CN FORUMS ARCHIVE Telescope Reviews: Power, objective size & resolution

The following originally appeared on Astromart’s two birding forums in 4 parts. I thought it might stimulate some lively discussion to revive this old favorite topic of power vs. resolution.

Here’s a re-hash of some of my old backyard mini-tests. They are not at all conclusive, nor scientific, nor meant to sway anybody’s opinions. It has amused me to do variations on this theme for several years, pitting whatever optics I have on hand against each other. I like to think that I’ve learned just a tiny, tiny bit by doing this kind of thing. One thing I’ve learned is that there is no one single answer for all situations.
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Power vs. resolution
I've often heard that "Z" power equates to the maximum detail resolution in handheld binos. Likewise, I've often believed that doubling the power halves the apparent distance. So I wanted to see how these two conflicting theories related to my actual experience. To do that I taped selected brightly colored supermarket fliers with varying print sizes and fonts to cardboard boxes and set them at measured distances and then attempted to read the finest level of print possible with various handheld binos.

I did this 3 times in varying light levels from full sunlight, to twilight. What follows are the results from the most neutral lighting test conducted between 6:30-7:30 pm 5/9/02 with neither direct light nor shadow. Before I begin with the results, let me make it plain that the numbers themselves are meaningless. They change with each user, and with each lighting condition and with each different level of contrast. What's relevant is their relationship. What follows is the best possible situation for each bino in even lighting. Both bright light and shadow make things worse.

15yd-
7x35 Nikon Naturalist-- 4mm, almost 3mm resolution
7x50 Steiner Night Hunters-- can't focus this close
8x24 Minox BD P-- barely 4mm, poor color separation
10x50 Leupold Wind River-- easily 3mm, not 2mm
12x50 Pentax PCF V-- easy 3mm, barely 2mm
12x50 Nikon SE-- 2mm if high contrast, easy 3mm phenomenal color separation

25yd-
7x35 Nikon Naturalist- barely 4mm only w/extreme contrast and concentration
7x50 Steiner Night Hunters -barely 4mm, but good color separation
8x24 Minox BD P -4-5mm but only with high contrast colors
10x50 Leupold Wind River--4mm, good even performance
12x50 Pentax PCF- very solid 4mm, shake evident
12x50 Nikon SE- 3mm across colors & contrasts, fantastic color separation

50yd-
7x35 Nikon Naturalist-10mm at best, 17mm at worst
7x50 Steiner Night Hunters - easily 10mm even w/poor contrasting letters
8x24 Minox BD P - 12mm at best, 17mm on average, 28mm w/poor contrast
10x50 Leupold Wind River - 10mm w/high contrast, 12mm w/poor contrasting letters
12x50 Pentax- easily 10mm, but not 8mm at all
12x50 Nikon SE- 9mm easy, 7mm w/high contrast letters

What I get out of this little exercise is that neither the bit about "Z" power being ideal, nor the bit about increasing the power to decrease the distance is always linearly true. Clearly the bit about seeing the most detail depends more on lighting, quality of optic and contrast of target subject than on power alone. But also, clearly, raw power can compensate for a few of those variables.

One thing worth pointing out is that the 7x50 Steiner Night Hunters were by far the most even and consistant performers across a range of lighting conditions. This doesn't mean that I think they're at all suitable for most (or any) birding. In most every instance that I can think of a set of center focus 10x50's is a better choice.

The other thing that really struck me was how close in performance the relatively inexpensive Pentax PCF V 12x50's were to the MUCH more expensive Nikon SE 12x50's. Make no mistake, the Nikon SE's have an edge in clarity and color definition that I've not seen in any other binos including the very finest phase coated roof prisms, but it's a subtle step up, not an overwhelming one.

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For this 2nd mini-test, I wanted to concentrate on the subtle differences of more average birding binoculars at more standard ranges from a braced position. In other words, I wanted to see what all I could wring out of these binoculars. It is merely a snapshot of what these particular, individual optics were capable of on this particular day. I started out attempting to test the resolution of various sizes of print on a brightly colored supermarket flyer in *shade* at 50yds. This proved to be so far beyond the capabilities of these binoculars, than I dropped the distance to 30yds and moved the target into a well lighted area out of either direct sunlight or shadow. What follows is the absolute smallest lettering that I could consistently read. Binoculars were handheld, albeit with my arms solidly braced on a picnic table.

Minox BD8x24P porro, 14mm wide eye lens-- 5mm high contrast lettering, much worse on low contrast lettering

Celestron Birdwatcher 7x35 porro, 17mm wide eye lens-- 7-9mm lettering max.

Cabela's Pine Ridge 8x42 porro, 18mm wide eye lens -- 3mm High Contrast lettering, Some 5mm medium contrast lettering

Optolyth Alpin 8x30 porro -- 4-5mm lettering including low contrast, NOT 3mm lettering of any contrast, good color separation

Tento 7x50 porro -- 5mm high contrast lettering only

Carton Adlerblick 10x50 porro -- 4-5mm lettering including low contrast and narrow font, good color separation

Swift Kestrel 10x50, 20mm eye lens -- 2-3mm high contrast lettering, any 4mm lettering including low contrast

At 50yds in shade ALL of the 7-8x binoculars were limited to 12mm BOLD lettering and none could even do 15mm low contrast lettering. Whereas, the Swift Kestrel and Carton Adlerblick 10x50s could make out 6mm BOLD lettering.

It's important not to read too much into these results. However, it's interesting that there's such an extreme cutoff in range somewhere between 30 & 50yds, and somewhere between bright light and shade. This was noted in the first series of these posts last year, as well.

It's also worth pointing out that none of the binoculars used in this little impromptu test represent the "best" or the most expensive. In fact, all binos in this 2nd test are available under $200US, (except the discontinued Optolyth Alpins, which were bought used for $50.) How would things change if this was a comparison of only the best and most refined binoculars? I don't claim to know, but I would speculate that amongst comparable binoculars, higher powers and larger objectives would continue to be able to resolve farther out.

It's also interesting that amongst low cost binoculars, the relationship between size, power and resolution is not always strictly linear. Clearly there are issues of quality of optics and coatings that have MUCH to do with how one perceives the overall view and how well each binocular does.

In this test, I felt like the Swift Kestrel 10x50s had by far the most relaxed and easy to enjoy view. None others were even close. However, there were some performance surprises. For instance, the little Minox BD8x24P is my most HATED binocular. Viewing through it is not pleasant for any length of time. Yet, it performed surprisingly well, as did the Optolyth Alpin 8x30. It had the best contrast and color separation of any of the 7-8x binoculars.

For me, I continue to believe that 10x50s will show me more of the image, at more ranges than any 7-8x binocular. What's more, is those 10x50 porros will do it cheaper than any 7-8x roofs. However, that's just a personal opinion, and is a moot point for those that can't hold the 10x steady.

So, the differences are lots more subtle than I would've imagined. Somewhere between 25 and 50yds they become more evident, but to me the differences were almost more meaningful in terms of high quality glass vs. lower quality than in terms of power. It'd be darned interesting to try this with some really top drawer mid-power binos such as the famous Swarovski El 8.5x42's.
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Since doing those little impromptu comparisons, I’ve done dozens more in all lighting conditions, and field conditions using a variety of targets from paper to birds. And the conclusion that I’ve come to is that for distance out to somewhere around 30-40 yards, binocular resolution between 7 or 8x and 10 or 12x is really more a matter of the quality of the optic, than strictly the power or objective size. However, out past 50-60 yards, the superiority of the higher powers and larger objectives becomes very evident, very quickly. At 75-100 yards, the amount of detail that is lost with the lower powers makes them unable to distinguish subtleties that are evident with careful observation through higher powers. The other extreme limiter of performance is lighting conditions. But, I’ll save that for a future post.

Mike Swaim

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18" Starsplitter II f/4.5
8" Hardin Dob f/6
C5 workhorse mini SCT f/10 or f/6.3
70mm TV Ranger dual purpose birding/astro
77mm Leica Televid APO
16x70 Fujinons on UA Deluxe Mt.
12x50 Nikon SE
8x30 Nikon E2s
and many others

Edited by lighttrap (04/23/04 06:55 PM)