Telescope Reviews: Small Binocular Illuminance

SEE MOST COMPLETE DATA IN ATTACHMENT added 1-25-08

Small Binocular Illumance as measured with a light meter.
Values are in Lux (Lumens/meter squared). Light source is a 60w light in a box with a hole cut to insert a tube, approx 3” dia and about 3’ long. Several layers of white paper are attached over the tube at the light source end, to diffuse the light. Light is directed thru the tube towards binocular objective which is inserted into the open end of tube.

Light measurement in open tube is checked before inserting objective and light exiting open tube almost always measured between 202-205 lux, for instance a few times I got a starting reading of 198 or 211. Ambient light outside tube light source measured less than 5 lux. Readings are taken with light meter probe centered on exit pupil and at exit pupil distance. That wasn’t easy, in fact it was extremely difficult. Alignment of binocular in tube was critical and position of probe centered on exit pupil and at optimum exit pupil distance was critical. It was easy to see when probe was not optimally placed. If held 5mm in front of or 5mm out past exit pupil distance, readings could vary from maximum by several hundred less when not in proper position. I had to do a lot of readings over after I discovered that just a slight position movement could change readings by a large amount.

As many as 15 to 20 readings were obtained for each binocular. I discarded all but the best 6 readings for each binocular. The average of the best three readings was compared to the best reading and in most cases did not vary by a lot, perhaps the most 10-20 Lux, but more often by only 5-10 Lux. I was simply looking to see if the maximum readings were a fluke or were repeatable. The best reading is reported here.

This is NOT a measure of transmission losses that you typically see measuring percent transmission. However, for similar sized binoculars (primarily similar sized exit pupil), which should have nearly equivalent lux readings, this may be used as a comparison of total brightness. Differences in readings may have several causes. Some reasons may be: differences in coatings, vignette or internal light loss, improper prism tilt or prism angle cut, premium glass vs other. Binoculars which have previously been noted as having reduced aperture, and therefore reduced exit pupil, clearly show up here as not being equal with the typical Lux range for the same nominal size binocular. Exit pupils and apertures have been verified elsewhere in the Small Binocular tests.

edz

Here's some of what I see in the results:

Generally, larger exit pupils produce higher Lux readings. That makes sense, since larger exit pupil correlates to brighter image. Differences of 1mm in exit pupil produce significantly different Lux readings.

Max *** Exit *** Effect
Lux *** Pupil *** Aper
630 *** 4.0 *** 32 *** Oberwerk Mariner 8x40
708 *** 5.3 *** 52 *** Oberwerk Mariner 10x60
773 *** 6.1 *** 43 *** Oberwerk Mariner 7x50

702 *** 4.0 *** 49 *** Nikon Action Extreme 12x50
845 *** 4.7 *** 48 *** Nikon Action Extreme 10x50
880 *** 4.8 *** 39 *** Nikon Action Extreme 8x40

Also several premium binoculars produced higher readings than others of similar size. Also, some correlation is seen between severe vignette and lower readings.

Max *** Exit *** Effect
Lux *** Pupil *** Aper
1063 ** 7.1 *** 50 *** William Optic 7x50 ED
902 *** 7.0 *** 49 *** Captain's Helmsman 7x50
773 *** 6.1 *** 43 *** Oberwerk Mariner 7x50

992 *** 4.9 *** 50 *** Fujinon FMT-SX 10x50
845 *** 4.7 *** 48 *** Nikon Action Extreme 10x50
715 *** 4.9 *** 48 *** Leupold W R Mesa 10x50
708 *** 5.3 *** 52 *** Oberwerk Mariner 10x60

The porros in my tests of same or similar size exit pupil as roofs produce significantly higher lux readings.

Max *** Exit *** Effect
Lux *** Pupil *** Aper

702 *** 4.0 *** 49 *** Nikon Action Extreme 12x50
670 *** 4.2 *** 50 *** Nikon SE 12x50
630 *** 4.0 *** 32 *** Oberwerk Mariner 8x40
574 *** 4.1 *** 41 *** Nikon Monarch ATB10x42 Roof
491 *** 4.0 *** 40 *** Pentax DCFHRII 10x42 Roof
538 *** 4.1 *** 42 *** Celestron Regal 10x42 Roof

751 *** 5.0 *** 40 *** Fujinon BFL 8x42
770 *** 5.0 *** 39 *** Pentax PCF WP II 8x40
770 *** 4.8 *** 38 *** Garrett Classic 8x45
880 *** 4.8 *** 39 *** Nikon Action Extreme 8x40
579 *** 5.1 *** 42 *** Celestron Regal 8x42 Roof
560 *** 5.1 *** 41 *** Bushnell Legend 8x42 Roof
660 *** 5.2 *** 41 *** Garrett DCF 8x42 ApoRoof

Edited by EdZ (01/25/08 04:55 AM)

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