115 objective focal length and f-number (1 Viewer)

[grackle314]

Taking the 115 objective by itself, I measured a focus point of 460 mm for a light source at 7000 mm which gives a focal length of 431 mm. Using the 431 mm focal length gives N = f/D of 431/115 = 3.74 . Can anyone share their knowledge of focal length and speed of the 115 objective for comparison? And when used in the BTX configuration how can I measure the reported objective reduction to 110?

 

[henry link]

Here's a short thread on the subject of the ATX/STX focal lengths.

ATX Scopes Objective Focal Lengths ?

Does anyone know the objective lens focal lengths for the 65, 80 and 90mm scopes? In the old brochure the ATM80 angled scope had a 460mm objective focal length. I can't see objective focal length mentioned in the new brochure only 'length', which seems different as the ATS 80 angled is stated...

www.birdforum.net

The 115mm focal length would be the same as the 95mm. Its focal ratio would be around f/4.8

There are a couple of ways to measure the clear aperture of the BTX-115. One way is to place a transparent ruler as close as possible to the front of the objective lens and examine the exit pupil with a loupe of about 10x. The clear aperture will be the part of the ruler that spans the diameter of the exit pupil. The other is to shine a flashlight through the scope from about a foot behind the eyepiece onto a flat surface placed a few inches in front of the objective lens. The clear aperture will be the diameter of the circle of light that emerges from the front of the objective lens.

 

[henry link]

Coincidentally I just ran into this interferometry report for a 95mm objective module on Cloudynights. Focal ratio is f/5.8, therefore focal length is 551mm.

Swarovski ATX 95 optical test - Refractors - Cloudy Nights

Swarovski ATX 95 optical test - posted in Refractors : I received optical bench test report of Swarovski ATX 95 spotting scope objective module. Here is some pictures. Setup: Summary: Star test: Ronchi and Foucault: A few comments from the tester. Probably some people...

www.cloudynights.com

 

[DRodrigues]

The X115 has about 1% longer focal length than the X95 X115 test

 

[etudiant]

So now I'm confused.
Is the focal length 1% more than 551 mm, as measured by Henri Link above, or 431 mm as measured by the OP?

 

[henry link]

Hi etudiant,

I didn't measure anything. I just passed along information from others.

We know Swarovski's magnification specs are the same for both the 95mm and the 115mm modules when they are combined with the eyepiece/prism module. That should indicate that they have the same focal length. If we accept the interferometry report of 551mm for a 95mm specimen and David's measurement of about 1% higher (556-557mm) for a 115mm specimen then the actual focal lengths must fall somewhere between about 550 and 560mm, presumably small enough to be within Swarovski's tolerances for magnification specs. I don't think 431mm could be accurate. That would be shorter than Swarovski's 65mm scopes.

Henry

 

[Binastro]

It seems the original 431mm may be a measurement of instrument length rather than focal length.

Where does one measure a multi element objective with possible telephoto design?

Even a simple doublet has thickness, so where does one measure?

The simplest way is image scale on photos of say the Moon, but even here the centre and edge results will differ.
Also the Moon varies a lot in angular dimensions and is given for centre Earth, i.e.geocentric, and actual size depends on location on the Earth.
But angular measures are published.

Star separations and short exposures probably give the best results.
Using the centre of the field to avoid distortion.

Regards,
B.

 

[etudiant]

It seems the original 431mm may be a measurement of instrument length rather than focal length.

Where does one measure a multi element objective with possible telephoto design?

Even a simple doublet has thickness, so where does one measure?

The simplest way is image scale on photos of say the Moon, but even here the centre and edge results will differ.
Also the Moon varies a lot in angular dimensions and is given for centre Earth, i.e.geocentric, and actual size depends on location on the Earth.
But angular measures are published.

Star separations and short exposures probably give the best results.
Using the centre of the field to avoid distortion.

Regards,
B.

Is there not some convention for this, such as center of the first glass surface to center of the focused image?

 

[Tringa45]

Where does one measure a multi element objective with possible telephoto design?
Even a simple doublet has thickness, so where does one measure?

Could one not take the coincident point of an incoming ray parallel to the optical axis and the refracted ray as being in the effective plane of the objective?
With a telephoto design this plane would of course be in front of all objective elements.

Regards,
John

 

[Binastro]

This cannot be the case, as the focal length of a lens depends on the image scale.

With a Maksutov Cassegrain, typically the primary is f/2.5 and the focal ratio is typically f/10 to f/15.

An SCT has a primary of f/2 or f/1.95 and a focal ratio of f/10 or a bit more.

A telephoto has a fast front end, maybe a doublet, and a negative rear group giving a slow final focal ratio.

In optics classes the elements are assumed to have zero depth but this isn't so.

Modern lenses can have numerous elements.
How can one measure from the front when the other elements can be vastly complex.

With a long focus doublet, say a cemented f/15, I take the mid point of the doublet, which is a close approximation.

But a Fraunhofer doublet has quite a large air space.

I am not sure how distortions and curved fields are handled regarding image scale and focal length.

With a standard Gauss 6 element 50mm standard camera lens, using the central part gives a good approximation of focal length.
But most 50mm lenses were actually 52mm.

With spotting scopes there is a desperate need to make the scope as short as possible, so these are clearly not simple doublets in most cases.

Regards,
B.

 

[etudiant]

This cannot be the case, as the focal length of a lens depends on the image scale.

With a Maksutov Cassegrain, typically the primary is f/2.5 and the focal ratio is typically f/10 to f/15.

An SCT has a primary of f/2 or f/1.95 and a focal ratio of f/10 or a bit more.

A telephoto has a fast front end, maybe a doublet, and a negative rear group giving a slow final focal ratio.

In optics classes the elements are assumed to have zero depth but this isn't so.

Modern lenses can have numerous elements.
How can one measure from the front when the other elements can be vastly complex.

With a long focus doublet, say a cemented f/15, I take the mid point of the doublet, which is a close approximation.

But a Fraunhofer doublet has quite a large air space.

I am not sure how distortions and curved fields are handled regarding image scale and focal length.

With a standard Gauss 6 element 50mm standard camera lens, using the central part gives a good approximation of focal length.
But most 50mm lenses were actually 52mm.

With spotting scopes there is a desperate need to make the scope as short as possible, so these are clearly not simple doublets in most cases.

Regards,
B.

Thank you for pointing out that we are not looking at simple refractors any more, even though optics remain stuck with measurements from that era.
You are of course spot on, scopes are not simple designs, so that even basic measurements are hard to find.
Swaro seems on course to keep its secrets.

 

[grackle314]

To add a little more detail to my initial comments. First, pardon my transcription error, the focal length of 431 mm should read 413 mm.

Second, I've done a second focal length measurement to compare. Consider the ATX/BTX 115 objective as a lens, one sees an object in front of the lens focusses to an image behind. As a first approximation, the objective is a convergent lens. From the front of the compressed objective shroud to the lens glass at the edge is 3.2 cm. The objective case (compressed shroud) is 30 cm to the back black metal face. An object placed 1,463 cm from the front lens glass showed an image focused at 15.7 cm behind the back black metal face. That calculates to (30 - 3.2) + 15.7 = 42.5 cm axially behind the glass edge.

Again, as a first approximation 1/f = 1/o + 1/i = 1/1,463 + 1/42.5 = 1/ 41.3 which gives f = 41.3 cm = 413 mm.

This is a direct measurement of the 115 objective. The first measurement at the start of this thread and the second one described here give the same physical result of a near infinity object focused to about 413 mm. The difference of focal length to image distance must be much less than the focal length since the object distances in the two cases measured were relatively large compared to image distances (factors of 7000/460 = 15; 14,630/463 = 31.6), the focal length must be near the image location in these two measurements.

It would be great to have further assessment, particularly if I can learn more about why this focal length measurement method of the objective is not seen as correct. Thanks for the continued interest and comments.

 

[Tringa45]

To add a little more detail to my initial comments. First, pardon my transcription error, the focal length of 431 mm should read 413 mm.

Second, I've done a second focal length measurement to compare. Consider the ATX/BTX 115 objective as a lens, one sees an object in front of the lens focusses to an image behind. As a first approximation, the objective is a convergent lens. From the front of the compressed objective shroud to the lens glass at the edge is 3.2 cm. The objective case (compressed shroud) is 30 cm to the back black metal face. An object placed 1,463 cm from the front lens glass showed an image focused at 15.7 cm behind the back black metal face. That calculates to (30 - 3.2) + 15.7 = 42.5 cm axially behind the glass edge.

Again, as a first approximation 1/f = 1/o + 1/i = 1/1,463 + 1/42.5 = 1/ 41.3 which gives f = 41.3 cm = 413 mm.

This is a direct measurement of the 115 objective. The first measurement at the start of this thread and the second one described here give the same physical result of a near infinity object focused to about 413 mm. The difference of focal length to image distance must be much less than the focal length since the object distances in the two cases measured were relatively large compared to image distances (factors of 7000/460 = 15; 14,630/463 = 31.6), the focal length must be near the image location in these two measurements.

It would be great to have further assessment, particularly if I can learn more about why this focal length measurement method of the objective is not seen as correct. Thanks for the continued interest and comments.

Just a few comments:-
Firstly, for all waterproof binoculars and scopes where the distance between objective and eyepiece is fixed, the focal length of the objective is variable.
Close focus is achieved by shortening the focal length of the objective, either by shifting a positive focussing lens forward or a negative lens rearward.
Consequently the focal length of the ATX/BTX objective module would be dependent on the focus setting.
Don't forget that in the above calculation object distance is negative if focal length and image distance are positive.

John

 

[Tringa45]

Example:-
Assume a birding scope objective had a focal length of 500 mm when focussed at infinity.
To achieve close focus at 5 m (not uncommon) the focusing lens would have to reduce the focal length to 455 mm.

John

PS:- The above assumes a viewer with normal vision, who would bring the image plane of the objective and the focal plane of the eyepiece to coincidence.

 

[Binastro]

The distance between the front lens and rear lens of a spotting scope or camera lens has nothing to do with focal length or final focal ratio.

The air spaces and air gaps in spotting scopes and camera lenses are critical to the design.
In fact some air gaps are essentially extra elements in the final design.

Measuring anything from the front surface of a spotting scope or camera lens serves no purpose, except in the simplest long focus telescope objective.

A top quality firm decided to buy back long focus aircraft lenses for £75 each from a dealer as new ones were £250,000 each.

Each old lens was dismantled and each element measured carefully. Thickness, focal length, refractive index etc.
Then these details were fed into the computer, particularly the exact separations needed.
In the best cases the resolving power was increased eight times from the results when initially measured when bought back.
These lenses had been cleaned and reassembled, but the technicians did not know the correct separations.

I have bought lenses disassembled and reassembled by tinkerers, with disastrous results.

As to varying focal lengths, small Maksutov Cassegrains, say 90mm aperture and nominally f/14 are about f/18 when prisms are added.

Lenses are usually computed for infinity and when working at close distances are not optimised.

However, top quality professional lenses are computed for their normal working distances.

The problem, I think, with modern spotting scopes is that the front ends are just too fast, and extreme care is needed in spacing and manufacturing tolerances.
This clearly doesn't happen, so many if not most are just not very good even ones costing thousands of dollars.

The lens or spotting scope designer has to design tolerances for each element, both mechanical and optical, but in practice these tolerances are probably not met.

What surprises me is the quality of compact camera lenses from say Canon, which all seem to be near the best possible for the front aperture size.

I have photos from the Canon A 650 IS, similar to a G9, that print well at A2 size from 12 megapixels.
It was difficult to get this quality from film cameras.

The zoom lenses for these cameras are made in vast quantities.

Regards,
B.

 

[Tringa45]

The problem, I think, with modern spotting scopes is that the front ends are just too fast, and extreme care is needed in spacing and manufacturing tolerances.
This clearly doesn't happen, so many if not most are just not very good even ones costing thousands of dollars.
The lens or spotting scope designer has to design tolerances for each element, both mechanical and optical, but in practice these tolerances are probably not met.

Some tolerances are indisputedly important, but I'm not sure that spacing is one of them.
A focussing lens is an integral part of the objective design and at its nominal focal length of 500 mm a Kowa 88 is f/5,7.
At 5 m close focus the focal length would be a mere 445 mm (f/5,2).
Interpolating from a cutaway drawing the travel of the -ve. focussing lens is about 40 mm!
Nevertheless, a decent sample should be diffraction limited from the intermediate distances up to infinity.

Regards,
John

 

[Binastro]

Hi John,

Depending on the position of the spacing, spacing can be absolutely critical to the lens.
In other cases not so much.

If you consider the position of a Barlow or negative component, it can vastly change the power from 2x to 5x or more.
This is entirely due to spacing and its effects.

In the front group of a complex lens, again the spacing can be critical, although between groups not so much.

In relay lenses again, the spacing can vastly change the lens.
In the case of some of Horace Dall's telescopes, more than one hundred times.
One might say that is not possible, but he used a 900,000 mm focal length to photograph Mercury to fill a 35mm frame, also using his atmospheric prism corrector and maybe stacking and unsharp masking.

The problem may be that the spotting scope makers are asking the designers to go beyond reasonable designs and design critical lenses that in practice don't give the intended results.
If enough care and time were given these lenses could be made, but clearly the results are very average instead of very good.

Regards,
B.

 

[Binastro]

1). I personally don't like the term diffraction limited, as it doesn't give an actual value as to how good the optics actually are.

But given this, to make a lens perform really well at close and far distance involves extra complexity.

I personally doubt that many spotting scopes perform as well close and far.

2). Spacing can be super critical is certain optics situations.

3). It can take six months for a designer to get spacings correct in some optics.

4). Generally, spacing is somewhat less critical than the thickness of a lens because of the refractive index of glass being say 1.5x or a bit more.

5). I am probably correct that spacing within groups is more critical than between groups.

6). This is a highly complex subject depending on each product, so there are no one size fits all answers.
But spacing is very important.

This, as you may guess, is the result of a discussion with someone who actually knows his stuff.

Regards,
B.