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How To Correctly Read The Aperture Value on a Manual Lens with Variable Aperture

Zoom-Nikkor 100-300mm f/5.6 Macro
Here is a Zoom-Nikkor 35-135mm f/3.5-4.5

Sometimes, reading the actual aperture value on a manual lens with variable maximum aperture can be confusing. I often get questions like: "If with a Nikkor 35-135mm f/3.5-4.5, the maximum aperture at 135mm is f/4.5... can I override this and make the aperture f/3.5 at 135mm by turning the aperture ring to f/3.5?"

The aperture value or 'f-number' of a lens is determined by 2 factors -- diameter and focal length.

N = f/D

f = focal length
D = diameter of the entrance pupil

The focal length of a lens is 60mm.
The diameter of the entrance is 30mm wide.
The aperture value is f/2

Therefore, if you want a lens with 60mm of focal length
to have a maximum aperture of f/2,
the width of the opening should be 30mm wide.
That's why lenses with longer focal length and wide aperture
are usually very big (eg. 85mm f/1.4)

Back to the question... the quick answer is no. There are usually 2 different colored dots on these lenses, right above the aperture ring, and these colors correspond to the colors of the focal length printed on the barrel.

Let's take our model here, the Zoom-Nikkor 35-135mm f/3.5-4.5. For newbies, let me explain this - it is a zoom lens that starts with 35mm and the longest range is 135mm. The maximum aperture at 35mm is f/3.5, and the maximum aperture opening at 135mm is f/4.5. That is that, and it will never change, 135mm will never become f/3.5 so don't let that confuse you.

See the photo below. 35mm is coded with color green, which corresponds to the green dot above the aperture ring. In this photo, if the lens is at 35mm, then the aperture value is at f/3.5

135mm is coded with an orange color, which corresponds to the orange dot above the aperture ring. Do you see the arrow I drew? It points at the position of f/4.5. Therefore, the way you read is that at 135mm, the current aperture is f/4.5

Likewise, if you turn the aperture ring, you will always look at what value the dots correspond to, depending on what focal length you set. Everything in between, eg if you set it to 50mm, you just have to mentally adjust/compute. But don't let it bother you too much.

Sometimes, some third party lenses don't do that and only have one dot, so it might add confusion. But this is the principle on how to read the aperture value on a manual lens with a variable maximum aperture. I hope my quick tutorial helps!

Image below shows a Zoom-Nikkor 35-135mm f/3.5-4.5 as our model. This image was shot with a Nikon D5200 and a Zoom-Nikkor 100-300mm f/5.6 Macro at 100mm macro f/16.

Zoom-Nikkor 100-300mm f/5.6 Macro

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