Focus lock on subject

[raider]

One of the tips given by experienced photographers for taking photos in a dim lit environment is to point the camera at an object that has more contrast than the subject of interest, then with the shutter button half-pressed, recompose the picture and then shoot.

Say I am going to take a portrait of a person but instead of focus locking on her eyes, I am focus locking on say another object of better contrast so that the AF can lock on. Would this technique not produce a shot whereby the person's eyes are not tact sharp, something which we all wanted to achieve in a portrait?

Sorry for asking this silly question. I am still a beginner.

[PDL]

Why insist on haveing the camera refocus on every shot? Focus once and shoot many times.
De-couple AF from the shutter button, search the forum for instructions on how to do this - everything from the *istDS to the K20D.

Like most things - never accept the default as the best setup. It is difficult to be creative if all you do is use defaults.

The Elitist - formerly known as PDL

[konraDarnok]

When I take pictures of people in very low light conditions, locking on to some high contrast area is often the only way to take the picture -- say the glint on a pair of ear rings. Sometimes focusing on the "line" formed by their head and the background will work.

Most of this is candid photos tho. If I was doing this in a controlled environment, I'd just use a flash light or some other light to acquire focus if I weren't using a flash.

And yes, you don't really get that 'focused on the eyes' look. But you get the picture.

[Wheatfield]

Originally Posted by raider

One of the tips given by experienced photographers for taking photos in a dim lit environment is to point the camera at an object that has more contrast than the subject of interest, then with the shutter button half-pressed, recompose the picture and then shoot.

Say I am going to take a portrait of a person but instead of focus locking on her eyes, I am focus locking on say another object of better contrast so that the AF can lock on. Would this technique not produce a shot whereby the person's eyes are not tact sharp, something which we all wanted to achieve in a portrait?

Sorry for asking this silly question. I am still a beginner.

Just switch off the AF and focus manually when the AF can't lock on.

[sewebster]

Originally Posted by Wheatfield

Just switch off the AF and focus manually when the AF can't lock on.

I wish it were that easy... when the AF is hunting usually the aperture is quite wide open and unfortunately it seems that the stock focusing screens show more DOF than you actually get, making MF rather hit and miss (FA50 at 1.4 on K200D for instance). I'm not sure I need a split prism, but a screen that actually showed the same DOF as the final picture would be nice.

[Marc Sabatella]

Originally Posted by raider

Say I am going to take a portrait of a person but instead of focus locking on her eyes, I am focus locking on say another object of better contrast so that the AF can lock on. Would this technique not produce a shot whereby the person's eyes are not tact sharp, something which we all wanted to achieve in a portrait?

Yes, if the thing you are focusing on is a different distance from the camera than the eyes, you will potentially have issues. This idea of substituting a different focus target is fine in situations where the *exact* focus point isn't too critical. If you re shooting at a relatively small aperture - say, f/8 - then the depth of field (DOF) is enough that if a person's shirt is focus, their eyes will be too. It's only when shooting at really wide aperture - say, f/2.8 and bigger - that the couple of inches difference in distance might matter. So I wouldb't be using this technique on portraits. Or if I did, I might expect to need to fine tune the focus manually after the AF did its job.

The technique you describe is really intended for less demanding applications that just happen to involve low-contrast subjects.