Why do 60mm and 105mm f/2.8 lenses have the same f-stop if their aperture diameters differ?
Asked 3/5/2015
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I'm comparing two macro lenses: a 60mm f/2.8 and a 105mm f/2.8. Using aperture diameter = focal length / f-number, the 60mm lens at f/2.8 has an entrance pupil around 21.4mm, while the 105mm lens at f/2.8 is about 37.5mm. That makes the 105mm lens's physical aperture larger, so why doesn't it gather more light per image area? Also, Nikon lists a minimum aperture of f/32 for both lenses. How can both lenses have the same maximum and minimum f-stops if their physical aperture sizes are different? What exactly does f-stop represent?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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"Therefore lens 2 has a larger maximum aperture than lens 1 and therefore capability to allow more light.
This is where your understanding is not quite right. The physical size of the aperture is indeed larger in the longer lens, but it does not allow in more light, because the longer focal length means that the field of view is narrower. This means that the cone of light entering the lens is also narrower, leading to a reduction in the overall brightness.
The reason aperture is expressed as a function of focal length is exactly this: so that the same number represents the same brightness. I'm not sure why you think the minimum aperture can't be the same for both lenses, so it's hard to address that point unless you clarify your reasoning. The 105mm lens will have an aperture diameter of ~3.3mm at f/32 and the 60mm lens will have an aperture diameter of 1.875mm at f/32. They will both let in the same amount of light for the reason given above.
When people talk about 'aperture' in photography they are almost always referring to the f/stop, not the physical size of the aperture, which is rarely relevant in exposure calculations.
Originally by user456. Source · Licensed CC BY-SA 4.0
user456
11y ago
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f-stop is the ratio of focal length to effective aperture diameter, not the diameter alone. So yes, a 105mm f/2.8 lens has a larger physical opening than a 60mm f/2.8 lens, but it also has a longer focal length and narrower angle of view. Those scale together, so both lenses produce the same image brightness at f/2.8.
In other words, the longer lens needs a larger opening just to maintain the same illumination on the sensor. A bigger physical aperture does not automatically mean a brighter image if focal length is also larger.
The same applies at the minimum aperture: f/32 on both lenses means the aperture diameter is different physically, but the ratio is the same. Roughly:
- 60mm at f/32 ≈ 1.9mm
- 105mm at f/32 ≈ 3.3mm
So both can be f/32 even though the actual openings are different sizes.
Why use f-stops? Because they let photographers compare exposure settings across different focal lengths. The same f-number represents the same exposure/brightness, regardless of lens focal length, assuming normal transmission differences are ignored.
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