The basic design of the refracting telescope is fairly simple. There are no mirrors used in refracting telescopes, as they employ the use of both an objective lens and an eyepiece. The objective lens is used to refract or bend light so that it converges on a focus point or plane. An eyepiece is used to assist the eye in gathering light.
What differing types of refracting telescopes exist?
Galilean Telescope - This type of telescope was invented in 1609 by, you guessed it, Galileo. Though this particular design did help Galileo view the some of the objects in our solar system, it did have some flaws. The flaw rested within the eyepiece, which caused images to appear distorted. This type of telescope is very much obsolete, but Galileo still achieved simply by being one of the first to create such a wonderful piece of technology.
Keplerian Telescope - Invented by Johannes Kepler in 1611, he employed the same principles as Galileo did, but with one key difference. Rather than using a concave eyepiece like Galileo, he employed a convex eyepiece. The downside to this design was that the image being viewed was inverted (upside-down). He did, however achieve a much clearer image and a wider field of view.
Achromatic Refractors - The key difference in an Achromatic Refractor is the use of an objective lens consisting of two separate pieces of glass. These are used to bring two wavelengths, blue and red, into focus on the same plane. These telescopes are widely available today for beginners and amateurs.
Apochromatic Refractors - Following the similar objective design as the Achromatic Refractor, the Apochromatic Refractor uses an objective lens able to bring three wavelengths (red, blue and green) into focus on the same plane. These telescopes are built with extra low dispersion materials to free them from chromatic aberration. These telescopes are geared more towards the more experienced astronomer, but are still relatively simple in usage.
Buy Phoenix - Lens - 500 mm - f/8.0 Mirror - Canon EF Order Today!
Design by : Phoenix
Over All Rating Reviews : Great Deal :
Date Created : Feb 02, 2011 08:21:11
Kit includes: ♦ 1) Phoenix 500mm f/8 Mirror Lens ♦ 2) Phoenix 2x Teleconverter ♦ 3) Precision Design T-Mount for Canon EOS (Autofocus) ♦ 4) Rear Lens Cap
Phoenix 500mm f/8 Mirror Lens includes: Filter Set (ND-2X, ND-4X, Skylight); Soft Pouch; Front and Rear Caps; Instruction Manual; Phoenix USA Warranty
This super-telephoto 500mm mirror lens and 2x teleconverter are incredibly compact and lightweight. Using multi-coated optics and an advanced reflex design, they're ideally suited for wildlife, nature and sports photography. The included 2x teleconverter will double the focal length to 1000mm, equivalent to the magnification a 20x high-power binocular and for additional control, a set of 3 filters (Skylight, ND-2X and ND-4X) are also supplied. Macro focusing capability provides an image to subject ratio of 1:2.7 and with the 2X teleconverter, incredibly detailed, almost life-size (1:1.35) images can be obtained. Designed to be easy to use, hold and transport, this super-telephoto lens kit offers incredible magnification and value.
Will mount to ANY Canon EOS AutoFocus Film or Canon Digital SLR Camera Including: Rebel X, Rebel XS, Rebel G, Rebel GII, Rebel 2000, Rebel Ti, Rebel T2, Rebel K2, Rebel S, Elan 7, Elan 7E, Elan 7n, Elan 7ne, EOS 1V, EOS 3, EOS 1Ds Mark II III, EOS 1D Mark II III, 10D, 20D, 30D, 40D, 5D, Digital Rebel (300D), Digital Rebel XT (350D), Digital Rebel XTi (400D), & Digital Rebel XSi (450D), Rebel XS (1000D) etc.
Buy Phoenix 1000mm f/8 Mirror Lens with Mount for Canon EOS 30D 20D 5D Digital Rebel XT XTI & Film SLR Cameras Order Today!
Design by : Phoenix
Over All Rating Reviews : Great Deal :
Date Created : Nov 01, 2010 11:17:08
Kit includes: ♦ 1) Phoenix 500mm f/8 Mirror Lens ♦ 2) Phoenix 2x Teleconverter ♦ 3) Precision Design T-Mount for Canon EOS (Autofocus) ♦ 4) Rear Lens Cap
Phoenix 500mm f/8 Mirror Lens includes: Filter Set (ND-2X, ND-4X, Skylight); Soft Pouch; Front and Rear Caps; Instruction Manual; Phoenix USA Warranty
This super-telephoto 500mm mirror lens and 2x teleconverter are incredibly compact and lightweight. Using multi-coated optics and an advanced reflex design, they're ideally suited for wildlife, nature and sports photography. The included 2x teleconverter will double the focal length to 1000mm, equivalent to the magnification a 20x high-power binocular and for additional control, a set of 3 filters (Skylight, ND-2X and ND-4X) are also supplied. Macro focusing capability provides an image to subject ratio of 1:2.7 and with the 2X teleconverter, incredibly detailed, almost life-size (1:1.35) images can be obtained. Designed to be easy to use, hold and transport, this super-telephoto lens kit offers incredible magnification and value.
Will mount to ANY Canon EOS AutoFocus Film or Canon Digital SLR Camera Including: Rebel X, Rebel XS, Rebel G, Rebel GII, Rebel 2000, Rebel Ti, Rebel T2, Rebel K2, Rebel S, Elan 7, Elan 7E, Elan 7n, Elan 7ne, EOS 1V, EOS 3, EOS 1Ds Mark II III, EOS 1D Mark II III, 10D, 20D, 30D, 40D, 5D, Digital Rebel (300D), Digital Rebel XT (350D), Digital Rebel XTi (400D), & Digital Rebel XSi (450D), Rebel XS (1000D) etc.
This is a catadioptric lens. That means that it is very similar to a reflex, or mirror, telescope, and that it mixes mirror elements with refractive elements. The light comes in the front of the lens, goes right to the back of the lens, where it hits a large (full lens circumference) concave mirror, which then focuses all that light more tightly back in the direction it came. In the middle of the business end of the lens is another mirror, which then sends the light down the center of the lens back toward the camera, where standard refractive optical elements further magnify and focus the incoming light prior to hitting the film or sensor. A lens of this type is not only similar to a reflector telescope, it can actually perform as one, and you can find eyepiece attachments for just that purpose.
One of the significant benefits of a catadioptric lens is that you get less chromatic aberration, and so with subjects like the moon, which you expect to be grey, you actually get a grey image. With a standard refractor telephoto, you're quite likely to get colored fringes on sharp bright/dark boundaries - this is an area where reflectors can and do excel, and this lens is no exception.
Here's why: Light bends when it crosses the boundary between materials of two different densities. With normal lenses, light goes from air, into the material of the lens, and back into the air. So it bends twice. But light of differing colors bends slightly differently, and so the more bending (also known as refraction) goes on, the more likely you are to see colored edges - typically at the blue or red end of the spectrum - or even both. But a reflector system is made with mirrored surfaces, and light doesn't enter these mirrors, it just bounces right off the mirror surface, and it bounces at the same angle no matter what color it is - hence, there is no difference in the path light of various colors follows. This is why a well made reflector system doesn't present chromatic aberration, or CA, which is just a fancy term for what I just described to you.
Something else to keep in mind is that the background blur, or bokeh, of a catadioptric lens is donut shaped, instead of spherical - this is because of the light blockage at the center of the lens by the second mirror. That in turn means that you won't be shooting many images with creative depth of field with this lens; either the subject matter is in focus and the rest, if anything, irrelevant to the shot, or you probably won't be happy. Shooting distant things with no foreground like the sky or scenery on the skyline? No problem. Try a portrait with out-of-focus point light sources... ouch. Donuts! Just a word to the wise.
This particular lens has a screw type fitting that is not, by itself, compatible with the Canon mount. Consequently, there is a screw-to-Canon adapter supplied as part of this kit as well.
The lens sports a 500mm focal length, so you're looking at about a 10x magnification at the rated f/8 fixed aperture. But the kit comes with a physically separate 2x magnification element, which effectively increases the focal length to 1000mm and about 20x magnification, although of course this cuts the amount of light considerably, raising the lens' effective f-stop and requiring either a slower shutter speed or a higher ASA to get the same relative exposure for any given scene.
You don't have to use the 2x adapter; you can attach the screw-to-Canon adapter directly to the Phoenix lens and use it as 500mm, f/8. The only downside to this is that it takes a physical act to change from one mode to the other, one that exposes both the camera and the lens to dust and other environmental gremlins. As the focal length is fixed, I strongly suggest you think carefully about what you're going to shoot before you shoot it so that you don't have to fool with switching the 2x adapter in and out in the field.
The quality of the 2x adapter - or at least, lets say the quality of photos taken with the 2x adapter attached to the Phoenix - is, in my opinion, unsatisfactory for any use. The Phoenix by itself seems fine, so I think we can safely point the finger at the 2x adapter.
The lens is manual focus, focusing is smooth and positive, which was a relief to me after purchasing a much more expensive lens (0) recently and running into some unexpected and entirely unwelcome backlash problems. The focusing ring is broad and well textured, providing a positive grip that you'll be thankful for when you're trying to get that last little bit of focus accuracy. The Phoenix is a very, very sharp lens for the money and the focal length; when you get the focus exactly right, I think you'll be delighted with the degree of telescopic detail achieved. I know I am.
Although you do have to stand off from your subject a bit, this lens can also serve as a perfectly reasonable macro lens. With the 2x adapter, you can get 1.35 macro shots on a film camera, and with a digital camera that uses a c-size sensor, you add an additional 1.5x so you're really getting close to a 1:1 full macro at that point. Again, here is an area where the catadioptric lens ability to basically do away with chromatic aberration serves to get you a considerably better image overall, and the lens' overall sharpness contributes a lot (as long as the object has a very flat focal plane, of course.)
There is visible, though not particularly offensive, vignetting that occurs around the center of the image. Sometimes this can work for you; so much so that it is a common effect you can add in many image processing packages. But there are times you won't want it, and with this lens, that means using post-processing to remove it, not impossible, but not trivial, either.
The lens is moderately heavy at 360 grams (12.7 oz), and with the approximate one and half inch 2x doubling tube attached, it sticks out pretty far so there's some leverage working on you along with the weight. It isn't so much that a standard tripod is overwhelmed, but if you're going to be shooting hand-held (presumably with a lot of light!) this thing will get your wrists tired in short order, no question about it.
One thing about any fixed f-stop lens, and this lens being no exception, is that the depth of field is limited. There isn't much you can do about it. For instance, at a distance of 12 feet, the DOF of this lens is about 1/4 inch. This has a number of consequences. The obvious one is the type of subject matter you will want to shoot with this lens will be limited to that which has a relatively shallow depth of field compared to the distance you'll be shooting at. Another, perhaps not immediately obvious consequence is that focus adjustment requires you to take great care or you'll find the focal plane is not collinear with your subject matter. In my case, I'm using the lens with a Canon EOS 40D, which provides for a live LCD view which can zoom the FOV in for a 1:1 pixel (LCD:Sensor) view; this allows me to see precisely how well I'm focused (or not.) In this sense, this lens is very well-suited to my camera (or vice versa.) If you are limited to a TTL viewfinder, you may find that focusing this lens perfectly is quite challenging. The lack of backlash helps, but even so, a very slight rotation of the focusing ring does move the focal plane a considerable distance. No question about it, this is one of the tricky areas for this type of lens.
As a manual focus lens and a fixed f-stop lens, it doesn't really tell the camera anything about itself. Your aperture readout will probably show a clueless 0.0 value; don't worry about it too much. If you set the camera to aperture priority, the camera will set the shutter speed appropriately and you'll get a properly exposed image. Or you can use full manual and tweak till you find what you're looking for. Given that shutter speed affects the blur of motion in an image as well as the available exposure time, and ASA (if your camera provides for adjustable ASA) affects the exposure time and the graininess and noise of the image, it may take some experimentation before you find the optimum combination of ASA and shutter speed that works best for your particular camera. Modern digital cameras have such large megapixel counts that in many cases, you can reduce noise after the fact by trading in some resolution; for instance, if you have a 10 mp camera like my EOS 40D, you can cut the resolution by half in both dimensions (proportionally) and this results in the light values of four adjacent pixels being combined into one new one in a final 2.5 mp image, which will serve to average out a great deal of sensor noise - at the cost of some detail, of course. In the case of a not perfectly focused image, though, which I think you'll find you take quite often with this lens because the DOF is so limited, there is little down side to such a reduction in pixel dimensions - and remember, noise occurs in an image regardless of focus because it comes from the sensor, not the image.
The kit comes with several filters which need to be mounted internally; they're more for film cameras which have fewer options for easily managing light (for instance, they can't adjust ASA without actually changing a film cannister.) They're quite inconvenient as they mount inside the lens at the adapter end, and so I have not even tried to use them. Nor do I really see a huge need for them in my situation. The only exception is related to the fact that filters can increase contrast as they block out ranges of light which may be obliterating certain features by virtue (or lack thereof) its abundance. So your mileage may vary; the filters are there if you choose to use them.
Last but not least, the lens comes with all the caps you need to keep dust off it, as well as a handy soft bag you can use to protect the fine surface features. It's a typical soft bag and it won't serve to protect the lens if, for instance, you were so unfortunate as to drop it. For that, you need a camera bag. As with all lenses, this baby is delicate, and the very first thing you should invest in is a bag that will keep the slings and arrows of outrageous fortune from lodging in your lens collection.
In summary, there can be no question that this is a special purpose lens, but when you are shooting distant objects on the ground or in the sky, it really comes into its own. It is all metal, at least on the outside, and feels very solid and frankly, it also feels expensive. Given the price, which is more than reasonable, and the build quality, which in my opinion is quite high, I think almost anyone who likes to really experiment with different types of images should have this lens in their kit bag. I recommend it enthusiastically and with great cheer. I'm very happy to have it myself, and I'd immediately and without any question replace it with the same model if something unfortunate happened to mine.
You can't put 500mm of telephoto capability in your camera bag at anywhere near this price point using standard lens designs; if you are willing to put out more money, you can get a better result, no question about it, but this is still a terrific way to start with telephoto photography.
In the next few days and evenings, I'm going to prepare some sample images taken with this lens specifically to go along with this review; please check the images section if you'd like to see them.
Physics: Introduction to the optics of lenses and mirrors. Concave, convex, converging, diverging; real, virtual; upright, inverted, magnified, shrunk. Sign conventions for focal length, image distance, object distance, magnification. The lens/mirror equation; the magnification equation. Introduction to ray tracing. This is arecording of a tutoring session, posted with the student's permission. These videos are offered on a "pay-what-you-like" basis. You can pay for the use of the videos at my website: www.freelance-teacher.com I offer tutoring via Skype. For more information, go to my website. For printable documents containing the "handout" and problems discussed in this video series, go to my website. For a list of all the available video series, arranged in suggested viewing order, go to my website. For a playlist containing all the videos in this series, click here: www.youtube.com (1) The lens/mirror equation. Focal length distance, object distance. Convex, concave, diverging, converging (2) Image distance. Real, virtual (3) Continued (4) Continued. Upright, inverted (5) Continued. Magnification equation (6) Continued. The lens/mirror chart (7) A problem (8) Continued. Ray tracing (9) Continued (10) Another problem (11) Continued
The basic design of the microscope has not changed that much over time. They have evolved, but the basic concept is still the same. There are several key parts that many types of microscopes have in common. All of the parts of a microscope must function properly for the microscope to work well. If one part is substandard, it can render the microscope useless. The major parts of a microscope are the lenses, the arm, the tube, the illuminator, the stage, and the adjustment knobs.
There are two kinds of lenses on a microscope. The eyepiece lens, also known as the ocular lens is at the top of the microscope. This is the part that people look through. The ocular lens is not adjustable on most models. The objective lens provides much of the microscope's magnification. A microscope usually has a few different objective lenses that vary in strength. The objective lenses are contained on a circular part placed between the eyepiece and the stage. Different objective lenses are chosen based on their strength. When someone wants to use a different strength of objective lens, they turn the circular disk to put another lens over the stage.
Other than the lenses, the other parts of a microscope are the tube, the arm, the stage, the illuminator and the adjustment knobs. The tube connects the ocular lens and the objective lens. People look through the ocular lens and tube and see out of the objective lens at the bottom. The arm connects the lenses and the stage. It protrudes to the side and provides a handle to carry the microscope as well. The stage is where the object is placed for examination. Stage clamps secure the microscope slides to the stage. The microscope slides contain specimens such as blood or other liquids. The illuminator is below the stage. This part provides light to make the specimen easier to see. The illuminator is either an actual light or a mirror.
Most microscopes feature two adjustment knobs to help focus the lenses. The coarse adjustment knob is the larger of the two and brings the lens and the stage closer together. The fine adjustment knob is smaller and is used after the coarse adjustment knob to provide any small adjustments to bring the item into sharp focus.
These parts of a microscope are common to nearly all models. Some microscopes use slightly different parts. For example, electron microscopes use electron beams instead of illuminators.
Getting used to inserting and removing contact lenses requires patience and practice until the process can be easily mastered. Initially, you will have the opportunity to both insert and remove your contact lenses whilst having them fitted at your opticians. However, when faced with this task without the support of the optician and his staff, the process may seem, at first, a little difficult but can be overcome if you persist.
When attempting to insert your contacts for the first time you may become frustrated as it can take a certain knack to be successful. In spite of your initial failures it is important to persevere and you will find that with each attempt the whole procedure will become easier and easier. Remember that very few are able to master the art of inserting and removing their contact lenses immediately so try to relax and be patient. To help you get started the information below should help as it contains some tried and tested ways to make life easier for you in this respect.
The first thing to remember before attempting to insert contact lenses is that your hands should be scrupulously clean and dry. You should then place the contact lens on the tip of your index finger in a concave position. Apply a little solution to the middle part of the contact lens. Using your thumb, pull down the lower lid of the eye into which you will be inserting you contact lens and look up.
You are now in a position to insert your contact lens that is sitting on the tip of your index finger. Avoid the impulse to blink but allow the contact lens to adjust to the eye, after a few seconds you may close your eye and your contact lens should feel secure and comfortable. If not, repeat the process until you are happy with the way the contact lens feels in your eye.
Removing contact lenses requires the same dedication to hygiene, so it is important to make sure that your hands are clean and dry. Remove the contact lens by using your thumb and forefinger, grab the sides of the lens and remove from the eye. This procedure can usually be accomplished instantly but also requires a degree of practice. To help lubricate your eyes and assist the process it may help to apply some eye drops.
After a few weeks you will find that inserting and removing your contact lenses has become easy and you shouldn't experience any difficulties. It is simply a matter of technique. Once you get the hang of it you will begin to enjoy the benefits that contact lenses offer as they are certainly a wonderful aid to correcting your vision.
