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Hello, “O”
Here are some general notes on bino’s that may be of help.
Classification:
Bino’s are classified by the diameter of the (front) lens, called the “objective lens” and their magnification. So,
Magnification and diameter of lens:
7x50 gives x7 magnification with a 50mm diameter objective lens.
8x42 gives x 8 magnification with a 42mm diameter objective lens.
8x24 gives x 8 magnification with a 24mm diameter objective lens.
The 50mm bino will be able to “grab” or “gather” more light as it is bigger than the 42mm and 24mm bino.
x10 magnification can be difficult to hold steady as the image will appear to wobble unless we use a tripod or rest our elbows on a wall or table.
At the eyepiece end – where you look through:
The next important attribute is the “exit pupil”. This is the small circle of light you can see at the eyepiece when holding the bino’s up to the bright sky. Why is the exit pupil important? Well, our physical eyes have an iris that expands in the dark and shrinks in the light. In a healthy young person, the diameter of the iris when fully opened is approx 7mm. In a middle aged adult, it is only 4 or 5 mm due to the ageing process.
For 7x50 bino’s the exit pupil is 50/7 = 7mm (approx).
For 8x42 bino’s, the exit pupil is 42/8 = 5mm (approx).
For 8x24 bino’s the exit pupil is 24/8 = 3mm
Image brightness:
Therefore, the 7x50’s will produce a bright image that will match the aperture of your iris on a dark night.
The 8x42’s will only illuminate the central 5mm of your eye’s 7mm iris aperture.
The 8x24’s will only illuminate the central 3mm of your eye’s 7mm iris aperture.
Eye relief:
This is a measure of how close you will have to place your physical eye to the eyepiece to see a fully illuminated image. If you need glasses to correct your vision, then long eye relief is good. I understand that if glasses are simply for e.g. close up reading, then they need not be worn for looking through bino’s as they are not correcting for optical defects such as astigmatism?
Notes:
WARNING – never, ever look at the Sun through bino’s or any optical instrument – you will be blinded immediately.
Traditionally, 10x50's or 7x50’s are classified as good astronomy bino’s because of their light grasp (50mm) and exit pupil (5mm & 7mm respectively) but they can be heavy and difficult to hold still.
What will you use them for?
If you would like to use the bino’s for bird watching, then models from Leica will typically focus close, to around 3 metres. All round or general purpose bino’s tend to only focus down to maybe 5 to 10 metres.
Shops in Brisbane:
York Optical in Fortitude Valley.
Sirius Optics at Underwood.
Photo Continental at Mt Gravatt.
I’d better stop here as I’m uncertain at your level of understanding and fearful of swamping you with too much stuff.
Cheers
Dennis
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