It also means if you try to photograph a white dog against a white fence, your in-camera meter will underexpose the image. And the opposite would be true with a black dog and black fence, leading to overexposure.

The distinction between objectives designed for use with or without cover slides is important for high numerical aperture (high magnification) lenses, but makes little difference for low magnification objectives.

In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, binoculars, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.

Nikki holding a combo light meter that allows for both types of metering by Casey Cosley, edited with Mastin Labs Kodak Gold 200 preset

Types ofmicroscope objectives

All these types of objectives will exhibit some spherical aberration. While the center of the image will be in focus, the edges will be slightly blurry. When this aberration is corrected, the objective is called a "plan" objective, and has a flat image across the field of view.

Armmicroscope function

Reflective Metering measures the light that bounces off of or reflects from your subject. These are most commonly found inside your camera or as an app for your phone. This is often called spot metering.

One of the most important properties of microscope objectives is their magnification. The magnification typically ranges from 4× to 100×. It is combined with the magnification of the eyepiece to determine the overall magnification of the microscope; a 4× objective with a 10× eyepiece produces an image that is 40 times the size of the object.

Numerical aperture for microscope lenses typically ranges from 0.10 to 1.25, corresponding to focal lengths of about 40 mm to 2 mm, respectively.

Low power objectivemicroscope function

Before we dive into these light measuring and exposure techniques, you'll first need to understand the two ways of measuring light for exposure and how they're different:

Reflective meters, like the one in your camera, are set to balance everything to mid-gray, also known as “Zone V” in the Zone System of Exposure. So what does that mean in everyday, practical terms?

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To take an incident meter reading, you'll need a hand-held light meter. (Many hand-held meters have the additional capability of taking a reflective measurement.)

Ocular lensmicroscope function

If you’re doing a portrait session on a bright, sandy beach and set your camera settings based on the exposure of the middle-gray card you can use those same setting in the same light and ensure perfect exposure and tonality. If you use your in-camera meter, be sure to set it to “spot metering mode” while using the gray card.

Though you can undoubtedly use spot metering in any situation to achieve an accurate reading, reflective meters excel in difficult or extreme lighting conditions, or when your subject is far away. If your subject is backlit, you can take an accurate reading of the subject without the light coming from behind affecting your reading. If you’re shooting a concert in a dark venue, you can take an accurate reading of the light on the performer and ensure proper exposure. And if you’re shooting landscapes and want to expose for shadows in the distance, you can easily do so.

A typical microscope has three or four objective lenses with different magnifications, screwed into a circular "nosepiece" which may be rotated to select the required lens. These lenses are often color coded for easier use. The least powerful lens is called the scanning objective lens, and is typically a 4× objective. The second lens is referred to as the small objective lens and is typically a 10× lens. The most powerful lens out of the three is referred to as the large objective lens and is typically 40–100×.

Most hand-held meters have multiple modes, including the ability to be used with a flash or strobes. In this case, we’re talking about ambient light. Make sure the meter is in incident mode. A little sun icon is usually used to represent this.

Some microscopes use an oil-immersion or water-immersion lens, which can have magnification greater than 100, and numerical aperture greater than 1. These objectives are specially designed for use with refractive index matching oil or water, which must fill the gap between the front element and the object. These lenses give greater resolution at high magnification. Numerical apertures as high as 1.6 can be achieved with oil immersion.[2]

Stagemicroscope function

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High power objectivemicroscope function

first surface mirror, front surface mirror.

A sure-fire way to obtain perfect exposure using a reflective or spot meter is to take a reading on a middle-gray card (also known as 18% gray card).

White dog. Black dog. It doesn’t matter to the meter. It wants it all to be mid-gray and will provide different settings in the same light to get you there. Luckily, you're way smarter than your meter!

In addition to oxide glasses, fluorite lenses are often used in specialty applications. These fluorite or semi-apochromat objectives deal with color better than achromatic objectives. To reduce aberration even further, more complex designs such as apochromat and superachromat objectives are also used.

It means if you’re photographing a medium gray dog against a medium gray fence on a medium gray day, your in-camera metering will be perfect. But how often does that happen?

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Camera lenses (usually referred to as "photographic objectives" instead of simply "objectives"[4]) need to cover a large focal plane so are made up of a number of optical lens elements to correct optical aberrations. Image projectors (such as video, movie, and slide projectors) use objective lenses that simply reverse the function of a camera lens, with lenses designed to cover a large image plane and project it at a distance onto another surface.[5]

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Condensermicroscope function

Instead of finite tube lengths, modern microscopes are often designed to use infinity correction instead, a technique in microscopy whereby the light coming out of the objective lens is focused at infinity.[1] This is denoted on the objective with the infinity symbol (∞).

Where you place the meter can also affect the reading. For instance, in a portrait, you can meter for shadows by angling the meter 45º downward. The angle effectively gives you the reading for the shadow beneath your subjects chin and will add about a half a stop of light to the reading.

Historically, microscopes were nearly universally designed with a finite mechanical tube length, which is the distance the light traveled in the microscope from the objective to the eyepiece. The Royal Microscopical Society standard is 160 millimeters, whereas Leitz often used 170 millimeters. 180 millimeter tube length objectives are also fairly common. Using an objective and microscope that were designed for different tube lengths will result in spherical aberration.

Input the ISO you’re shooting into the meter and place the meter in front of your subject, with the bulb facing towards the source of light. The meter will give you a reading of the suggested shutter speed and aperture settings, which you can also adjust.

Particularly in biological applications, samples are usually observed under a glass cover slip, which introduces distortions to the image. Objectives which are designed to be used with such cover slips will correct for these distortions, and typically have the thickness of the cover slip they are designed to work with written on the side of the objective (typically 0.17 mm).

An incident meter (sometimes called a bulb-meter) works by reading the intensity of the light that falls onto the subject, as opposed to reading the light reflected off of the subject as the reflective meter does. Usually, you take the reading from the position of the subject.

The objective lens of a microscope is the one at the bottom near the sample. At its simplest, it is a very high-powered magnifying glass, with very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus inside the microscope tube. The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample.

Incident Metering measures the light that falls on the meter, often directly from the source. These are always handheld and have a bulb-shaped sensor. This is often called ambient metering.

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Basic glass lenses will typically result in significant and unacceptable chromatic aberration. Therefore, most objectives have some kind of correction to allow multiple colors to focus at the same point. The easiest correction is an achromatic lens, which uses a combination of crown glass and flint glass to bring two colors into focus. Achromatic objectives are a typical standard design.

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Because incident meters are not as affected by reflections and color variance, they will record the light with high precision. In scenarios where you can be close to your subject, they are often the ideal choice because of their accuracy without having to consider other variables.

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No matter which meter you are using, each can provide an accurate reading as long as you take the proper variables into account.

The working distance (sometimes abbreviated WD) is the distance between the sample and the objective. As magnification increases, working distances generally shrinks. When space is needed, special long working distance objectives can be used.

The rule of thumb in a really white, bright scenario, like a fresh-fallen snowfield or a white sand beach, would be to add two stops of light to the given reading from a reflective meter. If you’re in a very dark setting, i.e., shooting in a coal field or dark, deep woods, you can subtract two stops of exposure.

Stage clipsmicroscope function

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In a telescope the objective is the lens at the front end of a refracting telescope (such as binoculars or telescopic sights) or the image-forming primary mirror of a reflecting or catadioptric telescope. A telescope's light-gathering power and angular resolution are both directly related to the diameter (or "aperture") of its objective lens or mirror. The larger the objective, the brighter the objects will appear and the more detail it can resolve.

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The traditional screw thread used to attach the objective to the microscope was standardized by the Royal Microscopical Society in 1858.[3] It was based on the British Standard Whitworth, with a 0.8 inch diameter and 36 threads per inch. This "RMS thread" or "society thread" is still in common use today. Alternatively, some objective manufacturers use designs based on ISO metric screw thread such as M26 × 0.75 and M25 × 0.75.

A reflective meter measures the intensity of light bouncing off the subject you’re photographing. Because the meter measures the light after hitting the subject, how reflective the subject is or isn’t will affect what the meter “thinks” is the proper exposure setting.

Point the bulb of the meter towards the camera to meter for the highlights. Point the bulb of the meter at a 45 degree angle to the ground to meter for the shadows.