Why does the lower f-stop in my camera makes the photo darker?

Question

I have a nikon d5100 camera with 35mm/f 1.8 lens. I'm not sure what's going wrong with my camera but my photo with larger f-stop numbers is brighter than the photo with smaller f-stop numbers (the ISO and the shutter speed remain the same). Isn't that the photo supposed to be darker on bigger f-stops?

Edit: I'm clearly understood with how aperture works. Just transferred the file into the computer and I have only realized that it was my fault for activating the "AUTO ISO sensitivity control" that messed up the shutter speed and the ISO for the photo I have taken.

This is the link that I have referred to: https://www.dummies.com/photography/cameras/nikon-camera/controlling-iso-with-a-nikon-d5100/

My camera is working fine after turning off it. Thank you to everyone who has given an effort in solving my problem.

Answer 1

... my photo with larger f-stop numbers is brighter than the photo with smaller f-stop numbers (the ISO and the shutter speed remain the same).

Since you refer to "f-stop numbers", it appears you know how F-numbers and aperture size are related to each other. You also appear to understand the effect that changing F-numbers should have on the image. When ISO and shutter speed are kept constant, it's normally expected that increasing the F-number reduces exposure, which causes the image to become darker. This is opposite what you describe.

Brighter images taken with smaller aperture sizes could be explained by:

• Changes in scene lighting.

• Settings that alter the processing of images, such as Active-D Lighting. The following page has sample images that show how Active-D Lighting affects the brightness of images:

  • Nikonites: Active D-Lighting For the D5100

• Changes to shutter speed and ISO. However, you state they did not change in your case.

Answer 2

I explain it like this: The apeture is like a faucet in a sink. The more you open it, the more water (light) you get. F1.8 is wide open,big hole, lots of light and so a bright photo. f22 is only a tiny hole, so only a trickle of light.... photo is dark.

Then, the shutter setting speeds up or slows down the flow of light... just like the pressure in a water line would increase or decrease the amount of water (or light). So 1/30 of a second doesn't flow as much water (or light) as 1/2 second.

Answer 3

The f-number is the denominator of a fraction.

An f/4 aperture is an opening that has a diameter one-fourth (1/4) the focal length of the lens. An f/8 aperture has an opening that is one-eighth (1/8) the focal length of the lens. One-eighth is smaller than one-fourth.

If we are using a 100mm lens, f/4 means the aperture, as seen through the front of the lens, has a diameter of 25mm because 100mm divided by 25mm equals 4 (100 ÷ 25 = 4). If we change the aperture to f/5.6, the diameter of the entrance pupil (the image of the aperture that is seen through the front of the lens) is reduced to about 17.86mm. Thus a ratio of 0.1786 (1/5.6) is smaller than a ratio of 0.25 (1/4).

So the lowest f-numbers have the widest diameters/largest openings when compared to the highest f-numbers with the smallest/narrowest openings on the same lens.