fixed zoom lens

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cadman777
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Joined: 25 Oct 2017, 15:40
Location: Florida

fixed zoom lens

Postby cadman777 » 16 Dec 2017, 23:49

Hi,
Can someone tell me if there's such a thing as a fixed zoom lens for use on an E700 cctv board camera?
I've been looking on the internet and in this forum, and haven't found anything.
Thanx ...

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some bloke
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Re: fixed zoom lens

Postby some bloke » 17 Dec 2017, 10:34

Yea, but without you indicating what size you want I copied this which is a bit open ended:

https://www.ebay.co.uk/sch/i.html?_from ... s&_sacat=0
Image

phoenix
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Location: Aberdeen

Re: fixed zoom lens

Postby phoenix » 17 Dec 2017, 15:11

cadman777 wrote:Hi,
Can someone tell me if there's such a thing as a fixed zoom lens for use on an E700 cctv board camera?
I've been looking on the internet and in this forum, and haven't found anything.
Thanx ...



Zoom lens https://en.wikipedia.org/wiki/Zoom_lens
Zoom lenses are usually specified by their range of focal lengths e.g 70-300, 5-100, 8-36 etc etc etc
So, a 5-100mm lens will have a focal length which can be adjusted between 5mm and 100mm
By definition a zoom lens is not a fixed focal length lens so there is, AFAIK, no such thing as a fixed zoom lens.
However, if our 5-100mm zoom lens is adjusted to a focal length of (say) 50mm and it is locked in that position, then you effectively have a fixed 50mm lens.
However a zoom lens locked at a particular focal length will not perform as well as a prime 50mm lens
I tried a 9-22mm zoom lens on an E700 add-on rig and locked the lens at 16mm and did a comparison with a fixed 16mm lens on the same rig.
The zoom lens was absolute rubbish compared to the fixed 16mm lens.

Cheers

Bruce
LAND ROVER - THE WORLD'S WORST 4X4 BY FAR

cadman777
Posts: 17
Joined: 25 Oct 2017, 15:40
Location: Florida

Re: fixed zoom lens

Postby cadman777 » 20 Dec 2017, 19:34

Hi SB & Bruce,

SB, thanx for the link.
Between your link and Bruce's 'white paper', I think I get it now.

So let me clarify:
A fixed 25mm lens will zoom-in less than a fixed 50mm lens, which will zoom in less than a 75mm lens, etc. on up, right?

If that's true, then I just need to figure out what zoom power suits my agenda, and get that lens for my NV setup.

Sound correct?

Two more questions:
Fact: My scope zooms 1.25-4.5.
1. What is the formula or method used to translate board lens focal lengths to scope zoom factors?
2. How does lens focal length interact with the optics on a scope that zooms?

Thanx! ... Chris

phoenix
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Re: fixed zoom lens

Postby phoenix » 21 Dec 2017, 13:21

Chris,
Apologies - this is going to be a long answer!!

I think you're mixing up zoom and magnification - they're not the same thing

When we speak about zoom lenses we mean they are lenses where the focal length can varied over a specified range e.g 5-100mm, 40-240mm, 15-45mm etc etc

The ratio between the longest and shortest focal length that the lens can be adjusted for is the zoom ratio, often just called zoom.

So a 5-100mm lens has a zoom ratio of 20 (100/5) and a 15-45mm lens has a zoom ratio of 3 (45/15)

Sometimes you'll see lenses sold as being x10 or x20, and this means the zoom ratio of the lens is a factor of 10 or 20 - it does NOT mean that the lens magnifies by a factor of x10 or x20

When a lens is used with a sensor such as a ccd or CMOS sensor found in an NV camera, the combination of the focal length of the lens and the physical size of the sensor sets the field of view and, to a large extent, the magnification of the system.

Wide field of view is equivalent to low magnification and narrow field of view is equivalent to high magnification.

The total magnification of a digital NV system is set by 4 factors. These are:

1. Focal length of the objective lens

2. Diagonal size of the sensor

3. Diagonal size of the display

4. Focal length of the lens used to magnify the display to a size where it can be comfortably seen by the human eye

The following equation can be used to calculate the actual magnification of a digital system:

Magnification = (focal length of the objective lens/diagonal size of the sensor) x (diagonal size of the display/focal length of the eyepiece lens)

all dimensions are in mm

In most cases it's difficult to know exactly the size of the display and the focal length of the lens which magnifies the display, but a ballpark range of values for that part of the equation is somewhere between 0.5 and 0.7

So, to get a reasonable estimate of the magnification of a camera/lens system, simply take the focal length of the lens and divide it by the diagonal size of the sensor and multiply your answer by somewhere between 0.5 and 0.7

FYI sensors which are sized as 1/3 have a diagonal size of 6mm. 1/2 sensors are 8mm diagonally and 1/4 sensors are 4.5mm diagonally

So, if you have a lens with a focal length of 50mm and you're using a camera with a 1/3 sensor, then the magnification is going to be around x3- x4
(50/8) x 0.5 = 3.125 and (50/8) x 0.7 = 4.375


Riflescopes are described by their magnification range and size of their objective lens, but for our discussion we'll stick to magnification.

Your Barska scope has variable magnification from x1.5 to x 4.5 - that's a zoom ratio of x3 (4.5/1.5) but nobody would describe your scope as a x3 scope or even a x3 zoom scope, it would simply be described a 1.5 - 4.5 x whatever it's objective lens diameter

Swarovski make different ranges of riflescopes based on their zoom ratios
the Z4 range has a zoom ratio of x4 and that range includes scope such as 2.5-10 x50 and 3-12 x 50 (10/2.5 = 4 and 12/3 = 4)
The Z5 range has a zoom ratio of x5 and includes scopes such as 3.5-18 and 5-25 (18/3.5 = 5 and 25/5 = 5)
The Z6 range has a zoom ratio of x6 and includes scope such as 2-12 and 3-18 (12/2 = 6 and 18/3 = 6)
You can see from the above that you can have several different scopes with the same zoom, but different magnifications

Mathematically, the magnification of a riflescope is simply the focal length of the objective lens assembly divided by the focal length of the eyepiece (ocular) lens assembly.

In a fixed magnification scope, the focal length of the objective lens assembly and the focal length of the ocular lens assembly are fixed - no adjustment is possible other than a small adjustment of the ocular lens assembly to accommodate variations in vision. This adjustment is called dioptre, and you'll often see scopes marked as having +/- 5 dioptres of eyepiece adjustment

In a variable magnification scope, the objective lens is fixed and there are lenses within the ocular assembly which are moved backwards and forwards which changes the focal length of the ocular assembly and hence the overall magnification of the scope.

Hope that helps

Cheers

Bruce
LAND ROVER - THE WORLD'S WORST 4X4 BY FAR

cadman777
Posts: 17
Joined: 25 Oct 2017, 15:40
Location: Florida

Re: fixed zoom lens

Postby cadman777 » 22 Dec 2017, 14:13

Bruce,

Thanx for the lesson in optics.
I think I get it now!

Let me clarify:
In my previous statement I said this:
"A fixed 25mm lens will zoom-in less than a fixed 50mm lens, which will zoom in less than a 75mm lens, etc. on up, right?"
But now I believe I should have said this:
"A prime 25mm lens will magnify less than a prime 50mm lens, which will magnify less than a prime 75mm lens, etc. on up, right?"

If true, then no 'zoom' exists with prime lenses, unless I pretend to 'zoom' by switching lenses?
I believe the thing I was looking for is magnification (of image in the eyepiece) instead of 'zoom'?
Thus, 'zoom' means that a lens has various magnifications, compared to a prime lens which has only one magnification.
That also means the eyepiece image size is a matter of personal preference, based on eye sight and ability to mentally process the image with the least amount of stress.

Conclusion:
Forgetting about 'zoom', the least expensive and best quality method for getting magnification is to use a longer focal length prime lens that suits my agenda and preferences.

Please correct me where I"m wrong.
Thanx!

PS: Yesterday while in the middle of reading this thread, I got bounced, and when trying to get back into the forum, it said my account was suspended. I couldn't get back on till the next day. What causes that to happen?

Silent Shooter
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Re: fixed zoom lens

Postby Silent Shooter » 22 Dec 2017, 14:19

Hi cadmam
Yes sorry about that the forum was off line for a few hours yesterday,and no it's not a usual accurance thankfully.

Cheers Geoff

cadman777
Posts: 17
Joined: 25 Oct 2017, 15:40
Location: Florida

Re: fixed zoom lens

Postby cadman777 » 22 Dec 2017, 16:54

Thanx Geoff, glad to see it wasn't something I inadvertently did!

phoenix
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Re: fixed zoom lens

Postby phoenix » 22 Dec 2017, 20:34

"A fixed 25mm lens will zoom-in less than a fixed 50mm lens, which will zoom in less than a 75mm lens, etc. on up, right?"
But now I believe I should have said this:
"A prime 25mm lens will magnify less than a prime 50mm lens, which will magnify less than a prime 75mm lens, etc. on up, right?"

As long as both lenses are used with sensors of the same physical size then you're correct!!


If true, then no 'zoom' exists with prime lenses, unless I pretend to 'zoom' by switching lenses?
I believe the thing I was looking for is magnification (of image in the eyepiece) instead of 'zoom'?
Thus, 'zoom' means that a lens has various magnifications, compared to a prime lens which has only one magnification.
That also means the eyepiece image size is a matter of personal preference, based on eye sight and ability to mentally process the image with the least amount of stress.

Again, magnification comes from both the focal length of the lens and the physical size of the sensor, so a zoom lens when used with a specific sensor can produce an image with a range of magnifications depending on how the focal length of the lens is adjusted. A short focal length will produce lower magnification and a long focal length will produce higher magnification.
Eyepiece image size has an upper and lower limit.
Too small an image will be difficult to see and cause eyestrain. Too large an image will allow the individual pixels of the display to be seen causing image quality to degrade.
There will be a range of image sizes that most people are comfortable with (that's where the 0.5 - 0.7 factor in my previous post comes in)


Conclusion:
Forgetting about 'zoom', the least expensive and best quality method for getting magnification is to use a longer focal length prime lens that suits my agenda and preferences.

Absolutely correct!!!

Well done for taking in all that technical stuff :thumbup:

Cheers

Bruce
LAND ROVER - THE WORLD'S WORST 4X4 BY FAR

cadman777
Posts: 17
Joined: 25 Oct 2017, 15:40
Location: Florida

Re: fixed zoom lens

Postby cadman777 » 22 Dec 2017, 21:06

Bruce,

Thanx for your replies.
Your summaries are appreciated!
Nobody on the internet whom I read said it as simply as you did, which is why I now understand the basics.
Otherwise I'd be brain dead reading everybody else's yapyap (which is what I came in here to avoid).

I'm curious about what prime lens focal distance is the typical choice for ratting at btw 5 and 15 meters, when used w/an E700 Sony CCTV cam and a 5" screen @ 800x480?

Thanx ...


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