- It's the opening in a lens through which light passes to enter the camera.
- You can shrink or enlarge the size of the aperture to allow more or less light to reach your camera sensor.
- It affects the depth of field.
- It also affects the exposure of your images by making them brighter or darker.
- e.g. aperture of f/1.4
- wide/large aperture
- a lot of light passing through the lens = bright image
- thin/shallow depth of field (focused object looks sharp, everything else looks blurry)
- e.g. aperture to f/22
- small aperture
- not a lot of light passing through the lens = dark image
- large depth of field (both near and far objects look relatively sharp)
- Depth of field is the distance between the closest and farthest objects in a photo that appears acceptably sharp.
- Factors that determine the depth of field:
- the aperture
- zoom (by moving further away from the subject, and zomming in)
- Inside diameter of the objective lens frame.
- With the binoculars designated with a numerical formula 8x42 7.0°, 42mm is the effective diameter of the objective lens.
- Given the same magnification, the larger the objective diameter, the greater the light-collecting power.
- A larger diameter results in higher resolution and a brighter image. However, large-diameter objective lenses make binoculars bigger and heavier.
- The diameter of the objective lens divided by the magnification -- determines the amount of light available to your eye. The larger the exit pupil, the brighter the image obtained will be.
- e.g. 3mm exit pupil
- generally sufficient for daytime use
- e.g. 5mm
- suitable for daytime and twilight
- e.g. 7mm
- generally necessary for maximum nighttime or astronomy use.
- This means that the view that is seen is either 8 times as large as with the naked eye.
- Image brightness decreases with higher magnifications.
- This will determine how well you can distinguish color and detail.
- The basic description of photographic lens in milimeters, but it doesn't refer to the physical length of the lens.
- It's actually a calculation of the optical length as measured from the lens (the point where the light-rays converge, the last/posterior glass piece in the lens), to the image sensor (the point where the image is in focus on the focal plane of the camera, where your eye sees the image being captured, whether an LCD screen or the sensor) when the subject is in focus.
- Describes two basic attributes of the lens:
- Angle of view: how much of the scene will be captured
- Magnification: how large individual elements in the image will be
- Effects:
- The shorter the focal length (e.g. 18mm), the wider the angle of view and the lower the magnification.
- The longer the focal length (e.g. 55mm), the smaller/narrower the angle of view and the higher the magnification.
- e.g. a zoom lens of 18–55 mm
- minimum focal length: 18mm
- maximum focal length: 55mm
- Informs us as to how much of a scene a lens can see; how much of the scene will be captured by the image sensor.
- It’s a more informative benchmark regarding what you can photograph with the specific camera and lens you’re using.
- The narrower field of view, the further you will have to move away from your subject to capture an image that’s roughly similar to one captured with the same lens on a full frame camera (i.e. without a crop factor).
- It changes as two factors change
- focal length of the lens
- the size of the sensor (or size of film) your camera uses
- On a full frame camera, a 50mm fixed lens has a 47-degree field of view. That 47-degree field of view is roughly what we see with our own eyes.
- On a camera with a crop factor of 1.6x, the field of view lessens to about 30-degrees (47 / 1.6 = ~30)
- e.g. 47-degree field of view (diagonal)
- There are two things to consider with a wide-angle lens:
- With a wider view, you are required to get closer to your subject if you want to fill the frame. If you don’t, there will be more background included in the shot.
- Wide-angle lenses have the appearance of more depth of field - that is, the area of the image that’s in sharp focus - than longer focal length lenses. As a result, the background (which, as noted above, there is more of) is more in focus).
- All binoculars are labeled with numbers delineating their size and magnification.
- These numbers might look like 8x42 (read as 8 by 42).
- The 8 in this example is the power or magnification.
- The 42 is the diameter (in millimeters) of the objective lens, the end through which light enters the binoculars
- The larger the objective diameter, the more light there is gathered to transmit to the eye. This translates to greater potential detail and greater color resolution.
- 8x magnification
- Porro prism
- Features BaK-4 prisms
- Linear Field of View (@1000 yds) / @1000 m): 305 ft (102 m).
- Objective lens diameter of 56 millimeters
- 7-millimeter exit pupil
- Orion SkyQuest XT8 Telescope
- SkyWatcher 8" Classic Dobsonian Telescope
- SkyLine 8" Dobsonian Reflector Telescope
- Apertura AD8 Dobsonian Telescope
- Zhumell Z8 Telescope
- Refractors
- Use lenses to bend the light and the focus
- Utilize specifically designed lenses to focus the light into an image
- Refractors are usually long relative to their size, as the light must flow in a straight path through the telescope tube to the eyepiece
- The larger the lens in the refracting telescope, the longer the optical tube has to be to bring the image into focus
- Variants:
- Doublet: when low-quality, suffer from Color Fringing (Chromatic Aberration, image with distinct colors on the edges)
- Triplet: no Color Fringing
- Reflectors
- Use mirrors that reflect light into focus
- This causes light to reflect at various angles within the optical tube, which extends the overall light path
- This makes Reflectors to be shorter (smaller) than Refractors of the same aperture (as the light doesn't need to go in a straight line to travel the same distance)
- Collimation (the process of mirror alignment/adjustment) may be required
- Variants:
- Dobsonian
- An altazimuth-mounted (rocker-box mount, simple left/right, up/down base mount) Newtonian telescope design popularized by John Dobson in 1965
- Newtonian
- Dobsonian
- Catadioptric (Compound Scopes)
- Choose a combination of both lenses and mirrors
- Schmidt-Cassegrain
- Maksutov-Cassegrain
- Choose a combination of both lenses and mirrors
Mirrors have a big advantage over lenses because all the wavelengths of light bounce off the mirror in exactly the same way but when light passes through a lens the blue light is bent slightly more than the red light and so the light doesn’t end up in exactly the same spot. This is why stars viewed through cheap refractors tend to blue halos around them. The geeks call this chromatic aberration.
The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English physicist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror.
- Spherical mirrors are easy to make but they are mostly rubbish
- Parabolic mirrors are harder to make but if done well they are fantastic