How Aperture Affects Sharpness

October 19th 2020

Brennan Kauffman

The Aperture is a portion of the lens that allows a certain amount of light in. This is measured as an F stop which is commonly marketed with a lens and referred to as a fast lens when it has a larger F stop. The larger the aperture is (f1.4,1.8,2.8 etc) the more light is let in allowing for photos to achieve a bokeh effect or allow for a photo to be taken with less noise at night. The downside to a larger aperture is they tend to not be as sharp when they’re wide open causing parts to be slightly out of focus even if the lens is set to infinity. The larger the aperture, the shallower the depth of field is. In landscape photography, especially in the day, you want a smaller aperture, most of the time you would want to use f8-f11 or find the sweet spot on the lens which varies by the lens, but is usually in that range, my Nikkor 85mm is sharpest around f8, while my Nikkor 24-120 is sharpest at f11, from my experience trying to avoid lens diffraction. The longer the lens is the less aperture plays a role in a blurry background because of compression. In animal photography, I tend to avoid wide-open apertures as they tend to leave parts out of focus though there were times where I used it wide open at f4 and you can achieve a similar bokeh effect by using a longer lens with lens compression. The two photos above used smaller apertures yet still achieved a properly blurred background.

There are situations where you need to use a wider aperture in such as Astrophotography which needs as much light as possible or if you want to achieve that “bokeh effect” on a shorter focal length. Bokeh is an effect where the lens creates circles or polygons in brighter lit areas when focused on a subject to create a smooth background which can create a great look for portraits in cities and brighter environments. My fastest prime lens is an f1.8 in which I rarely use at 1.8, I tend to aim for 2.2, 2.8 to achieve a similar effect while keeping the image at its sharpest.

The images above are all on the same lens but had different apertures showing the effect of a large and small aperture. as seen in the photos, the last image which was the smallest at f16 which has minor lens defractions but is noticeably sharper than the f1.8. When the lens is at f 3.5 it still achieves a similar blur but is quite a bit sharper. Lens diffraction occurs when the aperture is too small and shift out of focus on edges of a subject, hence why each lens has a sweet spot.