What is the relation between shutter speed and exposure compensation in aperture priority?

Question

I was using a manual focus lens with canon 600D and notice something in aperture priority mode. When I was applying the exposure compensation of -2EV, I had a shutter speed of 1/20 sec and when the exposure compensation was set to normal (0 EV), the shutter speed reduced to 1/4. Yes, shutter speed reduced with normal exposure compensation.

What voodoo magic is this? Why I have to deal with slower shutter speed for same ISO and aperture? How to solve this problem?

Further info:

Manual lens: SMC Takumar f/1.4 50mm
M42 -> EOS adapter: EMF AF chip (aperture can be changed)
ISO: 400

Answer 1

Fortunately, it's not actually voodoo magic. You have set a fixed ISO of 400, and you're in aperture priority mode, which means you're choosing the aperture and not giving the camera control of that.

That means the one variable the exposure program can change is shutter speed. When you tell it you want the exposure to be two stops darker, the only thing it can do to meet what you're asking is to shorten the exposure duration.

Two stops should mean halving the shutter time twice, so one would expect it to go from ¼ to ¹⁄₁₆ rather than ¹⁄₂₀, but there's probably some rounding going on, and maybe slight changes in the exposure in the scene.

You ask how to solve the problem, but I don't think it's clear what the problem is here. It might help to read What is exposure compensation?, but it might also help if you can explain more clearly what you're aiming to do, and what you expect to happen when you change the EV compensation.

Assuming you can't change the lighting, the only way you can get a brighter exposure is by increasing one of shutter speed, aperture, or ISO. If you've fixed two of those, there's nothing to do but change the third.

If you need a faster shutter for your exposure, you're going to need to raise the ISO or open the aperture more. If neither of those are possible, you need to increase the light. If that's not possible, you are out of luck.

Answer 2

The problem seems to be you don't know what exposure is. Exposure is the density of light that hits the sensor controlled by aperture and shutter speed. So in aperture priority mode exposure compensation changes shutter speed.

Now what about ISO. In film ISO was the sensitivity of film to light. It was therefore also part of exposure. A lot of people incorrectly describe ISO in digital cameras as the sensitivity of the sensor and thus party of exposure. That is incorrect. ISO is a gain (brightness) adjustment done after exposure. It is an analog form of post processing.

So why have ISO at all if you can just make it brighter in Photoshop. Good question. On most cameras increasing ISO adds less noise than increasing brightness in Photoshop because ISO is analog because ISO brightness is added before the analog to digital conversion. Some newer cameras are considered iso-less because ISO actually gives no advantage over Photoshop. The main advantage then is for previewing images. Camera makers have chosen not to let you preview underexposed images on an LCD by adding post processing brightness so your need to increase ISO to see the image.

Now, if you do not set a fixed ISO in aperture priority mode, exposure compensation may change both ISO and shutter speed, but only the change in shutter speed affects exposure, ISO affects brightness which is different.