-
Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.
Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
Why less F O V with larger aperture? (1 Viewer)
- Thread starter vop
- Start date
(I mean, that's what the lesser weight does!) 
vop
Well-known member
Thanks Rick, but this leaves me with new questions! |=)|
I can see why higher magnification gives less F o V, and longer focal length gives more magnification(?). But the magnification must be the same in a pair of 8x30 and 8x40 binoculars... or? What do I miss?
And apparent F o V, that has to do with the beam leaving the oculars, I think...
Maybe I just have to go back to wikipedia and study some more! :smoke:
I can see why higher magnification gives less F o V, and longer focal length gives more magnification(?). But the magnification must be the same in a pair of 8x30 and 8x40 binoculars... or? What do I miss?
And apparent F o V, that has to do with the beam leaving the oculars, I think...
Maybe I just have to go back to wikipedia and study some more! :smoke:
birdazzLED
Well-known member
That's what I want to know about if it's ok to inquire this thread - as it may relate to FOV as well.
Why are some bins longer than others? For instance the ZRS and ED2 - both being 8x43
Larger aperture objectives have longer focal length than small aperture, and so require longer focal length eyepieces to reach the same magnification. In order to show the same field of view, the longer focal length eyepieces must have larger field stops. So, if a larger binocular were to have the same magnification and field of view as a smaller binocular, its eyepieces would have to be quite a bit bigger around as well as in length. Additionally, it is harder to make a large eyepiece with as good edge of field correction as a smaller one, so the large eyepiece would also need to be more complex, with more elements. Furthermore, in order to sufficiently illuminate the larger eyepiece field stops, the prisms would have to be made bigger. So weight, size, and price are getting out of hand here, and impose the practical limits.
Ron
Ron
henry link
Well-known member
vop and birdazzLED,
Maybe this photo will help with the basics. I set up two simple binocular objectives, a 30mm with FL=110mm and a 50mm with FL=180mm. The image of the window you see projected on the wall behind the lenses is the image that forms in front of the eyepiece. The eyepiece then acts as a magnifier to enlarge that image to the desired magnification. Shorter focal length eyepieces are simply more powerful magnifiers, so if you want 8x from the small 30mm/110mm image you use a 13.75mm eyepiece (110/13.75=8). 8x from the larger 50mm/180mm image requires a less powerful 22.5mm eyepiece.
The metal ring between the projected images is an eyepiece fieldstop. The field you see when you look through the binocular eyepiece includes only the part of the projected image that fits inside the fieldstop. Because the image formed by the shorter focal length 30mm objective is smaller than the 50mm image it allows a wider FOV to fit into the same size eyepiece fieldstop.
Edit: I see Ron has already replied. Maybe between the two of us you will figure it out, or just get more confused.
Henry
Maybe this photo will help with the basics. I set up two simple binocular objectives, a 30mm with FL=110mm and a 50mm with FL=180mm. The image of the window you see projected on the wall behind the lenses is the image that forms in front of the eyepiece. The eyepiece then acts as a magnifier to enlarge that image to the desired magnification. Shorter focal length eyepieces are simply more powerful magnifiers, so if you want 8x from the small 30mm/110mm image you use a 13.75mm eyepiece (110/13.75=8). 8x from the larger 50mm/180mm image requires a less powerful 22.5mm eyepiece.
The metal ring between the projected images is an eyepiece fieldstop. The field you see when you look through the binocular eyepiece includes only the part of the projected image that fits inside the fieldstop. Because the image formed by the shorter focal length 30mm objective is smaller than the 50mm image it allows a wider FOV to fit into the same size eyepiece fieldstop.
Edit: I see Ron has already replied. Maybe between the two of us you will figure it out, or just get more confused.
Henry
Attachments
Last edited:
Thanks Henry, and, uh, my turn I guess!
The true and apparent angular fields of view are related to first order simply by the magnification. A 6deg true field, magnified 8x, makes an apparent field of 48deg. This is exactly correct only if there is no distortion, but gets close. So they are pretty much like the goose and the gander.
As Henry said, a shorter focal length lens is a more powerful magnifier (His nice photo of focused real images might seem confusing, since the image formed by the shorter lens is smaller. What the photo illustrates very well, is how the objective lens form a real image which is then magnified by the eyepiece and viewed directly by the eye, as a "virtual image". But if used as a visual "magnifying glass", it works like he says) and eyepieces work that way. The eyepiece gives a magnified view of the focused image from the objective, and also of the field stop, which delineates the black area surrounding the image. For a less powerful eyepiece to give a magnified view of a field stop that looks as big as the view of another field stop viewed in the more powerful eyepiece (ie, smaller binocular), its field stop must be bigger.
This is starting to sound complicated to me even though I wrote it!
Hope it helps though,
Ron
The true and apparent angular fields of view are related to first order simply by the magnification. A 6deg true field, magnified 8x, makes an apparent field of 48deg. This is exactly correct only if there is no distortion, but gets close. So they are pretty much like the goose and the gander.
As Henry said, a shorter focal length lens is a more powerful magnifier (His nice photo of focused real images might seem confusing, since the image formed by the shorter lens is smaller. What the photo illustrates very well, is how the objective lens form a real image which is then magnified by the eyepiece and viewed directly by the eye, as a "virtual image". But if used as a visual "magnifying glass", it works like he says) and eyepieces work that way. The eyepiece gives a magnified view of the focused image from the objective, and also of the field stop, which delineates the black area surrounding the image. For a less powerful eyepiece to give a magnified view of a field stop that looks as big as the view of another field stop viewed in the more powerful eyepiece (ie, smaller binocular), its field stop must be bigger.
This is starting to sound complicated to me even though I wrote it!
Hope it helps though,
Ron
Well, I'm glad this question got asked. It's something I've wondered about. My next question is why do compacts have narrow fields of view? Pretty much all the bins I've encountered with objectives smaller than 30mm have narrow AFOVs. Is it just because they use small prisms to keep compact bins compact? That makes sense for compact roofs, but you would think they could make a compact reverse porro with comparatively large prisms.
spitfiretriple
Well-known member
I'm going to pretend I haven't seen binoboy's question.
Instead, I'm going to throw something else into the mix re AFOV and FOV versus objective. Then I will turn to binocular length. I am a novice here, so what I suggest might not be correct. If so, perhaps someone would kindly point out my errors.
I first look for a metaphor to the car world: A manufacturer will produce a range of cars and a range of engines. In a BMW 3-series, you can choose engines from a 1.6 Liter straight-4 to a 4-liter V8. You can choose a compact 1-series to an executive 7-series. Depending on what you want (high performance or load-carrying comfort), you choose your car/engine mix accordingly. As a rule, matching your car size roughly to your engine size is a good idea. After all, a 7-series limo with a 1.6 Liter engine is going to struggle. And a compact 3-series with the 4 liter V8 is arguably poor value in terms of getting your family from A to B. (Fun though!)
Binocular manufacturers are similar. To save costs, they will often produce a model that shares many components, including prisms, but which has several different objective size options. The standard-throughout-the-model prisms will inevitably be optimised for a particular objective size - typically the smallest available in that model. This means the prisms are, to be honest, too small for the larger objective variants. Result: some of the light is cut off. Result: smaller FoV. Edit: Reading Henry's and Ron's explanations makes me doubt this explanation, but I find prisms easier to understand than multi-lens eyepieces! I am like the man looking for his wallet under the street-lamp, even though he knows he lost it elsewhere.
As regards, "Why are some binoculars longer than others, even though the spec is the same?"
Note: I'm going to ignore roofs vs porros, and indeed Zeiss with its Abbe-Koenig prisms vs the Schmidt-Pechans used by everyone else
If an objective lens has a given magnification (which is not the same as saying a binocular system as a whole has a given magnification), it will have a given Focal Length. A more powerful objective lens (with a shorter FL) bends the light more. All else being equal, the more a lens is required to bend a stream of light, the more aberrations will creep in. A cheap way (eschewing expensive glass) to minimise such aberrations is to use an Objective with a longer FL. This unfortunately means...roll of drums... a longer binocular.
Short, cheap, low-aberration. Pick any two.
Note: I offer no proof for any of the above, I just made it up, it might be nonsense!
Instead, I'm going to throw something else into the mix re AFOV and FOV versus objective. Then I will turn to binocular length. I am a novice here, so what I suggest might not be correct. If so, perhaps someone would kindly point out my errors.
I first look for a metaphor to the car world: A manufacturer will produce a range of cars and a range of engines. In a BMW 3-series, you can choose engines from a 1.6 Liter straight-4 to a 4-liter V8. You can choose a compact 1-series to an executive 7-series. Depending on what you want (high performance or load-carrying comfort), you choose your car/engine mix accordingly. As a rule, matching your car size roughly to your engine size is a good idea. After all, a 7-series limo with a 1.6 Liter engine is going to struggle. And a compact 3-series with the 4 liter V8 is arguably poor value in terms of getting your family from A to B. (Fun though!)
Binocular manufacturers are similar. To save costs, they will often produce a model that shares many components, including prisms, but which has several different objective size options. The standard-throughout-the-model prisms will inevitably be optimised for a particular objective size - typically the smallest available in that model. This means the prisms are, to be honest, too small for the larger objective variants. Result: some of the light is cut off. Result: smaller FoV. Edit: Reading Henry's and Ron's explanations makes me doubt this explanation, but I find prisms easier to understand than multi-lens eyepieces! I am like the man looking for his wallet under the street-lamp, even though he knows he lost it elsewhere.
As regards, "Why are some binoculars longer than others, even though the spec is the same?"
Note: I'm going to ignore roofs vs porros, and indeed Zeiss with its Abbe-Koenig prisms vs the Schmidt-Pechans used by everyone else
If an objective lens has a given magnification (which is not the same as saying a binocular system as a whole has a given magnification), it will have a given Focal Length. A more powerful objective lens (with a shorter FL) bends the light more. All else being equal, the more a lens is required to bend a stream of light, the more aberrations will creep in. A cheap way (eschewing expensive glass) to minimise such aberrations is to use an Objective with a longer FL. This unfortunately means...roll of drums... a longer binocular.
Short, cheap, low-aberration. Pick any two.
Note: I offer no proof for any of the above, I just made it up, it might be nonsense!
Last edited:
birdazzLED
Well-known member
I really have a hard time telling the difference between the ED2 and ZRS. I compared them all day and almost all night. The only difference in my eyes is, it seems like I focus less often with the ED2.
The FOV appears to be the same to me as well - whether looking at a chain link fence, counting the amount of trees in the field, or counting stars.
The FOV appears to be the same to me as well - whether looking at a chain link fence, counting the amount of trees in the field, or counting stars.
Last edited:
spitfiretriple
Well-known member
Sounds to me like the improvements made to the ZRS make it an even better value proposition than the ED2.
PS I'll leave my earlier post as it is, but a bit more thinking leads me to believe my "optimised prisms" logic is wide of the mark. I'm still reasonably happy with the blurb I posted about binocular length.
PS I'll leave my earlier post as it is, but a bit more thinking leads me to believe my "optimised prisms" logic is wide of the mark. I'm still reasonably happy with the blurb I posted about binocular length.
Similar threads
