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Dis­cover how your camera mea­sures the light and comes up with an ex­po­sure

Digital Camera World - - CONTENTS -

Dis­cover what’s go­ing on in the realm of in-camera me­ter­ing

YOUR camera’s me­ter­ing sys­tem mea­sures the in­ten­sity of the light re­flected by the scene that you’re pho­tograph­ing and de­ter­mines the ap­pro­pri­ate set­tings that will be re­quired to record that light.

This is known as a ‘through the lens’ re­flec­tive light read­ing. You can also take an in­ci­dent light read­ing us­ing an ex­ter­nal light me­ter, and set the sug­gested ex­po­sure man­u­ally on the camera. In­ci­dent light me­ter read­ings have the po­ten­tial to be more ac­cu­rate, but the process isn’t as con­ve­nient as us­ing your camera’s me­ter. Lightly tap­ping the camera’s shut­ter re­lease but­ton ac­ti­vates the me­ter, and a sug­gested aper­ture and shut­ter speed com­bi­na­tion – one that should give a ‘cor­rect’ ex­po­sure, based on the light­ing and the ISO sen­si­tiv­ity – is dis­played in the viewfinder. There are a range of fac­tors that can lead to the ex­po­sure not be­ing quite as cor­rect as it should be, which we’ll get to shortly.

The me­ter­ing sen­sors found in to­day’s dig­i­tal cam­eras typ­i­cally do more than just reg­is­ter the in­ten­sity or lu­mi­nance of the light com­ing through the lens. They can also take into ac­count the colours in the scene or sub­ject be­ing pho­tographed, and can pri­ori­tise the ex­po­sure for the parts of a pic­ture that are in fo­cus. These days, me­ter­ing sen­sors are densely packed with pix­els and are ef­fec­tively more like low-res­o­lu­tion imag­ing sen­sors; in some high-end cam­eras, they’re be­ing used in con­junc­tion with the camera’s aut­o­fo­cus sys­tem to im­prove the de­tec­tion and track­ing of ob­jects us­ing the colour in­for­ma­tion, as well en­abling face de­tec­tion through the viewfinder.

Your dig­i­tal camera might have a sin­gle me­ter­ing sen­sor, but there are a range of dif­fer­ent me­ter­ing modes or pat­terns that

you can choose from. These de­ter­mine how much of the scene is me­tered and whether the camera makes any ad­just­ments to the me­ter read­ing. When the camera’s set to an ad­vanced shoot­ing mode, such as Aper­ture Pri­or­ity, you can choose an ap­pro­pri­ate me­ter­ing mode.

The de­fault mode is ’pat­tern’ me­ter­ing, com­monly re­ferred to as Eval­u­a­tive, Ma­trix or Multi-pat­tern. In this mode, the camera splits the im­age into a se­ries of small zones from which it takes in­di­vid­ual me­ter read­ings. These read­ings are then com­pared be­fore the camera sug­gests an ex­po­sure set­ting that should give a bal­anced re­sult. It ef­fec­tively ap­plies its own ex­po­sure com­pen­sa­tion to take ac­count of vari­a­tions in bright­ness and where it de­ter­mines the sub­ject is.

The other me­ter­ing modes that you’ll find on your camera don’t an­a­lyse the scene in quite the same way, and are best re­served for oc­ca­sions when you have the time and ex­pe­ri­ence to take ad­van­tage of them. Take spot me­ter­ing, for ex­am­ple. This only takes around 1.5-3% of the pic­ture into ac­count, mak­ing it un­beat­able for tak­ing pre­cise read­ings from small ob­jects in a scene, or when you want to stop large ar­eas of dark or light tone from

The me­ter­ing sen­sors in to­day’s cam­eras typ­i­cally do more than just reg­is­ter the in­ten­sity of the light through the lens

fool­ing the me­ter. The draw­back is that the spot me­ter is con­fig­ured to pro­duce a mid-tone ex­po­sure value. Point it at a mid-tone sub­ject, such as lush green leaf, and you should get an ac­cu­rate ex­po­sure. But point it at an ob­ject that’s ei­ther brighter or darker than mid-tone and you’ll end up with an im­age that’s ei­ther un­der­ex­posed or over­ex­posed. (See ‘Me­ter­ing prob­lems’ op­po­site.)

Pat­tern me­ter­ing doesn’t al­ways get it right ei­ther. It can come un­stuck in high-con­trast con­di­tions for in­stance, and it won’t take into ac­count any cre­ative ef­fect that you’re try­ing to achieve. In both sit­u­a­tions you can use ex­po­sure com­pen­sa­tion to fine-tune the re­sult. Shoot­ing raw will en­able you to tweak the ex­po­sure when you process the re­sult, but you might as well try and get it close to per­fect in-camera.

light me­ter An in­ci­dent light me­ter can be used to mea­sure the light fall­ing on the sub­ject man­ual ex­po­sure The ex­po­sure is then man­u­ally set on the camera IN­CI­DENT

in­ci­dent light The light fall­ing on the sub­ject is re­flected to­wards the camera camera me­ter The camera mea­sures the in­ten­sity of the light en­ter­ing the lens RE­FLEC­TIVE

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