Now I am confused.
Different eyepiece designs with same focal length can have different eye relief. So how can you calculate focal ratio of objective by the eye relief?
What am I missing here? 🤔
-
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.
SFL 50mm Second Look - does first impressions really last? (12 Viewers)
- Thread starter HenRun
- Start date
Now I am confused.
Different eyepiece designs with same focal length can have different eye relief. So how can you calculate focal ratio of objective by the eye relief?
What am I missing here? 🤔
Ask Swarovski? That is how they told me to figure it out. I believe they are assuming the eye relief is close to the focal length of the eyepiece. I think it is more accurate to get the actual focal lengths of the binoculars from Zeiss which I am doing. It is too bad binoculars don't have the Focal Length printed on them like telescopes do, and then it would be easy to compute the Focal Ratio by just dividing the Focal Length by the aperture. You do it all the time with telescopes to figure out what eyepiece to use for the magnification you want to use, and the eyepiece has the Focal Length printed on it also. Magnification depends on the ratio of the telescope's focal length to the focal length of the eyepiece.
To calculate it, you have to divide the focal length of the telescope by the focal length of the eyepiece. The manufacturers probably figure that the Focal Ratio of most binoculars falls between 3.5 and 4.0 so it won't make a difference in performance, but I think they can differ more than that and the Focal Ratio can make a difference in CA control just like it does on the SFL 8x40 versus the SFL 8x50. That is why the older classic Zeiss Dialyt 7x42's for example were so long so they would have a slow Focal Ratio before they had fluorite glass to control CA. The old refractor telescopes were long and skinny for the same reason. Slower optics correct CA better, because the slower lens systems have more gently curved optical surfaces, which makes them easier to precisely shape and correct for optical aberrations like CA.
Last edited:
Yes, I know the eye relief of a given eyepiece design is proportional to the focal length, so I can see there is a connection. But because different configurations of binoculars within the same series use different eyepiece designs(for example 8x42 vs 10x42) it cannot be possible to calculate the focal ratio of any optical system just by aperture and eye relief. Another example is that I can use two different 5mm eyepieces to a telescope. The difference of eye relief can be big but they both give same magnification, and calculating focal ratio based on the two eyepieces gives very different results.
Please stay on topic. Think about it. When you go to a thread here or say on Cloudy Nights looking for information, do you want to filter through nonsense like this? No, you are looking for information pertaining to the thread topic. I hate it when I go over to Cloudy Night's researching something and there is maybe one post out of twenty that pertains to the subject of the thread. People from outside Bird Forum don't even get these comments like this, and it reflects badly on Bird Forum, especially when you are saying negative things about the forum.
henry link
Well-known member
Dennis,
Surely you can see that the method supplied by the Swarovski Customer Service Rep is complete nonsense. Time to put on your thinking cap!
Same for me.
I had a look at the 8x50 and the 10x50 today, first at the shop, later for a few hours out in the fields and woods.
Having read the various threads about the SFL x50s, I was worried about excessive CA - something I hate (users of the Canon 14x32 or the Duovid 10/15x50 - in the 15x position - know what I am talking about).
But I found CA much less prominent in the SFL than I had feared.
For my eyes, the 8x50 exhibits a bit more CA than the same size UVHD+, but basically only off-axis. In the center, I found CA almost imperceptible, even when looking for it.
The 10x50 exhibits clearly more CA than the 8x50, but again, for me, this becomes relevant mostly in the off-axis parts of the image; in the center, I could detect a little bit of CA when searching for it, but found it never intrusive, even in challenging contrast transitions.
For me, both models convinced me with a very nice image (wonderful display of the various green color tones in the spring forest), excellent ergonomics, beautiful focus mechanism - could they become a success like the smaller SFL models?? Will they „replace“ the large Conquest models which, according to my trusted source, don‘t sell ?
Last edited:
I didn't think their method was totally correct. As I said, the best way is to get the Focal Length from Zeiss on the SFL binoculars and then just divide by the aperture size. Here are a couple interesting methods to determine Focal Length in binoculars from Cloudy Nights. Do you agree with these methods?
"It is possible to have an estimate of the focal length without unscrewing a front barrel, if focusing is achieved by moving some external parts, like the eyepieces in most Porro prism binoculars.
If f is the focal length of the objective,
L the minimum focusing distance, measured between the target and the objective lens,
x the travel of the eyepiece between this focus point and infinity,
then we have the relationship:
f² = (L-f)x
Generally L is much greater than f, so this can be approximated by:
f² = L.x
The eyepiece positions for infinity and for the minimal focusing distance can be measured with a caliper, so x is readily given by the difference between these two numbers. The main difficulty lies in measuring precisely L, and setting precisely focus at infinity, because our eyes tend to accommodate and introduce some uncertainty. It's possible to improve precision by enlarging the image with an additional device, like a booster, a monocular, or another small binocular."
"FOCAL LENGTH IN BINOCULARS
Most Binoculars are Fast Achromats
Although this is not a particularly useful measurement in a fixed system, knowing how to calculate the focal length of binoculars may help you to understand a little more about how fast the optics are operating. Most common porro prism binoculars are fast Achromats. The short f-numbers of these systems certainly help explain why chromatic aberration can be present. Some binoculars are triplet apochromats, the Oberwerk BT100 for example. That would be relevant to the discussion if we were performing this calculation because we were concerned about chromatic aberration. This topic is focal length.
In binoculars, the same as in telescopes, magnification is focal length of objective (Fo) divided by focal length of eyepiece (Fe), or mag = Fo / Fe. Also, Fo + Fe is the total length of the light path. So, we know two formulas with the same terms and we know magnification. We need to find the length of the light path, Fo + Fe. Then we can solve the problem.
Measure the outside lens-to-lens distance of the binoculars. Focal length is generally accepted as taken from the centerline of a lens. So, the measurement should truly be from the center of the objective lens to the center of the eye lens. When I record this measurement, I correct for the thickness of the objective lens. I ignore the thickness of the eye lens. I have checked the thickness of the objective in several binoculars. For a Swift 8x42 it is 11m, the Nikon 10x50 and an Orion 10x50 is 13mm and the Oberwerk 20x80 Deluxe is closer to 16-18mm. Subtract one half the thickness for the glass out-to-out measurement.
Also, closely estimate the length of the light path through the prisms, then divide the length of the path thru the prisms by the glass index, and add this to the measured lens-to-lens distance. This will give you the total Fo + Fe.
If we use a common porro prism binocular, without any reducing lens in the light path, we can use these formulas. This probably refers to about 90% of the common binoculars on the market. It helps to see a diagram. Several of the books referenced in the credits at the end of this article (in the main article originally published in CN Reviews, see note above) show a cut-a-way diagram of a porro prism binocular. I have not included a diagram here. The formula can also be used for roof prism binoculars, but the light path through the prisms is quite a bit more complex to measure. Whether or not the binocular is a doublet achromat, or a triplet apochromat, has no effect on the outcome of this focal length calculation.
When light travels though glass, its angle changes. The convergence of the light cone from the objective, when it passes through the prisms, is slowed by a factor determined by the refractive index of the glass. While this occurs in all glass, this does not need to be addressed through thin lenses, but it has a significant impact in binocular prisms. Assuming Bak4 glass, the glass has a refractive index of 1.57. If the physical path through the prism is 100mm, the effective focal length through the prism is 100/1.57 = 64mm.
I have had several binoculars opened up and had the opportunity to measure the light path through the prisms. The prism light path in my smaller binoculars, those with mag. of 7x, 8x, 10x and 12x, measured 90mm to 100mm. The light path in my giant binoculars, 16x70, 16x80 and 20x80, with very large prisms measured 110mm to 130mm. I wouldn’t venture a guess at the length of the light path in the prisms of the BT100, but it’s longer. So the “effective length of the light path” through the small 10x50 prisms is about 65mm, and ranges from 65mm to 80mm, depending on the size of the binocular and the size of the prisms. A 10mm error in this actual size will produce a difference in the final outcome of only 0.1 in the f# for a 10x binocular and less for a higher power, not a significant error in the final outcome.
These two measurements added together, lens-to-lens + prism effective path, is the total effective light path, the total of effective Fo + Fe. For a Nikon (Earth and Sky) 10x50 the Fo + Fe = 218mm.
Fun With Math
Now we can use our two formulas. We will use the Nikon for our example. We know Fo + Fe = 218 and we have Fo / Fe = 10. Since magnification is given as 10x, it’s easy to understand this second formula will result in a Fo = 10x longer than Fe. If we solve this 2nd formula Fo / Fe = 10, then Fo = 10 Fe. Now substitute this answer for Fo into the first formula, so Fo + Fe = 218 can be written as 10 Fe + Fe = 218. Therefore, 11 Fe = 218. So Fe = 218 / 11 = 19.8. Now you know the focal length of the eyepiece = 19.8mm. Now then 218 – Fe = Fo, or Fo = 198. A check of the results is Fo / Fe = 10. Our check, 198 / 19.8 = 10 shows the answers are correct.
All of this just to get the f number of the binoculars? Yes, since now that we know the Fo, we can divide the Focal length of the objective by the Diameter of the objective, and we get f, the speed of the binocular system. In this example, it is Fo / Do = f, or 198 / 50 = 3.96. These Nikon 10x50 binoculars are operating at f4.0. Seven out of twelve binoculars I tested are operating within the narrow range of f3.9 to f4.3.
Other checks (exit pupil) will prove out these answers. Exit Pupil = Fe / f, so 19.8 / 3.96 = 5.0. Compared to Exit Pupil = Do / mag, = 50 / 10 = 5, it checks.
Two Things You Need to Know
Having gone through this whole exercise, you need to know only two things to help you determine the f of binoculars. First, the total length of the effective light path, which you must accurately measure, and second, that the shortest form of the formula above, for any binocular, will always be FL eyepiece = Length of Light Path / (magnification + 1). The rest is simple subtraction and division. To get the FL of the binocular, it’s Effective Length of the Light Path minus FL eyepiece. To get f of the binocular, it’s FL of the binocular divided by aperture."
Last edited:
In general the higher magnification - the shorter eye relief. This is not only for plossl eyepieces to telescopes but also the usual for binoculars when comparing different magnifications with same aperture and objective focal length. For example 6x30 vs 8x30, 7x35 vs 10x35, 7x42 vs 8x42 vs 10x42, 7x50 vs 10x50 vs 12x50. And so on. So there is a clear relation between eye relief and focal length of eyepieces.
But there are nowadays many advanced eyepieces with inbuilt barlow, which makes it possible to keep the same eye relief for a big range of focal lengths.
Interesting. We are already getting different opinions on the CA levels in the SFL 8x50. Almost every binocular has a little CA on the edge, even the Zeiss SF 8x32, which controls CA better than almost any binocular. Edge CA does not bother me that much either as long as the CA is not in the center. I think HenRun is one of those individuals that is very sensitive to CA and that is OK, but we must realize not everybody is as sensitive to CA as he is. The natural color tones of the SFL's are really nice and once you get used to them, it is hard to go back to the yellow/green tint of a Meopta or the green tint of the SF and FL. I get the SFL 8x50 Thursday and I will compare it for CA to a Swarovski EL 8.5x42 with fluorite lenses and an SFL 8x30 with a higher Focal Ratio.
I realize opinions are different on glare, but you keep repeating it in every post, how lousy the NL 832 is controlling glare. The NL 1052 is probably identical to the NL 832 in optical design, so there is no valid reason why the NL 1052 would have more glare than the NL 832 unless Swarovski changed the design for some reason. My problem is a lot of members will trust your opinion and won't even try this new binocular. They see you saying time and time again will I am not buying it so they won't either, and they might not even see any glare and miss out on a good binocular. Maybe Swarovski did change something in the NL1052, but you are the first reviewer I have heard that said they will not buy the binocular, and it is not worth the money. Maybe time will prove that you are correct that the new NL 1052 is a lemon. I will let you know what I think of it on Thursday.
henry link
Well-known member
Try applying their method to some other binoculars and you'll find out how totally incorrect it is.
For instance, try using it for 10x30 SFL with eye relief of 18mm. The result is a focal length of 180mm and a focal ratio of f/6.
Compare that to my Zeiss 8x56 FL with eye relief of 16mm. The result is a focal length of 128mm and a focal ratio of f/2.29.
Do those seem like plausible results to you?
JCB's method is the only one I would trust at all between the two you have copied, but it requires fine measurement accuracy and only works for external focusing binoculars.
Here is another interesting method for figuring Focal Length in binoculars. It requires some precise measurements also.
"You can calculate the optics focal-length by using the simple lens equation at two different object distances. 1/p + 1/q = 1/f where p=distance from lens of subject and q=distance where it focuses and f=focal length of the lens. Then at infinity, q=f but at some closer distance, q=1/(1/f - 1/p). Then, if you can measure precisely how far you had to adjust focus in the binocular from going from infinity to some closer object, you should have enough information to solve for the unknown binocular focal length. For example, at infinity 1/p1 + 1/q1 = 1/f but with p1=infinity, then q1=f. At closer object, 1/p2 + 1/q2 = 1/f and q2=1/(1/f-1/p2). Delta q would then be 1/(1/f-1/p2) - f. If you can measure delta q (call it d), you might be able to solve for f. I get the following with doing a little math. f = (-d + SRT(d^2 + 4dp)) / 2 where p=distance to the close object and d is the delta q that you had to focus the eyepiece from going from infinity to the object. I tried this equation with one of my refractors with a known f and got the following: Delta q (or d) for my refractor was about 32mm (with going from infinity to a close-by object, 8534.4 mm away. The math has f = (-32 + SRT(1093553.2) )/2 = 507mm for my refractor. My refractor should have a focal-length of 509mm according to specifications, so that is very close. The challenge is getting a good measurement of the delta q (especially for small F-number binoculars)."
Last edited:
There are a LOT of members and birders that have glare problems with the Swarovski NL, and it is well documented all over the internet. Jackjack talks about Swarovski's being glare monsters all the time. HenRun is the FIRST person that has said the SFL 8x50 has too much CA, and already a reviewer I trust more and with more experience Canip has said the SFL 8x50 is an excellent binocular, and he saw NO CA in the center and the CA on the edge was not that noticeable and Canip hates CA, so I would tend to trust Canip more than HenRun until more people have evaluated the SFL 8x50. I trust Holger Merlitz's reviews and when he says the NL has a glare problem they have a glare problem.
Holger Merlitz
"Stray light: The tendency to develop stray-light in some situations remains the only considerable weakness in both binoculars. In difficult light conditions, bright spots are emerging around the edges of the exit pupils, which tend to create partial whiteouts (in most cases a crescent-shaped glare in the lower half of the field) when the eye-pupils accidentally get in contact with them. A careful setting of eye cup positions and a certain discipline in the way and angle at which the instrument is held in front of the eyes go a long way to avoid these whiteouts in the vast majority of situations. Observer's reports vary wildly about the severeness of the glare, ranging from 'irrelevant' to 'irritating'. The fact is that there exist binoculars (including the Zeiss 8x32 SF) with a superior resistance against stray light. The stray light issue which has occasionally been reported to plague the EL WB has not been resolved with its successor, and this is going to remain a matter of dispute whenever the NL Pure's merits are discussed. Nonetheless, there exists only one binocular which could currently challenge its pole position, the Zeiss Victory SF. In comparison, the SF has the advantage of an even wider field, a lower weight and - yes - a superior stray light protection."
Last edited:
John A Roberts
Well-known member
Considering what we do know at this stage about the x50 SFL's . . .
Some obvious comparisons can be made between the 10x50 and 12x50 versions and those of Leica and Swarovski
(bearing in mind that the EL's were discontinued in June last year with the introduction of the x52 NL's).
Looking at the listed info for the 10x50 and 12x50 versions:
Comparing Zeiss to Leica, the Zeiss choices:
a) are 11% shorter and 14% lighter - with either a 4% or 5% greater FOV - and;
b) on introduction, have prices at least 35% less (on enquiry, BH Photo has a cheaper price for the Leica 10x50).
So before there's detailed comparative optical testing available, what should we reasonably expect in optical performance,
from a significantly shorter and lighter unit,
that uses less glass by employing thinner lenses,
while having a slightly larger FOV,
AND a much lower price point
The significantly shorter length shown in a quick mashup:
(It also indicates differences in handling and holding.)
John
Some obvious comparisons can be made between the 10x50 and 12x50 versions and those of Leica and Swarovski
(bearing in mind that the EL's were discontinued in June last year with the introduction of the x52 NL's).
Looking at the listed info for the 10x50 and 12x50 versions:
Comparing Zeiss to Leica, the Zeiss choices:
a) are 11% shorter and 14% lighter - with either a 4% or 5% greater FOV - and;
b) on introduction, have prices at least 35% less (on enquiry, BH Photo has a cheaper price for the Leica 10x50).
So before there's detailed comparative optical testing available, what should we reasonably expect in optical performance,
from a significantly shorter and lighter unit,
that uses less glass by employing thinner lenses,
while having a slightly larger FOV,
AND a much lower price point
The significantly shorter length shown in a quick mashup:
(It also indicates differences in handling and holding.)
John
Last edited:
tenex
reality-based
First reports suggest that apart from CA, SFL 50s seem to be performing surprisingly well, given all the doubts one could have for the reasons you list. But then again, many bins at the $1000 price level do too... so the question is how SFLs compare to them.
That's a good laugh, coming from the person who has derailed the most threads in BirdForum history -- including this one, with all your inanities about focal ratios.
Wow! That is a big difference in the length of the binocular. The SFL 10x50 would be much easier to hand hold for a longer period than the Leica 10x50 because not only is it lighter but much shorter, so the center of gravity is lower. Hunter's will absolutely love the SFL 10x50 and SFL 12x50. You could hold them steadier without a tripod longer. I wonder if they are brighter because the optics are faster, like a telescope.Considering what we do know at this stage about the x50 SFL's . . .
Some obvious comparisons can be made between the 10x50 and 12x50 versions and those of Leica and Swarovski
(bearing in mind that the EL's were discontinued in June last year with the introduction of the x52 NL's).
Looking at the listed info for the 10x50 and 12x50 versions:
View attachment 1642773
Comparing Zeiss to Leica, the Zeiss choices are:
a) 10% shorter and 13% lighter - with either a 4% or 5% greater FOV - and;
b) on introduction, have prices at least 26% less (on enquiry, BH Photo has a cheaper price for the Leica 10x50).
So before there's detailed comparative optical testing available, what should we reasonably expect in optical performance,
from a significantly shorter and lighter unit,
that uses less glass by employing thinner lenses,
while having a slightly larger FOV,
AND a much lower price point
To illustrate the significantly decreased physical length, a quick mashup:
View attachment 1642777
(It also indicates differences in handling and holding.)
John
Last edited:
No, that's not the same thing. That a telescope is faster is about that a shorter focal length gives lower magnification and therefore requires shorter exposing time for photography.
When an eyepiece is applied the brightness is determined by the exit pupil.
