Separate section on lenses?

[videobruce 0]

How about a separate section just on lenes?

After reading a document from Tamrons' "megapixel" lenes that can only resolve "200 lines" that sound as there is a major problem in the CCTV industry if their 'best' lens can't even do what wasn't even considered 'high resolution' back in the fifties.

[rikky 0]

interesting discussion point.

Where did you find that one on Tamron? As i only remember reading that Tamron and Pentax where the only ones available with good MPx lenses.

[videobruce 0]

Here is the link for the pdf (edit the 'dot');

 

tamron DOT com/cctv/prod/assets/pdfs/HiResCat.zip

 

High resolution of 200 lines at the center and 160 lines at the corners is achieved.

Is this the 1939 World Fair??

[phred 0]

It means the ability to resolve 200 lines per millimetre – not 200 lines total over the whole image.

 

With a (perfect) ½ inch square sensor that would allow you a horizontal 'resolution' of 5000 pixels. (It takes 2 pixels to resolve each line)

 

Although it is not clear from the above chart whether they are talking about 'line pairs' as they often do or just the black lines (hence the edits )

 

 

For ordinary cctv 70 lines per mm for a 1/3 ccd is very good.

Edited March 31, 2007 by Guest

[videobruce 0]

It surely doesn't say that! I just finished reading this test chart setup document (edit the 'dot') and discovered the same thing which I didn't know;

 

cctvlabs DOT com/TestChart/cctv_test_chart_instructions_latest.pdf

 

and on page 14 I see this;

 

in optics when counting lines per millimeter resolution only black lines are counted, as opposed to television where both black and white are

 

Comming from a television background resolution is always the whole screen, not per mm.

[phred 0]

The confusion is not helped by the omission of the units of measurement in the above.

It should say ' 200 l/mm at the centre'.

 

 

Lens resolution is always measured in lines per millimeter.

That means that this lens can 'see' separate lines when 200 are placed together in a space 1mm wide.

 

You can bet that this lens will be expensive.

Edited March 30, 2007 by Guest

[cctv_down_under 0]

I agree about the lense section, here is something I posted at another cctv forum.

 

Does anyone really even use non IR correct lenses anymore?

 

Most of the main manufactorers offer IR Correct lenses that are correct for IR Illumination between at least 400 - 950nm.

 

Normal lenses do not cater for the focal shift between the physical and IR parts causing a blurring effect, you can see this with the two attached images, Picture 1 is an IR Image with a normal lens and Picture 2 is an IR Image with a Compensating Lense.

 

Some of the advantages are:

Day/Night camera with IR illumination

- Colour image during daylight.

- Monochrome image is focussed using IR illuminator during night.

Monochrome camera (in outdoor lighting)

- Image stays in focus using IR illuminators

- Sharper Dusk/Dawn image because IR light part is focussed.

- Less car headlamp blooming caused by out of focus IR light

 

I do not know if you remember when you were a child in science class and they made you use a prism to refract the white rays of the sun, I am sure if you did this excercise you would have noticed the rainbow pattern that emerged on the other side of the prism, you also would have noticed that the red colour was at one end and violet at the other, that is because red has the longest wavelength or lowest frequency, Ultra Violet is even higher than Violet and therefore when refracted through the lens means the focal plane recieves different frequencies at the same point. This basically means the focal point will vary for different colours. This actuall effect is what is called colour distortion or more commonly known as Chromatic Abhorration (spelling?).

 

There are many and I do say many other distortions that can affect a lense for example; Barrel, Pincushion and Sperical, the actual name of the distortion directly relates to the effect they represent.

 

The dilema for lense manufactorers is that in order to produce a good lense they will need to add as many corrections to the lense as possible, but this means adding more elements to the lense making it more expensive to manufacture, keep in mind there are already a combination of two similar thickness convex and concave lenses that have to be manufactured just to make all the colours come together and form a single focussing point.

 

When shopping for a good lense you should consider the following (and much more) in order to determine the quality:

- Number of Elements - Abborration (spelling) correction in design - Type of Glass Manufactoring and process - Precision of grinding and polishing (ultimately important) - Anti Reflection and Internal Light Reflections but obviously these are not always available to find out.

 

The shorter the focal length of the lense the more optical elements have to be added to correct the above mentioned distortions which in turn makes the lense more expensive.

 

Expense in manufactoring is the major reason that good quality lenses are much harder to afford and one of the most common distortions that can affect a lense is Spherical Aborrhation, a lense that is spherically grinded and polished is simple to machine and therefore less expensive, however the focus is not a very precise single point.

 

Aspherical Lenses are much harder to grind because of their shape (kind of like a wave or bell shape) but these lenses offer a more precise focal point, wider angles of view, shorter minimum object distances and much less optical elements (less need to correct the lense), but beware some manufactorers (yes even big ones) have started to produce moulded aspherical lenses, and avoided the expensive grinding process and these lenses do not offer the same glass quality as properly machined aspherical lenses do.

 

In reality too many people choose lenses based on the F stop measurement which is a good guide but not the most important factor. The T Factor of a lense is much much more critical as it indicates the lenses level of light transmission. All lenses that encorporate glass will loose some level of light as it passes through the lense and those that loose less are usually of a much better quality glass than those that do.

 

You have probably heard the term "Japanese Glass" and this is for a reason.

 

~ I hope this helps

[videobruce 0]

Lens resolution is always measured in lines per millimeter.

I never really got into lens specs regarding resolution. I do vagely remember that tidbit, but completely forgot about it. I guess Tamron 'assumed' one would know this. Out of all the lens with specs listed (major manufactures), until this one, that would be a bad assumption on their part as no spec included this I have seen.

 

Anyone know of any lens spec that lists resolution?

Edited March 29, 2007 by Guest

[rikky 0]

Reading all of this stuff, it really interests me to know which kind of lenses you guys prefer.

I've got bad/good experiences with both Computar & Fujinon over time, but never used Tamron before.

[rory 0]

Well I dont know if we need a whole section on lenses, lets leave that up to the manufacturers .. but if enough of you show interest, then best bet would be to PM the admin, or see how this thread goes.

 

Personally, Ive used Computar, Tamron, and Fujinon.

Ive had good and bad results from all of them, but fujinon is my choice these days. I used a Tamron Aspherical Lens with a UF500 IR on a Sanyo D/N camera with great results, but that was a long time ago.

[videobruce 0]

For ordinary cctv 70 lines per mm for a 1/3 inch ccd is good.

What would that convert to for a full screen?

What would the formula be to convert lines per mm into a full screen number? IOW's for a 1/3" CCD with a 480l spec, how many lines per mm would I need to make the lens 'transparent' to the CCD (no degradation)?

 

Well I dont know if we need a whole section on lenses

How about a 'sticky'?

[phred 0]

There is no mathematical correlation between pixels and TVL. It is largely a subjective judgement.

 

As a rough guide

To convert horizontal lines of resolution to pixels, multiply by the aspect ratio and divide by the Extended Kell factor (usually taken as 0.7)

 

Also note: The l/mm resolution figures are subjective as they are judged by human eye using a test card like the one above - two testers will give different results.

 

No lens is optically perfect, all cause some degradation. There are many other factors which affect the quality so I would not judge a lens on the l/mm rating alone.

 

Some 1/3 format lenses that can resolve 480 TVL

 

Bosch 3.3 – 8mm F1.4

 

Panasonic 3.8 - 8mm F1.4

 

Fujinon fixed F1.2

 

There are many others from Pentax, Tamron and Ernitec that will also do

480 but Computar 3.5 – 8 F1.4 will only manage 460 TVL.

 

There is one popular vandal dome OEMed by many rated at 530 TVL and good low light - unfortunately it is usually fitted with a naff lens that only manages 390 by my tests.

Same is true for many other cameras on the market.

[phred 0]

Videobruce

To answer your question about a 480 TVL camera I have looked at the

detailed spec on a Sony 480TVL CCD (ICX084). In order to ensure the

lens is not the limiting factor in overall resolution, you would need a lens

capable of 50 l/mm.

 

Despite being a rather modest spec many cctv lenses will not reach this resolution.

 

This is why you need special lenses to get the best from megapixel cameras, unless they use much larger sensors.

[phred 0]

Does anyone really even use non IR correct lenses anymore?

 

... called colour distortion or more commonly known as Chromatic Abhorration (spelling?).

 

 

 

 

'Abhorration' may not be the correct spelling but is certainly a more

appropriate description for the abhorrent mess that spherical lenses can

make of an image.

 

Aspherical lenses do tend to perform better in almost every respect.

They first appeared in the 17th century - seems to have taken a while for

cctv to catch on.

[Lloyd 0]

Long as we're talkin about lens .....

 

Are these things any good??

 

[phred 0]

2X teleconverters, also known as range extenders, effectively double the

focal length of the lens, so an 8mm becomes 16mm.

 

As you might expect, there is a tradeoff.

 

With a converter fitted you are effectively only using the central portion of

the lens, so the amount of light that reaches the CCD is reduced - a 2X

converter will cost you 2F stops.

 

For the same reason the effective l/mm resolution is also reduced.

 

They are only worthwhile if they are good quality and fitted to a top quality lens.

I would limit their use to low TVL Cameras.

[forecamsales 0]

have you ever heard of RICOM

a mdel of high resousion lens from RICOM

 

EV0922DB.HR

9-22mm

board mount

1/2inch

[phred 0]

No