Rather than continuing to highjack the Nikon thread with an off-topic discussion I ‘m starting a new one for those who are interested.
Here’s a link to the original thread: http://birdforum.net/showthread.php?t=120290
I have a busy week ahead so this post will likely be my only lengthy contribution. Edz and I approach testing binoculars at boosted magnification in significantly different ways. Rather than continuing to dispute particulars I thought I would post a description of my methods as used to test 4 binoculars today. I picked these four because they demonstrate a wide range of results. They are Nikon 7x50 Prostar, Nikon 8x32 SE, Zeiss 8x42FL, and Pentax 10x50 PIF.
I used two test targets today: a USAF 1951 Resolution test pattern to measure resolution and an artificial star (silver Christmas tree ball in sunlight ) set up near by to evaluate aberrations and defects. The targets were placed 78’ from the binoculars. I used a Takahashi FC-50 telescope of known resolution to evaluate the test conditions, mainly air stability, and as a resolution reference to be sure that the test set up was producing accurate results.
I used one barrel of a Nikon 8-16x40 XL zoom binocular (stopped down to 20mm to prevent light leakage) behind the eyepieces of the test binoculars to boost magnification. The zoom allowed me to easily arrive at a magnification for each binocular that clearly showed it’s resolving power. In the end I equalized the magnifications of all the binoculars to about 80X, comfortably high enough to see the resolution in all of them.
Here are the results for raw resolution of the best barrel within about 6%. The elements of the USAF chart are in 12% steps. I didn’t bother to tweak the distance for each binocular to get a more accurate figure.
Nikon 7x50 Prostar - 3.06”
Zeiss 8x42 FL - 3.06”
Pentax 10x50 - 3.87”
Nikon 8x32 SE – 3.87”
These raw numbers may look similar but they actually indicate very different levels of optical quality; something that is quite obvious in the appearance of the image at 80X in the different binoculars. The Zeiss and Nikon SE images look quite impressively clean and contrasty. The Nikon Prostar and especially the Pentax look gauzy and soft with low contrast and insecure focus. It’s possible to assign a number to the optical quality with a simple formula: Resolution X Aperture in mm. A perfect telescope will show a number around 115-120 in this test. The higher the number the worse the optical quality. Using that formula the binoculars sort out like this:
Nikon SE – 124
Zeiss FL – 129
Nikon Prostar – 153
Pentax PIF – 194
By binocular standards two of these are exceptional, one is unremarkable and one is rather poor, but far from the worst I’ve seen. Now the question is: what is wrong with the Prostar and especially the Pentax? This is where the artificial star comes in. A quick 80X star test (with and without a green filter to evaluate spherical aberration) reveals slight miscollimation , a little pinching and overcorrection of spherical aberration in the Prostar. The Pentax is a mess with overcorrection of SA, severe pinching and miscollimation (the other barrel is worse with astigmatism added). The Nikon SE star test is truly excellent for a binocular with very little spherical aberration and only a mild defect from a prism edge impinging on the aperture. The Zeiss is nearly as well corrected for SA, shows no assembly defects and has lower chromatic aberration than any of the others.
After these results you might think the Pentax would be headed for the garbage can, but remember this is what you see on axis at 80x, not 10x. In normal use at low magnification the Pentax is quite acceptable, really quite good in the barrel without astigmatism in spite of its sloppy optical construction, and its field flattener gives it quite nice off-axis performance. The Prostar shows no problems at all in normal use at an even more forgiving 7x and its off axis performance is as good as it gets in binoculars. I’ve tested many binoculars this way over the past 20 years, so I long ago got used to how bad the optics of binoculars can look at high magnification. Now I use these tests as part of a total evaluation. When I see something bad at high magnification it simply alerts me to look very carefully to see if it really causes damage to the image in normal use. Sometimes it does, sometimes it doesn’t.
Henry Link
Henry,
Once again thanks for an interesting and informing post,you seem to have the knack of conveying information that us mere mortals can grasp without our eyes glazing over. Now all we need is to have you come along when we purchase a new scope or bins!!
fiddler.
Thanks Henry, I enjoyed seeing your process. Mine is remarkably similar and, at the same time, somewhat different.
I start by taking the objective size of the test optic (example: 8x32) and dividing that into 116 to get an expected max resolution (116/32=3.6”) and then divide the max expected into 200 (200/3.6=55.6x) to get to the minimum power I can use and then divide this by the bino power (55.6/8=6.9x) to get a minimum booster power and go to next larger size booster I have available. The 200 figure allows me about a 30% safety factor over my normal eyesight. When low power, big objective instruments require larger boosters than I have, I will cheat back towards the 150 apparent resolution mark.
My available boosters are:
A 3x at Inf. to 30x at 4 inches, inverted image commercial alignment scope (not used much).
A 4.7x dioptometer for some occasional curvature and astigmatism measurements.
A 5x fixed focus scope with a 1 and 3 degree reticle.
A 6x double collimator with a 5’ grid, inverted image, commercial.
A 8x Zeiss monocular.
A 10x LXL bino.
A few 30x commercial alignment scopes, but way to much trouble to use.
Instead of an open-air system, I use a closed system to control light and atmospheric conditions. I use a collimator with a 400 mm focal length with a USAF 1951 negative resolution reticle, supplied with about 14 or 15 EV of 5000K-6000K (usually 5200K) light through a diffuser. At this time I do not have an internal condenser lens between the light source and diffuser, but use an external one when needed. I just have not bought the lens or had the case machined yet. I have not noticed this having any effect on low power instrument measurements though.
The collimator always presents the test optics and boosters with an infinity image. The process is to make sure the boosters are focused on the collimator image, then the test optics are inserted between and then focused to bring the booster image to sharp focus. All components are then, hopefully, focused to infinity. This system allows for very close spacing of booster, bino and collimator to control light and air currents and controls power differences to those at infinity focus. Normally tests are made in a darkened room, enough light to write and work, but low enough to keep stray light problems down.
After making the readings of the USAF chart and reducing to a resolution figure I compare the results to the approximate expected figure I started with. I usually expect this to be about 110% to 150% larger than the estimated starting value. If the value exceeds 160% or otherwise does not appear acceptable, then like Henry, I start looking elsewhere.
Since this discussion pertains only to resolution I will quit here.
Best to all.
Ron