mirobolemeter vs Lens size

[Drift]

I don't quite understand the relationship between the microbolemeter sensitivity and lens size. Clearly, bigger lenses are better and more sensitive microbolemeters are better. But (if all else is equal) if I go from a 320 microbolometer to a 640 one, how much can I shrink my lens size and still keep the same resolution?
Or am I asking the question wrong?
My goal is picking the hogs out when they are intermingled with the smaller cows in 85 degree weather in grass tall enough so I cant tell leg length all at 70% humidity.

 

[Brian Shaffer]

The question you are asking is like trying to equate wheel size and engine displacement to get equal speed between two vehicles without taking into consideration vehicle weight, turbochargers, nitrous oxide, etc.

Well, it isn't just resolution, but other factors such as pixel microns (distance between pixels, smaller is better), firmware, etc. So it isn't just a matter of resolution versus lens size, but all of the factors.

Assuming everything else being equal, I am sure there would be some sort of physics formula to work out the solution, just like there is with film speed and lens configurations on cameras.

With that said, it sounds like the shooting situation you are describing would be decidedly unsafe for the cattle, even during a clear daylight situation.

 

[Ratdog68]

Have I read here (remembering correctly) that the 640 will suck batteries dry about twice as fast as the 320?

 

[Oso Grande]

This is a post I did on another site when this kind of question came up.

I see what appears to be a lot of misconceptions about lens sizes and focal lengths in regards to thermal and night vision scopes and handheld viewers.


One of my other hobbies is photography and this is a common thing for people to mix up, twist around and generally get lost in that world as well. I am by no means an expert, but I’m happy to see if I can help make things clearer for someone who’s looking and comparing different units.

Let’s get to the basics:
Focal length - The focal length of a lens determines the magnification at which it images distant objects.
Lens size - The diameter of the front lens, often referred to as the objective size
FOV - How much of the world can you see, ie: your field of view
Sensor size - The physical size of your sensor, ex: 320x240, 640x480

Ok, so why does any of this make a difference?

Starting with FOV (field of view)
This is how much of what is in front of you that you can see through the lens. Are you looking through a cardboard tube? Does it feel like one of your eyes is covered, or do you feel like you can see as you do normally with nothing in front of your face?
The FOV is usually measured in degrees. But it is tied directly to the focal length, which is measured in millimeters. The focal length can be looked at as the “zoom” amount. The higher the focal length number the more zoomed in you are. Compare it to looking through a 3x scope vs a 10x scope. It’s the focal length that’s changing when you zoom in on that day scope.

Here are some examples to give you a better idea as to what I’m talking about






Now keep in mind that those pictures are examples and not direct correlation to any scope or imager that you may be looking at. But it should help you understand the concept.

Two more things are the sensor size and the lens size.
Remember how I said that those pics were just examples…… well here’s why:

Each sensor size increase effectively increases the FOV if the focal length stays the same. For example a 19mm focal length on a 160x120 sensor has a FOV of about 12 degrees. Put that same lens on a 324x256 sensor and the FOV goes to about 24 degrees. Again with a 640x512 and that FOV climbs to 32 degrees.

So you can see, as you increase your sensor, it can show you more of the world in front of you.

Last but not least is the lens size, or objective size.

The size of the front lens is what collects the light on a night vision scope or collects the heat on a thermal. Think back to those pictures above, if you’re trying to see something small very far away you’d want the most sensitive lens doing your collecting. To get a more sensitive lens, that means going larger. If you look at scopes with large focal lengths, you’ll see they on average have much larger front lens sizes than their shorter focal length counterparts. That’s because it’s much easier to collect the light or heat from something that’s closer than it is for something much further way.

Ok, I didn’t want this to turn into a term paper but I did want to help clarify some things that I see people getting mixed up about when they’re referring to these toys of ours. Please feel free to add to this or tell me I’m full of it if I’ve screwed up somewhere. I’m not an expert, just someone who sees a lot of overlap between hobbies I've collected over the years…..

 

[Wildfowler]

Very informative Oso, thanks.

Do you or anyone else know why the scopes with largest lenses don't or maybe they "can't" have shorter focal length for less optical magnification? I'm guessing the size of the lens has a minimum focal length that needs to longer.

I think I would love a "variable" optical power scope with a really big lens. The highest digital zoom setting on my IR Hunter is really not very helpful at extreme distances to me.

Thanks

 

[Brian Shaffer]

Alright OSO, you are on a role! Now explain the changes produced when you go from 25 to 17 to 12 micron pixel pitch on the sensor, CMOS Noise levels, and Thermal Coefficient of Resistance differences between sensors on picture quality. My point here isn't actually to challenge you to answer so much as to hammer home the fact that even your basic explanation, simplified as it is, is only part of the total number of factors of producing a picture of sufficient quality for Drift to be able to pick out hogs intermingled with small calves in tall grass (never mind how much the variable of "tall grass" can differ between field situations). Never mind the addition changes produced by firmware interpolation, LOL. These factors are especially salient as there are different sensors being used by different companies with these different attributes.

The striking comparison here would be pitting the IR Defense's IR Hunter MKII 640x512 35mm BAE 12 micron core model against Armasight's (pick whichever is closest in lens size) Flir Tau2 640x512 17 micron cores where it is obvious that IRD is producing a better quality image with the same number of pixels and smaller lens.

 

[Oso Grande]

Alright OSO, you are on a role! Now explain the changes produced when you go from 25 to 17 to 12 micron pixel pitch on the sensor, CMOS Noise levels, and Thermal Coefficient of Resistance differences between sensors on picture quality. My point here isn't actually to challenge you to answer so much as to hammer home the fact that even your basic explanation, simplified as it is, is only part of the total number of factors of producing a picture of sufficient quality for Drift to be able to pick out hogs intermingled with small calves in tall grass (never mind how much the variable of "tall grass" can differ between field situations). Never mind the addition changes produced by firmware interpolation, LOL. These factors are especially salient as there are different sensors being used by different companies with these different attributes.

The striking comparison here would be pitting the IR Defense's IR Hunter MKII 640x512 35mm BAE 12 micron core model against Armasight's (pick whichever is closest in lens size) Flir Tau2 640x512 17 micron cores where it is obvious that IRD is producing a better quality image with the same number of pixels and smaller lens.

Absolutely Brian! My post was just to help further muddy the water with more info that can just lead to even more questions...... Not even close to claiming that I know or understand it all.

However, my understanding on the change of the microns of the sensor have a very similar effect as does sensor size. Smaller micron number acts like a larger sensor and requires a smaller objective lens to give the same resolution as a larger micron number.

There's no question that the software side of things plays a huge roll in all of this as well. How well the units process such basic things as contrast and brightness can have HUGE impact on the actual image you're seeing.

 

[Hard_ware]

Mrad is the resolution capability.
The pixel size and lens determine this.
A 12u (micron) core has the same resolution regardless of how m much of it you buy (640,320,160,80).
The amount you buy gives a larger or smaller field of view.
The reason 640 looks better is because when you display it on a 640 LCD screen it fills it.
A 320 detector would have to be expanded to fill the screen and would not look as clear.
Look at your tv, the entire tv is the same resolution. Now only look at 1/4 of the screen picture is the same, no less clear.
But if you expanded the 1/4 portion to fill the whole screen it would much lower quality, just like some tv s the have digital zoom.
The more you zoom in the worse the content looks. Take a photo get a magnifying lens look at it close looks very bad, just a bunch of dots. Hold it farther away and it looks sharp, it's all about dots per inch, or pixels per inch. The more pixels the sharper. The detector pitch/ lens ratio is what matters. Which boils down to pixels per inch, just like night vision lp/mm gives the resolution.

 

[Brian Shaffer]

And Hard_ware has now introduced another critical factor in the whole mess and that is the display screen used to show the image to the user. Not all displays are created equal as we commonly know from the TV industry. So if the display isn't sufficient on all levels to match what is produced by the camera end of the scope, then you will have loss of picture quality potential on the display end of the scope. IIRC, FLIR dealt with this on some of their scopes, improving the end-user satisfaction improving the display, but keeping everything(?) else virtually the same.

I will add another factor to all of this as well, lens coatings. I learned recently that ATN is (supposedly) changing their lens coatings (doing away with what they were using) because they found that while the coating being used did a better job of protecting the lens, it degraded the thermal image. Imagine that.

So getting around to the functional bottom line for Drift is that there is no easy solution for say what configuration is going to work best for your particular situation. There are a circus of features that contribute to the image that you get to see beyond simple lens size and microbolometer resolution.

 

[Hard_ware]

So far the IR hunter MKII and the IR Patrol have been the most impressive units I have had hands on with.
Side by side comparison against the flir ps24 ps32 and the armasight 336 75mm 60hz, the MKII 640 19mm and the Patrol 250 delivered a better picture and higher contrast. The armasight had a larger better lens (75mm) but a smaller 336 60hz core. Using the IR hunter and patrol the 2x and 4x digital zoom worked great the 8x was ok for precise shot placement but pic quality was bad.
The software and BAE core were really impressive. It's hard to compare apples to apples, when some apples are designed better then others

 

[BigRedDog]

And then there is the f-stop of the lens

 

[Drift]

Clearly, it is knowable. After all the technology exists and thermal is technology, not magic. A great attempt at head to head comparison between thermals was made on this site 18 months ago and that helped clear up a lot. But.... of course that let the door open for further questions.
Over time, I guess people will do further head to head testing and an answer will emerge.

 

[Brian Shaffer]

Let's approach this from another direction. Are you trying to work out the actual physics to make a 320 microbolometer perform equally to a 640 as an heuristic exercise or are you trying to shop for a 320 resolution scope that will give you equal performance to a 640 resolution scope?

If the latter, I assume you are trying to figure out if you can get the same performance out of something like an Armasight Zeus 336 3-12x75 as you would from a Armasight Zeus 640 3-12x42, the former being less expensive than the latter. On their native magnification, a cursory look may give you the impression of similar performance, though the eye can see a real difference between 320 and 640 pretty quick. Once zoomed, it would be no contest that the 640 is much better. And that is the problem. The scopes won't be comparable zoom levels.

 

[Drift]

Thanks a lot! That was just the information I was looking for.

 

[wigwamitus]

Some data on 320 vs 640 comparison ... (mixed up with lens size comparison) ...

I can see cows in a nearby pasture to the West from my hill tops ... the distance is between 1,200yds and 2,200yds, depending on where I am and where the cows are. And then some nights they are behind a ridge and I can't see them.
To the north, there are usually some bales sitting there, which radiate some heat and they are often visible to thermal as well. These bales are also average about 2,200yds away (2000-2400)

With my old Apollo 336 42mm I could see the cows out to 2,200 and the bales. They were little white blobs, but I could see them. And I could tell the cows were much whiter than the bales, so I could tell the difference.

Recently on a moderately poor thermal performance night, I could not see the cows with q-14 19mm 640 ... but I could see them with Zeus 75mm 640 ... so this must be a "big lens gets more data" difference.

 

[slim_shady]

I have a Zeus 640 3-12x42, and hunt hay fields sometimes with very tall Johnson grass. Around here I've never seen Johnson grass in a pasture, but can relate to the weeds that grow up in our pastures. We also have hay fields that border pasture with cows, and have experienced seeing hogs in and around the neighbors cows. Not being able to hunt the neighbors property I had a great opportunity to simply watch the hogs. Without using or even looking through any other thermal scope, the one thing I would suggest is paying very close attention to how the animals move around.

I understand that there is no absolute when it comes to animal movement, but from what I've experienced cows move around very little after dark. Hogs on the other hand will move around quite a bit unless they are feeding, and even then they will move around a fair amount in a small area when they are fixed on a single spot. With time and experience watching, its easier and easier to identify what your looking at simply by movement and behavior.

Last, even in the worst of conditions (high heat and high humidity), other then fog or rain which I've never hunted in. I have always had very good visual identification inside of 100 yards with my Zues.