Depth of focus/depth of focus field

Popular focused on a general lens is one point.

(Except for special lenses for precision measurement like telecentric lenses.)

There is a region with less focus blur before and after the point where perfect focus is on.

This is called depth of focus.

When it deviates from a point where it is completely in focus, it gradually blurs. Where is the practical range will be the subjectivity of the individual. By narrowing down the optical path, you can gradually reduce this degree of blurring. However, by narrowing, the image becomes dark, so you can not use it with a lens with a high magnification.



The left picture is our company’s USB microscope

USB microscope with aris



Compare the images when opened and narrowed with this irised microscope.

(When you narrow down the aperture, the depth of focus becomes deeper.)

<at 50x>
●Glass scale
Tilt the 0.5 mm pitch glass scale to 45 degrees and observe straight from the top

<Open the aperture>  <Close maximum aperture>
0.5mmピッチのガラススケールを観察 0.5mmピッチのガラススケールを観察

Since it is tilted to 45 degrees, it will be the depth of focus if multiplied by 1 / 1.41.

The focus on individuals depends on individual opinions.

If it is judged that 4 pitches (= 2 mm) are matched, 2 mm × (1 / 1.41) = 1.42 mm can be said as the depth of focus.

For board

Tilt the substrate at 45 degrees and observe it 50 times.

(1.6 mm × 0.8 mm electronic components are lined at 1 mm pitch.)


<Open the aperture>  <Close maximum aperture>
絞りを開放にした時 最大に絞った時

<at 100x>
●Glass scale

I also checked the times at 100 times for your reference.

Since the magnification is high, the glass scale has been changed to 0.2 mm pitch

<Open the aperture>  <Close maximum aperture>
絞りを開放にした時 最大に絞った時


if this range is judged as a practical range, focus range is 1.2 mm × (1 / 1.41) = 0.85 mm

Please note that narrowing the aperture will make the lens darker and the resolution will also decrease.

(For details, please refer to “NA (Numerical Aperture)”.)