When you buy a lens or search lens information on the Internet, you may find that some zoom lenses are particularly expensive, and unlike ordinary zoom lenses, there are 3.5-5.6 and other numeric strings. Instead, you only see a number such as f/2.8, f/ 4 Wait, do you know what is special about those lenses? Let’s discuss with you now.

Maximum aperture of zoom lens
As we all know that the numbers such as 1:3.5-5.6 on the zoom lens represent the maximum aperture of the lens at the widest angle and the farthest end. For example, a “NIKKOR 18-200mm f/3.5-5.6G IF-ED” at the focal length The maximum aperture that can be used at 18mm is f/3.5, and the maximum aperture that can be used when zoomed to 200mm is only f/5.6.

Can I keep the aperture constant when zooming?
Of course! This is the so-called “constant aperture lens” or “fixed aperture lens”. If you use a lens with a “constant aperture”, you can always maintain the maximum aperture that the lens can achieve, even if you adjust the focus to the farthest end, you can maintain it.” The maximum possible aperture of the lens”, such as SIGMA’s 24-60mm F2.8 EX DG, users can use the f/2.8 large aperture at the focal length of 24mm to 60mm.

Benefits of constant aperture
The aperture basically affects two aspects-(1) the depth of field and (2) the amount of light. If the aperture value is not maintained when changing the focal length (zoom), there is a chance that the shallow depth of field cannot be maintained, and because the aperture becomes smaller, the amount of light entering decreases accordingly, then the shutter must slow down or the ISO needs to be increased to keep the exposure normal. Once the shutter exceeds the “safe shutter”, it will easily lead to vibration and blurred photos, and The increase in ISO will cause the appearance of noise, making the photo unsmooth. Therefore, a lens with a “constant aperture” can make it easier for the photographer to control the exposure and depth of field of the photo, and focus on other things such as lighting and composition.

Why don’t all lenses also have “constant aperture”?
The lens structure of “constant aperture” is more complicated and the cost is dear. Therefore, manufacturers mostly launch some lenses with “constant aperture” in response to the market demand. Having said that, the price of DSLR is also very common nowadays, and some have “Constant aperture” lenses are not too expensive. For example, Tamron’s A16 model (SP AF17-50mm F/2.8 XR Di-II LD ASPHERICAL [IF] Zoom Lens) has a constant f/2.8 aperture, and the price is around GBP300. Really great value!

Do beginners need a constant aperture lens?
Beginners need to learn too much. Once they can often use large apertures such as f/2.8, beginners often forget the importance of aperture value for exposure, which hinders learning. Moreover, these lenses are generally expensive. Beginners can invest money in basic accessories such as external flashlights, tripods, polarizers, etc. Only when they need to upgrade the lens can they adapt to the actual use situation and see if they need to buy a lens with a “constant aperture”!

Written By Collin Smith @ remotes.works Holborn London