The one thing that enables the photographer to convey his photographic intent to the viewer is establishing the focus. What are the secrets to establishing the best focus? Let’s find out more about the special features of autofocus (AF) and manual focus (MF). (Reported by: Tomoko Suzuki)

After graduating from the Tokyo Polytechnic University Junior College, Suzuki joined an advertisement production firm. She has also worked as an assistant to photographers including Kirito Yanase, and specializes in commercial shoots for apparels and cosmetic products. She now works as a studio photographer for an apparel manufacturer.

Optical Coherence Tomography (OCT) is a powerful imaging technique used routinely by clinicians to aid in the diagnosis and monitoring of a range of diseases and medical conditions.  Imaging the layers of the retina in the eye is the main area of usage but it is also standard for imaging the cornea. It has also been developed and used to varying degrees in areas such the diagnosis of skin conditions, dentistry, and endoscopic imaging of the cardiovascular and gastro-intestinal systems [1, 2, 3, 4]. Outside of medicine it is becoming routinely used in areas such as industrial non-destructive testing [5], and art and historical artefact examination [6].

A: Foreground depth of field B: Background depth of field C: Focus position Expressing as a ratio the distance from the focus position to the foreground depth of field, to the distance from the focus position to the background depth of field, the focus ratio is said to be 1:2 foreground: background.

Exposure is one of the major factors that can make or break a picture. Let us talk about how we can go about make best use of exposure to get the best results from a shot (Reported by: Tomoko Suzuki)

Want to create photographs with a lovely background blur (bokeh effect), or perhaps ensure that everything in the image remains in focus? The Aperture-priority AE mode is a convenient mode to use for achieving those effects. Let’s look at this mode in closer detail. (Reported by Tomoko Suzuki)

When the f-number changes, it is not only the amount of light entering the camera that changes, but also the size of area in the image that appears in focus. The smaller the f-number, the smaller the image area in focus. Conversely, the larger the f-number, the larger the image area in focus. The latter results in a photo that is sharp all the way to the background.

When taking photographs, you want to have a good grasp of shutter speed and its effects on your photographs. What kind of effects can you create with a faster or slower shutter speed? Let us examine the effects of different shutter speeds with the help of the following examples. (Reported by: Tomoko Suzuki)

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Optical coherence tomography

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#1: The Relationship Between Lens Aperture and Bokeh #2: Create Background Bokeh for a Warm, Friendly Family Photo #3: The Wonders of f/2.2 in Still-Life Photography #4: Photographing Facial Expressions (f/2.8) #5: Camera Settings for the Perfect Outdoor Portrait (f/4) #6: A Useful Aperture Setting for Street Photography (f/5.6) #7: Aperture Settings for Sharp Depictions of Nightscapes (f/8) #8: The Ideal Aperture for Sharp Depictions of Natural Landscapes with Depth (f/11) #9: Getting Sharp Depictions of Landscapes from Foreground to Background (f/16)

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Spatial resolution in both the lateral and axial (depth into the tissue) is of key importance in OCT measurements. The axial resolution depends intrinsically on the optical bandwidth of the light source used. On the other hand the lateral  resolution is generally determined by the spot size of the light beam or confocal pinhole aperture used in the optical path for point scanning systems. In the case of LF-OCT the lateral resolution will depend intimately on the imaging quality of the spectrograph used. With the possibility of high axial resolutions OCT is proving to be an ideal tool for in-vivo imaging through relatively thick (several mm) sections of biological issue.

Optical Coherence Tomography (OCT) is the optical analogue of ultrasound and it allows for cross-sectional imaging of materials that are partially transparent to light signals. It operates by measuring the 3D location of light scattering and reflections, from scattering particles and interfaces where there are changes in optical refractive indices respectively [8]. Low coherence interferometry between a reference beam and the reflected signal beam is used to measure and differentiate the time delay, thus depth, of the many tiny reflections. The ranging of these signals by low coherence interferometry can be done using just a spectrometer to measure the monochromatic interference amplitude of each measured wavelength of light and then exploiting their combined Fourier mathematical properties. The lateral (sideways) locations of the reflections are found with the same basic principles of a camera. Figure 1 shows a schematic of an incident pulse interacting with the layers of the eye retina.

OCT can provide high resolution images in 2D and 3D of the layers of tissue in-vivo. This case note describes the work of a multi-disciplinary group at Liverpool University who are developing a new generation of ophthalmic instrument based on a technique known as Line Field – Optical Coherence Tomography (LF-OCT) [7]. This approach offers a number of key benefits over current clinically approved instruments including the simultaneous acquisition of complete 2D sectional images or ‘B’ scans, high data acquisition speeds, ultrahigh sensitivity and spatial resolution.

For more on individual f-numbers and the scenes they are commonly used for, check out our Aperture-Priority Technique series:

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A 300 g/mm grating and toroidal optics in the Shamrock spectrograph dispersed the interfered light at the spectrograph slit into multiple spectral interferograms across the output focal plane. These interferograms were captured on the 2D CCD or sCMOS detectors depending on which detector was being used at a given time. This spectral arrangement is analogous to high-density multitrack spectral or indeed hyperspectral imaging. Like these arrangements, the spatial resolution of these channels or tracks is dependent on the imaging quality of the spectrograph. An sCMOS Neo camera on the direct exit port camera was used for high-speed imaging and a slower back illuminated (BI) CCD camera on the side exit port was used for high-resolution imaging. A software application developed in Matlab using the Andor SDKs for the spectrograph and cameras allowed for automated control of the system and the real-time display of OCT images.

ISO speed plays an equally important role as aperture and shutter speed in its effect on exposure. Now let us learn more about the advantages and disadvantages of turning up the ISO speed. (Reported by: Tomoko Suzuki)

Most modern OCT systems only measure one lateral position at a time. The Liverpool group are using an alternative ‘Line Field’ approach, which measures all the lateral positions in cross-sectional image simultaneously, to image the cellular layers of the cornea and tear film, with a view to better understanding a range of diseases of the cornea such as corneal ulceration, Fuchs dystrophy, and refractive disorders [9].  Significant benefits should be realised for patients with the improved speeds and high resolution of LF-OCT.

Line Field OCT takes an increased parallelisation approach compared to the well-established scanning point method. A line of illumination is delivered on to the sample and the reflected signal is subsequently focused on the entrance slit of a multi-channel imaging spectrograph, which then measures the spectral interferograms of all positions at the same time. This allows for the illumination of one complete section through the sample resulting in the capture of one complete ‘B’ scan image in one acquisition. Essentially all the spectral information from each point along the line on the sample is captured in parallel – analogous to hyperspectral or high density multi-track spectral imaging. Parallel acquisition of A-scan data leads to much faster acquisition times. This has an advantage of reducing artefacts caused by motion for example when the patient’s heart beats or they breathe.

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Acknowledgement: The information and figures for this note were gratefully received from Dr S Lawman of the Dept. of Electrical Engineering and Electronics, & Dept. of Eye and Vision Science, University of Liverpool.

Figure 4: OCT images of a donor human cornea ex-vivo taken with two current commercial systems (SP-SS left, SP-SD middle) and Liverpool’s LF-OCT system (right). The ultra-high axial resolution of the system means that all 5 layers of the cornea and details of the stroma are better resolved than the current commercial systems [16].

The Program AE mode, a semi-automatic mode where the camera automatically sets the aperture and shutter speed, enables you to shoot quickly to capture sudden photographic opportunities, and yet still retain creative control over other settings such as white balance.

This illumination pattern enabled a complete B-scan to be acquired at the same time in contrast to the scanning point (SP) approach where multiple A-scans have to be acquired to build up the equivalent of the B-scan. The line image from the sample was imaged on to and aligned with the entrance slit of the spectrograph. A schematic layout of the experimental setup is shown in figure 3. As a first phase in the ‘proof of concept’ and development of an instrument, the group used off-the-shelf instruments which included an Andor Shamrock 303i spectrograph, a CCD camera and a Neo sCMOS camera.

Apr 29, 2024 — Hyperspectral imaging provides unmatched precision for tasks requiring in-depth material characterization, whereas multispectral imaging offers ...

When the aperture is widened, the f-number decreases. The in-focus area of the image decreases, and the bokeh gets more prominent (or "larger"). Conversely, when the aperture is narrowed, the f-number increases. The in-focus image area increases, and the bokeh becomes less obvious.

At the heart of any OCT system is an interferometer, which is based on the same principals as the Michelson interferometer of the famous Michelson-Morley experiment and the modern gigantic instruments used to measure gravitational waves. The particular type of interferometer used in the first Liverpool prototype is of the Linnik design [10]. This has identical optics in the reference and sample arms of the interferometer, which means the relative time differences that light of different wavelengths takes to travel through an object (i.e. the optics) are matched in both arms of the interferometer and has no impact on the interference signal. Without this matching, the mismatch of time differences between the different wavelengths of light (dispersion) blurs the ranging of the signals. The distinct benefit for the Liverpool system is the achievement of ultra-high (2.1 μm in air) depth resolution without the need for physical or data manipulation in the system.

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EOS 5D Mark III/ FL: 50mm/ Aperture-priority AE (f/1.8, 1/80sec., EV+0.7)/ ISO 100/ WB: Auto Shallow depth of field f/1.8

The LF-OCT setup at Liverpool is based on a Spectral Domain (SD) arrangement where an Andor Shamrock 303i imaging spectrograph is used to measure the spectral interferograms for all A-Scans simultaneously. The light source is an ultra-broadband Super-Continuum (SC) laser. High and low pass filters were used to define the used optical band from 700 nm to 1000 nm. To achieve the ‘Line Field’ geometry a cylindrical lens and an objective were used to form the illumination beam into a line focus on the sample.

The potential of LF-OCT for significant improvements in the performance for clinical OCT instruments has been clearly demonstrated in this work.  Instrumentation incorporating fast-frame-rate and sensitive imaging cameras (e.g. Neo or Zyla) along with high fidelity imaging spectrographs (e.g. Holospec f1.8) are required for the realisation of such  LF-OCT systems. Some of the advantages of LF-OCT compared with established OCT techniques are given below:

Ultra-high resolution was possible with their prototype instrument. A comparison of sectional views through the layers of cornea tissue acquired by different instruments is shown in figure 4. The image from the Liverpool system is shown on the right and is compared with  images from two current commercial systems. These sectional images or ‘B’ scans  illustrate the fine structure within the human cornea. This image is derived from measurements where the light is incident from the top and the image shows layers within the tissue in the axial direction down into the tissue; this explains in part the reduction in contrast towards deeper layers due to the attenuation of the light as it propagates into and out of the sample. Clearly the resolution and clarity for the image on the right is notably better as evidenced by the clearer visibility of the fine structure within the tissue. Operating in the conventional quasi-CW mode the group were able to achieve axial resolutions down to 2.1 mm, and lateral resolutions of 18 mm.

The metering function measures the brightness of a subject and decides how much exposure is best for the photo. Let’s take a look at each metering mode available, and get a better idea of which of them to use is best to use under which conditions/scene. (Reported by Tomoko Suzuki)

Exposure compensation is a function you can use to change the exposure set by the camera (camera-determined correct exposure) into something of your own preference. Here, we find out more about the function, and learn how to identify subjects that require positive or negative exposure compensation along the way. (Reported by: Tomoko Suzuki)

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With the Picture Style function, you can adjust the colour tone and the contrast to enhance the charm and appeal of the subject. By selecting the perfect Picture Style setting, you can get perfect results in expressing your shooting intent in a vivid photo. (Reported by Tomoko Suzuki)

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Shutter-priority AE mode is a shooting mode that is useful for when you want to ‘freeze’ subjects in action, or conversely, photograph moving subjects with motion blur. Read on to find out how you can use it! (Reported by Tomoko Suzuki)

The position and angle are two elements that greatly influence the outcome of your photos. Since they have such a significant impact, varying them ensures that you will be able to get a different effect in your photos. In the following, we go over 3 points each in relation to the position and the angle. (Reported by Tomoko Suzuki)

- The larger the aperture (i.e. the smaller the f-number), the larger the bokeh. - The smaller the aperture (i.e. the larger the f-number), the larger the area in-focus (depth-of-field). - The amount of light that enters The sensor can be controlled by widening/narrowing The aperture.

Exposure settings, also commonly referred to by photographers as “f-stops,” allow you to adjust the amount of light that enters the camera. These settings are also known as the “EV”, or exposure value. Increasing the aperture by 1 stop halves the amount of light entering the camera. Conversely, decreasing it by 1 stop doubles the amount of light entering the camera. For most DSLR cameras, in addition to the standard 1 stop, you can also set stops at 1/2 and 1/3 intervals. For example, if you set a 1/3 stop, the range of a full stop between f/2.8 to f/4 is divided into 3 parts, so it becomes f/2.8→f/3.2→f/3.5→f/4. The use of 1/3 stops allows finer adjustments to be made to the amount of light entering the camera.

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There are zoom lenses with a range of f-numbers given as f/3.5-5.6. These are known as “variable aperture zoom lenses”, where the aperture changes with the focal length. In the case of the EF24-105mm f/3.5-5.6 IS STM, the aperture (f-number) at the wide-angle end (24mm) is f/3.5, and the aperture at the telephoto end (105mm) is f/5.6. Lenses in which the aperture does not change even when the focal length changes are known as “fixed aperture zoom lenses”.

White balance is a feature that ensures that the colour white is reproduced accurately regardless of the type of lighting under which a photo is taken. At a very basic level, it is common to use the Auto White Balance setting. However, this setting is no one-size-fits-all solution. For a white balance setting that best suits the lighting source, choose one of the preset white balance settings on your camera.(Reported by Tomoko Suzuki)

When it comes to shooting, a vital part of the camera is the viewfinder. Nowadays, there are cameras that do not come with viewfinders, only with Live View shooting. However, as you get more experienced with photography, you will realise how much shooting with a viewfinder can affect your photos. In this article, we take a closer look at the viewfinder. (Reported by: Tomoko Suzuki)

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The implementation of Dual Pixel CMOS AF in Canon's latest camera models have vastly improved shooting conditions in Live View. Live View, which features fast AF speed that measures up to viewfinder AF, is gradually becoming the choice method of shooting for many photographers. In the following, we will explain more about the characteristics of Live View. (Reported by: Tomoko Suzuki)

The original approach was based on Time Domain (TD) measurements but this has now been superseded by Fourier Domain (FD) methods. Fourier Domain or FD-OCT can be distinguished into two main sub-groupings depending on whether a tunable light source (i.e. a tuneable wavelength output laser) is used to present a series of individual wavelengths sequentially in the measurement or if all the wavelengths (from a broadband source) are presented simultaneously and a spectrometer or spectrograph is used to capture the dispersed spectra. The former method is referred to as Swept Source (SS) and the latter as Spectral Domain (SD).

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When establishing focus on a subject, it is vitally important to anticipate the subject’s movement and capture it at the right moment. This means it is essential to know the appropriate autofocus (AF) mode to use for a stationary subject, and which to use when the subject is moving. Let us take a closer look at the 3 types of AF modes. (Reported by Tomoko Suzuki)

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The first thing to consider when taking photographs with a digital camera is the effect that the aperture can have on your pictures. How will the photograph finish change depending on the aperture is widened or narrowed? In this article, we study the effects of varying apertures on depth-of-field by comparing several examples, and learn about the concept of f-stops. (Reported by Tomoko Suzuki)

OCT developed from techniques such as low-coherence interferometry [11], and femtosecond optical ranging [12], with the terminology and development of the first widely used technique emerging in the early 90’s [13]. Since then there have been multiple variants and developments of the basic technique [14, 15]. The large amount of OCT variants comes from the fact that there are several methods to achieve depth ranging (Time Domain, Swept Source and Spectral Domain), several methods to get lateral information (scanning point, line field and full field), a number of functionalities which can be built in (e.g. polarisation sensitivity, OCT angiography and OCE), physical construction of interferometer (e.g. fibre Michelson, fibre Mach-Zander, free space Michelson, Linnik and common path) and where physically you implement your OCT sensor (e.g. on the end of an endoscope in endoscopic OCT). These different options can be put together in a multitude of combinations. LF-OCT is among the more  recent developments. Figure 2 shows the relationship between the main variants of the base approach including the common acronyms used.

In the common scanning point implementation of SS-OCT, a very fast single point detector is then used to measure the interference signal. This sequentialisation approach does give benefits in terms of image depth, speed and SNR but the limited optical bandwidth of source sweeping ranges means low depth resolution. Scanning point SD systems use a single channel input spectrometer with a dispersion element such as a grating to produce a single spectrum which is captured on a 1D-line detector such as a photodiode array or CCD.  In this case all the wavelengths from the broadband source illuminate the sample or tissue at the same time. Currently most standard clinical products are based on Scanning Point SS and SD systems, where the light illumination is focused to a point on the sample resulting in a single channel depth profile at that single point – which is referred to as an ‘A’ scan (a term borrowed from ultrasound imaging). To build up a 2D sectional image through the tissue sample, as illustrated in figure 1, the point of illumination has to be scanned laterally across the sample. The resulting 2D dataset is referred to as a ‘B’ scan image. This can be extended by doing further raster scanning (or a series of parallel scans) across the sample to build up a 3D imaging data volume.

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The first thing to consider when taking photographs with a digital camera is the effect that the aperture can have on your pictures. How will the photograph finish change depending on the aperture is widened or narrowed? In this article, we study the effects of varying apertures on depth-of-field by comparing several examples, and learn about the concept of f-stops. (Reported by Tomoko Suzuki)

When running the Neo camera at 100 full frames/s, 3D volume imaging speeds up to 213 kA-Scans per second were achievable representing a very large increase in speed compared with current clinical instrumentation.  The group are also working on a range of techniques for improving the image quality further by for example using Fourier phase analysis to handle the degradations in image quality due to artefacts [5].

Learn the best ways to create amazing images and videos, share your works with the community and be inspired by our community.

If you want control over both the aperture and the shutter speed, Manual exposure mode is the way to go. It might be quite a tough mode to conquer for a beginner, but also can be very convenient to achieve certain shooting intentions. In this final article in our Camera Basics series, we take a closer look at this mode and what it can be used for. (Reported by Tomoko Suzuki)

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oct中文

The aperture allows us to control the amount of light entering the lens. When the aperture is widened, more light can enter, and conversely, when the aperture is narrowed, less light can enter the lens. The numerical values of the difference in aperture size is known as the f-number. The standard f-numbers are: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8… etc. Widening the aperture reduces the f-number whereas narrowing the aperture increases it.

At the smallest f-number, you achieve “maximum aperture”. This allows the greatest amount of light possible to enter, and is also when you can achieve the most prominent ("biggest") bokeh.

Phase detection AF (also known as phase-difference detection AF) is the autofocus system used in viewfinder shooting on DSLR cameras. Its main feature lies in its rapid autofocusing speed. In the following, we will explain more about phase detection AF, and how Canon’s Dual Pixel CMOS AF utilizes the latest AF technology to enable phase detection AF even in Live View. (Reported by Tomoko Suzuki)

The bokeh also gets more prominent the closer the focusing distance. The range of focus (how much of the image is in focus) is known as the “depth of field”. When this range is small, it is known as a “shallow depth of field”. Likewise when the range is large, this is a “deep depth of field”.

The compound microscope has two systems of lenses for greater magnification, 1) the ocular, or eyepiece lens that one looks into and 2) the objective lens, or ...