Focal length versus focus! Minimum distance for sharp image?

[Brenner]

Concerning my article here: http://www.ipcamtalk.com/showthread.php/1413-New-Hikvision-Bullets-DS-2CD2T32?p=31939&viewfull=1#post31939
i want to know you experience with different focal lengths and the necessary minimum distance to get a sharp image.


I ordered a 16mm cam for my driveway, but my driveway is to short to get a sharp image all over. Now I want to order a 12mm, 8mm or 6mm cam but I don´t know the minimum sharpness distance. There is no hint in the datasheets of the Hikvision cameras.

My experience with the 16mm is that you need a minimum distance of 11-12 meter to get a sharp view.

What is it with 12 or 8mm?

 

[aster1x]

The IPVM calcuator at
http://ipvm.com/calculator
is probably the best way to visualise the optical effects of various focal lengths and spatial dimensions apart from the depth of field of the lens. The depth of field can not be easily simulated over the typical monitor resolution which is 72 dots per inch. Thats why most marketting (advertising) material exagerate the effect of the depth of field or variable plane focusing in order to promote the optical quality of the advertised item.

 

[Brenner]

The calculator doesn´t tell me the minimum distance for a sharp image :-(

 

[aster1x]

Yes unfortunately as I also mentioned. I do not believe that you will find such info anywhere. Think about this "how much optically focused is a focused object?" Focusing is not a black or white evaluation. It is an infinitely variable evaluation related to the object distace from the lens and is also related to the optical resolution capabilties of the lens and the optical CMOS sensor of the camera. Do not expect a justifiable answer from any forum. Your best bet to understand the limitations of your quest, search in photography forums and in photography lenses reviews.

The ideal solution to your question would be a very wide angle (short focal length) lens and a very high CMOS resolution ( say 100Mpixels!!!) that will allow you to maximise the depth of field and also enable you to digitally zoom at the far objects with clarity. Obviously such an item does not exist :-(

As I suggested before, select a varifocal lens.

 

[devastator]

Interesting discussion.
I'm doubting between a varifocal cam or one of the EX-IR cams, but these are not varifocal, so I would need the minimum distance for sharp images :-/

 

[Brenner]

We have the first value from me:

-16mm: minimum distance of 11-12 meter
-12mm:
-08mm:
-06mm:





I continue to hope that somebody else who has a 12/8/6mm Cam does the test and fill up the information in here.

 

[devastator]

Regarding the angle of view ...
If I enter the specs from this cam (sensor, pixels, lens mm), I get an angle of approx 62 degrees, while the specifications mention 70.
Whats the deal there ?

 

[Brenner]

It has noting to do with the topic but you have different angle of views. With 1920x1080 (2MP) you have a little more in the width, with 2048x1536 (3MP) setting a little less.