EV compensation question

back in the day, you would set the ASA on your camera's meter (or hand-held) to give it some reference to the sensitivity of your film. it would then give an indicated exposure for that film 'speed'...

on my AE-1, and on the other 'automatic' cameras, there was an exposure compensation button that would alter the actual exposure settings by 1-1/2 stops (i THINK)...

this is how i understood it. what ever light you had and what ever film speed you had dialed in might give you an exposure combination of f5.6 and 1/125 of a second. if you then hit the button it would cause the aperture to be closed to only f3.2 resulting in a 1-1/2 stop increase in exposure. correct?

with the DSLRs does dialing in EV compensation alter the aperture (or shutter speed depending on shooting mode selected) does it alter the ISO setting (behind the scenes) or does it just screw with the sensors sensitivity?

i shot some hockey over the weekend and dialed in an extra 2/3 stop but i didnt pay any attention to whether or not it had any impact on my aperture (shooting Tv mode)...

just curious...

Until recently DSLRs worked like the film cameras; they had to work that way because ISO was fixed for them as much as it was for film as long as you didn't change the setting. Today, however, we have Auto ISO which you can enable or not. If it is enabled, the EC changes the ISO in most models, although there can be some cameras in which in Av a +EC will cause the shutter speed to decrease to a certain point based on the focal length of the lens and only if additional exposure increase is needed, the ISO will increase.

RIght. It all depends on how you have the camera setup. If you are in Av mode, it will compensate with changes to the shutter speed. If you are in Tv, it will make changes to the aperture. This is assuming the auto-ISO is disabled. I am not a fan of having a second floating variable, so I never use it. Maybe someone else knows for sure how it chooses a setting in auto mode. For instance, in shutter priority at 1/125 the exposure could correct at f8 and ISO 400. But it could also be correct at f5.6 and ISO 200, or f4 and ISO 100. How does it choose? I really don't know but I would assume that the exposure compensation just changes the EV value it needs to calculate, and then generates the settings using the same algorithms.

On the couple of occasions that I've fooled around with it, Auto ISO will run the ISO up to the boundary of that particular camera before changing the other floating variable. This is one of the reasons I've hated to use it.

kfreels wrote in post #15068176
RIght. It all depends on how you have the camera setup. If you are in Av mode, it will compensate with changes to the shutter speed. If you are in Tv, it will make changes to the aperture. This is assuming the auto-ISO is disabled. I am not a fan of having a second floating variable, so I never use it. Maybe someone else knows for sure how it chooses a setting in auto mode. For instance, in shutter priority at 1/125 the exposure could correct at f8 and ISO 400. But it could also be correct at f5.6 and ISO 200, or f4 and ISO 100. How does it choose? I really don't know but I would assume that the exposure compensation just changes the EV value it needs to calculate, and then generates the settings using the same algorithms.

that's how it works with the rebels, I'm also not a fan of auto ISO so I'm not sure how it works with EC

This must be a shutter speed/aperture adjustment then (in my camera) since I've never had it in auto ISO...

Behavior of Auto ISO is likely to vary based upon which model Canon you are experimenting with, as Canon has altered Auto ISO from a scarcely usable feature with absurd choices to a somewhat useable feature with not so many absurd choices!

Using my 40D as the testbed...

• In Av mode and fixed ISO, if I alter EC the aperture remains fixed and the shutter speed changes.
  
• In Tv mode and fixed ISO, if I alter EC the shutter speed remains fixed and the aperture changes.
  
• In Av mode and Auto ISO, if I alter EC the aperture remains fixed and the shutter speed changes.
  
• In Tv mode and Auto ISO, if I alter EC the shutter speed remains fixed and the aperture speed changes.