焦点深度

focal depth (depth of focus)

[目次：レンズ系]

物体に焦点を合わせたとき、その焦点面の前後で十分鮮明な像が得られる距離。焦点深度は実際の空間分解能と倍率の２乗に比例し開口角に逆比例する。電顕の場合は可視光の場合にて比べて開口角が小さいので、焦点深度は深い。倍率が１万倍、開口角が1×10⁻³rad、分解能が1nmとすると焦点深度は100mになる。

The "focal depth (depth of focus)" is the range of distances for which the object is imaged with an acceptable sharpness on the image plane. The focal depth is proportional to the spatial resolution of a microscope and to the square of magnification, and inversely proportional to the aperture angle. In the case of a TEM, since the aperture angle is smaller than that of an optical microscope, the focal depth is large. For example, if the magnification is 10,000×, the aperture angle is 1×10^-3 rad, and the resolution is 1 nm, the focal depth reaches 100 m.