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Aperture is the first common photography term you should learn. Simply put, aperture is the size of the opening in the lens. Think of the lens as a window—large windows or wide angles let in more light, while small windows let in less light. A wide open aperture will let more light into the image for a brighter photo, while a smaller aperture lets in less light.

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The ISO determines how sensitive the camera is to light. For example, an ISO of 100 means the camera isn’t very sensitive—great for shooting in the daylight. An ISO 3200 means the camera is very sensitive to light, so you can use that higher ISO for getting shots in low light. The trade off is that images at high ISOs appear to be grainy and have less detail. ISO is balanced with aperture and shutter speed to get a proper exposure.

This article presents the applicational advantages as well as the functionality of object space telecentric lenses. Finally, it should be noted that image space telecentric lenses can also offer benefits. These are less sensitive to the exact position and location of the sensor in the camera. Furthermore, the radiometric effect described in the cos4 law is avoided: the intensity of the light will decrease from the center away to the image edge with the factor cos4(CRA), where CRA stands for the image space chief ray angle. For a telecentric lens, this angle is 0°, so the factor is 1. Provi­ded that the lens does not have mechanical vignetting in conjunction with the sensor used, no apparent drop in relative illumination will therefore be observed. A final advantage of image space telecentrics kicks in when using precise optical filters installed between the lens and the camera. These filters are usually designed for an angle of incidence of 0°. The more one deviates from this angle, the more the filter response shifts to shorter wavelengths. The filter transmission would thus not be uniform over the sensor area, which is usually not acceptable for precise measurements in e.g. fluorescence or hyperspectral imaging.

In the field of optical metrology, the factor of the lower sensitivity to the exact sensor position is usually the most important. However, in order to achieve the best possible measurement results, it is recommended to use bi-telecentric lenses, which combine the advantages of both concepts. ¢

14.04.2022 - How do telecentric lenses work? And how exactly do they differ from fixed focal length lenses? This article introduces the peculiarities and explains why telecentric models are predominantly used in optical metrology.

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Depth of field is a photography term that refers to how much of the image is in focus. The camera will focus on one distance, but there’s a range of distance in front and behind that point that stays sharp—that’s depth of field. Portraits often have a soft, unfocused background—this is a shallow depth of field. Landscapes, on the other hand, often have more of the image in focus—this is a large depth of field, with a big range of distance that stays sharp.

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A long exposure is an image that has been exposed for a long time or uses a long shutter speed. This technique is useful for shooting still objects in low light (used often by landscape photographers), or rendering moving objects into an artistic blur.

Similar to human vision, fixed focal length lenses (also referred to as entocentric lenses) have an angular field of view, resulting in objects further away being imaged smaller and vice versa. In a metrology setup, this property is undesirable because the measurement result, e.g. the diameter of a component, is not meant to change even if the test specimen is not positioned exactly but only within a certain tolerance range in front of the test system. In optical terminology, this phenomenon is referred to as parallax or perspective error, meaning the change in magnification depending on the working distance.

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If you’ve ever printed images before, you’ve probably noticed that an 8 x 10 usually crops from the original image. That’s due to aspect ratio. Aspect ratio is simply the ratio of the height to width. An 8 x 10 has an equal aspect ratio to a 4 x 5, but a 4 x 7 image is a bit wider. You can change the aspect ratio in your camera if you know how you’d like to print your image, or you can crop your photo when you edit it to the right ratio.

How is telecentricity quantified? In the data sheets, telecentricity is often specified as an angle, usually together with a wavelength. This angle describes the residual angle of the object space chief ray. Since the ideal target is 0°, these are usually very small angles, say in the range of < 0.1°. An exact 0° angle is usually never achieved, because the angle ultimately depends on the wavelength of the light and most lenses are designed for an entire range of wavelengths, not just a single wavelength. It is therefore necessary to find the best possible compromise between the residual angle and the desired wavelength range at which the lens is to be used. Figure 2 shows an example of data of a telecentric 1X lens where the angle of the chief ray relative to the optical axis is shown for three different wavelengths. The y-axis demonstrates the position of the chief ray on the sensor. By definition, the angle at the center of the image is 0° – here is where the chief ray coincides with the optical axis.

A single lens reflex camera has a single lens that forms an image which is reflected to the viewfinder. Digital single lens reflex cameras or DSLR cameras are the most versatile of the digital cameras.

You can take photos one at a time. Or, you can turn the burst mode on and the camera will continue snapping photos as long as you hold the button down, or until the buffer is full (which is a fancy way of saying the camera can’t process anymore). Burst speeds differ based on what camera or film camera you own, some are faster than others. Just how fast is written in “fps” or frames (pictures) per second. This will give you a wide selection of which close-up you’ll ultimately select of your dog!

Noise is simply little flecks in an image, also sometimes called grain. Images taken at high ISOs have a lot of noise, so it’s best to use the lowest ISO you can for the amount of light in the scene.

When your eyes focus on an object that’s close to you, the objects far away will appear blurry. The common photography term “focus” has the same meaning. Something that is in focus is sharp, while an object that is out-of-focus isn’t sharp. Different focus areas determine if the camera is focusing on multiple points or one user-selected point.

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The focal length describes the distance in millimeters between the lens and the image it forms on the film. It informs the angle of view (how much of what is being shot will be captured) and the magnification (how large things will appear). Essentially, the focal length is how ‘zoomed in’ your images will appear. For example, a Canon (or Nikon or Olympus) 35mm lens will create images that appear more ‘zoomed in’ than a Canon 18mm.

Exposure compensation is a way to tell the camera that you’d like the exposure to be lighter or darker. Exposure compensation can be used on some automated modes and semi-automated modes like aperture priority. It’s measured in stops of light, with negative numbers resulting in a darker image and positive ones creating a brighter shot.

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It is often mistakenly assumed that telecentric lenses have a greater depth of field than fixed focal length lenses. Ultimately, ­however, the depth of field is determined by the f-number of a lens; telecentricity only has an indirect impact. The f/# is, among other things, decisive for the diameter of the lens. This diameter is anyway large with telecentric optics compared to a fixed focal length lens, all other parameters being equal. Therefore, it is common for telecentric lenses to start only at f-numbers of about f/6. This is to reduce the diameter, design complexity, but also the cost of a telecentric lens. Fixed focal length lenses can often be opened up to f/1.4 – which is unattainable for telecentric lenses.

When moving from here towards the image corner, the chief ray angle is in most cases described by a monotonic function – i.e. the further away from the image center, the larger the angle. In the example, the angle at 5.5 mm (corresponding to a sensor with 11 mm diagonal, 2/3" format) reaches a value of ~0.1° for blue light of wavelength 486 nm. However, the function does not always have to be monotonic and may differ, especially for other wavelengths. For red light (656 nm), the angle at 5.5 mm is only about 0.015°, and the curve even shows a sign change. At approximately 4.4 mm image height, the angle is 0° again. As a rule, it is therefore advisable to use a graph of this type instead of referring to a single value in a spec sheet, which may not be representative of the parameters relevant to the application. Furthermore, it is recommended to recalibrate an optical measuring system when working monochromatically and changing the wavelength of the light.

After all, those great how-to guides and classes to improve image quality or depth of field are full of new terms and concepts. While there are hundreds of terms associated with photography, beginners should add these 25 terms to their vocabulary to get a good start on mastering the basics. After learning these basics, watch our comprehensive Fundamentals of Photography series, taught by John Greengo.

FOV tofocal length

If you look at a round object with telecentric optics and leave the focus area, the object will become symmetrically blurred. An algorithm can therefore still determine the correct position of the center of mass. With fixed focal length lenses, the object would be distorted asymmetrically into an ellipse depending on its position in the image, and it would not be possible to determine its exact position. In this respect, it is possible to extend the nominal depth of field in special situations without necessarily having to accept losses in the quality of measurement results. Depending on the application and algorithm, it may even be advantageous to work with a certain amount of blurring. If the transition of an edge is distributed over several pixels instead of only two in the extreme case,  a measurement system may provide more ­repeatable results.

The shutter speed is the part of the camera that opens and closes to let light in and take a picture. The shutter speed is how long that shutter stays open, written in seconds or fractions of a second, like 1/200 s. or 1”, with the “ symbol often used to designate an entire second. The longer the shutter stays open, the more light that is let in. But, anything that moves while the shutter is open will become a blur, and if the entire camera moves while the shutter is open the whole image will be blurry—that’s why tripods are necessary for longer shutter speeds.

You probably know that the flash is a burst of light—flash sync determines when the flash fires. Normally, the flash fires at the beginning of the photo, but changing the flash sync mode adjusts when that happens. The rear curtain flash sync mode, for example, fires the flash at the end of the photo instead of the beginning.

It should also be noted that when comparing the f-numbers of telecentric and entocentric lenses, it is easy to confuse two different parameters. For fixed focal length lenses, the value usually engraved on the lenses denotes the image space f-number for infinite working distance. The choice of this working distance makes sense for several reasons, a quite pragmatic one being that fixed focal length lenses can be used over a large range of working distances and there is no other "universal" working distance that is appli­cable to all lenses regardless of focal length or manufacturer. For telecentric optics, however, this choice does not make sense; after all, the working distance is finite and fixed for these lenses. Accordingly, in order to ensure comparability, a parameter known as working f-number is used. It is calculated by weighting the theoretical f-number based on infinity with a factor that depends on the specific magnification of the respective lens. For a correct comparison with the f-number of a fixed focal length lens, the value engraved on this lens must be converted by the same factor.  There is another interesting property of telecentric lenses that can give them advantages over entocentric optics in some appli­cations. Due to the lack of perspective, objects become uniformly blurred when you reach or go beyond the limits of depth of field.

Manual mode allows the photographer to set the exposure instead of having the camera do it automatically. In manual, you choose the aperture, shutter speed and ISO, and those choices affect how light or dark the image is. Semi-manual modes include aperture priority (where you only choose the aperture), shutter priority (where you only choose the shutter speed) and programed auto (where you choose a combination of aperture and shutter speed together instead of setting them individually). Manual can also refer to manual focus, or focusing yourself instead of using the autofocus.

Your eyes automatically adjust to different light sources, but a camera can’t do that—that’s why sometimes you take an image and it looks very blue or very yellow. Using the right white balance setting will make what’s white in real life actually appear white in the photo. There’s an auto white balance setting, but like any automatic setting, it’s not always accurate. You can use a preset based on what light you are shooting in like sun or tungsten light bulbs, or you can take a picture of a white object and manually set the white balance.

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That’s the hole you look through to take the picture. Some digital cameras don’t have one and just use the screen, but all DSLRs and most mirrorless cameras use them.

Aperture is measured in f-stops; a small f-stop like f/1.8 is a wide opening, a large f-stop like f/22 is a very narrow one. Aperture is one of three camera settings that determine an image’s exposure, or how light or dark it is. Aperture also affects how much of the image is in focus—wide apertures result in that creamy, unfocused background while narrow apertures keep more of the image sharp.

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Learning digital photography seems like a tough task—especially when you’re met with all kinds of technical jargon that leave you clueless and itching for a dictionary. Or worse, leaving you trying to explain what you just learned by using phrases like “that hole you look through” or “that one button you press to take the picture.” Understanding the common photography terms, definitions, and lingo is a crucial first step towards improving your skill as a beginner photographer. Whether you’re shooting with your very first digital camera or want to learn more complex terms like chromatic aberration, f-number or image sensor, read on to see how you should change your perspective (or field of view!) when approaching digital photography or iso photography.

In telecentric lenses, this undesirable property is eliminated or greatly reduced by their design. Figure 1 shows two identical objects at different distances from the respective measurement setup. The left image shows the scene taken with a fixed focal length lens, for the right image a telecentric lens was used. Despite the significantly different distance of the two cubes from the lens, they are shown the same size in the right image. This visualization is achieved by ensuring that the chief ray runs parallel to the optical axis in the design of the lens. This is the defining characteristic of an object space telecentric lens. However, this also means that at least the first lens element must be at least as large in diameter as the workpiece itself to be inspected.

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Exposure is how light or dark an image is. An image is created when the camera sensor (or film strip) is exposed to light—that’s where the term originates. A dark photo is considered underexposed, or it wasn’t exposed to enough light; a light photo is overexposed or exposed to too much light. Exposure is controlled through aperture, shutter speed and ISO.

Using manual mode isn’t all guesswork—a light meter built into the camera helps guide those decisions, indicating if the camera thinks the image is over or under exposed. Metering is actually based on a middle gray, so having lighter or darker objects in the image can throw the metering off a little bit. Metering modes indicate how the meter is reading the light. Matrix metering means the camera is reading the light from the entire scene. Center-weighted metering considers only what’s at the center of the frame and spot metering measures the light based on where your focus point is.

RAW is a file type that gives the photographer more control over photo editing. RAW is considered a digital negative, where the default JPEG file type has already been processed a bit. RAW requires special software to open, however, while JPEG is more universal. Typically, it’s better to shoot in RAW because the image retains more quality making it better for editing.

Focal length

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A time-lapse is a video created from stitching several photos together taken of the same thing at different times. Don’t confuse a time lapse with a long exposure, which is a single image with a long shutter speed.

Bokeh is the orbs created when lights are out of focus in an image. It’s a neat effect to have in the background of a photo, created through wide apertures. It will have an interesting effect on your image quality. Check out our ultimate guide to Creating Backgrounds With Bokeh for everything you could want to learn.

The file format is how your camera lens will record the image or image file. Raw files contain more information than JPGs, which makes them more suitable for photo editing in various editing software.

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Digital and optical are important terms to understand when shopping for a new camera. Digital means the effect is achieved through software, not physical parts of the camera. Optical is always better than digital. These terms are usually used when referring to a zoom lens (on a compact camera) as well as image stabilization.