EDIT: not sure how to delete my post but I just realized the numbers represent the minimum f number, so this makes sense!

I recently purchased both a Canon M50 and Sigma 18-300m f3.6-6.3

I understood that this lens has a narrow aperture range (f3.6 to f6.3).

However on the camera (for example in manual mode and in Av aperture priority mode I can select the entire range of aperture from f3.5 to f22.

Does anyone know exactly what is happening here? I tried searching for an answer but could not find a similar thread. Thank you!


2 Answers 2


You have misunderstood the meaning of 18 – 300 3.5 -6.3 The zoom range of this lens is 18mm thru 300mm – quite an impressive zoom 16.67 X range. The maximum lens opening is f/3.5 (widest opening allows the most light in thus the brightest image) can only occur if the lens is operated as a wide-angle. When you zoom to telephoto territory, the max light gathering power wanes to f/ 6.3. In other words, we are taking max aperture, not minimum, The minimum (tiny lens opening is f/22. You can achieve this tiny opening over the entire zoom range. The f-number is derived by dividing the focal length by the diameter of the aperture (iris). As you zoom to higher and higher focal length, a toll is taken as to the light gathering ability of this lens. It takes a very expensive lens to maintain full aperture throughout the entire zoom range. This one can’t do that.


Almost all lenses have a device called an iris inside. It's that iconic circle of small interlocking metal leaves with a polygonal hole in the middle, often used in logos of photo accessories and software.

The more you close it, the higher your aperture number gets.

Lenses have a wide-open aperture, meaning the iris is invisible, fully retracted into the sides of the lens body. This is what is printed on the lens nameplate. For some zoom lens types (such as yours), the wide-open aperture is not constant across focal lengths, neither is the aperture you get for a given opening diameter of the iris.

It is physically impossible for a (conventional) lens to have a smaller diameter of the front lens element than (focal length divided by wide open aperture). So, a zoom lens with 300mm focal length and aperture 3.5 at that focal length will need to have a front diameter of 300/3.5 ~ 86mm, which is not compact, light, or cheap. 300/6.3 is a mere 48mm - MUCH smaller.

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