MJHamilton
Member
Hi everyone, I was wondering if someone could define what it means when a binocular is described as 8x40? I think the 8x means magnification, right? But then what does the 40 mean? Thanks in advance for your help!
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Meaning of 8x40? (1 Viewer)
- Thread starter MJHamilton
- Start date
brocknroller
porromaniac
The "40" number means the objective lenses are 40 millimeters in diameter (assumed too much prior knowledge on the part of the OP, as Ron's post elucidated, but correcting here in case this gets picked up on Google).
An 8x40 or 8x42 is pretty typical for birding binoculars, because many people start seeing "the shakes" when they try to hold more magnification, and 40/42mm because more than that and the bins start to become heavier and unwieldy.
Eye relief, the distance from the lens to the focal point, is also measured in mm. It gives you a rough idea if you can use the bins with eyeglasses, but eyecups and lens recession detract from the specs, so it's not always an accurate measurement of useable eye relief for eyeglass wearers.
Brock
An 8x40 or 8x42 is pretty typical for birding binoculars, because many people start seeing "the shakes" when they try to hold more magnification, and 40/42mm because more than that and the bins start to become heavier and unwieldy.
Eye relief, the distance from the lens to the focal point, is also measured in mm. It gives you a rough idea if you can use the bins with eyeglasses, but eyecups and lens recession detract from the specs, so it's not always an accurate measurement of useable eye relief for eyeglass wearers.
Brock
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MJHamilton
Member
Ah thank you so much!
Surveyor
The more I understand, the more I understand why I
The 40 in 8x40 is the objective lens diameter (the large lens at the front of the binocular) measured in mm's.
normjackson
Well-known member
8x40???
The 8 is the number of days a week and the 40 is the hours a day that some folks spend on Birdforum. You have been warned...:-O
There are three pages of "primer" on binoculars here with illustrations :
http://monkoptics.co.uk/binoculars-explained.html
Something similar on the Nikon sports optics site.
BTW looks like the National Geographic shop site is down at the moment hence lack of comment on your other thread.
The 8 is the number of days a week and the 40 is the hours a day that some folks spend on Birdforum. You have been warned...:-O
There are three pages of "primer" on binoculars here with illustrations :
http://monkoptics.co.uk/binoculars-explained.html
Something similar on the Nikon sports optics site.
BTW looks like the National Geographic shop site is down at the moment hence lack of comment on your other thread.
One more point to consider. The relationship between magnification and objective size is known as the exit pupil and that is computed as diameter / magnification. You can see the exit pupil in the eyepiece if you hold the bins away from your face. That number typically ranges from 2mm to 7mm although it can go outside that range. A smaller number 2 to 3 indicates the bins will deliver the best images during day time. A larger number indicates more light is coming to your eyeballs and the bins could also be used in dimmer conditions or for observing the night sky.
Bins with an exit pupil of 4 to 5 are useful in a wide number of applications. Your 8x40 should serve you well.
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Well said. We recommend a minimum 4mm exit pupil (objective lens diameter divided by magnification power) when choosing your binoculars. So 8x40 has EP of 5mm, which should suit you well.
Charles
mooreorless
Well-known member
An exit pupil of 2 or 3 in a binocular doesn't necessarily mean a better view in the daytime, a 5-7 mm exit pupil binocular [8x40=5 mm 7x50= 7mm 8x56=7mm EP] stopped down by your eye during the daytime to say 3mm could mean a better view.
http://www.birdforum.net/showpost.php?p=840895&postcount=1
I thank Henry for his posts on here and other sites!
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An exit pupil of 2 or 3 in a binocular doesn't necessarily mean a better view in the daytime, a 5-7 mm exit pupil binocular [8x40=5 mm 7x50= 7mm 8x56=7mm EP] stopped down by your eye during the daytime to say 3mm could mean a better view.
http://www.birdforum.net/showpost.php?p=840895&postcount=1
I thank Henry for his posts on here and other sites!
Brock I am glad you fixed your post.
Not to mince words but I think it is correct to say a binocular with a relatively small exit pupil is best suited to daytime viewing.
There's been lots of discussion on exit pupils over the years, including this: http://www.birdforum.net/showthread.php?t=175494&highlight=Exit+pupil
mooreorless
Well-known member
Here is what I was referring to and helps explain why using 8x42 etc. binoculars in daytime the view looks better than 8x25/8x20 etc.
http://www.birdforum.net/showpost.php?p=2492973&postcount=10
A single quote from that link:>" most 7mm exit pupil binoculars show very low aberrations when stopped down to 2-3mm by the pupil of the eye in bright light."<
Thanks RonE [Surveyor] for posting this link on brightness thread!
http://www.birdforum.net/showpost.php?p=2527082&postcount=9
Surveyor's reply on that thread.
Fugl you have some great pictures on here and on Flickr!:t: Esp. the birds of prey!
http://www.birdforum.net/showpost.php?p=2492973&postcount=10
A single quote from that link:>" most 7mm exit pupil binoculars show very low aberrations when stopped down to 2-3mm by the pupil of the eye in bright light."<
Thanks RonE [Surveyor] for posting this link on brightness thread!
http://www.birdforum.net/showpost.php?p=2527082&postcount=9
Surveyor's reply on that thread.
Fugl you have some great pictures on here and on Flickr!:t: Esp. the birds of prey!
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