Tag Archives: exposure

Explaining Exposure

Now, I’ll talk a bit about your camera and exposure. For any given light situation metered, there is an ideal exposure value (0) with it’s corresponding aperture, shutter speed and ISO settings. By default, a camera’s light meter will seek to expose anything being metered to appear as 18% gray, or Zone V in the Zone System [I'll later be posting some info on the Zone System]. To your camera, a perfect exposure is making everything look like 18% gray.

 
Meaning, that if you point your camera at something primarily dark, or spot meter on something dark, the camera will give you a reading that makes those dark areas appear 18% gray. This will over-exposure your photo, blowing out whites and losing all detail there.

 
The same is inversely true for metering on something primarily white in a scene. The entire photo will be under-exposed with a lot of detail on whites, but with those white areas appearing as 18% gray. All detail in dark areas is then lost.

 
 Essentially, what is happening is this:

For this reason, 18% gray cards exist. Metering on an 18% gray card will give you the most ideally balanced amount of detail possible that your sensor or film can capture at both the dark and light ends of the spectrum. The exposure you are aiming for is likely to be something like this:

That is the ideal shot. Artistically, you may want to shoot high or low key over-exposing white or under-exposing darks respectively.
  
  
In any case, if you don’t have an 18% gray card and you seem to be getting photos that are way too dark or too bright you need to pay attention to the scene you’re shooting. Is it primarlily light or primarily dark? Are you spot metering on something that deviates far from a middle gray? If that is the case, there are faster, easier ways to compensate for it than to pull out an 18% gray card for every shot. Your camera has a EV scale that looks something like this:
 

 
You’ll need to do some Exposure Compensation. If your photo is too dark then you need to over-expose from the camera’s “correct” reading. so adding one [+1] or two [+2] stops might help.
 
If your photo is too bright then you need to under-expose from the camera’s “correct” reading by stopping down one stop [-1] or more.
 
If you plan on doing post processing, on digital you may want to under-expose the photo a bit [-1/3] and for film you may want to over-expose a bit [+1/3]. If you want to know more on why you should do this refer to my previous post [Film vs Digital Tidbit: On Exposure].

Film vs Digital Tip: On Exposure

When you plan on post processing your photos there is a trick you can use to get more detail in the lights and darks in one exposure. For digital, underexposing a bit [up to -2/3 of a stop] is best for retaining details in the highlights. It’s not difficult to pull detail from dark areas in digital photos with an image editing program, but blown out highlights can’t be recovered. For film, overexposing [+2/3 of a stop or even more] works best for capturing and retaining details.

The reason comes down to the way each format captures images (and how as well, don’t rule out film as irrelevant just yet). If your capturing negatives, then the area that the light hits on the film turns dark. These dark areas on the film are actually the highlights when transfered as a positive print. Exposing it longer captures more detail throughout and there is more to work with than just clear film. The opposite is true of digital because it’s not capturing a negative. The area the light hits is corresponds to the highlights. Overexposing will lead to less detail to work with.

In addition to that, digital sensors don’t have the ability to capture a wide range of values in the highlight end of the spectrum. Negative film is special in that it is able to retain remarkable amounts of detail even in somewhat extreme overexposure. Details can later be recovered when printing or converting to digital. A white digital image is basically the same as a clear negative…if that all makes any sense…

So why should you care about this? No more washed out skies in landscape photos! :-)

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