Sharp enought ? Focusing point precision.

I read today a post I can't find about the focusing point which I've been searching for some minutes.

As long as I have understood, the photographer was telling that using the center point for focusing, compose and then shoot was not a good issue.
We should use the correct focusing point for each picture.

I have a focusing problem which irritates me very much.

Today I did use the center point - I promess to correct from now on - recompose and shoot and the photos are not sharp as they should be.

Then I wonder how shall I shoot people moving all the time if I don't have time to change to the correct point I'm needing ?

Shall I set the points to automatic ? Shall I use AI Servo ?

I thing auto is the best option (is it really ?). At least for non sport situations...

I don't want to disguise the unsharpness with Unshap Mask and other "instruments" !

Teeps needed, please.

4 pics are here not as sharp as I wanted !

Antonio,

What camera are you using and what images had problems?

When one uses the center point and then recompose, the focus point swings to where you want but the plane of focus will now be in front of the object of interest because oblique angles yield longer distances as the are the hypotenuse of a right angled triangle.

The trick is to find something in the same plane as the object of interest very close by and then if needed be switch to manual focus and bracket the focus.

Hopefully, the 1DIII should do better in lower contrast areas especially with more light with larger aperture lenses.

Asher

Hi Antonio,

As an architect, you could do a couple of scale drawings. Roughly speaking, if you use centre focus point, and you are about 12 foot from the subject, and set the focus, but then swing the camera, so that the centre point has moved about two foot, the plane of focus will have moved back one inch wrt the subject. In other words, use the camera centre focus point, but focus on the nose. If you then point the camera at the 'belly button', the eyes will be in focus. If you are closer than 12 ft, then you have to compensate more.

In most of your images, the background is some distance away, the overalls are white, I can't see where the focus plane is, and I guess you were not twelve foot away.

(I did say 'roughly speaking')

Best wishes

Ray

Here's a link to an article regarding the pitfalls of "focus and recompose". Note that this causes problems mostly in situations with very narrow dof - close shots with wide apertures.

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

A very interesting article thanks for the link! I have always had an inkling that focus-recompose is not ideal (or even, it sucks, in the author's words) and I now understand why. This is the sort of stuff I expect on a photography forum - not debates about television programmes as was so often the case on the forum I boycotted.

I would like to thank everybody for the answers posted here.
Only today, received a mail of warning.
The link is interesting.

I would like to say to Ray West that I had thought of the focal plane before.
This problem is bigger if you are using a large aperture.

Thank you.
Cheers.

Hi Antonia,

The problem is the same, whatever lens, whatever aperture. However, if the angle changes (i.e. closer is less acute angle, and plane of focus will move more) and as always a wider aperture will mean shallower depth of field - so you need to be more precise in your compensation.
I am not too certain, but there may be another effect, in that the plane of focus is itself not flat, which would depend on the type of lens.

On the 20D, the centre point is about the only one that works, - well its far better than the others, - and it is very rare that I use any other.

Best wishes,

Ray

Ray West said:

Hi Antonia,

The problem is the same, whatever lens, whatever aperture. However, if the angle changes (i.e. closer is less acute angle, and plane of focus will move more) and as always a wider aperture will mean shallower depth of field - so you need to be more precise in your compensation.
I am not too certain, but there may be another effect, in that the plane of focus is itself not flat, which would depend on the type of lens.

On the 20D, the centre point is about the only one that works, - well its far better than the others, - and it is very rare that I use any other.

Best wishes,

Ray

Click to expand...

Hello Ray,

I'm Antonio and not Antonia. Antonia is the feminine of Antonio. It does not matter because it does not offend me at all, but it simply is a little correction. You did un-voluntarily, I know. It's OK.

But in your post you said that "On the 20D, the centre point is about the only one that works, - well its far better than the others, - and it is very rare that I use any other."

I am missing something here. I think I am supposed to understand that the center point is very good compared to the others and because of that you use it all the time.
Is that right ? Did I understand correctly ?

Thank you Ray. Comments and helps much appreciated.

Oh ! I still want to say that I want/wish, from now on, to use all the points on manual mode. I mean changing the focus point to the most convenient position after composing. Mainly if I am using wide apertures and medium millimeters lens, like th 24-70 zoom lens.

Hi Antonio,

sorry about the previous name typo.

For the 20d, the centre af sensor is a 'cross sensor' the 8 others are linear sensors. The cross sensor will be better at f2.8 and below. (or is it above i.e. not much better at f4) here is an explanation - http://photonotes.org/articles/beginner-faq/cameras.html#cross

I've been looking for a chart that was published - the sensor area used is bigger than the little red box you see, and often does not line up exactly. They need a sharp edge/high contrast boundary to lock on. the centre one, any direction, the others vertical lines only.

Best wishes,

Ray

Hi Antonio,

I still can't find the chart - it was not provided by canon, but by a user, however after an hour or so with google, I expect this may give you something more to think about....

http://www.fredmiranda.com/forum/topic2/241524

(it's archived, but you should be able to get it.)

I think, if you weigh it all up, you may decide to use just the centre focus point too. I don't know if it says in the fm piece, but if you use multi point af, it focusses on the nearest point. It all starts getting too complicated, even for clever chaps like us ;-)

Best wishes,

Ray

Ray West said:

... sorry about the previous name typo... Best wishes, Ray

Click to expand...

Ray,
There is nothing to sorry about !

Tim Gray said:

Here's a link to an article regarding the pitfalls of "focus and recompose". Note that this causes problems mostly in situations with very narrow dof - close shots with wide apertures.

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

Click to expand...

Thank you Tim
Interesting article to read.
I knew already something about the subject, but I always learn something more ...

Ray West said:

Hi Antonio,

sorry about the previous name typo.

For the 20d, the centre af sensor is a 'cross sensor' the 8 others are linear sensors. The cross sensor will be better at f2.8 and below. (or is it above i.e. not much better at f4) here is an explanation - http://photonotes.org/articles/beginner-faq/cameras.html#cross

I've been looking for a chart that was published - the sensor area used is bigger than the little red box you see, and often does not line up exactly. They need a sharp edge/high contrast boundary to lock on. the centre one, any direction, the others vertical lines only.

Best wishes,

Ray

Click to expand...

Thank you Ray.
I already knew photonotes. But you gave me a very good link/tip to it.
I can remember the split circle viewfinder screens of the Nikons back to 1974.

Unfortunally there is no such item for the 20 D.
Focusing problems are in fact a most common issue with photographers, as far as I know.
I wouldn't mind to change my screen for a split one, but I can't.

At the moment I own a 20 d and a 350 D.
Last week I asked my daughter if she wanted to sell me her 20 D and she told me that she didn't.
I think I don't want to buy it anymore.
I have rather a 5D when I can aford it.
Then, I'll be able to change the screen for better accuracy in focusing.

I have read also that "How good are EOS cameras and lenses for manual focus work? Frankly, not great. EOS cameras and lenses are both completely optimized for autofocus operation." (quoting from photonotes).

Anyway, as long as I will be using both my Canon - 350 and 20 - I'll keep an eye on focusing and will be very carefull when using both cameras.

On the other hand I have all L lenses and it is a waiste not to use them with a better body than the 20/350 Ds

Thank you very much for your comments and time spent.