Survey: Practical Magnifications in the Field (1 Viewer)

Useful binocular magnification is usually limited by the unsteadiness of hand-holding and the concensus is that the upper limit is 10x for most people, where one would be able to discern details around seven times the distance as with the naked eye.

Assuming that a scope is equipped with an adequate tripod, unsteadiness should not be a problem most of the time and image quality will be affected by the atmospheric conditions, the size of the exit pupil (which diminishes with increasing magnification) and, of course, the quality of the scope.

I thought it would be interesting to hear which scopes others are using, what and where they observe, and at which magnifications and distances. Who wouldn't want 50x capability with a 2,5 mm exit pupil, but no-one is going to be willing to carry a 125 mm scope in the field and few of us could afford one if such were available. I recall Troubador's interview with Gerald Dobler (the man behind the Zeiss Harpia), where he referred to the popularity of 65 mm scopes and said that they could give good images up to about 40x. I have used 65 mm scopes since 2005 and think they are a very good compromise for mobile use in hot or temperate conditions. Exceptions are the one BF member, who needs high magnifications to read rings and some of the Scandinavians, who often enjoy stable air and good lighting to make use of 70x in the field.

My interests are primarily waders, so living inland (51° N) my observations apart from occasional trips to the coast are largely confined to inland stretches of water at migration. Distances are usually under 250 m (measured). A couple of years ago I was observing a Ringed Plover and invited a much younger birder to take a look through my Swarovski ATM 65HD with 30x eyepiece. He identified it as a Little Ringed Plover by its yellow eye ring, which I could no longer see. It was obvious that I "needed" more magnification and aperture and I bought a Kowa 883 with 25-60x zoom. However I found myself using the Kowa most of the time at 25x and seldom profited from magnifications above 50x so that I sometimes doubted its capabilities and had to reassure myself under optimum conditions. I have since tested the Kowa at 1,42". The diffraction limit for an 88 mm scope would be 1,32" and it might have managed this if I had tested at a longer distance than 23 m and not neglected to give it time to cool down. The little Swarovski btw managed 1,78", which is the diffraction limit for a 65 mm scope and I still take it out in good lighting conditions where I know observing distances are not going to be that large.

Some years ago a dealer here carried out some tests with customers to determine the minimum useful exit pupil size for terrestrial observations. IIRC he used a Diascope 85 with astro adapter and came to the conclusion that the minimum was around 1,3 mm. This would correlate very well with 50x on a 65 mm scope, or 70x on a 95 mm scope. I just don't think that many of us are regularly going to be able to use 70x magnification.

I await your input.

John

It is more dependent from the quality of the scope (glass etc.) than magnification. Almost everybody uses 20-60x zoom in the scope and 10x or 8x binoculars.

Orniwelt.de is a pretty good reference to good optics popular among German birders.

For terrestrial observations, but not particularly for bird watching, I have used the following.

A fixed Erfle eyepiece 95x on a custom 150mm Maksutov on a very robust Ex gov Aluminium square leg tripod, easily able to support a 75kg. man.
Long chain wrapped around the bottom of the legs for securing them. Very noisy set up.
Kept on the back seat of the car, for transport.
Beautiful bright images in daytime.

135mm f/16 observatory refractor, anything up to 200x or more, terrestrially day or night depending on Seeing and transparency. 5 miles, sometimes 40 miles targets.

120mm f/8.3 high quality doublet refractor.
250x used regularly at 3 a.m. terrestrially with great care over temperature control.
Observing position 10 metres above ground at targets 7.5 km away, across suburban parks and houses.

20x80 Celestron binoculars hand held. Used for many years. Day and night.

12x50 binoculars. My standard for 50mm binoculars hand held.
12x45 Russian binocular hand held daily for 10 years.

However, I would always try to brace on anything available, and from indoors rest the objective covers against double glazing and also window side wood.

Double glazed windows open, central horizontal hinge, rest scope or binoculars on open window rotating to any angle elevation.

25x to 135x 80 zoom binocular. Fine up to 80x, but scope gives better images.

95x 80 Acuter scope through double glazing. 4 Jupiter belts seen.

Canon 18x50 IS used for more than 15 years.

80x filar micrometer eyepiece 3 inch f/14 refractor, used to follow and measure distance of aircraft up to 250 miles away. Some heights measured at 63,000 ft.

Skywatcher 90mm Maksutov. Up to 190x on crows at 120 metres sitting on chimney pots. Bright sun 20 degree elevation behind me.
But best at 125x.
If the sun was not bright then contrast and bightness not sufficient for high power.

140mm approx aperture De Oude Delft military Maksutov. 200x used for testing. Beautiful top quality instrument.

100mm MTO used at 100x often, probably with six examples. Usually temperature effects from instrument.

100mm 1950s Soviet Maksutov absolutely brilliant at any magnification. An essentially perfect scope.

100mm f/12 Pentax refractor 200x during daytime. An essentially perfect scope.

Ross 100m f/15, another essentially perfect triplet objective.

Zeiss 20x60S Superb central resolution but curved field.

Yukon 30x50 folded refractor binocular, very good resolution equal to Zeiss 20x60s, but needs firm support. Poor coatings, poor transmission.

Yukon 6x-100x folded refractor lightweight plastic spotter. Good images at 100x, but scope probably fragile.

BAS 90mm Maksutov rather poor at 100x. Optics not the best.

Questar 3.5 inch good at about 120x.

Horace Dall's 108mm f/30 camera obscura corrected for four colours. Incredible.

Horace Dall's 8 inch Maksutov. Similar.

Etc. etc.

Practical magnification is mostly 25x for my type of birding. 50x is used occasionally but the air needs to be stable in those cases if it's a long distance subject.

Hi John,

I use my scope a lot like you: Kowa 883, for waders, up to 60x.

Tringa45 said:

Distances are usually under 250 m (measured).

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Since I'd like to know the actual observation distances too, how do you measure them?

Regards,

Henning

Hauksen said:

I use my scope a lot like you: Kowa 883, for waders, up to 60x.
Since I'd like to know the actual observation distances too, how do you measure them?

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Hi Henning,

I use a cheap Nikon Aculon laser rangefinder of which I posted a short review on the binoculars section. I do have some doubts about its accuracy though. When I was doing the resolution tests it indicated 24 m and I measured 23 m with a yard stick. For these tests I consider accuracy important and will have to do some calibration checks.

Regards,

John

Hi John,

Tringa45 said:

I use a cheap Nikon Aculon laser rangefinder of which I posted a short review on the binoculars section. I do have some doubts about its accuracy though. When I was doing the resolution tests it indicated 24 m and I measured 23 m with a yard stick. For these tests I consider accuracy important and will have to do some calibration checks.

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Thanks! I bought an even cheaper Aculon-lookalike rangefinder earlier this year, and that one at least seems to run out of steam at around 200 m, depending on conditions. I do often observe at far longer distances than that. Not ideal, but geography can be annoyingly uncooperative! ;-)

I believe my rangefinder has a resolution of just 1 m, which at short ranges translates in a considerable error. I have not checked it at long ranges, as I'm more interested in the order of magnitude of the distances involved than in the exact range. (The North Sea coast around here often lacks anything one could use for traditional rule-of-thumb ranging.)

To check distances below 50 m, wouldn't a "carpenter" type laser be better? These are really accurate, and they don't shoot a diffuse wide-angle beam, but a picture book laser dot. I picked up one at a supermarket recently for something like 20 EUR.

Regards,

Henning

Tringa45 said:

Some years ago a dealer here carried out some tests with customers to determine the minimum useful exit pupil size for terrestrial observations. IIRC he used a Diascope 85 with astro adapter and came to the conclusion that the minimum was around 1,3 mm. This would correlate very well with 50x on a 65 mm scope, or 70x on a 95 mm scope. I just don't think that many of us are regularly going to be able to use 70x magnification.

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I think for terrestrial viewing an exit pupil of about 1 mm is just about the limit - provided the scope is a cherry AND the seeing is exceptionally good, for instance an hour or so before sunset on a clear day. My Nikon ED III has a maximum magnification of 60x with the zoom, and I certainly use that once in a while to get an ID. Similarly, I also sometimes use 75x on my ED82. If the conditions are right, I can get some detail at such high magnifications I can't get at lower magnifications.

However, an exit pupil of around ~1,3 mm will in most cases be enough, and it's also going to be more comfortable, no matter how stable your tripod is. In a strong wind exit pupils below ~2 mm are almost impossible to use IME.

But once again, such high magnifications only work if the scope is a cherry and the seeing is exceptionally good.

Hermann

I use an old Leica laser rangefinder 7x20? 1100m, 1200 yards and
The Leica Disto x310 for shorter distances.
The Disto is probably very accurate?
The 7x20 less so, but it gives consistent readings at 124metres.
It might vary slightly with battery voltage.

I also use O/S large scale maps for longer distances.

The cheap Garmin yellow Etrek? is accurate to 5 metres, maybe better with repeated readings. It is much more sensitive than the 15 year? old one I gave to a friend who went across the Sahara desert alone on a BMW? motorbike.

I also use triangulation using a telescope.

Surveyors theodolites would be accurate.

Professional GPS I think works to a few centimetres but is a specialist activity.

I also use telescope fields measured with accurate star separations to target objects of known size.

Photography is fairly accurate.
The image size is calculated photographing the Moon. The diameter is given in ephemerides for a known time in UT. Or photographing star separations.
The centre of a low distortion lens is used. Macro lenses are accurate but so are long lenses when using just the centre.

I have used filar micrometers on astro scopes also.

Long tapes can be used, correcting for temperature or Invar? tapes. Sag etc. should be accounted for.

But the Leica 7x20 1200RF? is what I usually use.
One must not target a leaf in the way on a tree, as the distance to the leaf will be measured not the target one wants to measure.
Different lighting and different targets means the maximum distance varies, but I have used it at 600 metres and more.

There are about 190 benchmarks, maybe across Britain, some maintained some not, for accurate work.
The sea levels are measured to a few mms. on a regular basis. Britain has some very large tidal changes compared to other countries.

I think the Garmin GPS is prrobably accurate to maybe 5ft in height.

The Minox 10x25? altimeter binocular is accurate to 2 ft height, but there is no way of getting an accurate barometer fix.
One needs to visit a weather station or airfield, as broadcast hectoPascals are only given to 1hPa or 28ft.

Arlanda airport has a 0.01 hPa device.

The distance to the Moon is known to a few centimetres using laser reflectors there. More accurate than most measurements on Earth.

P.S.
Using the moon to calibrate a lens is not super accurate, because the diameter may be given from the Earth's centre, rather than the Earth's surface. It will also vary in apparent size depending on its position in the sky.
Using star separations is very accurate. The only variables are the lens distortion and possible atmospheric refraction changes with low elevation stars.

Ordnance Survey maps have relative accuracy tolerances at 95% level of less than +/- 2 metres for points 100m apart in rural areas, a bit better for towns.

If the ground is flat and paved I would think that a 50 metre tape or even 20 metre tapes would be accurate for most purposes up to 100 metres.

In towns I count paving stones, which are about 2 feet wide with an allowance for the gap between paving stones.

Pedometers I have found to be useless and very inaccurate.

Hermann said:

I think for terrestrial viewing an exit pupil of about 1 mm is just about the limit - provided the scope is a cherry AND the seeing is exceptionally good, for instance an hour or so before sunset on a clear day. My Nikon ED III has a maximum magnification of 60x with the zoom, and I certainly use that once in a while to get an ID. Similarly, I also sometimes use 75x on my ED82. If the conditions are right, I can get some detail at such high magnifications I can't get at lower magnifications.

However, an exit pupil of around ~1,3 mm will in most cases be enough, and it's also going to be more comfortable, no matter how stable your tripod is. In a strong wind exit pupils below ~2 mm are almost impossible to use IME.

But once again, such high magnifications only work if the scope is a cherry and the seeing is exceptionally good.

Hermann

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That's an interesting viewpoint. Indeed, under ideal conditions a scope which still offers a sharp image at 1 mm exit pupil is likely to be a good example.
I had however interpreted the dealer's tests to be more concerned with terrestrial viewing comfort just as amateur astronomers consider o,5 mm to be the bottom limit. Perhaps though the 1,3 mm determined by the dealer was due to a poor example of the Diascope 85, which was known to suffer from inconsistencies.

What I really wanted to hear from other BF members was what magnifications they use most of the time in the field. A few years ago the big guns from Leica, Nikon and Swarovski had 77, 78 and 80 mm objectives but the aperture war has taken us up to 95mm and I question whether the additional cost and weight is justified for the majority.

Three or so years ago I met a birder at my local patch with one of the first ATX 95s. I was almost shocked when he said he used it most of the time at 30x and thought he must have bought a lemon. Since acquiring the Kowa 883 though I can understand that. I certainly appreciate the large exit pupil at 25x but that suffices most of the time. I had thought I needed more magnification than the 30x on my ATM 65, but mainly I don't.

Hermann, if you were to use 25-30x magnification most of the time, would the ED82 be much better than the EDIII and which gets more use?

John

PS:- Regarding tripod stability, I took an old aluminium Gitzo Series 4 with me on my last trip to the North Sea coast. It was rock steady at 60x, however not something I would wish to carry far.

Hi,

my old Kowa TSN 3 usually stays at 53x which is the maximum magnification with the SDLv2 zoom on it. Image is very nice that way (and beyond that with extender) and I can aim easily with a cable tie sight.

I do turn down the magnification a bit in very bad light.

Joachim

I use ~30x magnification >95% of the time. My goal is bird ID, not digiscoping. The distances at which I pay attention to birds depend on the size of the bird, so I watch bigger birds at greater maximum distances. The largest birds for which I regularly use a scope are waterfowl/lake birds (e.g. loons/divers, grebes, gulls/terns) and perched birds of prey. At 30x, and with stable air and good light, most such big birds can be identified fairly easily at 500 m. I rarely (but regularly) attempt to ID birds beyond 500 m.

--AP

My big learning curve with magnification came with sea-watching. With such large distances, surely the more mag, the better?
For the first year i was constantly cranking up the zoom to max, and put the 1.6x extender on the Kowa 883 to get more.
However, as time has passed, i am much more inclined to stay at around 30-35x - perhaps zoom when a target is located and there is a tricky i.d. detail.
However, the often-fluctuating light conditions on the North Sea, the reduction in FoV and the difficulty of locating a bird with bins, then going to scope only to find you've left it on max zoom have all made me zoomaphobic.
Best sea-watching conditions where i am is usually a NE wind, thus blowing obliquely across the scope/tripod. At 60x plus, even the smallest tremble affects the view.
So, the top end of the zoom is now a very briefly used facility when essential.

Now, a summer's evening on a placid estuary in good light might be different, but i'll still regard around 40x as 'operational level'.

Thanks Joachim, Alexis and Paddy for your input.

John

The birds I observe on our local lakes in the winter are similar to what Alex mentioned, but at one favored overlook I have to deal with much longer distances, from about 1 km to almost 3 km. I use a 92mm Astro-Physics Stowaway that has sensibly perfect optics so only aperture limits its resolving power.

The way I deal with such long distances is to pay attention to the water and air temperature, wind conditions, sunlight and time of day so that I arrive at the lake when I'm pretty sure conditions will allow effective high magnification/long distance scoping. Ideally that would be during periods of calm water and low wind when the air and water temperatures are close to equal. Late afternoon in the winter or early morning in the summer tends to be the best. Elevation also matters. The spot I use benefits from being elevated about 20' above the water level, but the higher the better. On the very best days I can ID many species by shape, color or medium to large plumage patterns at 2.5- 3 km using magnifications between 75x and 120x. Trickier IDs, like separating Lesser and Greater Scaup have to be closer, but at 100x, are still possible at much longer distance than at 50-60x. 30x I only use for scanning.

henry link said:

I use a 92mm Astro-Physics Stowaway that has sensibly perfect optics so only aperture limits its resolving power.

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Hi Henry,

you are making me a little bit envious!

Enjoy that great glass!

Joachim

Tringa45 said:

Indeed, under ideal conditions a scope which still offers a sharp image at 1 mm exit pupil is likely to be a good example.
I had however interpreted the dealer's tests to be more concerned with terrestrial viewing comfort just as amateur astronomers consider o,5 mm to be the bottom limit.

Click to expand...

Viewing comfort always increases with the size of the exit pupil. Always. In an ideal world I'd like to have something like 2,5 mm - 3 mm with a scope at all times (and >4mm with binoculars, ideally 6mm - 7mm). Once you get below 1,5mm the going gets tough, especially in "real world conditions". Even a light wind makes things more difficult with small exit pupils because the body moves even if the scope is rock steady on a heavy tripod. For really small exit pupils I prefer to sit behind the scope, especially in windy conditions, like at the North Sea or on Helgoland.

As an aside: König/Köhler wrote 40x was the maximum useful magnification for terrestrial viewing. Nowadays, with modern scopes and perhaps especially with modern eyepiece designs that seems me to be rather on the low side.

Tringa45 said:

What I really wanted to hear from other BF members was what magnifications they use most of the time in the field. A few years ago the big guns from Leica, Nikon and Swarovski had 77, 78 and 80 mm objectives but the aperture war has taken us up to 95mm and I question whether the additional cost and weight is justified for the majority.

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IMO what we've got here is the law of diminishing returns - the big guns give you higher resolution (provided you've got a cherry) and better ease of use because of their larger exit pupils at any given magnification. But they're bigger and heavier, which means you'll need a larger and heavier tripod and head as well. And they're, of course, even more expensive than smaller high quality scopes.

In the end only you can decide what works best for you. Or get more than one scope. That's what I did, but then I got my ED 50 and my ED82 at really competitive prices. And I had all the eyepieces I needed already.

Tringa45 said:

Hermann, if you were to use 25-30x magnification most of the time, would the ED82 be much better than the EDIII and which gets more use?

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Yes, it would be better. Not much better, but better. Even at my favourite magnification for general birdwatching of ~25x. (I prefer that magnification simply because it gives me better depth of field, I don't have to refocus so much.) I've actually got all three Nikon Fieldscopes:

50mm: Mainly used on holiday and on trips when I don't expect I'll need the scope so much or on strenuous hikes. Small size, light weight, works really well on a lightweight tripod (or a monopod like the good old Monostat). Magnifications used between 16x and 40x, most of the time somewhere between 16x and 20x, especially when used on a monopod. Often the only scope I take when going on holiday.
Usage: 25-30%.

60mm: My main scope, the one that sees the most use by far. Reasonable size and weight, works well of a reasonably light tripod (Gitzo Carbon Series 2 with a lightweight Gitzo head) and on a monopod from the car. Can also be used on a monopod on longer hikes at magnifications up to about 30-35x. Magnifications used (on a tripod) between 20x and 60x, usually somewhere between 20x and 30x. I actually like this size of scope so much I got myself another, straight ED 60 for use from the car because I don't like using angles scopes from the car.
Usage: >50%.

82mm: The big boy. Despite its optical quality I only really use it when I expect to do a lot of watching over large distances at higher magnifications, e.g. when watching geese in the winter or at the coast (e.g. at the Meldorfer Speicherkoog) or when I know I won't have to walk longer distances. Needs a larger, heavier tripod and ideally also a larger, heavier head. Too heavy for longer hikes - not a scope I'd like to lug around in the Alps or the Norwegian mountains. Magnifications used between 30x and 75x, usually about 30x.
Usage: ~20%

I think all my scopes are cherries (in fact, the old Nikon Fieldscopes don't seem to suffer quite as much as some other scopes from sample variation), but the differences between the three sizes are quite obvious in the field - the larger the scope, the better the optical quality and, crucially, the ease of use.

However, if I had to get a new scope today, I'd probably go for a decent 65mm scope. That size seems me to an ideal compromise between weight and size on the one hand and resolution and ease of use on the other. I quite like the "normal" Swarovski 65 mm, for instance.

Hermann

henry link said:

I use a 92mm Astro-Physics Stowaway that has sensibly perfect optics so only aperture limits its resolving power.

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I only once had a chance to look through a Stowaway, and it's an amazing scope. Fantastic optical quality but you definitely need very good seeing to make use of its exceptional optics.

henry link said:

The way I deal with such long distances is to pay attention to the water and air temperature, wind conditions, sunlight and time of day so that I arrive at the lake when I'm pretty sure conditions will allow effective high magnification/long distance scoping. Ideally that would be during periods of calm water and low wind when the air and water temperatures are close to equal. Late afternoon in the winter or early morning in the summer tends to be the best. Elevation also matters. The spot I use benefits from being elevated about 20' above the water level, but the higher the better. On the very best days I can ID many species by shape, color or medium to large plumage patterns at 2.5- 3 km using magnifications between 75x and 120x. Trickier IDs, like separating Lesser and Greater Scaup have to be closer, but at 100x, are still possible at much longer distance than at 50-60x. 30x I only use for scanning.

Click to expand...

Under such conditions you can certainly make use of the quality of your scope. I usually don't have such conditions, expecially not when going after a rarity. In my neck of the woods the atmospheric conditions more often than not limit me to using magnifications up to 40x at most - no matter which scope I use.

Hermann

paddy7 said:

However, as time has passed, i am much more inclined to stay at around 30-35x - perhaps zoom when a target is located and there is a tricky i.d. detail.

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I do the same - I prefer scanning the sea at ~25x, than cranking up the zoom whenever I see something "interesting". Works much, much better that viewing at high magnifications at all times, especially in the often bad conditions on a seawatch.

paddy7 said:

Best sea-watching conditions where i am is usually a NE wind, thus blowing obliquely across the scope/tripod. At 60x plus, even the smallest tremble affects the view.

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You need a larger, much heavier tripod ... Seriously, when I go to the coast because a storm is forecast, I take a *very* heavy, wooden tripod. Carrying that monster for a mile or so is OK, and it really makes a difference in a howling gale.

Hermann