Lens

[Thomas 0]

Time for a new question. Camera lens.

 

Tell me if I'm even in the ball park with these statements.

 

1. The larger the milimeter the lens, the further I can see?

2. The 1/3 1/2 2/3 should match the size of the CCD, but if I do a larger lens with a smaller CCD I get a wider view?

[Cooperman 0]

Hi Thomas

 

1. The larger the milimeter the lens, the further I can see?

O.K. on that one. The greater the focal length (i.e. the millimetres), the narrower the angle, so the lens makes objects appear larger at distance.

 

2. The 1/3 1/2 2/3 should match the size of the CCD, but if I do a larger lens with a smaller CCD I get a wider view?

You don't HAVE to match the size of the lens to the CCD; in fact it's generally an advantage to use a larger format lens on a smaller format camera.

 

The lens coverage means that a 1/3" lens will give a full image on screen with a 1/3" or 1/4" CCD (or MOS) camera; in the majority of cases, it will not cover the whole of a 1/2" or 2/3" imager (the corners will be missing, or you see a circular image).

 

You can successfully use 1" or 2/3'" coverage lenses on any of the major formats.

 

When you use both large and small format lenses side by side (you know what I mean), if the focal length is the same (or equivalent) then you will have virtually the same degree of coverage;

 

For example, a 6mm 1/2" lens or a 6.5mm 1" lens will both provide about the same effect on a 1/3" camera, but the optical performance on the 1" lens will be vastly superior.

 

That said, if you use a 6mm lens on a 1/2" camera it will be a normal 'wide angle', but if you put the same lens on a 1/4" camera it becomes a 'standard'.

 

In this situation, using a 'larger lens with a smaller CCD' actually gives you a narrower view.

 

And I used to think that life was confusing

[Thomas 0]

And how does C/CS fit in with all of this?

[cctv_down_under 0]

C Mount lenses use and adapter ring to mount on CS mount cameras, CS mount is pretty much the new standard, really I would not bother with C mount anymore

[Thomas 0]

So the adaptor rings don't alter focal length?

[cctv_down_under 0]

Yes they will, but I do not know anyone that still sells Cmount lenses, most these days are CSmount

[Cooperman 0]

DVR Expert Australia,

 

Sorry but I have to disagree with you.

 

C and CS mount have nothing to do with the focal length of a lens. The focal length of a lens is fixed, and cannot be changed (basic optical principles).

 

When lens designers calculate the point at which the image is brought into sharp focus, they try and engineer this effect to a common point.

 

The difference between a 'C' mount lens, and a 'CS' (C-Short) is simply that without the 5mm spacer ring on a 'C' lens, the image will be brought into focus 5 millimetres behind the face of the CCD imager, so you end up with a picture that cannot be focussed.

 

C lenses focus 17.625mm from the back focal point (technically known as the secondary focal point), whilst CS lenses do the same at 12.5mm.

 

That is why you can fit a spacer adaptor onto a 'C' to make it work on a CS camera.

 

You cannot do it the other way round, as a CS lens on a C camera, would produce a sharp image 5mm in front of the CCD, so again you'd have an image which cannot be brought into focus.

 

In practice the longer back focus 'C' lenses are generally much better corrected than their 'short' cousins.

 

So if you want the best quality, C mount will generally outperform a CS everytime.

 

For my own work, I will rarely if ever use CS lenses, unless there is no alternative (for example, ultra wide CS lenses on 1/3" format).

[cctv_down_under 0]

Sorry, i dont doubt your right, I never have used Cmount lenses! werent used much when I started doing this, I would have assumed because of the different sized cmount adaptors that I have seen, many different thread measurements and tapers, that it would affect focal legth after all the difference between lens and sensor would have to adjust the focal distance???

 

I know your going to say they all the same but I have at least three different ones on my desk in front of me, some of which do not ven seem to work with the old cmount lenses that I have here

[Cooperman 0]

DVR Expert Australia,

 

We never stop learning in this game

 

The first CCD and CMOS cameras all used C mount as standard. I remember the first CS mount lens I saw (from memory, I think it was early to mid 1980's) looked like a plastic toy. The funny thing was, the rep. wasn't that impressed with it, and he had to sell them!

 

I have seen lenses with various mounts over the years, but to be honest a 'C' had always (previous to CS) been the standard for CCTV work.

It was based on cine camera lens mounts, which were the norm, pre the invention of zoom lenses.

 

If you ever get a chance to set up two identical 1/3" cameras, one with an 8mm CS mount manual iris lens, and the other with an 8 or 8.5mm 2/3" C mount manual iris lens, I'd be amazed if you don't see a difference. Close the iris a couple of stops and the picture just keeps getting better (make sure they're quality lenses though).

 

I'm not sure what lenses you have on the desk, but if they don't screw on to a standard CCTV camera, they're definitely not 'C' thread. C-D maybe?

 

Just to clarify one point though; when I said that you can't change the focal length of a lens, I actually meant ... lenses with fixed focal lengths

 

Quite obviously, varifocals and zooms both offer a variable focal length, albeit that they achieve this in a slightly different way.

 

Life used to be so easy when it was just 1" Vidicon C mount cameras!

[cctv_down_under 0]

Showing you age coops!!

Thanks for the heads up, I dont know any of the old stuff at all, I apreciate the info!

There are always two thngs that amaze me, the new things you learn on the job everyday and the wy Manchester United manages to screw up inthe premier league!

[Cooperman 0]

Anytime DVR.

 

They don't call me the silver fox for nothing ... it's either the grey hair, or my deep love of animals

 

Last I heard Man U. were the subject of a takeover bid; if you've got any shares you want to get rid of, I've heard of a guy who might be interested

[cctv_down_under 0]

Yeh read that, always drams with them, bloody should concentrate on their game a little more than the outside world, so many stars and so few goals.

 

Oh well come Monday morning I will either be hung over from drowning my sorrows from loosing to Arsenal or Hung over from celebrating.. either way I will be hung over.

[Cooperman 0]

2 - 0 to Man. Utd.

 

May all your hangovers be little ones