Confusion with field of view differences?
Posted 25 November 2003 - 02:58 AM
As a more specific example, Pentax have two roof prism binoculars with specs as follows:
Pentax DCF SP 8x42 (RFOV 6.3 degrees, AFOV 50.4 degrees)
Pentax DCF SP 10x42 (RFOV 6.0 degrees, AFOV 60 degrees)
How does the 10x42 which has a narrower real FOV end up with an apparent FOV greater than the 8x42? Does this mean a wider visual view through the binoculars (i.e. more sky)?
My apologies if this is repeating old material, but despite my best efforts I am struggling to work this one out... Help?
Posted 25 November 2003 - 06:45 AM
Posted 25 November 2003 - 01:22 PM
In your example the Pentax 8 x 42 has R.F.O.V of 6.3 degrees
This is more usually described as TRUE field of view(TFOV)
What does 6.3 degrees actually mean ?
One degree can also be expressed as 52.4 feet / 1000 yards
Or as 17.4 metres / 1000 metres
What this means is that if you looked through a binocular with a ( very narrow )one degree TFOV , then at a distance of exactly 1000 yards from where you were standing you would only be able to see a stretch of land or object that was 52.4 feet wide . Anything wider than that would be out of the area which you can see through the binocular.
Or , to convert to metric measures , if you looked through the same binocular , then at a distance of 1000 metres from where you were standing , you would only be able to see a stretch of land or object that measured 17.4 metres wide.
So with a 6.3 degree TFOV , to get the equivalent figure in feet per 1000 yards or in metres per kilometre , you would multiply the figures quoted above by 6.3.
So at 1000 yards you would be able to see a stretch expanding 6.3 x 52.4 which = around 330 feet at 100 yards
In metric , 6.3 x 17.4 = around 109m per 1000m.
In addition to , or as an alternative to degrees such as 6.3
you will often find one or the other of these expressions engraved on the data plate of a binocular and in the technical specifications literature or website info.
Any of these is a true indication of field of view.
But binocular eyepieces themselves are ( most commonly ) designed with a fixed magnification in mind.
The Apparant Field of View simply means the TFOV as described above MULTIPLIED by the MAGNIFICATION.
So a 10 x 50 with a 5 degree TFOV provides a 50 degree AFOV
In this case , knowing the requirement is for a 5 degree TFOV with a 10 x power factor , the lens designer selects what is universally accepted as a 50 degree eyepiece of focal length sufficient to provide 10x magnification within that binocular system.
Thus is why telescope eyepieces are always described by their APPARANT field of view rather than by their TRUE field of view.
For example a plossl e.p usually offers around 50 - 52 degrees AFOV whereas a Televue Nagler can offer as wide as 82 degrees AFOV .
The True fields of views offered by such e.ps in a telescope depends on the focal length of the telescope which in turns affects magnification.
The greater the magnification the narrower the TFOV and the lesser the magnification the wider the TFOV.
A TFOV that would be considered annoyingly narrow for say a 10x binocular( for example 4 degrees TFOV )only really appears so because the AFOV would be only 40 degrees ,arrived at by multiplying 10 x 4 .
However if you had say a 16x binocular with a TFOV of "only" 4 degrees -- the AFOV would be 64 degrees(16 x 4)
and as such would appear to be perfectly acceptable because the extra magnification is making up for the narrow field.
What is an "acceptable" AFOV depends very much on individual choice and experience .
One can and often does "get used to" a particular AFOV.
Personally , regardless of magnification , I find anything less than a 55 degree AFOV to be less than satisfying , yet some of the most revered binos , such as for example the Takahashi 22 x 60 , provides only around 45 degrees AFOV.
After years of experience one gets a very accurate idea of what to expect with any given TFOV / AFOV configuration before even looking through a binocular.
I hope this helps you to understand this often mentioned but rarely explained matter.
Kindest regards -- Kenny .
Posted 25 November 2003 - 11:33 PM
Once again, I think I am back to the drawing board in my hunt for binoculars (my ideal bino with wide FOV, and perfect optics at a bargain basement cost still eludes me).
Posted 26 November 2003 - 12:49 AM
Frilby, what are you wanting your binocs to do for you? General purpose day and night? astronomy only? hand holdable or mounted?
Posted 26 November 2003 - 02:17 AM
Astronomy only (I will no doubt use them during the day, but I am seeking binos specifically for astronomy).
They will be for hand holding only to complement an 8" scope.
Primary use will be for general sky orientation, locating areas of the sky, location of objects, etc for viewing with my scope.
I originally started my "quest" looking for 10x50's (based on smaller exit pupil as majority of usage will be from a light polluted metro area) but the few I have had an oppourtunity to view have disappointed me with their edge performance.
More recently I have been looking at roof prism 8x42's as they appear to have far better edge quality (and $$$ unfortunately).
This triggered my query regarding FOV as I had a look through a pair of Pentax PCF WP 10x50's and 7x50's and from an image perspecticve they are the best I have seen to date, but the FOV was only 5 degrees and it looked like you where looking down a tunnel with a small window at the end, rather than a wider window style view.
Further to this I have been debating the performance differences between 40mm and 50mm aperture and weather I would be shooting myself in the foot by moving away from my original 10x50 preferences...
My difficulty is that I am unable to "try before I buy" many of the binos recommended in this forum as they are international order only (I'm in Australia) and over here noone will let you buy them and return them if you are unhappy, and if I order them from stores in the US, I can return them, but get hit with $30-40US (about $70-80 Aussie) for the freight each way!!! Not something I want to do if I can avoid it.
So any ideas on my best options would be most appreciated...
Posted 26 November 2003 - 12:50 PM
I've been giving some thought(usually a bad sign!)to your predicament and hope that what has resulted helps you make the right decision , which is a difficult one to make for several reasons.
For starters you have a couple of opposing factors adding to the major ,all too familiar problem of being unable to easily try before you buy .
These two factors would seem to be :
1. You WANT top quality -- but at relatively LOW COST .
This is probably what everyone apart from Bill Gates would desire -- and from my experience you will be exceptionally fortunate to find both in the same package, the only remote possibilities being buying USED or from the back of a dodgy truck!
You say you have been dissapointed with the edge performance of some porros you've tried , and I don't know which make or type they were , but there is no technical reason I am aware of that would pre-determine a porro prism binocular having worse edge performance than a roof prism model per se.
Remember that dollar for dollar a porro prism will almost always outperform a roof prism bino and that roughly speaking from personal experience , the VERY best porros equal the VERY BEST roofs available , but at around 70% of the price of the roofs.
Also in my experience , most of the "advantages" of waterproof , nitrogen -purged , phase -corrected etc roof prism binos are relevant to terrestrial use rather than astro-use , which you specify as your main use.
2.You want WIDE -FIELD but good edge performance .
As a rule one is a compromise against the other , so what you must ask yourself is , within reason , which of these two aspects are the most important for YOUR intended use ?
From what you have described as your main intended use I would lean towards a wider field than you will get from the Pentax , which offer about as flat a field and decent edge performance for the money as any binos I've tried.
If that 50 degree AFOV frustrates you as much as it does me then you have your answer to that option.
So thus far I am steering you towards Porro prism binos with a wide-ish FOV .
Next important detail is magnification / objective size.
Given your main intended useage , I would give serious consideration to anything from 7 x 35 through 8 x 40 to no more than 8.5 x 44.
Four reasons here :
1.No matter what ANYBODY tells you -- don't be fooled -- anything above 8.5x is TOO high a magnification for truly satisfying hand -held binocular astronomy -- UNLESS you go for Image Stabilised binos ( more on those in a moment )
2.The LOWER the magnification the more likely you are to find a quality bino with a WIDE FOV that will be much easier to find things with.
3.Anything above around 44m usually comes with a weight factor that can cause discomfort for extended hand -held viewing , especially when angling binos skywards -- which is a different kettle of fish from holding them straight whilst standing or squatting during typical terrestrial glassing. Consider WEIGHT .
4.Quality SMALLER binos tend to COST LESS than quality LARGER binos.
One reason I include the figures 8.5 and 44mm is that I have yet to find a better value for money all round binocular than the Swift Audubon non -ED porro prism
8.5 x 44 -- THE original 8.5 bino ( pre Swarovski et al ) and one which comes with a 8.2 degree TFOV and some edge distortion that can be forgiven with such a wide field.
The "sweet spot" is so large that you still have around 7.0 degrees of on-axis quality that seriously competes with that of my beloved Zeiss 7 x 42s , which cost £400UK MORE !
As far as 7 x 35s go --don't rule them out !
Nikon Criterion E or the E11 range are well worthy of consideration .
Yet the answer to your problem COULD be the Canon 10 x 30 IS
The latter offer a 60 degree AFOV with excellent edge performance -- and no problems with the dreaded shakes -- which is a far more noticeable problem at night than in daylight --trust me !
So , my "short -list" would be :
1. Nikon E series 7 x 35 porros
2. Swift Audubon 8.5 x 44 porros ( even 8 x 40 Ultralites)
3. Canon 10 x 30 Image Stabilised
Likely ANY of these will cost more NEW than what you thought you wanted to pay -- but please don't make the mistake of millions before you ( self included ) and end up buying something you don't really like --for the sake of $100
I get the impression Frilby that you are a very patient person -- which is a very useful attribute to have when it comes to astronomy -- but even more so when it comes to buying the RIGHT bino .
I hope my suggestions help you , but I will not be offended if you completely ignore all that I've said .
I simply love good binoculars , hate bad binoculars , and enjoy trying to help people.
Good luck once again with your choice !
Clear skies -- Kenny.
Posted 26 November 2003 - 01:40 PM
Frilby, You realize that I am heavily swayed by Kenny's opinion, just wanted to put that out there.
Your intended use is exactly what mine is, plus I use mine when hiking so weight is a factor for me as well. This is what I did and am going to do.
I bought a pair of Nikon Action 8x40 binocs for $65 (they had a $35 refund from Nikon). They are very acceptable binocs for now and allows me to get out and view. I use them as my first finder scope (or a quick look at M31) with my 8" Dob. These can be purchased on eBay for under $100.
I plan to purchase a pair of the Swift Audobon 8.5x44's because they maximize everything I want in a binocular. Everything is a trade off and this hits my sweetspot. This are available online, eBay, etc.
I haven't had a chance to see the Canon IS glasses and would like to sometime. Good luck, Tom
Posted 26 November 2003 - 07:55 PM
Thankyou... Thankyou... Thankyou...
Your feedback and amazing detail in your response was fantastic and has helped so much.
Yes, I try to be patient, but I must say that I'm getting to that point where my urge to buy is reaching my threshold for restraint! :-) (This has been an investigative process spanning nearly 3 months so far...)
To date, I seem to be going around and around in circles... seemingly narrowing down the choice and then - BAM! - another issues crops up and everything I though was right and the direction I was heading all seems wrong and I start the process again.
Well... at least I don't have to worry about all the ones I finally eliminate hey!
Anyway, once again my genuine thanks for your help... I'll let you know how I go (and I'm sure you will be hearing more from me yet!).
All the best...
Posted 26 November 2003 - 08:01 PM
What is the specific model for the Swift Audobon 8.5x44's?
When I search on these there are a few different types (roof prism, rubber armoured, standard, etc).
Posted 26 November 2003 - 10:13 PM
The roofs have a narrower FOV also. I need waterproof because I am in the US Northwest and will take them hiking also (we don't let rain stop us, otherwise we'd be in the house all the time!)
Posted 27 November 2003 - 12:36 AM
Found this link interesting: http://www.telescope.../Swift10x50.htm
It refers to the 10x50 Audubon, but aparently the waterproof version for a period of time was not the same bino! Is this the case with the 8.5x44 you are looking at (or is the waterproof version identical optically to the non-waterproof version)?
I would hate to order one, only to find out that the waterproof one is an entirely different (and less performing) model?
Posted 27 November 2003 - 01:11 AM
Posted 27 November 2003 - 02:49 AM
First ,thanks to both of you for such kind words.
One's efforts being so well appreciated is what makes not just bino ramblings but life itself worthwhile.
The three of us intertwined in this particular thread could hardly be more spaced apart in terms of international time zones so I'm sorry if the delay with this response seems longer to you both than the few hours sleep I've just managed to get does to me !
I may have mentioned this before somewhere but I wish that Swift had decided other than to use the name "Audubon" on so many of it's binoculars.
The "original" and so deservedly such a popular favourite glass amongst a whole generation of "birders" was the
8.5 x 44 non -waterproof PORRO prism model , the one I tried to describe in my previous post.
A notorious problem with this model however, which presumably would still crop up , was that after a few years of hard useage , moisture tended to get inside , clouding the image somewhat.
Two other minor "quibbles" with this model are that eye-relief may be slightly shorter than is officially stated and is certainly marginal for wearers of eye -glasses and, presumably to keep the already slightly bulky package to a minimum , the length of objective tube that overlaps the actual glass is too short to cut out all stray light as ell it could do.
Neither is the model likely to win any awards for "sexiness"
I hasten to add that for years neither of these issues has proved a serious problem for me.
At some point in time a 10 x 50 "replica" of this classic porro glass was introduced , a fine example of which I currently own and enjoy using.
This was given the name "Audubon Kestrel" model 826 , and has an outstanding 7 degree TFOV BAK 4 prisms , and is fully multi-coated. It is a VERY good binocular , one of my all time favourites which I would never part with.
From what I've read about the subject , which admittedly does not amount to very much , for reasons better known to the the company , this model was temporarily withdrawn and a newer , completely DIFFERENT model was ( unfathomably ) also designated the title " Swift Kestrel 10 x50 "
This model had a narrower FOV amongst other things and as ought to have been predictable from it's much lower price bracket, was ,simply put ,not a good as a binocular -- yet some people did make the mistake of buying this model under the impression they were getting the Audubon Kestrel.
This confusion was very quickly put right by the friendly people at Swift and the 10 x 50 Audubon Kestrel Porro that I like so much was indeed re -launched .
To confuse matters further there are at least two other binoculars , nothing to do with Swift , that I've actually seen for sale bearing the name " Kestrel" .
Over the past 3 or 4 years or so , Swift seem to have complicated the issue much further by introducing several new binoculars using the name "Audubon" in the title.
Even the Porro prism versions have been modified in a few ways , included "waterproofing" which I would personally take with a pinch of salt.
A good review of this modified 8.5 x 44 version, comparing it with the Swarovski 8.5 x 42 EL can be found on the Better View Desired web site , which has so painstakingly been put together over many years by Stephen Ingrahams.
There are now also several Swift Roof prism models bearing the name "Audubon" , each I'm sure very fine glasses in their own right in both ED and non -ED versions.
I think one of such is what Tom has been looking at.
I have never even tried any of these Roof prism versions.
I hope this helps to clarify the admittedly somewhat confusing situation with Swift "Audubon" binoculars.
Clea skies --Kenny .
Posted 27 November 2003 - 03:22 AM
Do you use a finderscope? I find using a pair of binoculars with the same power as my finder very useful. (i just turn the binoculars over to orient the image with the finder. )
I would not agonize too much about the binoculars. The major distributors, Swift, Nikon, Vixen, Minolta, Pentax, etc, have excellent products. Porro-prisms will be much cheaper than roof prisms. It is really a personal preference which is better. Cheaper units can use plastic optics, but I have never known anyone who can spot the difference.
I use a pair of very old Swift 7X50 porros at the scope (I have a 7X50 finder) and Minolta 10X42 roof-prisms when I'm traveling light. I like them both, but 10X is a little more satisfying and the 50mm objectives brighter. I have large hands so the porros are more comfortable to hold
Posted 27 November 2003 - 04:18 AM
Between the two Swift binos (8.5x44 and 10x50) which have you found to have the better view in lighter skies?
How do they compare optically with regard to image quality at the edge of the FOV?
I guess the ultimate question is: "If you could only chose one pair would it be the 10x50's or the 8.5x44's"?
Finally, as far as you are aware, are the waterproof models for both the 8.5x44 and 10x50's the same optically as their non-waterproof counterparts?
P.S. I must agree, it is amazing when you think about it... we are spread around the globe, yet happily chat away as if we were right next door and the best of friends! Pretty amazing!
Posted 27 November 2003 - 11:42 AM
Tom beat me to the punch but I also would like to add how very helpful your posts have been to read. Thanks, Nick
Posted 27 November 2003 - 11:59 AM
Wilash is right, though. Don't fret over it too much...you hit a certain level of quality and it all starts to be good!
Posted 27 November 2003 - 01:47 PM
It was 6.30 am , I was only half awake and had to rush it out before heading out to work.
Indeed I am presently so pre-occupied that this one might turn out little better in that respect !
I hasten to add that my single typing finger appears to have suffered an invasion by a tiny piece of metal so I'm really struggling with my pinky finger here !
If I had no other binoculars ( which is far from the situation nowadays !) and I could only choose ONE between the Swift Audubon porros in ether 8.5 x 44 or 10 x 50 then I think I would go for te 8.5 x 44s , even though the edge performance is slightly more noticeable IMHO.
If I had the 7 x 42s and could choose only one it would be the 10 x 50s ( for variety )
That's how close it is -- and remember that my main use for binoculars is daylight terrestrial stuff.
The main reason for taking such a tough decision would be ability to hold the image steady.
There are times ( especially for astro use ) when I just have to attach the 10 x 50s to a tripod , handy post or a steel stand that I threw together for the purpose for standing -up whilst looking overhead -- all these options made possible by the low -cost acquisition of a heavy duty L -bracket and car window mount -- the most useful two items I've ever bought to enhance my hobby.
I would go with the 8.5 x 44s --but as Wilash as just added to the discussion --notwithstanding what I said previously about the folly of buying what dissapoints you , within reason there is no need to worry overmuch about this decision.
Only 2 weeks ago I bought as a Xmas present for one of my daughters a brand new Bushnell H20 8 x 42 waterproof nitrogen purged porro prism bino with 8.2 degree TFOV with a lifetime guarantee for the equivalent of $100US.
It's not in the same league as my Zeiss , Swarovski or even Swift , but I wouldn't have bought it if I didn't think it to be superb value for money and very useable.
It is a far better bino than most that would have cost more than that 25 years ago and I would rather own just that than my 15 x 70 Little Giant for example.
Go with the flow Frilby !
Clear skies --Kenny.
Posted 28 November 2003 - 03:15 AM
I've been reviewing the feedback on Swift binos and they do indeed have some great reviews. I will investigate the best way of getting a pair in Australia...
Will let you know...
Posted 28 November 2003 - 09:23 PM
Posted 01 December 2003 - 12:34 PM
Good Advice! and very clearly explained.
I am happy to read the good comments you and some others have written about the Swift Audubon 8.5x44. I've recently recommended that binocular to two of my personal friends for their spouse as a birding binoc. These are people that will use these binocs 80% day 20% night.
I own the Swift Ultralites as you know and I like them for their compact size and lite weight. The glass is excellent. I also own some waterproof binocs. Without question on a night when dew brings viewing with all the lenses to a quick ending, the waterproof binocs are the last instrument left standing. They take a lot longer to dew up.
So, I'm feeling better that I can stand behind my 8.5x44 recommendation.
Posted 02 December 2003 - 08:37 PM
I've read a number of comments that praise these binos for daytime usage, but comment on the rapid degradation of image quality due to field curvature at the edges (an inherent side effect of a wide FOV).
I guess the question is: Do these binos provide an equivalent "sharp" FOV size to other binos before they begin to break down?
I am so tempted with these binos given Kenny's (and others) feedback, but have also had a few comments (Barry Simon comes to mind) stating that strictly for astro use they would not be the preferred first choice binocular.
I like the idea of a wide FOV (70 degree apparent). The "window" effect is very appealing given that I have had a look through a pair of 10x50 PCF WP's (50 degree apparent) that gave the impression of looking down a tunnel - although it was quite sharp to the edge of the 5 degree FOV. So this in conjunction with the added appeal of smooth rounded shape, waterproof design (although not fogproof apparently?), and overall "best general purpose binocular out there" type of feedback... It really does seem like - THE ONE
If only I could actually get my hands on one to have a look
Anyway... comments and feedback most appreciated!
Posted 03 December 2003 - 01:55 PM
I really do wish that perhaps two or three different commentators with experiences of the Swifts in question and other binos would reply to your queries.
This is not least because I would hate to be even partly responsible for someone making such a big decision and ending up dissapointed in any way.
Ironically it is only about 2 months ago that on this very forum I actually recommended the Pentax 10 x 50 to another person in a very similar situation as you are in now and held my breath a little until he responded with positive thanks. The slight difference with his situation was that his budget was considerably lower than that of the price of a Swift Audubon in the UK , and he seemed to indicate a preference for a flat -field and waterproofing .
There are so many little things about binoculars that can make the difference being being enjoyable and satisfactory to use and not quite so.
This is compounded by the fact that everyone is different and not everyone wants or expects the same qualities.
Actual feel in the hand -- the weight , ergonomics ,types and material of oculars,eye -relief , smoothness and retainment of focus , brightness , contrast , false colour , edge performance --the list goes on and on -- BUT to my mind , particularly for celestial use ,two VERY important factors are "ease of view " -- improved by a wider field and "steadiness" of view --- improved by lower magnification and good balance.
Even the VERY best binoculars have their faults --none are perfect --and most of even the VERY expensive models do not have perfect edge performance or perfectly FLAT fields.
Some astro -bino users have an obsession with these two factors -- I for one do NOT.
Those that DO pay for this in other ways , but it is a human weakness not to admit to such things after one has splashed out a significant amount of money ( to them ) for their chosen glass.
For example the highly praised Takahashi 22 x 60s have barely a 45 degree AFOV -- almost impossible to FIND anything with in a sky full of stars without a finder of some sort. The very popular Fujinon 7 x 50 FMT SXs are ridiculously heavy and have only just over 50 degree AFOV --facts hardly ever mentioned by their delighted owners but specifications which would really annoy me personally.
The edge softness and distortion with either of the Audubons in question is there if you are looking for it -- but to me is completely irrelevent -- and more than worth putting up for the pleasures of all the other qualities.
Put it this way :
I would NOT EVEN CONSIDER swapping my Swift 10 x 50 Audubon Kestrels for TWO pairs of Pentax 10 x 50s.
IF you want as flat a field as you can get for around $200 you may well struggle to find any better than the Pentax.
If you like an extra -wide field 70 degree AFOV good quality binocular -unless you buy a Zeiss Victory for FOUR times the price - I don't think you will find better than the Swift Audubon.
But I repeat -- DO NOT UNDERESTIMATE THE DIFFICULTY OF HOLDING A 10x BINO STEADY BY HAND AT THE NIGHT SKY -- nor the improved resolution of that same bino when mounted.
Think about it -- very carefully !
Clear skies --and good luck with other advice - Kenny