AF points and focus accuracy

[Drake]

I've been using my Rebel XS the other day to shoot couple dozens of pages with text to e-mail to my friend, as my scanner has died recently. I was pretty angry when I found out that quite a lot of the shots were out of focus. We're talking about 30% here. Some were sharper, but there was a visible amount of blur.

It couldn't be camera shake as I was shooting with the lens at 28mm f5,6 and the shutter speed of about 1/100, which with IS should provide sharp images. First thing I thought about was FF/BF, so I pulled a focusing chart from the drawer and made a series of tests. Turns out FF/BF is not a problem. The focusing is just not very consistent.

Since about always I've been using only the center AF point and recomposing after focusing. Of course some of the real shots, like portraits or landscapes were out of focus, but I never realized it was that much of a problem before I shot the sheets of written paper parallel to the focusing plane of the lens.

I know the 7 point 1 cross type AF of my Rebel XS isn't very sophisticated, probably comes from the old Rebel XT, and that the 18-55mm IS lens is no pro-grade either, but I think it should perform a little bit better.

After that I tried shooting the pages of text with all the AF points enabled. Immediately there was a difference, as the camera used 3-5 points to focus, the number of out of focus images was minimal.

And that got me thinking - is using the multiple AF points a good way of making up for the not so accurate focusing system? Of course that wouldn't work for portraits when you want to focus on the eyes, but for general shooting of subjects that are pretty much parallel to the focusing plane, like landscapes etc.

I am also pretty curious how exactly does the AF system work. Does the camera actually use a few AF points to focus the lens, or it just uses one, focuses, and then lights the other AF points in the viewfinder to show what else is in focus?

 

[gsgary]

It could be camera shake, i always use 1 focus point

 

[Drake]

It could be camera shake, i always use 1 focus point

Maybe in the case of a few shots, but generally using more AF points improved the results significantly, so I assume it was also the AF system.

 

[table1349]

It could be camera shake, i always use 1 focus point

Maybe in the case of a few shots, but generally using more AF points improved the results significantly, so I assume it was also the AF system.

Why do you believe that using more AF points improves results? Good tutorial to read: Understanding Camera Autofocus

I normally use one point and choose that one point from the 45 I have available that is on the exact spot I want in focus.

 

[Overread16 4]

Just a point but think of the depth of field that you are focusing as being like a sheet of paper parallel to the front of the lens. Now if you focus on something and then recompose the shot the angle between you and the point you focused on changes, that means your depth of field (bit of paper) is going to shift as well. That can result in your focused subject moving out of focus as you recompose the photo. That is why focus recompose works best with a lens that allows all the time manual focus so that you can use AF for the quick lock and then manually refocus the small adjustment most times needed to ensure that the subject is still in focus after recomposing.

You can also compose the shot and then use the AF point closest to the subject to get the focus lock - that can be more limited with entry level bodies that have far fewer AF points. In addition in the rebels only the middle point is a cross sensor type so its the most accurate - the outer points are ok, but not as good nor as fast as the middle point (This becomes more apparent when shooting in lower light and/or when shooting a moving subject).


As for how AF works the typical kind works by finding the closest point to the camera (in line with the AF point/s) which has a difference in contrast between one area and the next, This is why AF can often not work if you are trying to focus on a totally single coloured surface - eg shooting a white wall which has no light changes/shadows or markings can confuse the AF because there are no contrast changes for it to detect and focus on.

 

[Rocan]

yeah... I choose one point as well. lets say if your taking a picture of a person, and the camera is not paralel to the ground; multi point AF will find a compromise between the distances that the AF points are picking up, but one point will make the image sharp at the one distance you want, which would be the eyes.

 

[Drake]

As for how AF works the typical kind works by finding the closest point to the camera (in line with the AF point/s) which has a difference in contrast between one area and the next, This is why AF can often not work if you are trying to focus on a totally single coloured surface - eg shooting a white wall which has no light changes/shadows or markings can confuse the AF because there are no contrast changes for it to detect and focus on.

I know the AF system is looking for contrast. What I meant asking how it works was: does it use all the enabled AF points at once to focus or does it just use one and then light up all the other ones that should be in focus?

 

[Derrel]

Drake,
A picture is worth a thousand words!!! We could tell from a picture or two, or a 100% crop, if the blur is from camera shake, or simple front- or back-focus. You were using IS; but was the camera on a tripod? Using IS when on a tripod can, at times, actually cause some blurring as the camera's IS tries to quell miniscule vibrations, which can set up a "feedback loop", and actually ruin the shot instead of helping!! Anyhooo...

When working at close distances, such as when copying pages from a book or typewritten/printed 8.5x11 inch sheets, the depth of field is pretty skinny, so if the pages are not parallell with the back of the camera (ie, withy the sensor plane), the top or bottom or edges of the frame can and will quite often be soft or out of focus...so, the camera and lens need to be aligned really well for the best possible whole frame sharpness when doing copy work. Quite often, the tendency is to set the lens to a wide-angle setting, lean over, and snap the shot from a pretty close range,with the focal length in the "short" range---and so, there's plenty of keystoning of the page,and parts of the page look like cvap.

With the simpler cameras with more-basic AF systems, like the Rebels, and the baby Nikons, using more than one AF point will give the camera mote information, and that can, many times, lead to better autofocusing. One problem when shooting text/books is that the central AF bracket is the closest measured distance, but the edges of a page are FARTHER away, so focus and recompose will lead to errors in focusing that are greater than the depth of field at close ranges, unless the lens is well stopped down (to f/11, f/13, maybe even f/16 or f/22). Why???? MANY lenses, and I mean MANY lenses suffer from curvature of field...they do NOT come to focus in a flat plane, but instead, they focus the image in an arc!!! Literally, the center of the frame will be in good,sharp focus, but the edges of the frame will actually come into their best focus behind the center point; this is called curvature of field,and as I said, many lenses suffer from this optical flaw, and it is usually at its absolute worst in the close-range focusing distances. This is one of the critical,key features of a macro lens or a process lens: it delivers a perfectly flat plane of focus from the center of the frame, and across to the edges, and that leads to sharp copy work,edge-to-edge, as long as the depth of field is sufficient for the image magnification in use.

Again, a picture or two would be helpful in diagnosing the image faults; there are about four, or five factors that could be at work in this type of scenario...the lens itself and curvature of field; insufficient depth of field for the magnification; insufficient DOF to cover the curvature of field; focus shift between wide-open and shooting aperture; and a couple more things.

 

[KmH]

..... does it use all the enabled AF points at once to focus or does it just use one and then light up all the other ones that should be in focus?

The camera can only focus in a single plane that is parallel AND EQUIDISTANT to the image sensor. Unless all the AF points are on that plane AND EQUIDISTANT to the camera, the camera can only use one of the focus points to focus.

As far as focus accuracy, cross-type focus points are more accurate than regular focus points.

Also, if the secondary mirror or the window in the bottom of the mirror box over the AF module are dusty, AF performance will suffeer.

 

[imagemaker46]

I use single point focus the majority of the time and even with autofocus and using the Nikon D3s or Canon 1D mkll, both with fast lenses, not everything is in focus, autofocus doesn't always stay in focus, there are lots of times when it just pops out in a split second. As far as camera shake goes, you can get camera shake at pretty much any speed, again, just because it uses has IS technology doesn't mean you still can't have shake, at 100th of a sec, it's pretty easy to get some shake without even realizing it.

I think hitting on having 70% sharp is pretty good, I've had lots of shoots where the percentange is lower than that, most of the time it depends on what you are shooting. If it's something that is sitting still I would guess that the percentage be closer to 90% plus, if it's a moving subject odds are that it will be lower. As good as all the technology is, it's not perfect.

 

[Drake]

Thanks Derrel, that really helps a lot. I'll post a few shots. As said before, I've used the central cross type AF point to focus on the closest to the center line of text. Images shot in Large Normal JPG, for the smaller size.

The first one is almost perfect for the given settings. It's a bit unsharp in the corners, but I guess that's just the way it is with the kit lens in close focusing, or it could be the curvature of field Derrel has written.
http://dl.dropbox.com/u/17041033/IMG_2546.JPG

Now there's one with a problem. The upper right part seems to be ok though, so I guess it's just a result of a small DOF and the sheet not being perfectly parallel to the sensor.
http://dl.dropbox.com/u/17041033/IMG_2514.JPG

Here's a serious one. The text is readable, but if it was a 'normal' photo, I'd probably just delete it as unacceptable.
http://dl.dropbox.com/u/17041033/IMG_2575.JPG

Some of the photos were even more blurred, but they got deleted and reshot, so I can't post them.

 

[bottguardo]

I shoot with an xs as well, and it could just be the auto focus. Mine has a tendency to back focus a lot.

 

[Drake]

The camera can only focus in a single plane that is parallel AND EQUIDISTANT to the image sensor. Unless all the AF points are on that plane AND EQUIDISTANT to the camera, the camera can only use one of the focus points to focus.

This is what I was looking for, thanks a lot KmH.

 

[Derrel]

Your camera is not squared up with your pages...you've GOT to get the lens located centrally,and thus ensuring that the pages are parallel to the sensor. Also, the lens suffers from chromatic aberration, which is a problem with many kit zooms.

 

[pdq5oh]

One thing to remember is, if you're hand holding the camera and close focusing, any movement to or from the subject will cause focus to be off. It's pretty hard to do this and not have that movement affect some shots. The closer you focus, the more critical correct focus becomes.