understanding "ev compensation"

[arkansas72762]

When using ev compensation, where does the compensation take place. Is it an adjustment to ISO, shutter speed,??? Something has to give right? Does this lead to increased image noise in low light? Thanks for the help.

[pasipasi]

In Av mode, ev compensation affects the shutter speed, with Tv mode it affects the aperture and in TAv mode it affects the ISO speed etc

[embedded]

As on film camera's it really is telling the metering system to add or subtract exposure. On previous film Pentax, Canon A1 it was under the rewind and on the same multi-level dial as the ISO. It was considered + or - an F stop by 1/4 F-stop.

Kodak added this on the first point and shoots It can be adjusted in a very Pentax way on a Kodak CX7410 with the front thumb dial using the Kodak jog shuttle front thumb wheel. (I like that by the way you click the front thumbwheel in until the triangle appears over the item you want to adjust through the viewfinder.)

On my K20D e/v appears to add or subtract the number of f-stops I asked it too.

It does not increase or decrease noise of itself. If you asked it to add +1 ev it will may push up or down the "apparent auto iso" for shooting and increase noise in low light.

Most people think the K20D and K10D optimize about -1/2 to -1 ev. Compared to Kodak digital point and shoot they are right. Compared to Tri-X pan in a Canon A1 or Pentax Spotmatic I think the K20D is right because it gives more dynamic room in post processing. I used to take advantage of that in spades on an enlarger to dodge out and burn in parts of my shot to get some shots that showed the crispness of Ansel Adams. (Not nearly the lovely composition though!) Now you do that in Photoshop...(with the Tri-X plug in)

[Ron Boggs]

pasipasi gave the nice short and sweet answer.

The long winded extra explanation is that the type of metering will change the amount of needed compensation. In spot and center weighted metering, you'll need more compensation, especially for lightening. Snow scenes may need as much as +2 compensation or as low as +.7 depending on the reflectivity and light conditions. Beach scenes, sand, wheat colored fields, water all commonly require exposure compensation to the plus side. Dark forests, pine, firs, cedars etc generally need some compensation to the darker side. But not that different than when in matrix metering...

If you shoot in matrix metering mode, you'll still need to use some compensation, but usually not as much as with spot or cw metering. Even bright snow generally requires +1 or less and many dark forested areas work out at -.3 or maybe -.7. This is because matrix metering is designed to precompensate for subjects that are not exactly mid-tone. It just isn't perfect at the task and you can "bump" it a bit with ev comp.

Any subject (and background) that aren't mid-toned will probably benefit from a bit of exposure compensation. When in doubt I do a 2 shot bracket--first shot at the ev I expect to work and a second shot .7 stops either brighter or darker based on my best guess that if my chosen exposure is wrong, it would likely be brighter or darker not both. This method worked very well for me in the film era and still serves admirably with digital.

Sorry I'm no help with indoor subjects. Just not part of my experience...but the need for ev comp exists indoors just as outdoors--think white wall background requiring exposure compensation to the plus side...