Focus Settings?

[Overread]

What do you do if you have 2 people in a shot and want both sets of eyes in focus?

I have a 50mm I'm trying this on and I'm having a hell of a time. Shooting with a D5000. Any advice?

If you have two subjects you have to do two things to get them both in focus. The first is to have the subjects at the same distance from the lens - when at the same distance the focus should (in theory) cover both of the sets of eyes. However the other side is to use a smaller aperture (a bigger f number). This increases the depth of field (area of photo in focus) and a deeper depth of field means that you can get more in focus in the shot. Remember a 50mm f1.8 can shoot very wide, but at f1.8 you can find it hard to get both sets of eyes on a single person in focus - stop down to a smaller aperture like f4 or f5.6 and you have more depth to work with. Often many studio photographers are shooting at f8 not just for sharpness but for the heightened depth.

Now if you recompose the shot by holding that paper in that position (holding the focus) you run the risk that when you move the lens angle, the paper will also change to follow it and in doing so the plane of focus will shift off the area you selected before.

I thought that is why you select AF-S (single) as opposed to AF-C (continuous) or AF-A (auto, where the camera decides if it wants to use single or continuous).

But if I use AF-S the focus will not change off where I had it set?

Remember the paper idea - the focus isn't picking up on a subject within your photo, instead its finding contrast differences* and then setting the focus so that that point of contrast difference (that you select with the manual AF point selection or that it selects if you let it pick the AF points to use) is within focus. Essentially its calculating the distance between the lens and subject and then setting the lens to the point on its focus scale where that distance is within focus.

In single mode AF the camera does this once each time you start the AF mode (either with the dedicated AF button on the back of the camera or from a half depression of the shutter button) and once its set the "distance/focus" it holds that position until you tell it to find the focus again. This means that the focus/bit of paper remains at a fixed point/distance - so if you recompose the distance from lens to that specific point you started with will change - if it changes enough it might slip out of focus - which is why you often have to adjust the focus manually.

In AF-A or continuous AF modes the camera keeps detecting at the selected AF point and continues to measure the distance and set the focus to that point.

This can be confusing if you are moving from cameras which have so called face detection and other auto features - thus far the DSLRs are not doing this and are just using points and measuring the distance before setting the focus.

*contrast differences is why if you try getting the focus using AF, on say a single coloured subject with no contrast differences (eg just a blank blue sheet) then it can sometimes fail.

 

[ababysean]

OHHHHHHHHH

Is this why my camera would NOT focus on her WHITE dress at all yesterday?? I wanted to focus on just her belly in some shots but the camera would not give me the beep or green button. Her dress was white, I'm wondering now if that is why?

***the contrast focus deal****

 

[Overread]

I would think so = if the dress was close enough that it was dominant and there was nothing for the AF to lock into in contrast differences surrounding it then yes it might very well be unable to get a lock on. This is why backbutton AF and USM/HSM lenses are popular choices since then you simply don't press the backbutton AF button and use the alltime manual focus to get the focus manually - of course with regular lenses you just flick the AF on/off switch on the lens to get manual focus back.