To put it simply, the EV button allows you to quickly underexpose (darken) or overexpose (brighten) your image. How it works is pretty simple. When you’re taking a photo, the camera’s job is to adjust itself by changing the shutter speed and/or aperture to properly expose your shot so that it’s not too bright or too dark. Some cameras do this better than others but that’s another story.
When you play with the EV button, what you’re doing is telling the camera to either brighten or darken the photo from the optimal exposure it perceives. You can use the EV button in P (programed auto), S or Tv (shutter priority) or A (aperture priority) modes.
In P mode, the camera will adjust the EV by changing the shutter speed and/or the apperture. In S/Tv mode, since you set the shutter speed manually, it will adjust the aperture to compensate. In A mode, the camera will change the shutter speed since you manually control the aperture.
NB: You cannot use the EV button to under or overexpose your photo in M (manual) mode since you control both the shutter speed and aperture manually.
Look at an example. The 1st shot is without EV compensation, in other words how the camera sees proper exposure. It is shot in Aperture Priority so the aperture stays the same. This shot is at 1/640 sec.
Using the EV button, select +1 EV and you'll have this shot at 1/320 sec.
At +2 EV the shutter speed was at 1/160 sec.
It is then underexposed by -1 EV and this is the result. The shutter speed went to 1/1250 sec.
At -2 EV the shutter speed was at 1/2500 sec.
So, as you can see, the camera adjusted the shutter speed to let in more or less light to fulfill my request.
When to Use EV Compensation
You’re probably thinking to yourself: “Great! Now I understand how to use the EV compensation button. Super! OK… when do I need to use this? You say you use it all the time? You don’t think the camera is smart enough for you?” Alright then. When to use it?. You can’t go through all the situations but here are few explanations.
Your camera has a tendency to over/underexpose
You may have this issue with your Nikon D200. The camera seemed to overexpose by roughly 0.3 EV most of the time. Fix the problem by setting your EV at -0.3 and the problem will be solve for general optimal exposures. Simple as that.
You need more shutter speed
If you oftentimes shoot birds and those suckers move pretty fast, to freeze their movement you need as high a shutter speed as you can. And if their also far away and you're at your full 400mm on your Nikkor 80-400mm VR you need speed to reduce or eliminate blur from camera shake. The first thing you need to do is go into A mode and set your aperture wide open (smallest number) to get the most light. Then bring your EV down by roughly 0.7. You would rather have a crisp darker shot to easily recalibrate in post processing than have a properly exposed blurry shot.
Your subject is brighter/darker than your background
Sometimes your subject will be smaller, like a bird in a tree. Let’s say you’re shooting a bright yellow bird perched in a dark green tree and the bird only takes up 1/10th or less of the frame because you’re too cheap to buy that Canon 800mm IS, The Sigma 800mm or that Nikkor 600mm, what your camera does is get a general metering of the frame and adjusts the EV accordingly.
What will happen is the your dark green tree will be properly exposed since it takes up most of the frame which means your little bird will be overexposed and therefore lose all it’s detail. You’ll have a little white spot where the bird is. Not exactly what we want. So with the flip of a button you then underexpose your shot by -1 EV and see if you get the details back. If it’s still not enough bring it down lower until your bird is properly exposed. It’s quick and easy. And of course you can apply this to a dark subject on a bright background to get details back by bringing up your EV.
Top photo is normally exposed. Bottom photo is exposed at -1.3 EV
Bright sky
So you’re shooting this lovely landscape with a beautiful blue sky and poofy white clouds and you forgot your graduated ND filter. Shoot! Ah, but you do have your tripod so you set it up, frame your shot and take the 1st shot at normal exposure. Most of the time (depending on your composition) the land will be properly exposed and the blue sky turns white (overexposed). Darn! What to do? Underexpose your shot (by using the EV button of course) until your sky is nice and blue. Having used a tripod, my composition is the same so
You can easily stitch the land and the sky together in Photoshop™ to make the perfectly exposed photo. Or use the HDR technique. You can also do this by setting up your camera to bracket your exposure but that’s way too long to do in the menus compared to just pressing a button and turning a dial. So there you go! The mysteries of the EV compensation button are no more.
