Something wrong with this photo, can't fix it

[fazhar]

There are 8 combinations of shutter speed, lens aperture and ISO settings that all deliver the same exposure.

you can change the settings based on what is known as a stop of exposure. A stop of exposure is a doubling or a halving of the amount of light.

For now we will ignore the ISO setting.

If you change the shutter speed from 10 seconds to 5 seconds, that is 1 stop of shutter speed change and reduces the shutter time such that it will let in 1/2 as much light.
To keep the exposure constant we can double the area of the lens aperture making it bigger so the lens aperture lets in 2 times more light. Opening the aperture from f/18 to f/13 doubles the area of the lens aperture.
Since the shutter speed was adjusted to let in 1/2 as much light each change balances the other so there is no change in the exposure.

F/13 will likely still degrade sharpness from diffraction so we need to open the lens aperture 1 more stop, from f/13 to f/9 to maintain the exposure.
We also need to speed up the shutter another stop by changing it from 5 seconds to 2.5 seconds.

Using f/9 diffraction should no longer be an issue.

But, by changing the shutter speed from 10 seconds to 2.5 seconds the car light trails in the photo will be 4x shorter

So now lets again consider the ISO setting.

If we change the ISO setting by 2 stops from say ISO 400 to ISO 200 for 1 stop of change, and then from TSO 200 to ISO 100 for the 2nd stop, you can then return the shutter speed to 10 seconds and still have the same exposure you started out with and have no change in car light trails.

Of course you could have just left the shutter speed set to 10 seconds and just Changed the ISO setting, if the ISO you started out at allowed doing so.

I see the settings you used were 10 seconds, f/18. and ISO 200. The 600D (T3i here in the US) has a base ISO setting of ISO 100 so you could only change the ISO by 1 stop.

10 seconds, f/13. ISO 100 is the same exposure as the 10 seconds, f/18, ISO 200 that you used.

Changing from f/18 to f/13 lets in 2 times more light, while changing from ISO 200 to ISO 100 makes the image sensor 2 times less sensitive to light. A net change of 0.

Verrry interesting. I had always let my camera adjust these factors while I modify only one or two of them. I definitely need to move to manual mode.

So to confirm my understanding, to increase shutter speed, reduce ISO and lowering F-stop, the overall exposure will remain the same with a zero net change, but it will eliminate defraction from the photos. Right?

 

[KmH]

Diffraction is a function of the lens aperture, and a larger aperture (smaller f/number) will reduce the focus softness caused by diffraction. A larger lens aperture will also reduce the size of the diffraction spikes seen emanating from bright light sources in the photo.

A net zero change in the exposure, but as mentioned changing the shutter speed will affect the light trails.
changing the lens aperture will affect the depth-of field (DoF), but the larger point of focus distance used for landscape/cityscape photography limits how much the DoF will change.

On most photography forums, lens aperture is often described as being the only factor that affects DoF.
In fact, point of focus distance has to be considered right along with lens aperture. To a lesser extent lens focal length and the camera's image sensor size also have to be considered.

Having a good understanding of the exposure triad - shutter speed, aperture, and ISO - helps you keep track of any settings the camera is choosing for you.
The bottom line is o the camera is just a stupid machine that follows rote programming and cannot actually think.