Low light bins

To: "Peter Shute" <>, <>, <>, <>, <>
Subject: Low light bins
From: "Roger Giller" <>
Date: Wed, 9 Mar 2011 23:35:46 +1100
Peter et al,

Firstly, my apologies if this rambles a bit and covers some items already explained. I started composing it before some of the later postings. The thread is active enough that it presents a moving target.

I am also an astronomer ("amateur", in the same sense that most members of this list would be amateur birdwatchers, i.e. do not engage in it as a paid profession) and have been watching this thread with interest. I would like to add my bit - starting with the suggestion that most of the light enters through the centre portion of the objective. I suspect your astronomer was attempting to point out that the increase in light gathered is proportional not to the diameter of the objective but to the area of the objective and proportional to the square of the diameter.

Consider an objective 42 mm in diameter collecting 1 unit of light. Increasing the diameter to 50 mm is like adding a 4 mm wide ring of extra glass. The area of the objective is 50^2/42^2 = 1.42 times greater, now collecting 1.42 units of light. of this, 1 unit or 70% is collected by the inner 42 mm, and the remaining 30% is collected by the 4 mm wide "ring" of extra glass.

The next bit in the optical train is the prism. There are various types which I won't go in to except to say that one reason very cheap binoculars are is that the manufacturers use a prism too small for the job, resulting in vignetting. This is familiar to the photographers among us. Basically it means the edge of the field of view receives less light than the centre.

The next bit in the equation is the eyepiece or ocular. It is essentially a magnifying glass to magnify the image that is formed by the objective. There are various designs used my manufacturers, all with there good and bad points. The magnification is equal to the focal length of the objective divided by the focal length of the eyepiece. All else being equal a higher magnification means a dimmer image as you are trying to spread a smaller amount of light (from a smaller part of the image formed by the objective) over the same area (the eyepiece's apparent field of view.) This means 10X42s will give a dimmer view than 8X42s.

Much has been said about exit pupils. This is the image of the objective, formed by the eyepiece, and it lies a short distance outside the binoculars, behind the eyepiece. This distance is known as the "eye relief" and depends primarily on the eyepiece focal length, but also on the eyepiece design. Shorter focal length (higher magnification) means shorter eye relief, which may be a problem if you wear glasses. The diameter of the exit pupil is equal to the objective diameter divided by the magnification. Theoretically all the light collected by the objective, (42 or 50 or whatever mm in diameter) is "funnelled down" to the diameter of the exit pupil, then into your eye making the image brighter than if seen by the naked eye alone, which only utilises the light collected by the eye's pupil, normally never more than 7 mm in diameter and usually considerably less. Because of other factors this increased brightness is not readily noticeable in normal daylight viewing, but becomes apparent under low light conditions. Turn your binoculars to a starry sky to gain a good appreciation of this.

The final bit is the eye itself. After all of the above, the light still has to get in to your eye through the pupil, which varies in diameter depending on the amount of light entering the eye. It is generally accepted that the maximum pupil diameter of a fully dark adapted eye is 7 mm. For this reason many older binoculars were designed to achieve a 7 mm exit pupil to match, hence the almost ubiquitous 7X50s. the important point to note here is DARK ADAPTED. Think looking for the silhouette of a ship against the ocean in the middle of the Pacific ocean with only the light of the stars. This is when young healthy eyes can open up to 7 mm. In most practical birding situations, even a rainforest understory on a cloudy day, there is way more light than that, and we are deliberately trying to make it brighter by using the best combination of aperture and magnification. I see no need to go for an exit pupil of more than 5 mm. If you want 10X magnification then 10X50s are OK. 8X40 should give similar brightness. the reduction in lens diameter means the rest of the instrument, both body and internal prisms, can also be smaller with a significant reduction in weight.

Close focus is one consideration that is very important for birders but rarely presents a problem for other users such a sailors, sports watchers and astronomers.Focusing depends on a mechanical system to vary the distance between the objective and the eyepiece and has nothing to do with the optical specification. Close focus is achieved by moving the eyepiece further away from the objective.

I had a $200 pair of 10X50s which were great for astronomy but when I took up birding I quickly realise their limitations. Heavy, smaller field of view and unable to focus closer than about 12 metres. I now use a $250 pair of William Optics 8X42 SEMI-APO with 2.5 metre close focus and weighing 675 gm. The optical quality is very good, but obviously not in the class of Swarovski or Leica, but I consider them excellent value for money.

<Prev in Thread] Current Thread [Next in Thread>

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the birding-aus mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU