Question about Focusing

[zebulus]

Okay, im really curious. I have read in many places that the way to focus a portrait is to use the center AF point on the eye, recompose, and then take the shot.

Ive also had people tell me that you should never do that as as soon as you recompose, you lose your focus. I dont see how that is possible, because to my understanding, focus is determined by the distance of the element(s) from the subject. But then I could be wrong, thats why im asking.

I also read THIS which leads me to think my initial thought was correct.

David Hobby actually says

(Just show them how to grab focus on your nearest eye, recompose and shoot.)

But then there is also THIS and THIS that say different.

What is your thought/opinion on the matter? I want to hear some varying opinions on the matter.

Thank you!

-Zeb-U-Lus

[CaptainTom]

Your camera focuses on a flat plane parallel to the sensor at a specified distance from the sensor. Think of this as a piece of paper that intersects the eye of your subject and is connected to the lens.

When you put the paper on the eye (focus) and then move the camera to recompose, the paper moves away from the eye and unless your DOF is sufficient, you will lose focus.

The best way to assure good focus with a small DOF is to compose your photo and then pick the focus point closest to the subjects eye.

[zebulus]

Originally Posted by CaptainTom

Your camera focuses on a flat plane parallel to the sensor at a specified distance from the sensor. Think of this as a piece of paper that intersects the eye of your subject and is connected to the lens.

When you put the paper on the eye (focus) and then move the camera to recompose, the paper moves away from the eye and unless your DOF is sufficient, you will lose focus.

The best way to assure good focus with a small DOF is to compose your photo and then pick the focus point closest to the subjects eye.

Exactly what I needed to know. Thanks Tom. I guess that's what makes having a TON of AF points valuable.

[CaptainTom]

Originally Posted by zebulus

Exactly what I needed to know. Thanks Tom. I guess that's what makes having a TON of AF points valuable.

If we could only convince Canon that more AF points are a good selling point.

[John Godwin]

It has been long time since I studied either geometry or trigonometry, yet if my memory is okay the second example is either not correct or misleading. Granted the distance from the focal point to a spot on the sensor will change when rotating the camera, but the distance from the center of the sensor to the focus point will not change. That is why they use arcs for things that rotate, not straight lines.

BTW either of the first two methods (recompose or shifting the AF point) will give more than satisfactory results in almost 100% of the cases where you compose the shot.

[CaptainTom]

Originally Posted by John Godwin

It has been long time since I studied either geometry or trigonometry, yet if my memory is okay the second example is either not correct or misleading. Granted the distance from the focal point to a spot on the sensor will change when rotating the camera, but the distance from the center of the sensor to the focus point will not change. That is why they use arcs for things that rotate, not straight lines.

BTW either of the first two methods (recompose or shifting the AF point) will give more than satisfactory results in almost 100% of the cases where you compose the shot.

Light usually travels in a straight line and not in an arc.

If you are shooting with a very fast lens and a very short DOF you will see what I mean. The focal point will no longer be on the spot you originally chose unless you move the camera on the exact plane of the original focus.
Look at the example provided by the OP.

[brad]

Great diagram, Tom.. that's exactly what happens.

The distance seems negligible, but if you are shooting with a DOF measured in millimeters, it makes a world of difference. PLUS there is the assumption that when you move the camera like that, you don't lean slightly forward or slightly away as you moved to recompose.

You can minimize the potential for such problems if you choose a focus point that's nearest to your subject.. and thus have to move the camera the least amount between locking the focus and recomposing.

Of course, a deeper DOF will also help.

[zebulus]

Originally Posted by brad

locking the focus

Okay, heres another question:

I know its different on every camera, but how do I go about locking my focus?

[CaptainTom]

refer to pages 73 and 74 of your owners manual.

[zebulus]

Originally Posted by CaptainTom

refer to pages 73 and 74 of your owners manual.

Dang tom, you were dead on!

It just says keep the shutter depressed, lol. I thought there was actually like a lock to where I could take my finger off the shutter. Oh well.

[CaptainTom]

Originally Posted by zebulus

Dang tom, you were dead on!

It just says keep the shutter depressed, lol. I thought there was actually like a lock to where I could take my finger off the shutter. Oh well.

There's always that tried and true method.

Manual Focus

[zebulus]

Originally Posted by CaptainTom

There's always that tried and true method.

Manual Focus

Manual Focus dosent scare me at all, but todays modern lenses just arent designed to be manually focused very well.

[CaptainTom]

Originally Posted by zebulus

Manual Focus dosent scare me at all, but todays modern lenses just arent designed to be manually focused very well.

I agree completely. I yearn for the nice split prism focusing screen of old manual cameras. So far, the ones available seem to hinder the auto focus, so aren't viable for me.

[Dokk]

Originally Posted by CaptainTom

I agree completely. I yearn for the nice split prism focusing screen of old manual cameras. So far, the ones available seem to hinder the auto focus, so aren't viable for me.

The one I always liked best was the diagonal split prism screen in my old Olympus OM-1. Many years ago, I remember focusing with magnifying glass and a ground glass plate. I've tried manually focusing with my E series cameras but my eyes are older now and it seems I can't quite get there most of the time.

[zebulus]

I love my digital SLR, but when I pick up one of my Pentax K1000's, the difference really shows. I dont have to squint to look through the viewfinder. Its nice.