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Binoculars review

Delta Optical Discovery 8x40

Delta Optical Discovery 8x40
13 November 2021
Optyczne.pl 
Till June 2014 the cheapest series in the line-up of Delta Optical was called Voyager, and the second less expensive was the Entry series, with binoculars costing around 200-250 PLN. At the beginning of June 2014 the producer offered a new, cheap series of binoculars called Discovery. They feature Porro prisms made of Bk7 glass and a metal casing with a rubber armour. The new series is not supposed to replace any older one, it's just a kind of gap-filling line, fitting right between the Voyager and the Entry models.

Very good fields of view of the new binoculars are certainly worth your attention. For example the 8x40 model features a field of 8.2 degrees, the 10x50 specimen has a field of 6.6 degrees, the 12x50 instrument that of 5.5 degrees and the field of the16x50 model amounts to 4.1 degrees. As you see, in all cases the producers used eyepieces with an apparent field of view of 66 degrees.

The Discovery binoculars come with a 2-year guarantee period of the producer and in the box you get, along with the device: two separate caps for eyepieces and objectives, a strap, a case, and a cleaning cloth.

Pictures
  • Lornetka Delta Optical Discovery 8x40
  • Lornetka Delta Optical Discovery 8x40
  • Lornetka Delta Optical Discovery 8x40
  • Lornetka Delta Optical Discovery 8x40
Manufacturer data

Manufacturer:

Delta Optical
web site

Distribution / Sales:

Delta Optical
web site

Magnification Lens diameter Angular field of view Prisms Eye relief Weight Price
8 40 143/1000(8.2o) Bk7/Porro 18.3 mm 724 g 259 PLN
Results of the review
Real front lens diameter Left:   40.1+/- 0.05 mm
Right:  40.1+/- 0.05 mm
8 / 8.0 pkt
Real magnification 7.94+/- 0.05x 3/3.0
Transmission 75.7+/- 1% 8/25.0
Chromatic aberration Medium in the middle, high on the edge. 3.3/10.0
Astigmatism Sensible result. Stars are a bit sparking but it's not a huge problem. 5.5/10.0
Distortion The distance between the first curved line and the field centre compared to the field of vision radius: 40% ± 3% 4/10.0
Coma Appears in a distance of 50% of the field of view radius and is big on the edge. 2.8/10.0
Blurring at the edge of the FOV The blur occurs in a distance of 67% ± 3% from the field of view centre. 2/10.0
Darkening at the edge the FOV Significant. 2/5.0
Whiteness of the image A slanted transmission graph with the highest values for red light so images come with slight orange-red colouring. 3.3/5.0
Collimation Perfect. 5/5.0
Internal reflections
Left: Right:
Delta Optical Discovery 8x40 - Internal reflections - Left Delta Optical Discovery 8x40 - Internal reflections - Right
Without any disnticnt arcs or false pupils. Area around the exit pupils is overall brighter than the rest.
2.9/5.0
Housing A classic Porro padded with very decent rubber armour that doesn't have any unpleasant smell. Rubber is pleasing to the touch and slightly rough to make your grip even firmer.All looks very stylish but rubber next to objectives sticks out a bit and if you squeeze objectives a bit you hear squelching and cracking. Rubberized folded-down eyecups. Produced in China. 6/8.0
Focusing A big central wheel with a very good access – it moves evenly with an appropriate resistance. Its focus throw amounts to 550 degrees. The eyepieces move sideways and their work is slightly uneven. If you put a lot of pressure on the bridge the instrument might defocus slightly. Individual focusing moves the whole right eyepiece. 1.7/5.0
Tripod There is a comfortable access to a tripod socket. 3/3.0
Interpupilary distance from 56.4 to 73mm 4/6.0
Closest focusing distance 3.3 meters. 1/2.0
Eyepieces FOV Apparent field of view of 64.4 degrees (according to the simplified formula) and 58.7 degrees (according to the tangent formula). 13/20.0
Field of view Measured by us amounted to 8.11 ± 0.04 degrees and it was a tad narrower than stated in official specifications. A very wide field for this class of equipment. 7/8.0
Quality of the interior of the barrels Dark and mat tubes near objectives but without any ribs. If you look deeper they become shiny – it's obvious you see metal not covered by anything. Bottoms are grey and not covered by anything either. A lot of dark-grey glue near the prisms and a few specks of dust. 2.7/5.0
Vignetting
Left: Right:
Delta Optical Discovery 8x40 - Vignetting - Left Delta Optical Discovery 8x40 - Vignetting - Right
OL:2.20%, OR: 2.02%
5/8.0
Prisms quality Made of Bk7 glass but the quality is very poor. You can spot distinct 'diamonds'. OL: 15.3%, OR: 16.7% 0/8.0
Antireflection coatings Greenish on the objectives but only from the exterior.Inside there are no coatings. Green-blue on eyepieces but one air-to-glass surface is left uncovered. Prisms without any coatings. 2.5/5.0
Warranty [years] 2 2/6.0
Final result
51.4%
97.7 / 190 pkt
Econo result 0pkt.

Summary


Pros:
  • nice casing of sensible quality – especially at such a price point,
  • wide angle of view of eyepieces,
  • sensible correction of astigmatism,
  • affordable price.

Cons:
  • distinct coma,
  • significant chromatic aberration on the edge of the field,
  • weak image quality on the edge of the field,
  • small prisms made of Bk7 with noticeable brightness loss,
  • distinct light fall-off on the edge of the field of view,
  • very unstable eyepiece bridge.

If you only begin your adventure with observation equipment you might think that a paltry sum of money is more than enough for a pair of binoculars. People often comb different auction sites where there are many bargains starting with just several dozen PLN.

After looking at all these trashy binoculars (I don't hesitate to call them so) products of a more renowned manufacturer, offered for 200-300 PLN, seem really solid instruments, with good optical and mechanical quality.

Of course if you are a sensible person you might remember that 200-300 PLN is usually not enough to buy you a pair of good quality spectacles with frames. It would be foolish to expect that you can buy a good quality pair of binoculars for equal amount of money – after all even a simple optical system features7-8 elements in each tube and all of this is joined with a bridge and a focusing mechanism. It has to cost more.


Delta Optical Discovery 8x40 and Delta Optical Chase ED 8x42.

However, if there is such a market need, there will be always producers that will try to fullfil it, even if it means cutting costs as much as possible. Typically, in cheap binoculars you get an ordinary achromatic objective lens, Porro prisms made of inferior Bk7 glass, and 3-element Kellner or RKE eyepieces. Anti-reflection coatins are very simple and not all air-to-glass surfaces are covered by them.


X-ray photo of Delta Optical Discovery 8x40 binoculars. Phot. Obserwatorek

The Delta Optical Discovery 8x40 is exaclty such a product even though the x-ray photo, presented above, shows that its eyepiece is not a simple Keller as it features more elements. I suppose, a wide field of view forced the producers to employ a more complex optical system.

The list of cons, however, doesn't leave any doubts and is quite self-explanatory. You might add that the transmission values of both tubes are not the same and small wonder – in case of such a piece of equipment quality control is very limited and a difference of 5% in transmission between two tubes might be considered something perfectly acceptable.

What are the assets? The Delta Discovery 8x40 is an instrument made by a renowned producer, not just stuff sold by a flight-by-night, active mainly on different internet auction sites. A renowned producer means good technical support and an enforceable guarantee; also real parameters of the binoculars are in accordance with these officially stated. The Delta does feature an 8x magnificantion, the diameter of its objective lenses amounts to 40 mm, its field of view is over 8 degrees, and the eye relief distance reaches comfortable 18 mm. What's more, the product is physically a bit lighter than its official weight, always a positive thing– our scale showed 704 grams.

If you are really determined to buy a new product for less than 300 PLN, the Discovery series is certainly worth your attention as it might satisfy a not especially demanding beginner. Still, you should be aware of the fact that if you want to own a set of binoculars that will last several years of wear and tear, with a bit better mechanical and optical qualities, your spending budget has to be increased.