Pentax 62593 8×25 DCF SW Binocular – Black

The Pentax 62593 8×25 DCF SW Binocular – Black
is the most recent release in the Pentax DCF series (launched in February 2008) and is very similar to the Pentax 62594 10×25 DCF SW Binocular, with dual hinge and all, but slighty heavier at 13.6 ounces. It actually has the same dimensions (1.6 x 4.1 x 4.3 inches). It trades at a lower price, also available at a huge discount at Amazon.

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Pentax 62594 10×25 DCF SW Binocular – Black

With dimensions of 1.6 x 4.1 x 4.3 inches and weight of only 10.6 ounces, the PENTAX DCF SW 10×25 binocular is indeed ultra compact and a true pocket binocular. It also has a distinctive advantage with the dual hinge design, making it exceptionally portable (can fold up closer than any roof prism pocket with only one hinge).
Lens coatings: Fully multi-coated lenses increase light transmission and color quality.  Weatherproofing: Waterproof for up to 3.3 feet submersion and fogproof (nitrogen-filled).
Eyepieces: Helicoid eyepieces ring with four click stops offer the most comfortable eye relief. The exceptionally long eye-relief (20 millimeters!!) allows for viewing with or without glasses.
Diopter: The click-stop adjustment locks in the right eyepiece diopter.
Ruggedness: The rubber-covered exterior provides a sure grip and protects the internal mechanisms.
Like the Pentax 62599 9×28 DCF LV Binocular – Black the Pentax 62594 10×25 DCF SW Binocular is also trading at a huge discount at Amazon.

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Pentax 62599 9×28 DCF LV Binocular – Black

Although the objective lenses of the the PENTAX 62599 9×28 DCF LV binoculars are slightly bigger (28 mm) than what could expect from a pocket binocular, with dimensions of 1.8 x 4.5 x 4.6 inches  and weight of only 13 ounces it will definitely fit into a jacket pocket.
Optics: Coatings: High-resolution phase-coated and super-reflective coated roof prisms for high contrast images. The multi-coated lenses’ increase light transmission and color quality.
Weatherproofing: Waterproof for up to 3.3 feet submersion and fogproof (nitrogen-filled). Eyepieces: Helicoid eyepiece rings offer comfortable eye relief.
Ruggedness: Rubber covered exterior provides a sure grip and protects the internal mechanisms.
The Pentax 62599 9×28 DCF LV Binocular – Black trades at a huge discount at Amazon and has received excellent customer reviews (5 out of 7 gives it a 5 stars rating).

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Image brightness in dim light

Three factors determine the brightness of the image in dim light: The quality of optical coatings, a factor determined by the size of the exit pupil and finally a factor determined by the objective lens size and magnification.

Lens coatings: Not all the light arriving at the objective lenses at the front actually passes through – some is reflected. This happens with the light finally arriving at the viewer’s eyes at the other end of the binoculars as well – not all the light travels through the final two lenses either. On top of this the other 10+ glass surfaces inside the instrument contributes to scattering of light as well. This is where optical coatings come into their own: They reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast. In this way lens coatings make a huge contribution towards a better image, in particular in dim light.

Size of exit pupil: The size of the final lens through which the image has to go is crucial, in particular as compared to the size of the human pupil. The smaller the exit pupil, the less light passes through. In bright daylight this is no problem at all, but in dim light, when the human eye needs as much light as possible to see an image clearly, the situation changes altogether. The ability of a binocular to transmit light in terms of the exit pupil is expressed as its performance on the relative brightness index (RBI). It is computed by squaring the exit pupil. For example, 7×35 binoculars have a 5mm exit pupil (35/7=5). So their RBI is 25 (5×5=25). A RBI of 25 or greater is considered good for use in dim light.

Objective lens size and magnification: Some manufacturers claim that a combination of the magnification and objective lens size makes a greater contribution towards image brightness in dim light. A factor, known as the twilight factor, is calculated by taking the square root of the product of the aperture and magnification (aperture x magnification). According to this approach a binocular with 10×40 specifications (twilight factor: 20) will render a clearer image than a 7×35 (twilight factor: 15.4), even though it has a smaller exit pupil (4 mm) than the 7×35 (5 mm).

A final remark on brightness of image in dim light: The bigger the objective lenses, the more light is gathered and the better the chances of getting a brighter image. For this reason binoculars used for star gazing have really big objective lenses. For the same reason pocket binoculars with their small apertures will not be as effective in dim light as normal size binoculars.

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Depth of field

The “depth of field” of a binocular refers to its ability to keep objects further towards the background and closer towards the foreground in focus without you having to adjust the focus. Stated in another way: Once you’ve focused on an object in the distance, how far behind this object (in other words, further away into the background) and how far in front of the object (in other words, closer to you, the viewer) is still in focus, without you having to adjust the focus. A special category of binoculars with super depth of field features is the so-called “auto focus” binoculars. They always stay in focus from 10 meter away to infinity.

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Focus speed

If you need the binoculars for watching wild animals, in particular birds, the speed of focus is important. While following a bird in flight, you need to change the focus all the time if the bird gets closer or further, and if your binoculars do not allow for quick adjustment, you will struggle all the way.

Obviously, this problem does not exist in the case of “auto focus” binoculars: They stay in focus all the time, from infinity to about 10 meters away. The bird can fly as fast as he can, straight at you, and it will stay in focus. Very convenient.

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Close focus

Focusing at a distance is no problem for any pair of binoculars; focusing at close range, however, is a totally different matter. This is important, since sometimes you will be in a situation where close focus is important, in particular if you start getting into birding or even watching butterflies and you would like to appreciate minute details. Clkose focus is the domain of compact binoculars and pocket binoculars in particular, although not all of them specialize in this feature. Some pocket binoculars can focus as close as 1 meter, but the ultimate close focus binocular is the Pentax papilio (see links elsewhere on this website), which can focus as close as 46 centimeter, 18 inches.

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Field of view

Another set of technical detail which is imprinted on the binoculars, could be something like this: 367 ft. at 1000 yds. This is the field of view (FOV), the width of the view at the particular distance. In this case 367 feet at 1000 yards. Could be designated in meters or in degree. Field-of-view is determined by magnification and eyepiece lenses. It is not true that binoculars with small objective lenses like pocket binoculars will inevitably have a narrow FOV. This can be addressed by the way the optics inside the eyepieces have been designed. Example: The Swarovski Optiks Pocket Binoculars 8×20 are very small pocket binoculars (dimensions: 3.98 x 1.5 x 2.3 inches) and have an aperture of only 20 mm and power of 8, but an excellent FOV of 345 feet @ 1,000 yards. Compare this to the FOV of 330 feet of the full-size NIKON 7294 Monarch ATB 8×42 Binocular, with 8×42 specifications.

FOV can also be expressed as degrees, e.g. 5.6 degrees, or 6.7 degrees.

Finally: FOV is handy, but not crucial, except in the case of young children, who still struggle to find and follow moving objects with a binocular.

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Coating of lenses

Not all the light arriving at the objective lenses at the front actually passes through – some is reflected. This happens with the light finally arriving at the viewer’s eyes at the other end of the binoculars as well – not all the light travels through the final two lenses either. On top of this the other 10+ glass surfaces inside the instrument contributes to scattering of light as well. Anti-reflective optical coatings reduce internal light loss and glare and ensure even light transmission, resulting in greater image sharpness and contrast.

These coating differ substantially in quality: “Coated” simply means a single layer anti-reflection coating on the two objective lenses; “Fully coated” means that all air-to-glass surfaces are coated, whereas the ideal coating, “fully multi-coated” means all air-to-glass surfaces have multiple layers of coats.

In the case of roof prism binoculars the lenses also have to be “phase-corrected” (PC) to ensure that the loss of image brightness and contrast is corrected.

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Optical features of a binocular

All binoculars have the same set of optical features, whether small, normal size or big. These include lens coatings, close focus ability, field of view (FOV), focus speed, depth of view and ability to produce a bright image in dim light. These will be discussed below.

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