How do I calculate the aperture diameter for a 35mm f/1.8 lens to make a DIY bokeh mask?
Asked 8/7/2013
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I want to make a DIY custom bokeh/aperture mask that mounts on the front of my lens. My lens is 35mm and its widest aperture is f/1.8, but I may also want to use shapes sized for other settings such as f/2.8. How do I calculate the aperture diameter in millimeters for a given f-number? Also, when making a front-mounted mask, should I use the physical diaphragm size or the effective aperture size?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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Pretty simple to calculate: the way the aperture is written tells you the diameter. Remember f refers to the focal length:
f/2.0
Means the diameter is the focal length divided by 2.0.
f/4.0
Means the diameter is the focal length divided by 4.0.
Warning: the effective aperture diameter is not necessarily the same as the diameter of the part of the front lens element in use. The aperture in a lens is placed at a point in the lens array where the image is as defocused as possible so as to minimise vignette. Placing a shape over the front lens element will not get you exactly the same effect as placing that same shape in the same plane as the aperture, but it will approximate it. Furthermore if you place the shape in front of the front lens element but you stop down the lens aperture, your effective aperture will still be round.
Thus, for what you are planning to do, the aperture diameter is probably not as relevant as the size of the front lens element.
Originally by user3422. Source · Licensed CC BY-SA 4.0
user3422
13y ago
0
Generated from our catalog & community — verify before relying on it.
Use the effective aperture diameter (the entrance pupil), not the physical iris size inside the lens. The standard formula is:
D = F / N
where D is aperture diameter, F is focal length, and N is the f-number.
For a 35mm lens:
- f/1.8: 35 / 1.8 = 19.4 mm
- f/2.8: 35 / 2.8 = 12.5 mm
- f/4: 35 / 4 = 8.75 mm
So for your DIY mask, size the opening based on the f-number you want to shoot at.
Important: a shape placed on the front of the lens is only an approximation of the actual aperture shape, because the real aperture is inside the lens and its apparent size is modified by the front optical elements. For custom bokeh, the mask should cover the visible entrance pupil as seen from the front. If you stop the lens down, the mask opening must still fully cover the visible diaphragm at that setting or it may clip the image in unintended ways.
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