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by Aditya

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I am a noob in photography and don't have any idea of lenses and optics...

Recently though I was looking out for some camera modules and couldn't understand some of the specifications mentioned in the data-sheet.

One of them stated that the Lens construction is 4P + IR, what does this mean?... To an extent I understand the IR part indicates to some filter in the lens.

I searched for the term "4P Lens" on the web but it didn't return relevant results either?

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2  
One answer appears in the first hit when you Google "4P + IR lens": "4P = four plastic lenses; 1G = one glass lens; IR = infrared filter" –  whuber Oct 14 '10 at 14:51
    
@whuber: Thanks, so why exactly do we need these four plastic lenses or glass lenses? Is there some reference that explains the construction and understanding of lenses. –  Kevin Boyd Oct 14 '10 at 17:56
1  
You might start at the Wikipedia article and then explore whatever interests you in the links from it. en.wikipedia.org/wiki/Camera_lens –  whuber Oct 14 '10 at 18:00
    
It might be useful to add a link to the data-sheet you mention –  Rowland Shaw Oct 15 '10 at 18:48
    
@Rowland Shaw: trulydisplays.com/ccm/specs/5.0M%20CM5326-B500BA-E%20Spec.pdf Look at page4 in the lens section –  Kevin Boyd Oct 16 '10 at 9:07

2 Answers 2

I think f/4P should be read "f/4" + "P". f/4 is clear, "P" according to this documentation means:

P lenses are manual focus AI-s lenses that have had the electronic contacts of an AF lens added to them.

There are only a few of these: the 500mm f/4 P from 1988, the 1200 - 1700 mm f/5.6-8.0 P ED and the new 45mm f/2.8 P.

They allow Matrix metering and I believe the addition of all the automatic exposure modes on AF cameras.

Again, these are manual focus lenses that are unique in their ability to take advantage of exposure and metering modes usually reserved only for AF lenses on AF cameras. You still have to focus them by hand.

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I'm a bit confused now after reading this, whuber mentions that 4P means a 4 element plastic lens. The datasheet of this particular camera states that it is fixed focus, I don't know about manual focus adjustment and the electronic contacts of AF as said in your answer. What conclusions should I come to? –  Kevin Boyd Oct 15 '10 at 15:33
    
The naming is very confusing. The same letter has several meanings. Maybe if you show the full name, someone will give you a better answer. –  asalamon74 Oct 15 '10 at 16:48
    
This wasn't a question about Nikon lenses, I think. –  James Youngman Mar 4 '12 at 16:37

I don't know about 4P, but at least in CCTV IR lenses are lenses that are corrected for infrared wavelengths, which means you can use them with day/night cameras that switch to IR when it goes dark (see Tamron site for examples of these).

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I may be wrong, but my thinking was that the IR part was a filter attached behind the lens and it was to remove the IR component of light entering the camera. –  Kevin Boyd Oct 14 '10 at 17:58
    
Tamron explanation at tamron.co.jp/en/data/cctv_ir/ir_series2.html suggest that at least for Tamron it works the way I described. –  che Oct 14 '10 at 19:04
    
I think you're both correct: you need to remove the IR filter from the sensor in order to shoot IR, and having an IR-corrected lens helps with focusing. –  Reid Oct 14 '10 at 22:44
    
Kevin's link actually says 4Plastic + IR; so I'd say you're spot on. –  Rowland Shaw Oct 16 '10 at 14:14

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