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Posted: 7/19/2014 11:43:46 AM EDT
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My question relates to lens sizes with Thermal units. Am I correct in my thinking that the larger the lens size, the better the heat signature detection?
An example being a 50mm lens unit compared to an 18mm unit. The 50mm unit should pick up a heat signature better than the 18mm unit, correct? Thanks! |
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Quoted:
My question relates to lens sizes with Thermal units. Am I correct in my thinking that the larger the lens size, the better the heat signature detection? An example being a 50mm lens unit compared to an 18mm unit. The 50mm unit should pick up a heat signature better than the 18mm unit, correct? Thanks! I can't say for sure, but I don't think it works that way. If the "glass" is of the same quality, the same amount of thermal IR should pass through. The larger the lens, the lower the FOV / more magnification which results in increased detection range at the expense of FOV. |
| with the same detector core , the larger lens would give more magnification so you would see a hot object further away but lose some field of view over the smaller lens. ( sorry thehorta - you just said that before me - i did not mean to but in with the same stuff ) |
| I have a 60mm lens on my FLIR T-50 with an older Tau-1 320 core, it is way way more powerful than my PS-32 with the Tau-2 336 core with a 19mm lens and much more powerful than my FLIR PS-32R with 336 core and 35mm lens....the lens is 1/2 or more of the equation. |
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So with what's been said, then it's actually a combination of what the core is as well as the lens size. Would that be correct?
The resolution would only be related to what the display is rated at, and would not have much to do with what lens size is or the core type? Also correct? |
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Quoted:
So with what's been said, then it's actually a combination of what the core is as well as the lens size. Would that be correct? Lens "size" is actually the lens focal length. In old 35mm camera terminology, a 50mm lens is approx 1X ... a 35mm lens is 0.7X wide-angle magnification ... a 200mm lens is 4X telephoto magnification ... etc. When camera manufacturers first converted to digital, the "core size" was usually smaller than the the original 35mm of the film itself. The result was that a 50mm lens now had the magnification of the wide-angle 35mm lens. Again in camera jargon, the light gathering property of a lens is called it's "f/number". This is really related to physical lens "size", and an f/1.4 lens gathered more light than a smaller diameter f/2 lens. When thermal technology shrunk core sizes, focal lengths got shorter for the same magnification, and the lens diameter got smaller for the same light gathering capability, but I've never seen an "f/number" of a thermal optic. Someone will certainly correct me if my analogy is incorrect. :) |
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Quoted:
Lens "size" is actually the lens focal length. In old 35mm camera terminology, a 50mm lens is approx 1X ... a 35mm lens is 0.7X wide-angle magnification ... a 200mm lens is 4X telephoto magnification ... etc. When camera manufacturers first converted to digital, the "core size" was usually smaller than the the original 35mm of the film itself. The result was that a 50mm lens now had the magnification of the wide-angle 35mm lens. Again in camera jargon, the light gathering property of a lens is called it's "f" rating. This is really related to physical lens "size", and an f 1.4 lens gathered more light than a smaller diameter f 2 lens. When thermal technology shrunk core sizes, focal lengths got shorter for the same magnification, and the lens diameter got smaller for the same light gathering capability, but I've never seen an "f rating" of a thermal optic. Someone will certainly correct me if my analogy is incorrect. :) Quoted:
Quoted:
So with what's been said, then it's actually a combination of what the core is as well as the lens size. Would that be correct? Lens "size" is actually the lens focal length. In old 35mm camera terminology, a 50mm lens is approx 1X ... a 35mm lens is 0.7X wide-angle magnification ... a 200mm lens is 4X telephoto magnification ... etc. When camera manufacturers first converted to digital, the "core size" was usually smaller than the the original 35mm of the film itself. The result was that a 50mm lens now had the magnification of the wide-angle 35mm lens. Again in camera jargon, the light gathering property of a lens is called it's "f" rating. This is really related to physical lens "size", and an f 1.4 lens gathered more light than a smaller diameter f 2 lens. When thermal technology shrunk core sizes, focal lengths got shorter for the same magnification, and the lens diameter got smaller for the same light gathering capability, but I've never seen an "f rating" of a thermal optic. Someone will certainly correct me if my analogy is incorrect. :) not true flir does this to confuse the issue. in normal lens nomenclature the size is the diameter. |
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I think this thread is getting off of the OP's original question, which either I am misunderstanding or he is asking if the size of the lens determines how efficiently the lens "gathers light" (using old photography parlance),
If thermal optics function anything like photography optics, an f/1.4 lens is always an f/1.4 lens whether it's 24mm, 50mm, or 85mm. An 85mm lens does not gather any more light than a 24mm lens at the same aperture. Increased focal length serves only to narrow FOV at the same focal plane and same distance-to-subject. That said, using the above reasoning, a 50mm lens on a thermal camera will not "pick up a heat signature" any "better" than an 18mm lens, but would rather narrow the FOV by a factor of roughly 2.9 times, thereby increasing the image size of any thermal signatures by that amount within its field of view (FoV). By comparison, perspective issues aside, if you were to move close to the subject with the 18mm lens to approximate the FOV of the 50mm lens at greater distance, it should show a similar signature provided both lenses are of similar quality and operate at the same aperture. So, while it may appear as though the longer focal length is providing a "better" heat signature, it's really a matter of focal length, just as a longer lens on an SLR will appear to provide a "better" visual image of a subject, if that makes sense. |
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Read this from FLIR: http://www.flir.com/uploadedFiles/Eurasia/Cores_and_Components/Technical_Notes/uncooled%20detectors%20BST.pdf Their explanation seems incomplete, but given that they don't have a lot of space to fully explain every aspect of the optics, it would seem as though thermal optics follow the same principles as those in the photographic world. What might be a point of confusion in their explanation is the idea of f/ratio and focal length. While they're related they are not the same. To my untrained brain the blurb in the blue sidebar makes them sound the same -- i.e. if you have a bigger lens you have a bigger relative aperture (lower f/number), where that's only the case of focal lengths being the same (a 200mm f/2.0 will be much larger than a 200mm f/2.8, even though it's "only" one f/stop). However, a 50mm f/2.0 will transmit as much light as a 200mm f/2.0 to the same focal plane, even though the 200mm objective will be many times larger. DOF and FOV are all considerably LESS on the 200mm lens, of course, and it seems like this holds true in the thermal optics world as well. |
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Depth of focus is also an important aspect too, many of these high priced big lens thermals already have a tiny FOV at range, add that to a tiny depth of field for actual focus and it become problematic, ie trying to take out a moving hog at a distance and it going out of focus....
It pays to have a matched lens to the particular microbolometer, another reason I purchase FLIR products, since they make the lens and microbolometer and match them together. |
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Quoted: Am I correct in my thinking that the larger the lens size, the better the heat signature detection? An example being a 50mm lens unit compared to an 18mm unit. The 50mm unit should pick up a heat signature better than the 18mm unit, correct? |
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A lot of great info! This helps me understand better how it all works together, and what to expect the end result to be. Hands on use is one thing, but having a good basic knowledge of what happens to get you to what you see, is equally important IMHO.............
Thanks! |
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Quoted: Depth of focus is also an important aspect too, many of these high priced big lens thermals already have a tiny FOV at range, add that to a tiny depth of field for actual focus and it become problematic, ie trying to take out a moving hog at a distance and it going out of focus.... It pays to have a matched lens to the particular microbolometer, another reason I purchase FLIR products, since they make the lens and microbolometer and match them together.  The first deer is just under 50 yards and the last is 239 yards away |
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