[Marc Sabatella]

Interesting! I might try disabling that red focus point indicator, and get used to using the the circle for a while. That would be less misleading visually. Does anybody else do that?

I do. If you're letting the camera select focus point, I can see wanting the indicator to show you which one, but if you're using center point as I always do, the red squares do nothing but mislead.

[Sean Nelson]

...trying to shoot our doggie by aiming her eyes with center AF point selected. This randomly results in hitting the nose with focus instead of eye (nose would be just on the edge of the center circle)

I'm not 100% sure about this, but I *think* that the AF system is designed to focus on the closest thing within it's field of view. It would make sense, since under normal conditions you would prefer the AF system to lock onto a foreground subject rather than the background.

I'm pretty certain this is how the AF system chooses between the various focus points when you have multiple AF points enabled - what I'm not sure about is whether it applies to different objects within one AF point's field of view. It may work that way, or it may be that it just locks onto the highest contrast object or some such thing.

[Sean Nelson]

I might try disabling that red focus point indicator ... Does anybody else do that?

Like Marc, I leave my camera on center AF only, so I find the red LED to be redundant and distracting.

[wasim_altaf]

I'm not 100% sure about this, but I *think* that the AF system is designed to focus on the closest thing within it's field of view. It would make sense, since under normal conditions you would prefer the AF system to lock onto a foreground subject rather than the background.

I'm pretty certain this is how the AF system chooses between the various focus points when you have multiple AF points enabled - what I'm not sure about is whether it applies to different objects within one AF point's field of view. It may work that way, or it may be that it just locks onto the highest contrast object or some such thing.

What I've learned so far, from another thread recently and some trials, is that in distant focus situation AF -- all, selected, central -- becomes problematic when the subject is smaller than the sensor itself. Center-AF may very well be affected most if it happens to be the largest sensor by area. I've seen cases when the camera went right past the intended subject and locked on a bigger surface in the background. You don't see the difference on the viewfinder or on the LCD, but very visible when magnified to 50%.

This doesn't seem to happen in close range (to me, at least) when the subject usually occupies a greater portion the viewfinder.

[HermanLee]

This is indeed a good finding and really useful.
Thanks, Paul for the trial & Sean for the image.

I found out that my focus area to be similar to this.
This actually will answer a lot of people's confusion about Front Focusing and Back Focusing that happen when we do the test with the paper on 45 degree too.

I'd like to recommend this to be made as sticky by moderator.

[FHPhotographer]

So that would mean that any point within that central focus circle as marked below as orange blocks might be chosen by the camera to lock on even though the hollow red square is dead center of the circle? The problem then isn't with the AF, it's working fine, the problem is that the camera processor isn't rendering the focus point correctly in the viewfinder?
Brian

[NeverSatisfied]

Brian I don't think this specifically addresses your comment, but it seems that in your example, if any one of those orange blocks is higher contrast than the rest of whatever's in the zone, that's what the camera picks for focus.

I took a couple quick informal snapshots of the dog. Plain old brown dog laying on the couch. Three-fourths of the center-AF "circle" contained her head, but about 6 inches behind her, yet still inside the top of the circle, was a black-and-white striped blanket. The camera focused on the blanket.

[Sean Nelson]

The problem then isn't with the AF, it's working fine, the problem is that the camera processor isn't rendering the focus point correctly in the viewfinder?

You have to understand that this is an optical viewfinder. It's not an LCD where the camera can display any mark anywhere on the screen.

The LEDs that light up to show you which focus point has been selected are just that - they're little lamps that are fixed in place. Their purpose is to let you know whether a focus lock was achieved using the center AF point, the upper left one, etc. And as it's been discussed in this thread, the AF sensors cover a lot more area than the lamps would indicate. If the lamps were the size of the AF areas they would be incredibly distracting.

So it's not a fault of the camera that the LEDs can't mark the focus point exactly, it's a hallmark of the basic DSLR design. It's really just incumbent on the photographer to understand how the tool works.

[Marc Sabatella]

To be more specific, the viewfinder *always* displays the red square for the center AF point in the exact center of the viewfinder - it doesn't try to be more specific than that. Similar for the other focus points it just lights up to tell you which AF point was used, but it can't be more specific than that.

Really, I don't think it is possible to learn so much about the AF system that you can anticipate its every decision every time and ever expect to get it to be able to exactly what you want every time. There's too much of its that going to be remain an element of chance. In simple cases, sure, it's simple. But in complicated cases, it's hopelessly complicated.

Which is, again, why my AF lenses all have quick shift - I let the camera try, but I look for myself in the viewfinder not for red dots but at the actual image itself ot see if it looks in focus, and if not, I immediately override it manually - it's practically guaranteed to be far faster and more than multiple attempts to convince the AF system to do what I want.

[torneol]

You have to understand that this is an optical viewfinder. It's not an LCD where the camera can display any mark anywhere on the screen.

The LEDs that light up to show you which focus point has been selected are just that - they're little lamps that are fixed in place. Their purpose is to let you know whether a focus lock was achieved using the center AF point, the upper left one, etc. And as it's been discussed in this thread, the AF sensors cover a lot more area than the lamps would indicate. If the lamps were the size of the AF areas they would be incredibly distracting.

So it's not a fault of the camera that the LEDs can't mark the focus point exactly, it's a hallmark of the basic DSLR design. It's really just incumbent on the photographer to understand how the tool works.

This is the way, how things must explain for beginners, like myself ! Thanks for very helpfull writing for everyone involved.

[FHPhotographer]

Brian I don't think this specifically addresses your comment, but it seems that in your example, if any one of those orange blocks is higher contrast than the rest of whatever's in the zone, that's what the camera picks for focus.

I get the difference between the VF analog view and the LCD digital view, but still and all despite my technical ignorance, I'm dubious that a company that can cram 14.2 million pixel photosites plus microlenses onto a sensor smaller than a postage stamp can't figure out a way to get a couple of hundred points onto a viewfinder screen and get those points to light up the actual AF spot rather than a general "you guess where it is within this area" point.

Imagine the marketing advantage (assuming Hoya/Pentax can ever figure out a good marketing plan) of being to claim, and demonstrate, that your camera AF point in the VF is the "real" focus point and not just an approximation,
Brian

[Maffer]

I get the difference between the VF analog view and the LCD digital view, but still and all despite my technical ignorance, I'm dubious that a company that can cram 14.2 million pixel photosites plus microlenses onto a sensor smaller than a postage stamp can't figure out a way to get a couple of hundred points onto a viewfinder screen and get those points to light up the actual AF spot rather than a general "you guess where it is within this area" point.

I'm with you on this one. One could compare this situation with a rifle which has scope attached in a way that it isn't in line with the gun barrel but few degrees off instead. Then you can try shooting and guess where you gotta aim in order to hit the target.

[monochrome]

Which is, again, why my AF lenses all have quick shift - I let the camera try, but I look for myself in the viewfinder not for red dots but at the actual image itself ot see if it looks in focus, and if not, I immediately override it manually

I was waiting to read that - I would have posted had you not. If I could see better (given that almost all my lenses are Manual anyway) I wouldn't use AF at all. I just go slowly - and since I don't have to charge myself by the hour for a hobby, time is pretty cheap.

[NeverSatisfied]

I was waiting to read that - I would have posted had you not. If I could see better (given that almost all my lenses are Manual anyway) I wouldn't use AF at all. I just go slowly - and since I don't have to charge myself by the hour for a hobby, time is pretty cheap.

Thank you both for yet another excellent point! This is a factor that will surely influence any future lens purchases. (Oh yea I forgot, I was done buying lenses! )
Paul

[wasim_altaf]

Which is, again, why my AF lenses all have quick shift - I let the camera try, but I look for myself in the viewfinder not for red dots but at the actual image itself ot see if it looks in focus, and if not, I immediately override it manually - it's practically guaranteed to be far faster and more than multiple attempts to convince the AF system to do what I want.

I think it helps to know when and where the camera could focus on the wrong subject, so that one can recognize such a case and learn to avoid it or switch to manual-focus. That said, with the stock focusing-screen, smaller (to some) viewfinder, and lack of nose-relief (for me, at least), manual-focusing is no cake-walk, either.