16 replies to this topic

[TimD]

I was looking for an entry level 100mm pair of binoculars. I have a friend getting rid of a pair. His are 20x100mm Apogees. I know theat Apogee started making them 25x100mm also. Does anyone have an opinion on 20x100 vs 25x100? I am wondering magnification wise which would make an overall all around astronomy binocular. Thanks.

[Erik D]

There are three reviews of the Apogee 25X100s in the CN bino review section:
http://www.cloudynig...y_id=60&pr=2x48

I recall some of the earlier lower cost 100 mm Chinese binos, Oberwerk 22X100, Burgess 25X100, Celestron 25X100 have collimation and other mechanical issues. Since it is your friend offering a pair of sale I suggest it's best to have a look thru that pair yourself.

Speaking in general terms, I prefer giant binos with ~4 mm exit pupil for astronomy. Thus 20x80, 25X100 or 28X110. Tests show 25% increase in magnification has a bigger impact on limiting magnitude than 25% increase in objective size. My guess is you will see bigger difference moving up from 20 to 25X than 80 to 100 mm.


ERik D

[Ad Astra]

20x will give a wider field, better eye-relief, and a brighter (more contrasty) image. Better for nebula & extended objects.

25x will give a slightly narrower field, less eye relief (probably only an issue if you have to wear specs when observing like I do). Higher magnification means that you will see fainter point sources = more stars in all star fields; better for globulars etc.

Comes down to your observing preferences and personal preference. I went through the same thing thinking about Garrett's 20x110 or 28x110. Being a confirmed spectacle wearer, and enjoying nebulae and galaxies more than globulars (I have a refractor for that!) settled me on the 20x version. The better eye relief also makes it easier for other family members to use (more forgiving eye placement). Now all I have to do is save up enough $$$ to purchase them!

I hope that helps,

Dan

[GlennLeDrew]

20x will give a wider field, better eye-relief, and a brighter (more contrasty) image. Better for nebula & extended objects.

Dan,
You're the first that I know of to say that lower power delivers a more contrasty view. The common 'wisdom' has it that higher power improves contrast.

Actually, neither is technically true. As the exit pupil gets smaller, all objects dim to the same degree; sky, nebulae, galaxies, etc. Therefore contrast doesn't change with magnification/exit pupil. What gives the impression that it does is the fact that (1) fainter stars are seen, and (2) smaller 'fuzzies' are detectable and more detail is seen in them.

[TimD]

Thanks to all. Just spoke to my friend and the binoculars are labeled as 20x100mm and FOV 4.9 deg. I think I will heed the advice here and take a look through them and see how it looks.

[Erik D]

......the binoculars are labeled as 20x100mm and FOV 4.9 deg.


I think I will heed the advice here and take a look through them and see how it looks.

If this pair is indeed 20X with 4.9 deg Tfov they would have Ethos like98 deg Afov!!

One of the CN reviewers mentioned he paid $299 intro price for his Apogee 25X100s. 98 deg FOV is outstanding achievement for 100 mm binos in this price range.

Glenn L, Take note!

ERik D

[TimD]

here is a picture of the back of the binos. Going to try them out this evening...

Attached Thumbnails

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[Ad Astra]

20x will give a wider field, better eye-relief, and a brighter (more contrasty) image. Better for nebula & extended objects.

Dan,
You're the first that I know of to say that lower power delivers a more contrasty view. The common 'wisdom' has it that higher power improves contrast.

Actually, neither is technically true. As the exit pupil gets smaller, all objects dim to the same degree; sky, nebulae, galaxies, etc. Therefore contrast doesn't change with magnification/exit pupil. What gives the impression that it does is the fact that (1) fainter stars are seen, and (2) smaller 'fuzzies' are detectable and more detail is seen in them.

I realize that technically the change in exit pupil applies equally to all objects in the field; and "contrasty" isn't exactly a scientific term, but the eye does not detect or respond to light in a linear manner. Increasing the size of the exit pupil will increase the amount of light on the retina, and faint/extended objects become bright enough for the eye/brain to detect them against the background. When this threshold of detection effect occurs we can now detect more extended objects that just weren't visible before - in effect increasing the perceived contrast between faint fuzzies and the dark background. You need a big enough bino to achieve this - and we probably require more aperture to get there as we get older... at least that's what I tell the LOML!

The essence of the message is still the same; lower power is often preferred for faint, extended objects such as galaxies and nebulae, while higher power allows you to reach 'deeper', split more double stars, and get more detail, see more (and fainter) stars visible from clusters and star fields (bright, point-like objects).

Maybe the best advice is to look up the local astronomy club, attend a star party, and see what works best for you!

Have fun,

Dan

[GlennLeDrew]

Maybe the best advice is to look up the local astronomy club,

I'm a member of the RASC, and I've chaired our centre meetings for several years.

attend a star party

I've been to numerous star parties.

and see what works best for you.

What works best for me are the binos I modify, or design and build myself.

And to top it off, I've been an active observer for over 30 years and have peered through countless telescopes.

[GlennLeDrew]

Tim,
A 20X bino with a 4.9 deg. TFoV?!? That would imply an AFoV of more than 90 deg. (98 by simple multiplication)!! Methinks the real TFoV would be closer to 2.9 than 4.9., maybe 3.5-ish.

@EdZ,
Remember the 22X100 whose 3.8 FOV was closer to 2.8? Here's a likely more egregious candidate for outright exaggeration. Unless of course the eyepieces are truly ultra-wide in design...

[TimD]

Glenn I guessing the claimed FOV is an exaggeration. I picked them up today since the daytime views were pretty stunning. Will try them out tonight and attempt to estimate what the true FOV is. Thanks to all for your input!

[aa5te]

Hi Tim,

FWIW, my Apogee 25x100s are advertised as 3.5 degrees TFOV, but actually measure right at 2.25 degrees.

[TimD]

Took them out this evening. they are the 20x100mm ones and I gotta say they are at about 4 degrees at least. Maybe 4.25. All i can say is wow. fit items in the field that my ST-80 can't! More to come later on what I think overall.

[GlennLeDrew]

Tim,
That sounds rather exciting! The AFoV must then be at least 70 deg., perhaps 75, and possibly closer to 80. Nice! Let us know how they perform off-axis.

In the meantime, you can get a pretty accurate measurement of the AFoV as outlined below, all from the comfort of indoors.

First, practice looking at the exit pupil from about 1.5-2ft away. Slowly move your eye off axis until the exit pupil is suddenly clipped by the edge of the field stop. You'll know at what point that is because only a very small amount of further movement will cause the exit pupil to disappear completely. For this test, the 'key moment' is when the field stop edge is about halfway across the (usually somewhat elliptical) exit pupil. You will be taking two sight lines--from opposite sides--toward the center of the partially clipped exit pupil. Ready to proceed?

1) Set the bino on a table, about a foot or so from the edge, with the eyepices facing you.

2) Take a piece of paper and line it up as follows. If you are offset to the right, line up the long edge on the right side of the sheet. As best you can estimate, place the paper edge in line with the exit pupil at the point when it's half clipped by the field stop. Be sure to place the paper so that the corner farthest from you is partly under the eyepice (you'll see why when marking the second sight line).

3) Anchor the paper with tape or a weight so that it can't easily move.

4) Move so that you are now looking at the exit pupil from the opposite side, until the opposite side of the field stop clips the pupil.

5) Line up a ruler along this new sight line, and make a mark on the paper. Extend the mark so that it crosses the paper's edge which served as the first sight line.

6) Measure the angle with a protractor. No protractor? Work it out from either geometric or trigonometric principles.


Forgot your high school math? Do this:

1) From the crossing point (vertex) where the line and paper edge meet, place a tick mark on the marked line (the second sight line) about 6-8" distant.

2) Do the same along the paper edge, as measured from the vertex. Make sure both tick marks are at the identical distance from the vertex.

3) Mark a line connecting these two tick marks, and measure its length. Divide it by two. We'll call this half-line, 'X'.

4) At the the mid-point of the line you just made, mark a tick, and draw a line from this mark to the vertex and measure its length. We'll call this, 'Y'.

The AFoV of the eyepiece is = (ATN(X / Y)) * 2

'ATN' is the inverse tangent, which on most calculators is entered by pressing something like "Shift" or "2nd fn" keys followed by the "TAN" key.

[KennyJ]

If it's as simple as that Glenn , one has to wonder why instructions stating precisely what you stated above don't come gift wrapped with every binocular ?

The more cunning manufacturers / vendors could even provide a free sheet of paper , pencil , protactor and calculator with each purchase , along with the cleaning cloth and lens caps that don't fit properly ! :-)

Kenny

[GlennLeDrew]

Kenny,
We all know that the manufacturers/vendors don't want us to be privy to the real workings of their wares. I feel like I've just divulged forbidden knowledge, along the lines of a certain 'cookbook' for those who might foment disorder.

[pjsemail1]

Here we are, fat, dumb and happy. Now you make use look in the mirror, point out our lack knowledge and now we are sad :o