F/2, F/4 or F/8?

 

85mm, F/2, 1/200, ISO 100

We compare an expensive f/2 lens to another lens which can only achieve f/4 of f/8 by shooting two pictures on my 85mm f/1.8 with different f-stops. The goal is to state as clearly as possible the benefits and drawbacks of a faster lens. A variable "kit lens", like the Nikon z 24-70 f/4 is so good nowadays, that we really need to have a clear idea why to use anything different.

85mm, F/4, 1/2000, ISO 400

Have a look at the images on this page. They were taken with manual settings and developed in Lightroom with only very gentle tweaks. 

The f-stop between the images changed from f/2 to f/4 by two stops. With the same shutter speed, the exposure will reduce to 1/4 of the previous exposure. Note, that the exposure is the amount of light per area on your sensor. You could counteract by taking a 4-times longer shutter, i.e., 1/500. 

But you can also compensate by setting a two stops higher ISO number, which is essentially a brightening of the image data. I did the latter in the image above, because there was some wind and I needed to keep the short shutter speed of 1/2000 to avoid motion blur in the flower.

When compensating the change in f-stops with the ISO as I did while keeping the shutter speed, the following differences occur.

• Since increasing the ISO essentially is a lifting of the brightness values, the noise will become brighter too. But we have reduced the exposure. Thus the signal has got closer to the noise, i.e., the signal to noise ratio became worse. However, you won't detect much more noise in these images. That is partly to more noise reduction and partly to the already very low noise levels of a full frame camera at this exposure. The noise reduction will indeed destroy a bit of the details. But you will have a hard time to actually see this in this instance.
• The depth of field is smaller. You can spot this on the rose in front. It is more apparent in the f/8 version below. For portraits, I find the DOF of f/2 sometimes too narrow. For group shots or even two faces, it is definitely not sufficient. Note, that switching to another focal length and changing the distance does not help if you fill the frame with the subject. The DOF will almost only depend on the f-stop.
• The background became more blurry. That is the most prominent effect of a wider lens. It is the dreamy and smooth isolation effect we want. I would argue that is is not always the best solution. In fact, the f/8 image has something too. The value of this isolation effect depends on the subject and the background. Moreover, we can often achieve more background blurriness with a longer lens much more easily, and there will also be less of the background in the image.

85mm, f/8, 1/2000, ISO1600

Below is a portrait with a very wide aperture. It demonstrates the special effect of a very shallow depth of field. I prefer f/4, or at least f/2.8, for portraits.

85mm, f/1.8