Teleside / Telephoto Conversion Lens Optics Question - Page 1
Forum section: electronics / general

author: frogg
author_rank: Regular Contributor
timestamp: August 03, 2019, 06:57:22 pm
content: Here's a dumb question.Dave Prutchi wrote a good instructables on a DIY telephoto converter for a Seek Revealpro:https://www.instructables.com/id/Diy-Thermal-Camera-Telephoto-Converter/He describes a Keplerian system using Edmunds Optics or Thorlabs parts. Anyways, he builds a working telephoto converter using a 25mm diameter, fl=50mm objective and a 12.7mm / fl=15mm eyepiece. Using only two elements reduces his cost, and overall it's a good idea.The downside of the Keplerian telephoto converter (apart from aberrations - let's ignore those, please.) is that the image is inverted.Nowadays, there are some cheaper options using Ebay CO2 laser singlets and I thought I would take a look at the options.The largest lens with the shortest focal length on ebay is D=25mm, fl=25.4mm ($35.00)The cheapest eyepiece I could find is an uncoated PCX ZnSe D=12.7mm, fl=12.7mm ($165.00) from Edmund Optics.It then got me thinking:Three Element non-inverting telephoto converter:a. objective: 1 x ebay: D=25mm, fl=38.1mm ($35.00)b. eyepiece: 1 x EO: uncoated PCX ZnSe D=12.7mm, fl=12.7mm ($165.00)c. inverter: 1 x ebay: D=25mm, fl=25.4mm ($35.00)Four Element non-inverting telephoto converter:a. objective: 1 x ebay: D=25mm, fl=25.4mm ($35.00)b. inverter element a: 1 x EO: uncoated PCX ZnSe D=12.7mm, fl=12.7mm ($165.00)c. inverter element b: 1 x ebay: D=20mm, fl=25.4mm ($18.88)d. eyepiece: 1 x ebay: D=20mm, fl=25.4mm ($18.88)Let's say, hypothetically, we're choosing between a three-element non-inverting vs a four-element non-inverting telephoto converter. Given the same OD of the objective, is it actually better to use a longer focal length objective with only three lenses, or is it better to use a shorter focal length objective with four lenses?

author: Uho
author_rank: Frequent Contributor
timestamp: August 03, 2019, 08:07:03 pm
content: Hey. In your reasoning, you forgot about the loss in the lenses. The more surfaces the more losses. A low focus zinc selenide lens is not recommended. Image distortions at small lens radii are large. Therefore, a lens made of germanium is placed in the eyepiece. GE has a refractive index of twice as much. So many people dream of making an afocal non-flipping lens. Give it a try. Maybe you get lucky.P.s. Dave Prutchi forgot to write that the thermal imager matrix needs to be turned over. To do this, you need to disassemble the thermal imager. In none of the Seek Reveal I was able to unscrew the screws. The screws had to be drilled. In the photo, my version of adding a lens.

author: frogg
author_rank: Regular Contributor
timestamp: August 04, 2019, 02:37:04 pm
content: Thanks UhoOn your Seek picture, when you say, "the thermal imager matrix needs to be turned over", you are talking about fixing the inverted image problem by turning the sensor upside down? I believe Prutchi didn't do that because he just lived with the inverted imageOn light loss - that's actually the root of my question...is it better to have a larger f-stop aperture, or is it better to have fewer lens elements? In some sense, I'm not particularly bothered by resolution or aberrations or even light loss - because most people who want telephoto for infrared cameras would rather have rather range at the expense of almost everything else. The low resolution of thermal sensors is in some ways an advantage...and moving to smaller micron detectors is again another advantage.I was under the impression that larger radii lenses were actually more difficult to correct aberrations on, which is why it is significantly more expensive to buy sharp large-aperture lenses, or why "lens groups" are more common on large optics? I'm probably mistaken...It also seems that a rudimentary simple four-element telephoto zoom using 25mm lenses only can be built. But the downside is that 25mm focal length means the assembly will be quite long.If there was a cheap source of 12.7mm focal length PCX lenses, it appears a fairly small (less than 4 inch long) zoom telephoto could be built...but is there a good cheap source of 12.7mm FL lenses out there?

author: Uho
author_rank: Frequent Contributor
timestamp: August 04, 2019, 03:16:44 pm
content: Hi!If you want to get at least some kind of image. I mean to see not only objects with a temperature difference of 50 degrees. The first lens should be of maximum diameter. Desirably quality CVD. My opinion - better fewer lenses.How did you manage to get a parallel bunch of light in the eyepiece? Without applying a negative lens?

author: Uho
author_rank: Frequent Contributor
timestamp: August 04, 2019, 04:05:09 pm
content: If there is no lens with a small focus, then it is possible to put two lenses. Losses will be greater. In the figure, I showed how. Better from Ge.

author: Ultrapurple
author_rank: Super Contributor
timestamp: August 05, 2019, 01:40:43 pm
content: Good quality LWIR Keplarian telescopes are available on the used market for reasonable prices. All the difficult stuff has been done for you and the image quality is excellent. The downside is you can expect to pay maybe US$200 or more for a good one. Beware - there is a similar surplus "telescope" that's actually just a long focal length lens. It sells for much less than the Keplarian one and can't easily be used with an existing camera (unless you happen to have the right, obsolete camera of course). The Keplarian has a large lens at the rear. The 'wrong' one has a thumbnail-size rectangular aperture plate at the back.This Keplarian x3 telescope is temporarily adapted to a Therm-App camera using an old water bottle and duct tape.And rubber bands.

author: frogg
author_rank: Regular Contributor
timestamp: August 05, 2019, 02:35:48 pm
content: Thanks Ultrapurple and Uho...Uho your ray tracing is awesome and I'm still noodling on it.Ultrapurple - It makes sense then that a source of short focal lens germanium lenses would be very helpful for a DIY market given that these telescopes already exist.By the way - you have been rocking that plastic water bottle for too long...are you sure you don't want me to 3d print you something??One other thing - what about using an IR fresnel as the eyepiece? Again, forgetting the chromatic aberrations, IR fresnels of 12.7mm x 9.4mmFL with 250 lines/mm and 25.4mm x 14.22mmFL with 200 lines/mm are available from EO very economically.

author: Ultrapurple
author_rank: Super Contributor
timestamp: August 06, 2019, 07:46:52 am
content: I did some playing with plastic Fresnel LWIR lenses a while ago. Here's the thread.https://www.eevblog.com/forum/thermal-imaging/plastic-lenses-for-lwir/msg1306037/Spoiler alert: I got images but they weren't terribly good.

author: Bill W
author_rank: Super Contributor
timestamp: August 07, 2019, 02:55:36 pm
content: Quote from: frogg on August 04, 2019, 02:37:04 pmThat is why a number of cores and sensors offer image flip as a function.Bill

author: Bill W
author_rank: Super Contributor
timestamp: August 07, 2019, 03:06:38 pm
content: Quote from: frogg on August 04, 2019, 02:37:04 pmFor light loss, it is simply maths. Each surface will lose 5 to 15 % depending on coatings, there is also some loss in the materials.Making up for it with aperture is just sorting the f number.However smaller apertures also mean better depth of field and reduced aberrations, as that avoids high divergence in the rays. That is where large aperture large diameter optics have problems.Equally, a long focal length is usually easier to make, often simple spherical surfaces and large radii. Just a lot of germanium.There may be a lot to be gained by using another Seek lens as the 'eyepiece', perhaps from a dead one off eBay ?You then get a perfect second image plane to focus at with the long focus lens.Bill

author: Uho
author_rank: Frequent Contributor
timestamp: August 07, 2019, 09:17:46 pm
content: It is not possible to use a ready-made telephoto lens with all thermal imagers. I was unable to replace the Seek lens. The shutter makes it difficult to bring the eyepiece closer to focus. The only successful replacement for the T10-M. But I had to put an additional filter. Using the filter increased the focusing distance.https://youtu.be/gUOvAg0dGxY

author: Vipitis
author_rank: Frequent Contributor
timestamp: August 11, 2019, 04:53:07 pm
content: Once I get my cameras working (been a while, I know) I will get into collecting lenses. And getting those big ones for low money would be really nice.There is a slight chance I can get my hands on a optical bench soon to prototype lens designs and combinations. I also hope to pickup some optical physics at University.My current experience is just trail and error. But it helps to have a working lens at hand so you aren't lost. Keeping the same groups and orientations helped, as all I had to achieve focus was to change distances between them. Building from scratch can be done and will likely give you better results. But you need to know what you are doing and plan it correctly. Don't we love to save money and jank with our hobbies more?@Ultrapurple: that 3D model to hold a lens onto the ThermApp is somewhere in my old folders - I am not sure at what stage that project was at.

author: TE-V1
author_rank: Newbie
timestamp: August 16, 2019, 02:17:34 am
content: Quote from: frogg on August 03, 2019, 06:57:22 pmHow long do you want to go? I have a 19mm f1.0 lens on my 640 x 480 Thermal Expert TE-V1 and the FOV is too narrow for most of my uses...I am going to get a Umicore 8.5mm f1.2 for it. For long range Telephoto work have you considered buying a ready made lens? Umicore go up to 100mm focal length but Shalom EO offer up to 180mm, and at f1.2!!! But if your prefer to try and build your own, and save money of course, then both Shalom EO and Umicore offer both standard off the shelf, and custom made at customers request, thermal lens elements, in various focal lengths and in various materials...Even in Sapphire glass, which is virtually indestructible.