An interesting thread that I am sure will be of interest to many
I am no expert on optics as I have stated previously. I have bought various books in an attempt to better understand optical block designs. Please bear this in mind when reading my comments. I am writing this in less than ideal circumstances on an iPad so it is bound to be full of typos and weird auto corrections. Sorry about that and I will correct the text later
Firstly, I do not know anything about the 75mm lens you are using, or its intended original host. Such information is invaluable as I may know a little about the host camera and its optical systems.
The lens in your E40 will consist of at least two lens elements. Sometimes thermal cameras have a ‘doublet’ design that looks like a single lens when, in fact, the ray trace indicates two distinct lens actions are in play. The E4 has such a lens. The two lenses can be separate, as in the Exx series, but they work in harmony with each other as a matched pair to create a low distortion image that illuminates the sensor array. It is not normally possible to just replace the objective lens element to change the optical blocks characteristics for telephoto operation. A mismatch beteween the lenses causes the effect you have seen. You description of the effect experienced, (close-up effect) makes me think you are using a 75mm lens from certain models of scanning thermal camera made by Inframetrics or Agema. Such lens barrels contain only one or two lenses, the objective and a focus element, as the rear lens is a fixed component in the scanning cameras lens mount. That fixed lens is usually a negative concave type.
The back focus distance of a thermal camera is a critical specification of the lens block design and can be completely wrong if the lens block contents are modified without consideration of the effect on the back focus distance and correct illumination of the sensor array.
So what can I offer as positive comment .........
I personally would avoid any modification of the excellent quality FLIR Exx lens block. Such is fraught with complications that severely reduce the imaging performance. You really have to be a master of ray tracing simulation and have access to both the specifications of the original lens elements and those you are hoping to replace them with. Practical Experimentation is possible but can be fruitless and frustrating without a good understanding of optics and interpretation of issues that can occur.
A safer/easier way to enhance a thermal camera optical performance is to go down the path of supplimentary optics. In the same way that a close-up lens is like placing reading glasses on the camera, you can effectively place a monocular telescope in front of the objective to achieve the desired telephoto effect. There are losses involved in such an approach but a calibration offset may be applied to account for such where needed. For wildlife observation, calibration is not often required.
Inframetrics, NEC-AVIO, AGEMA, FLIR and FLUKE all use afocal, non inverting supplimentary telescopes and wide angle Adaptor’s in order to avoid the need to remove the cameras objective lens. This is often a requirement due to the lens block design (sealed or internal motorised focus). Supplimentary lenses are not to be scorned or feared and they are very suitable for experimentation.
A quick guide to supplemental lenses.... These are normally Afocal when intended for use on a conventional camera optical block. That is to say, they receive rays at their input and ‘process’ them to exit the output in parallel ray format suitable for illumination of the cameras fixed objective. Some cameras are focussed at finity when used with supplemental lenses. The output of a supplemental lens can be an inverted version of that which entered them. This is due to the optical design and is normally dealt with in the host camera the lens was intended for. It is a purely electronic processing change in the camera to re-invert the image. Sadly when using an inverting supplimentary lens on a camera that does not offer image inversion, the user is faced with either modifying the supplemental lens by inserting an erector element, inverting lens pair, or post image capture manipulation in software (host PC)
Are all supplimentary lenses the same in terms of how they work ? In short, NO.
As an example, the supplimentary lenses for the Agema PM series, FLIR Exx series and FLUKE cameras are non inverting and may be mounted in front of many LWIR cameras that have a matching, or similar FOV and lens diameter. Using a small diameter supplimentary lens on a large diameter objective is not a great idea due to vignetting. The use of a large diameter supplimentary lens on a small diameter camera objective works fine however.
Now the tricky types of supplimentary lens........
Agema used to offer alternative lenses for their scanning cameras that, at first glance, appeared to be supplemental types. Upon seeing how they mount on the camera it becomes clear that it is not a supplimentary lens system, but rather a split lens block with half the elements in the removable lens barrel, and half permanently mounted in the camera mount on the camera chassis. This approach effectively reduced lens barrel cost as one Germanium element was not required in the barrel..... the one fixed in the camera. This also facilitated the required vacuum sealed scanning mirror assembly as the fixed Germanium element is sealed into the side of the vacuum module.
Inframetrics took different approaches to the challenge of interchangeable lenses. Some are the same principle as that used by AGEMA, a spilt lens block, whilst others are a complete lens block in the camera, with true supplimentary telescopes or wide angle adapters that mount in front of the scanning cameras input window. These are the lenses that I sought out for experimentation as the range includes both MWIR and LWIR coated telescopes of decent power. x3 is a common magnification factor for the telescopes found on eBay. The Inframetrics supplimentary lenses are different to those ones from FLIR, FLUKE etc. They are intended for operation into the large window input of a scanning mirror imaging system and those systems were capable of image inversion. As such, a standard Infametrics x3 supplimentary lens will be inverting and will not optically match into another cameras objective without some experimentation. Please see the experiments by Ultrapurple for detail. He had to place his cameras objective quite some distance behind the telescopes output. These Inframetrics lenses are superb quality thermal optics though, so worth the effort.
Well that is enough from me for now
Fraser
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