black crow
Well-known member
I'm sure this has been answered many times but I must have been away those days. Why is it that in a binocular line the 42mm will often have a smaller FOV than the 32mm?
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32mm vs 42mm fov question (1 Viewer)
- Thread starter black crow
- Start date
WJC
Well-known member
Field of view is not based on aperture; it’s based on magnification and field stop. Those things are based on what the manufacturer has determined to be his or her market for the particular instrument. Bigger is not always better. Unless, of course, you are talking about ... ice cream. :cat:
Bill
Alexis Powell
Natural history enthusiast
You two haven't answered black crow's question. A refined version follows: why are bins of the same magnification with 32 mm objectives more often engineered with wider angle oculars than are bins with 42 mm objectives?
--AP
--AP
black crow
Well-known member
Thank you Alexis. lol
black crow
Well-known member
henry link
Well-known member
Maybe the principle will be clear if I describe the simplest case: an 8x42 and an 8x32 using the same prisms, with the same size eyepiece field stops, but different focal length objective lenses and eyepieces.
The short focal length objective of the 8x32 focuses a small image of the FOV at the plane of the eyepiece field stop. The longer focal length objective of the 8x42 focuses a larger image at the plane of the same sized eyepiece stop. Because the 8x42 image is larger the eyepiece field stop in the 8x42 can't include as wide a FOV as the same sized field stop in the 8x32, so the FOV is smaller.
The last wrinkle is that, because the objective lenses have different focal lengths, the eyepieces must also have different focal lengths for the two binoculars to both be 8x. The 8x42 requires a longer focal length eyepiece which magnifies its eyepiece field stop less than short focal length eyepiece in the 8v32, so the eye seems to see a smaller field stop and therefore a smaller apparent field in the 8x42.
Best I can do just before going to bed.
The short focal length objective of the 8x32 focuses a small image of the FOV at the plane of the eyepiece field stop. The longer focal length objective of the 8x42 focuses a larger image at the plane of the same sized eyepiece stop. Because the 8x42 image is larger the eyepiece field stop in the 8x42 can't include as wide a FOV as the same sized field stop in the 8x32, so the FOV is smaller.
The last wrinkle is that, because the objective lenses have different focal lengths, the eyepieces must also have different focal lengths for the two binoculars to both be 8x. The 8x42 requires a longer focal length eyepiece which magnifies its eyepiece field stop less than short focal length eyepiece in the 8v32, so the eye seems to see a smaller field stop and therefore a smaller apparent field in the 8x42.
Best I can do just before going to bed.
tenex
reality-based
Would they really use the same size prisms in 32 and 42mm glasses?
Wide FOV is one reason I've remained happy with a 32mm for years. I always assumed that 42s had a narrower view because this is the most popular size among birders who carry them all day, and want them kept small and lightweight. Some are barely taller than 32s. One can only make so many compromises.
Wide FOV is one reason I've remained happy with a 32mm for years. I always assumed that 42s had a narrower view because this is the most popular size among birders who carry them all day, and want them kept small and lightweight. Some are barely taller than 32s. One can only make so many compromises.
WJC
Well-known member
One of the many, many ways to save money? Why? Because only us opto-geeks would ever see a reason not too. And even THAT one would be nonsensical because most birders are interested in viewing birds as opposed to reinventing the proverbial wheel. :cat:
Bill
Last edited:
black crow
Well-known member
OK thanks.
An 8x32 may have objectives of 120mm focal length, so the eyepieces are 15mm focal length.
An 8x42 may have objectives of 160mm focal length, so the eyepieces are 20mm focal length.
For the same size field stops, the 15mm focal length eyepieces will have a much wider AFOV and FOV than the 20mm eyepieces.
However, the 15mm eyepieces may be more complex and have more lens elements than the 20mm eyepieces.
The 15mm eyepieces may also have less eye relief than the 20mm eyepieces.
An 8x42 may have objectives of 160mm focal length, so the eyepieces are 20mm focal length.
For the same size field stops, the 15mm focal length eyepieces will have a much wider AFOV and FOV than the 20mm eyepieces.
However, the 15mm eyepieces may be more complex and have more lens elements than the 20mm eyepieces.
The 15mm eyepieces may also have less eye relief than the 20mm eyepieces.
How about the FOVs of f4 42mm binoculars? Most 7x42s have 24mm eyepieces and a real FOV of 8º while most 8x42s use a 21mm eyepiece and have real FOVs of around 7.7º.
If you want wider FOVs do you manipulate the focal lengths of the objectives or fiddle with the ocular lenses or do both? For instance, the Zeiss Victory 7x42 FL has a real FOV of about 8.5º and the Zeiss 8x42 SF has a real FOV in that neighborhood.
Bob
If you want wider FOVs do you manipulate the focal lengths of the objectives or fiddle with the ocular lenses or do both? For instance, the Zeiss Victory 7x42 FL has a real FOV of about 8.5º and the Zeiss 8x42 SF has a real FOV in that neighborhood.
Bob
I would guess the answer depends on what you want to achieve with the eye relief too.
Lee
Hi Bob,
Normally one fiddles with the eyepiece.
However, something like the 8x30 Nikon E11 has short focal length objectives and short focal length eyepieces, which enable it to have a wider FOV than say the Nikon 8x32 SE, which I don't like because I get blackouts not wearing glasses.
I just wish older type Minolta Standard MK 8x40s with around 9.5 degree FOV were made nowadays as I have no problem with the shorter eye relief.
I do struggle with some binoculars that have extremely short eye relief.
Normally one fiddles with the eyepiece.
However, something like the 8x30 Nikon E11 has short focal length objectives and short focal length eyepieces, which enable it to have a wider FOV than say the Nikon 8x32 SE, which I don't like because I get blackouts not wearing glasses.
I just wish older type Minolta Standard MK 8x40s with around 9.5 degree FOV were made nowadays as I have no problem with the shorter eye relief.
I do struggle with some binoculars that have extremely short eye relief.
henry link
Well-known member
I used the simplest example, but there are many possible variations. In the old days of simple Porros a binocular series was frequently based on a single prism housing with various objectives lenses and eyepieces of different focal lengths and apertures mixed and matched to produce different models. The Nikon E and SE and the Swarovski Habichts are examples.
Of course if no consideration was given to cost, size and weight it was always possible to make a very wide field 8x42 or even 6x42 like the 12º WW2 Sard 6x42 below.
Of course if no consideration was given to cost, size and weight it was always possible to make a very wide field 8x42 or even 6x42 like the 12º WW2 Sard 6x42 below.
Attachments
NDhunter
Experienced observer
You have a very good point, and that is many mfrs. make their top
8x32 models with a wider FOV than their 8x42's.
And 42mm models have larger prisms, usually.
We should include a discussion of the Nikon SE porro models.
These have the same size eyepiece and prisms, with a different size
objective set. These are a masterpiece in interchangeable design.
I hope Henry will chime in.
Jerry
black crow
Well-known member
Another question, Meopta has Meostar and Meopro 10x32. Meostar being the alpha of the lineup. Why is the Meopro got the larger fov?
Does it have something to do with wide fov being harder to keep sharp as you go out towards the edge? The wider fov of the Meopro is too soft for an alpha hunter?
Does it have something to do with wide fov being harder to keep sharp as you go out towards the edge? The wider fov of the Meopro is too soft for an alpha hunter?
Hello black crow. I'm late to the discussion, and certainly no expert, but I'm here so I might as well offer my 2 cents.... First, as has been pointed out, an 8x42 would often need wider field stops in its eyepieces, compared to the field stops in an 8x32's eyepieces, in order to match the field size of both binoculars. There is a direct relationship between an eyepiece's focal length and the size of the field stop necessary to produce an given apparent field size. The 8x42 usually needs a longer focal length eyepiece, so it would also need a wider field stop to maintain the 8x32's field. So does the binocular's body have enough space to accommodate the wider field stop? Maybe, maybe not. But why wouldn't the binocular's designer make sure that it does? Maybe because the market doesn't demand it. If you go back a decade or so, 6.3* was a respectable field size for an 8x42. Nowadays, customers want at least 7.5*, preferably closer to 8*. So binocular designers respond. Consider for example the Bushnell Legend M 8x42 which has an 8* field and recently sold for under $200. So, it's not that there is some theoretical reason that prevents an 8x42 from having the same wide field as an 8x32, it's just a design decision. Now, maybe the 8x42 is a bit larger than it might have been otherwise, but that's just a compromise the designer chose to make.
About your Meostar vs Meopro HD question, I've got both models in 8x32. Yes, the obvious optical difference between the two models is that the Meostar's field edges are better. But the Meostar also has advantages in size, brightness (slight), and glare control (by a country mile). In my opinion the Meostar and Meopro HD are well differentiated products. A great many users would be perfectly happy with the Meopro HD as their main binocular, but there would be others who would appreciate what the Meostar brings to the table.
About your Meostar vs Meopro HD question, I've got both models in 8x32. Yes, the obvious optical difference between the two models is that the Meostar's field edges are better. But the Meostar also has advantages in size, brightness (slight), and glare control (by a country mile). In my opinion the Meostar and Meopro HD are well differentiated products. A great many users would be perfectly happy with the Meopro HD as their main binocular, but there would be others who would appreciate what the Meostar brings to the table.
black crow
Well-known member
Yeah that's something I've certainly noticed about the Meopro. My $189 Celestron Trailmaster 10x handles glare much better. Go figure. In fact if the Trailmaster had a better FOV I'd have likely returned the Meopro. However it is a very nice binocular in all other ways and the fast smooth focuser is the best I've had on any of my binoculars including a Swarovski.
