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05-05-2013, 07:00 AM   #1
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Telephoto lens to duplicate binocular FOV

I have Canon 15x45 image stabalized binoculars that are supposed to offer 15x magnification and a 67 degree FOV, according to the specs I can dig up on the web. I am always impressed with what I can see through these, it opens up a whole new world when you put them up to your eyes. The optics are amazing. I can pick out the individual hairs on a bee at 30 feet.

Is there some way I can calculate what dslr lens would somewhat reproduce this kind of view? I don't understand optics that well so I'm not sure what all the variables you have to take into account to figure this out. I'm looking at buying a Tamron 70-200mm anyway, would this do it for me?

05-05-2013, 07:25 AM   #2
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A 67 degree angle sounds impossibly wide for binoculars.

Most manufacturers will indicate the angle of coverage in the lens specs.

Here is a sample from the Pentax database.

SMC Pentax-DA 18-270mm F3.5-6.3 ED SDM Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database
05-05-2013, 07:35 AM - 1 Like   #3
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An example from the Pentax XCF 16x50 binocular:

Specifications 16X
Apparent Field Of View 56 degrees
Real Field Of View 3.5 degrees

Another example: Canon 15x50 IS Binoculars:
Field of view: 4.5 degrees

What you listed is the apparent field of view, which is the size of the circle you see with your eyes in the binoculars. You will be looking for a 300mm lens on a APS-C camera to get the same FOV.

SMC Pentax-DA* 300mm F4 ED [IF] SDM Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
Angle of view horizontal: 4.6 degrees (APS-C)
05-05-2013, 07:53 AM   #4
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Well, I've had these for about 15 years now and it's hard to find any official specs.

I got those numbers from Canon 15x45 IS Binocular

Perhaps they are missing a decimal point, 6.7 degrees? This one says 6.5: Canon 15x45 IS.

I looked at the canon site and can't find the specs there. I have found links to manuals that 404 when I click on them.

Here is another one that says 67 degrees: LISTSERV 15.5 - ATM-OBSERVERS-L Archives

Let's say 6.5 - 6.7 degrees if that's more reasonable. As I said, I don't know optics all that well so when I see odd numbers they don't stick out to me.

05-05-2013, 08:02 AM   #5
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QuoteOriginally posted by JinDesu Quote
An example from the Pentax XCF 16x50 binocular:

Specifications 16X
Apparent Field Of View 56 degrees
Real Field Of View 3.5 degrees

Another example: Canon 15x50 IS Binoculars:
Field of view: 4.5 degrees

What you listed is the apparent field of view, which is the size of the circle you see with your eyes in the binoculars. You will be looking for a 300mm lens on a APS-C camera to get the same FOV.

SMC Pentax-DA* 300mm F4 ED [IF] SDM Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
Angle of view horizontal: 4.6 degrees (APS-C)
Thanks, that's making more sense. So my 200mm won't cut it then.
05-05-2013, 08:04 AM   #6
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It won't be that far off, a little cropping and you get there.
05-05-2013, 08:07 AM   #7
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Try the Pentax DA 55-300mm.
If that doesn't work, get a sigma that goes over 300mm
05-05-2013, 09:09 AM   #8
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QuoteOriginally posted by mvsjes2 Quote
I have Canon 15x45 image stabalized binoculars that are supposed to offer 15x magnification and a 67 degree FOV, according to the specs I can dig up on the web. I am always impressed with what I can see through these, it opens up a whole new world when you put them up to your eyes. The optics are amazing. I can pick out the individual hairs on a bee at 30 feet.

Is there some way I can calculate what dslr lens would somewhat reproduce this kind of view? I don't understand optics that well so I'm not sure what all the variables you have to take into account to figure this out. I'm looking at buying a Tamron 70-200mm anyway, would this do it for me?
Stick this on a 100-200mm lens and you'll be all set

PENTAX Monocular Converter K reviews - Pentax Camera Accessory Review Database

If you want to reproduce the same image on your DSLR's big sensor, you'll need a much longer lens.


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05-05-2013, 02:37 PM   #9
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You can work out which lens to use at a given distance to cover a given width, but you will have to do some measuring of distance and image width with the binoculars. Let's say that at a distance of 2500mm you want to cover a width of 120mm. You have a camera with a 24mm wide sensor.

Divide the width of the view by the width of the sensor: 120/24 = 5. Your subject will be reproduced on the sensor at 1/5th actual size.

Divide the distance by the answer to the last sum: 2500/5 = 500. In this case you would need a 500mm lens.

Of course, you can also find the width you will cover, when you know the distance, lens and sensor size. Let's say you are taking photos of a 130mm long bird at a distance of 3600mm with a 400mm lens and a 24mm wide sensor.

Distance divided by lens: 3600/400 = 9

Multiply the answer by the sensor width: 9 X 24 = 216

The width covered is 216mm. The height covered will be 2/3 of 216mm, 144mm. (We know the proportions of the APS-C size sensor) The bird will fit in nicely.

Since you are going from a round image to a rectangular one, you may wish to use the shorter dimension of the sensor, which would mean a shorter lens.
05-10-2013, 12:32 PM   #10
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Thanks for the math, that's a great post!
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