A Work Commenced June 12 2022
Product: Opticron Adventurer T WP 6.5 x 32
Country of Manufacture: China
Chassis Material: Rubberised Aluminium & Polycarbonate
Exit Pupil: 4.9mm
Eye Relief: 18mm
Field of View: 161m@1000m(9.2 angular degrees)
Coatings: Fully Multicoated on all glass surfaces
Prisms: Porro BAK4
ED Glass: No
Close Focus: 3m advertised, 2.56m measured
Dioptre Compensation: +/- 4.0
Nitrogen Purged: No
Tripod Mountable: Yes
Accessories: tetherable rubber objective lens caps, ocular caps, padded neck strap, soft carrying case, microfibre lens cleaning cloth, warranty card & instruction manual.
Weight: 549g advertised, 556g measured
Warranty: 2 Years
Dimensions LxWxD (cm): 10.9 x 16.9 x 5.0
Recently I put the Opticron Adventurer T WP 8 x 32 through its paces. This neat little porro prism binocular greatly exceeded my expectations, based on its excellent price to performance ratio. But I was keen also to test drive its lower power sibling, the Opticron Adventurer T WP 6.5 x 32. So I ordered up a unit from Amazon and spent a couple of weeks using it in a variety of environments. Since many of the basic features on both the 8 x 32 and the 6.5 x 32 are identical, it provided a good opportunity to investigate a phenomenon known as depth of field, and the factors which might govern its behaviour, which I shall elaborate on shortly. For now, I want to briefly summarise my findings of the 6.5 x 32 in relation to the 8x glass in the same series.
Like the 8 x 32 model, the 6.5x glass showed excellent control of internal reflections, diffused light and diffraction spikes(i.e. none seen). It was extremely clean, as judged by my Iphone 7 torch test.
Collimation was good as tested under the stars and also by checking a horizontal electricity cable in the distance.
Having a look at the exit pupils, I noted only slight truncation in the left barrel, but in general, the results were very good:
The first big surprise for me was its much superior glare suppression compared with the 8 x 32 model. For some reason that still escapes me, the 6.5x produced noticeably higher contrast images than its 8x sibling. I can only surmise that newer coatings were applied to these units at some point. Veiling glare was also much better controlled in the 6.5x unit. Eye relief is good: I was able to image the entire field with my glasses by rolling down the rubber eyecups, but it’s a fairly tight squeeze!
Close focus was measured at just over 2.5m, considerably better than the advertised 3m setting. The 6.5 x 32 delivers a huge field of view – fully 9.2 angular degrees! The sweet spot is quite large too, remaining very sharp in the inner 60 per cent or so of the field .After that, mild field curvature sets in, becoming progressively more severe as one approaches the field edges. To my eye, about 75 per cent of the field was acceptably sharp, with more pronounced blurring occurring in the last 25 per cent before hitting the field stops. Sometimes I would notice a ‘fish bowl’ effect while panning large swathes of landscape. The image is very bright; noticeably brighter than the 8x glass in fact, especially in low light conditions, at and after sunset. Colour correction is excellent in the centre, but does show a bit of lateral colour as the eyes are moved off axis, but it was no more than I’ve seen in instruments costing ten times its retail cost. In terms of colour balance, I judged the image as quite neutral.
The image through the Adventurer T WP 6.5 x 32 is very stable and quite immersive. I can easily understand why an instrument like this would be ideal for a younger individual or an older observer wishing to minimise image shake while glassing. For me though, I felt the 6.5x lacked those little details I’ve come to pick up more easily in 8x and 10x instruments. In other words, it lacked a little bit of reach. But that’s an entirely personal judgement and your mileage may vary.
Depth of Field & Stereoptic Comparisons Between the 8 x 32 and 6.5 x 32 Models
Comparing the two instruments on an open landscape in bright sunlight, I judged the image plasticity(3D effect) to be noticeably more pronounced in the 8x model. This was in keeping with my previous study on stereopsis which can be seen in this link. Indeed, the only two factors which influence image plasticity are the IPD, the separation between the objectives and the magnification, increasing linearly as these variables increase.
The opposite was true when I made some depth of field measurements, that is, when focused at infinity, how close could I keep an object focused sharply in the foreground. Borrowing my son’s laser rangefinder, and being careful to only image objects in the centre of the field to avoid the spurious effects of field curvature creeping in at the bottom of the field(which gives the impression of tightening up the focus at closer ranges), I measured the close focus at infinity of the 6.5x glass to be 33.9 yards, while that of the 8x glass gave a result of 44 yards.
So depth of field increases as magnification decreases. The question remains though, how does magnification scale with this phenomenon? To what power? And do any other factors determine the outcome?
It would be nice to know.
This is a rather complex and interesting question for sure, but I did find a reliable source that could give me a head start. Way back in 2004, a German professor of computational physics, Dr Holger Merlitz, based at the Leibniz Institute for Polymer Research, Dresden, posted an interesting communication in Cloudynights Binocular forum, where he adopted a very interesting quantitative approach to this question. I will quote the relevant part here for interest:
Your results on DOF for a binocular is in agreement with whatever I was able to figure out so far. In fact, magnification and (effective) exit pupil appear to be the dominating parameters. Here, ‘effective’ means the smaller of both, the observer’s eye-pupils and the exit pupil. I must admit that not all aspects are clear to me. The following approach to analyse this problem was suggested by Walter E. Schoen on a German discussion board:
The thin-lens equation
1/F = 1/G + 1/B
relates the distance of the object to be observed (G) with the focal length (F) and the distance of its image (B). A telescope is essentially made of two lenses, and the above relation is valid for both of them, the objective, and the ocular, for which we shall write
1/f = 1/b + 1/g
Now we assume that the binocular is focused to infinity. This means that the ocular is positioned in a way that the focal plane of the objective is on top of the focal plane of the ocular. Each object with large distance produces a sharp image in this particular plane, and the image ‘B’ of the objective coincides with the object ‘g’ of the ocular. Now we assume the object is coming closer. Its image ‘B’ is therefore shifting away from this plane, and since we keep the telescope focused on infinity, the ocular’s image ‘b’ of the ‘object ‘B’ becomes unsharp. One approach is to calculate the distance, to which the eye has to focus in order to get this image ‘b’ back into focus. The reciprocal value of this distance is the diopter-value the eye has to accommodate. With some arithmetic, and using V = F/f (magnification) and f+F = g+B = distance between objective and ocular one can obtain
b = G/Vˆ2 – f – f/V
(actually, when I tried to verify this relation, I got the opposite sign, but, being no professional, I may have messed up some conventions used for optical computations).
The result he got seemed to suggest that the main factor determining depth of field is indeed magnification. However, Dr Merlitz didn’t flesh out the details of how he arrived at this result.
Trust but verify.
So I had a go this afternoon and was able to derive the same formula, the details of which are reproduced below in my own handwriting:
So the result appears to indicate that depth of field in binoculars scales as 1/v^2, and this appears to be the predominant factor determining this effect. Incidentally, it also agrees with the findings in an article published on binoculars on Wikipedia, though no reference is given, and I always take such sources with a pinch of salt:
With increasing magnification the depth of field – the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image – decreases. The depth of field reduces quadratic with the magnification, so compared to 7× binoculars, 10× binoculars offer about half (7² ÷ 10² = 0.49) the depth of field. However, not related to the binoculars optical system, the user perceived practical depth of field or depth of acceptable view performance is also dependent on the accommodation ability (accommodation ability varies from person to person and decreases significantly with age) and light conditions dependent effective pupil size or diameter of the user’s eyes.
Thus, the increase in the depth of field of the 6.5x glass compared with the 8x instrument should be about 8^2/6.5^2 or 1.51. Comparing this result to the numbers I measured, I get 44^2/33.9^2 = 1.68.
Not bad at all!
Of course, other inter-individual factors may also contribute to greater or less perceived depth of field, when two different binoculars of the same magnification are employed, such as accommodation, field curvature, or the size of the exit pupil etc.
Conclusions & Recommendations
The Opticron Adventurer T WP 6.5 x 32 is an excellent bargain for the rock bottom price paid. In keeping with the results reported by the reviewer showcased in the preamble above, it performs very well indeed, and should delight the owner with sharp, contrast-rich details in a very impressive and immersive field of view. Its minimum IPD of 53mm will make it especially attractive to those who have smaller faces, and the ultra-stable views at 6.5x will likely delight individuals who suffer from significant handshake.
Dr Neil English is the author of seven books in amateur and professional astronomy. His 8th title, Choosing Binoculars: A Guide for Stargazers, Birders and Outdoor Enthusiasts, will be published sometime in late 2023 by Springer Nature.