8x25 compared to 8x40 (1 Viewer)

[Aquaman]

[EDIT:/] Eye placement greatly affects viewing comfort and subjective impression of the field. The bulk of my posts are largely founded on a mental error.[/EDIT]

Was attempting to determine the FOV of my new Bushnell H2O 8x25's. I found something interesting that may be obvious to others.

For these particular compacts, Bushnell's stated FOV is all over the place - website says 341/114, Focus dial reads "FOV 360 ft.", and back of package states 428'/142m. Not sure I care about the actual numbers, but I thought it worth investigating.

My Nikon Action 8x40's state a real FOV of 8.2 degrees.

The 8x25 field is only a little smaller than the 8x40's as best I can tell in neighborhood testing. The interesting thing, besides that, is the 25mm view is a smaller circle yet containing almost everything found in the 40mm view. Yeah it's dimmer, everything is smaller despite the same magnification, and the edges are harder to see due to the small exit pupil requiring quite a bit of accommodation to scan from one edge to the other.

The view in the 8x40's is, I suppose, what you'd expect from a 40mm image magnified 8 times. The 8x25's view is similar, but it's only a 25mm image to start with, so everything is smaller, including the circle of view, even if the angular size is similar..

Now I am wondering if a smaller objective makes an inherently smaller image to start with, regardless of magnification. It's not just dimmer, it's smaller. So to see the same size level of detail in both you would need more magnification the smaller the objective, compared to whatever reference glass you had. Yet the image can only tolerate so much magnification before detail is lost to magnified aberrations. Which comes right back to "bigger is better," optically speaking.

I have compared 7x35's to 7x50's and noticed more, dimmer stars visible in the larger objective, but the FOV of the two were quite different. With my 8x40 and 8x25 the mag and FOV are close, making the difference in view from the objective size easier to witness. Or so I surmise.

Anyone else notice this?

 

[Binastro]

Hi Aquaman.
It sounds as if the 8x25 might be 7x25?
If the apparent fields of view are the same then the circles should look the same size.
I would use alternate eyes with alternate barrels to find out.

The bottom 2 stars of Ursa Major bowl are 7.9 deg apart, so it is easy to measure the real fields.
A problem arises if you cannot see the whole fields, perhaps wearing glasses.

With the 7x50 and 7x35 it depends on your pupil sizes and the transmission etc. of the binoculars and if they have haze inside etc. etc. And whether they are really 7x and really unvignetted 35mm and 50mm.

One cannot believe what is written on binoculars, boxes, websites etc. You have to actually measure things.

 

[typo]

The H20 8x25 may well have smaller apparent FOV than the Action and appear dimmer. I thought the little Bushnell was pretty horrible when I tried it with no phase coatings, apparently poor AR coating, both field curvature and astigmatism and poor effective resolution if my memory serves me well. However there is absolutely no inherent reason why a small objective would have a smaller AFOV than a big one, have a poorer effective resolution or would appear dimmer, until light levels fall sufficiently to make the exit pupil limiting.

David

 

[Aquaman]

Hi Aquaman.
It sounds as if the 8x25 might be 7x25?
If the apparent fields of view are the same then the circles should look the same size.
I would use alternate eyes with alternate barrels to find out.

The bottom 2 stars of Ursa Major bowl are 7.9 deg apart, so it is easy to measure the real fields.
A problem arises if you cannot see the whole fields, perhaps wearing glasses.

With the 7x50 and 7x35 it depends on your pupil sizes and the transmission etc. of the binoculars and if they have haze inside etc. etc. And whether they are really 7x and really unvignetted 35mm and 50mm.

One cannot believe what is written on binoculars, boxes, websites etc. You have to actually measure things.

The bold underlined above is the idea that spawned my original post.

I checked using one barrel on each eye, then switched eyes, and the 40mm circle is clearly larger than the 25mm circle of view. Aculon 7x35's show very similar to the 8x40's but the FOV is much larger.

The different numbers I have found "written on binoculars, boxes, websites etc." is what initiated the testing and revealed the difference in the size of the image circle.

The H20 8x25 may well have smaller apparent FOV than the Action and appear dimmer. I thought the little Bushnell was pretty horrible when I tried it with no phase coatings, apparently poor AR coating, both field curvature and astigmatism and poor effective resolution if my memory serves me well. However there is absolutely no inherent reason why a small objective would have a smaller AFOV than a big one, have a poorer effective resolution or would appear dimmer, until light levels fall sufficiently to make the exit pupil limiting.

David

I don't think there is any debate the 8x25's have a smaller FOV than the 8x40's, and detectably dimmer when comparing the views of, say, a yellow taxi in the sun. These aren't night hunting glasses or aimed at the astronomy market, they are light and compact roof prisms.

I don't know what vintage of the H20 8x32's you are trying to recall, but after looking at just about every model in the case under $600, I chose the cheaper Bushnells because they had a combination of FOV, eye relief, and compactness I was looking for. If I intended to be comparing test charts in a lab I might have chosen differently That said, I tested a few larger phase-coated glasses that had worse sharpness, even in the center. The lack of phase coatings has not been an issue yet, but again, I chose these for carrying when size and weight are premium considerations. I did not notice any correlation in the store between price and suitability for a given task.

I began this investigation-of-sorts to identify the differences between these compact binoculars and the binos I already own and use. It's not about better or worse, it's about an observation of the image circle in each. If I had more subjects I could test my idea that the apparent size of the image circle is related to the size of the exit pupil.

I just thought it was curious, is all. Thanks for the input

 

[Binastro]

The Nikon Aculon and Action VIIs have similar apparent fields and I think aspheric eyepieces, giving slightly undulating magnification across the fields.
Nikon specs are usually accurate.
The Bushnell Xtrawides have smaller mags than stated. 4x21 is 3.5x21. 5x25 is 4.4x25 etc.
Artistic licence? Marketing mumbo jumbo?
They are useful low price binoculars, so I am not complaining.
But don't take anything written as accurate unless you know otherwise.
The FOV is dependent on the eyepieces.

 

[jring]

I don't think there is any debate the 8x25's have a smaller FOV than the 8x40's, and detectably dimmer when comparing the views of, say, a yellow taxi in the sun. These aren't night hunting glasses or aimed at the astronomy market, they are light and compact roof prisms.

Hi,

actually I would expect an 8x25 in bright sunlight to give as bright an image as an 8x40 of comparable build and quality. At least my little 10x25 Zeiss does when compared to larger roofs - at dusk, things change quite a bit though...

The exit pupil of both bins is larger than the eyes pupil size in bright sunlight, so the amount of light getting into the eye is only limited by the eyes pupil size.

So if your 8x25 is noticeably darker in bright sunlight than the 8x40 porros, it seems to be a good example why cheap roof bins are best avoided.
At the top end the transmission difference between porro and roof models is 5-6% which I guess is hard to notice visibly.

Joachim

 

[Aquaman]

...

So if your 8x25 is noticeably darker in bright sunlight than the 8x40 porros, it seems to be a good example why cheap roof bins are best avoided.
At the top end the transmission difference between porro and roof models is 5-6% which I guess is hard to notice visibly.

Joachim

With all due respect, these "cheap roof bins" were lighter, more compact, and give a decent or better view, at least the ones I chose do B

This thread is not at all about what is better. Since you brought it up, how relevant is the top end of the market to the bottom end? Wouldn't it seem that - given a difference in light transmission even when money is no object - one ought to be able to see that difference most pronounced when comparing inexpensive compact roof prisms to full-sized porros of the same price?

I'm not trying to be contentious I just trying to figure out how things really work.

 

[Aquaman]

Yeah, so, I had been wondering this and now have my answer - if feeling a bit foolish. The 8x25's have smaller diameter eyecups, which tends to make me place the glasses farther inside my eyesocket. This shows me more of the inside of the binocular and the image circle is somewhat smaller at least subjectively. I'm sure I'm placing the projected image somewhere beyond ideal to get the view I was getting. Ergonomic issue between me and the binocular, or alternatively a technical issue with my face

Moral of the story: some measurements are useless no matter carefully you take them...

Anyway, liking my small cheapies regardless.

 

[jring]

Hi,

ok let me elaborate a bit - it seems my previous answer was not received well, sorry for that.

First, it's a well known fact that Porro prism bins have a lot of advantages optically - but cosmetics are lacking for most, which lead to the unfortunate situation that Porro bins are mainly found in the lowest price brackets where building decent roof bins is hard.

Roof bins need a lot of expensive high tech in order to almost catch up optically to porro bins due to these disadvantages:

- porro prisms work by total reflection only, no mirroring and thus light loss needed. In the most widely used Schmidt Pechan roof prisms one prism side needs to be mirrored which will always induce some light loss. The best dielectric mirroring variant is quite expensive to make.

- two prism sides in Schmidt Pechan prisms reflect and transmit the light beam at the same time which leads to the problem that improving transmission by multicoating this side will make the reflection worse and vice versa. The least bad solution is to single-coat those sides - another complication because not all sides are coated the same way.

- the reflection on the roof-edge splits the light beam into two differently polarised beams. Those will create interference effects which result in a loss of contrast if there is not a phase correction coating applied.

- Schmidt Pechan prisms must have a very precise air-gap between them. Porro prisms can be cemented together or not and if used with an air gap, the position tolerances are several orders of magnitude less strict than for Schmidt Pechan prisms.

In high end bins all those problems of Schmidt Pechan prisms are mostly ironed out and they reach a transmission of a bit above 90% vs the best Porros at 96% - this difference is hard to detect without measuring it.

In very low end bins (say below $100) dielectric mirrors, phase coating or using different AR coatings on different sides of the prism might not fit the bill and thus result in a lot lower transmission which makes them noticeably darker than other bins. That's why I said they are best avoided.

A difference in objective lens aperture does NOT explain a darker image in bright sun light.

Regards,

Joachim

 

[Troubador]

Hi,

ok let me elaborate a bit - it seems my previous answer was not received well, sorry for that.

First, it's a well known fact that Porro prism bins have a lot of advantages optically - but cosmetics are lacking for most, which lead to the unfortunate situation that Porro bins are mainly found in the lowest price brackets where building decent roof bins is hard.

Roof bins need a lot of expensive high tech in order to almost catch up optically to porro bins due to these disadvantages:

- porro prisms work by total reflection only, no mirroring and thus light loss needed. In the most widely used Schmidt Pechan roof prisms one prism side needs to be mirrored which will always induce some light loss. The best dielectric mirroring variant is quite expensive to make.

- two prism sides in Schmidt Pechan prisms reflect and transmit the light beam at the same time which leads to the problem that improving transmission by multicoating this side will make the reflection worse and vice versa. The least bad solution is to single-coat those sides - another complication because not all sides are coated the same way.

- the reflection on the roof-edge splits the light beam into two differently polarised beams. Those will create interference effects which result in a loss of contrast if there is not a phase correction coating applied.

- Schmidt Pechan prisms must have a very precise air-gap between them. Porro prisms can be cemented together or not and if used with an air gap, the position tolerances are several orders of magnitude less strict than for Schmidt Pechan prisms.

In high end bins all those problems of Schmidt Pechan prisms are mostly ironed out and they reach a transmission of a bit above 90% vs the best Porros at 96% - this difference is hard to detect without measuring it.

In very low end bins (say below $100) dielectric mirrors, phase coating or using different AR coatings on different sides of the prism might not fit the bill and thus result in a lot lower transmission which makes them noticeably darker than other bins. That's why I said they are best avoided.

A difference in objective lens aperture does NOT explain a darker image in bright sun light.

Regards,

Joachim

NIce explanation Jo.

Lee