Pupilaperture

All lenses have a maximum and minimum aperture, expressed as “f-numbers”, but it is the maximum aperture that is most commonly quoted in lens specifications. Take the Sony 35 mm F1.4 G as an example. This is a 35 mm F1.4 lens: 35 mm is the focal length (we’ll get to that later) and F1.4 is the maximum aperture. But what exactly does “F1.4” mean? See the “F-number maths” box for some technical details, but for a practical understanding it’s enough to know that smaller f-numbers correspond to larger apertures, and that F1.4 is about the largest maximum aperture you’re likely to encounter on general-purpose lenses. Lenses with a maximum aperture of F1.4, F2 or F2.8 are generally considered to be “fast” or “bright.”

Many of us have looked though the eyepiece of a department store microscope and seen a fuzzy looking “something” with the highest magnification objective lens. It’s not completely surprising that an inexpensive lens would give a blurry image. There are many optical aberrations that need to be corrected to manufacture the expensive lenses that are used on research grade microscopes.

The aperture in a lens—also known as the “diaphragm” or “iris”—is an ingenious piece of mechanical engineering that provides a variable-size opening in the optical path that can be used to control the amount of light that passes through the lens. Aperture and shutter speed are the two primary means of controlling exposure: for a given shutter speed, dimmer lighting will require a larger aperture to allow more light to reach the image sensor plane, while brighter light will require a smaller aperture to achieve optimum exposure. Alternatively, you could keep the same aperture setting and change the shutter speed to achieve similar results. But the size of the opening provided by the aperture also determines how “collimated” the light passing through the lens is, and this directly affects depth of field, so you’ll need to be in control of both aperture and shutter speed to create images that look the way you want them to.

In extreme examples of narrow depth of field, the in-focus depth might be just a few millimetres. At the opposite extreme, some landscape photographs show very deep depth of field with everything in sharp focus from just in front of the camera to many kilometers away. Controlling depth of field is one of the most useful techniques you have for creative photography.

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aperture中文

“Depth of field” refers to the range of distances from the camera within which photographed objects will appear acceptably sharp.

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f-stop vsaperture

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The standard f-numbers you’ll use with camera lenses are, from larger to smaller apertures: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 and sometimes 32 (for you mathematicians, those are all powers of the square root of 2). Those are the full stops, but you’ll also see fractional stops that correspond to a half or a third of the full stops. Increasing the size of the aperture by one full stop doubles the amount of light that is allowed to pass through the lens. Decreasing the size of the aperture by one stop halves the amount of light reaching the sensor.

f-stop是什么

High magnification without high NA does not give the resolving power that most people expect from a research grade microscope.  Using a high NA objective lens means that you are most likely sacrificing working distance (how deep into the sample that you can focus) for higher optical resolution. In most instances this is a very acceptable trade off.

The “cost” of obtaining a higher NA is that the working distance (WD) of the lens becomes much shorter. Working distance is “… the distance between the objective front lens and the top of the cover glass when the specimen is in focus. In most instances, the working distance of an objective decreases as magnification increases.” (1) A smaller working distance can be a problem when you cannot see an object with a high magnification lens, even though you could see it with a low magnification lens. A 10x objective can have a WD of several millimeters (4-10mm, or 4000-10,000um). A well corrected, high NA 20x dry objective will have a WD of slightly less than 1mm (1000um). Most well corrected, high NA 40x and 60x oil objectives have working distances on the order of 0.1mm (100um).

Numerical Aperture (NA) is “… a critical value that indicates the light acceptance angle, which in turn determines the light gathering power, the resolving power, and depth of field of the objective.”(1) As light passes through a sample, the information describing the highest resolution information in the sample is diffracted at a very wide angle. Low magnification lenses typically have low NAs, meaning that they cannot capture the highest resolution information. To capture the widely diffracted information, high NA lenses move the front of the lens closer to the sample (increases the light acceptance angle). Dry lenses can only have NAs of up to 1.0. By using specially formulated oil and oil lenses, NAs of up to 1.4 can be achieved.

F-stops

There’s actually more to shooting images with beautifully defocused backgrounds than simply choosing a bright lens and opening the aperture up all the way. That’s the first “key”, but sometimes a large aperture alone won’t produce the desired results. The second key is the distance between your subject and the background. If the background is very close to your subject it might fall within the depth of field, or be so close that the amount of defocusing isn’t sufficient. Whenever possible, keep plenty of distance between your subject and the background you want to defocus. The third key is the focal length of the lens you use. As mentioned above, it’s easier to get a narrow depth of field with longer focal lengths, so take advantage of that characteristic as well. Many photographers find that focal lengths between about 75 mm and 100 mm are ideal for shooting portraits with nicely blurred backgrounds.

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Aperture

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The f-number is the focal length of the lens divided by the effective diameter of the aperture. So in the case of the 35 mm F1.4 G lens, when the aperture is set to its maximum of F1.4, the effective diameter of the aperture will be 35 ÷ 1.4 = 25 mm. Note that as the focal length of the lens changes, the diameter of the aperture at a given f-number will change too. For example, an aperture of F1.4 in a 300 mm telephoto lens would require an effective aperture diameter of 300 ÷ 1.4 ≈ 214 mm. That would end up being a huge, bulky and very expensive lens, which is why you don’t see too many long telephoto lenses with very large maximum apertures. There’s really no need for the photographer to know what the actual aperture diameter is, but it’s helpful to understand the principle.

Reviewed & updated 06/16/2017. Creation of this web page was originally supported as part of the Southwest Environmental Health Sciences Center at the University of Arizona, NIEHS P30 ES006694.

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F-stop

[1] Effective aperture (size of the entrance pupil)  [2] Aperture  [3] Focal length  Note: Aperture and focal length values in the illustration are approximate.

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f-number

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Basically, larger apertures produce narrower depth of field, so if you want to shoot a portrait with a nicely defocused background you’ll want to open up the aperture wide. But other factors come into play. Lenses of longer focal lengths are generally capable of producing narrower depth of field (partly because, as we learned above, an F1.4 aperture in an 85 mm lens, for example, is a lot larger than an F1.4 aperture in a wide-angle 24 mm lens), and the distance between objects in the scene being photographed will have an effect on the perceived depth of field as well.

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Light microscopes can, under the best conditions, resolve objects that are approximately equal to half the size of the wavelength used. In the real world this comes out to objects that are 250-300nm in size, if you are using a NA=1.4 objective lens (under optimal conditions). This means that you can make out two adjacent objects in this size range, assuming that you can see at least a 25% dip in intensity between them (Rayleigh criterion). Sample preparation is especially important when you want to resolve structures this small.

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