle humans have with their vision.
In order to calculate exact angle of view, both horizontal an vertical, you have to know both focal lenght an exact dimensions of detector you are using.

I'm trying to dig up information on the CMOS sensor.

This is what i have so far:

-- http://forums.quickcamteam.net/showthread.php?tid=265
The QuickCam Pro 9000 has a CMOS-sensor with a resolution of 1600x1200 and its size is 6mm x 4mm (I meassured it). [trying to confirm this persons information].

-- http://forums.quickcamteam.net/showthread.php?tid=107
Sony ClearVid sensor is possibly the CMOS-sensor used in this webcam. If i can find it's specifications then maybe it will indicate it's dimensions for comparison to what that one person measured.

-- From someone on the "QuickCam Team": http://forums.quickcamteam.net/showthread.php?tid=107
"I've talked to one of our optical engineers and I was told that our lab tests confirmed the following values: DFoV = 75° and EFL = 3.7 mm."

**S.Vasojevic** · Nov6-09, 10:07 PM

Originally Posted by FocusedWolf

-- http://forums.quickcamteam.net/showthread.php?tid=265
The QuickCam Pro 9000 has a CMOS-sensor with a resolution of 1600x1200 and its size is 6mm x 4mm (I meassured it).

Thing is that you will have to know active area of cmos, which is not always easy to measure. It is best to find factory specs, then you can go to:
http://imaginatorium.org/stuff/angle.htm

and easily calculate angles.