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Auto Focus Tips

Wed Jul 24, 2002 12:18 am

In the past, I've had some problems focusing correctly on aircraft approaches or aircraft take-offs.

Many times after I auto focus, the photo becomes out of focus because the plane is not fixed and is moving closer to me, thus a different setting of auto focus is need. So my photos are sometimes turning out soft as a result of this. I use an 80-320MM lens, and sometimes I have to adjust the zoom AFTER I focus, and as a result it will lose focus.

I use a Pentax ZX-7 which is acclaimed to have a very advanced auto focus system for an SLR of it's price range. It does have predictive autofocus according to the manual and Pentax's website. The manual says it automatically kicks in when it senses the object is moving.

Does anyone have any tips on how to keep the plane perfectly in focus throughout the entire phase of panning with it? It seems too tedious to constantly press the shutter button half way. Plus, that's a great way to drain out the batteries aswell.

Any help would be greatly appreciated.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
LGW
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RE: Auto Focus Tips

Wed Jul 24, 2002 12:24 am

I just keep half pressing the shutter button on the a/c whilst it approaches - i have done since I got my first AF camera so its habit whther or not its the best/right way!

You could use sports mode? i know my D30 ya just continusly hold down the shutter button and it changes focus as the object, in this example a plane moves.

LGW
 
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RE: Auto Focus Tips

Wed Jul 24, 2002 12:29 am

LGW,

I use the Action Mode which I guess is the same as the sports mode. Although, in the manual it just says the action mode focuses on selecting the fastest shutter speed possible with at the same time a good aperature.

My problem is focusing, so I'm not too sure.

Thanks for your reply.

Regards
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
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RE: Auto Focus Tips

Wed Jul 24, 2002 12:44 am

Okay, I just tested to see if the predictive auto focus works or not.

I put no film in my camera since I just want to test to see how it works. I targetted an object from a distance, and started walking towards it, I pressed the shutter button half way and it focused on it, however I continued walking to see if the focus would stay in place or if I would lose it. The focus slowly started to get worse and worse. The AF system didn't do anything to focus on the object while I was walking towards it.

I walked towards it to simulate a plane on approach getting closer and closer, however since it didn't work on the object, it means it won't work on the plane approaches.

Anyone have any idea why the predictive focus isn't kicking in? It's supposed to be automatic.

Any help would be greatly appreciated.
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
BA
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RE: Auto Focus Tips

Wed Jul 24, 2002 12:54 am

Okay this is what it says in the manual:

Predictive Autofocus Mode
When the camera senses subject movement during the autofocus operation, the camera will automatically switch the focus mode to the predictive autofocus mode to measure the speed of a moving subject, and predict where it will be at the moment of shutter release to maintain sharp focus on the subject.

Does that include walking towards the subject to simulate as if it's moving towards you?

If so, then the predictive autofocus isn't working.

Any feedback would be appreciated.

Regards
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
Skymonster
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RE: Auto Focus Tips

Wed Jul 24, 2002 1:11 am

Predictive AF should work whether you move, or the subject. Essentially its the same thing - the distance between you and the subject is changing.

Either (a) there's something wrong with the camera or (b) you need to switch the predictive AF mode on - on Canon's there's a setting for predictive AF and it doesn't just activate by itself - maybe the same on the Pentax???

Andy
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RE: Auto Focus Tips

Wed Jul 24, 2002 1:32 am

Skymonster,

From what I can see, it's automatic. There is no switch to turn it on.

Any help would be appreciated.

Regards
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
BA
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RE: Auto Focus Tips

Wed Jul 24, 2002 2:50 am

What is the difference between continuous autofocus and predictive autofocus?

Regards
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RE: Auto Focus Tips

Wed Jul 24, 2002 6:18 am

Does this mean nobody has ANY tips about Autofocusing?
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wietse
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RE: Auto Focus Tips

Wed Jul 24, 2002 6:21 am

Well..... what I do, (seeing I don't have AI servo...) Is keep on pressing the autofocus button.... 3x a second or so, that usually keeps it kinda in focus...

Wietse de Graaf
 
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RE: Auto Focus Tips

Wed Jul 24, 2002 6:29 am

I've been thinking about doing that Wietse. But doesn't that eventually cause wear and tear to the camera? Plus, I'm sure it drains the batteries quickly.

But if I have no choice, I'll do that.

What SLR body do you have?

Regards
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wietse
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RE: Auto Focus Tips

Wed Jul 24, 2002 6:35 am

an EOS 300. Sometimes it does pretend like it has AI servo, but I don't know if I have it or not.... (no way to turn it on/off)

Don't think it'll really wear the cam down.

Wietse
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KingWide
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RE: Auto Focus Tips

Wed Jul 24, 2002 6:40 am

It is possible for some predictive systems to be fooled into thinking the subject isn't moving when in fact it is. This usually happens with a subject which has large expanses of low-contrast such as, for example, the fuselage of an airliner. The camera detect subject movement by looking for contrast changes in an area it had previously focussed correctly. If these changes are within some tolerances set in the AF system, then the camera will assume that the subject hasn't moved and not refocus. Sharp contrast changes would indicate to the camera that the subject has moved in some way and that a refocus is possible.

The maximum aperture of your lens can have a big effect on your camera's ability to detect these contrasts. Lenses with smaller apertures let in less light and so the AF system needs to be much more sensitive to function as effectively as when a lens with a wide maximum aperture is placed on the camera.

I'm not saying that this is the case here, but it may be worth considering.


J
Jason Taperell - AirTeamImages
 
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RE: Auto Focus Tips

Wed Jul 24, 2002 6:51 am

I think I will take Wietse. I guess I will just have to constantly press the autofocus button to keep the plane focused. That seems to do the trick.

OR, if I want to save on battery usage, I can simply focus once while far from the plane. When I want to take the picture, I'll move my finger off of the shutter button, then press it again, but these time all the way (not half way). This way it will focus first, then take the picture.

Problem is you can increase camera shake this way.

Thanks for everyone's help.

Regards
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Bruce
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RE: Auto Focus Tips

Wed Jul 24, 2002 4:39 pm

How often do you guys use manual focus?
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wietse
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RE: Auto Focus Tips

Wed Jul 24, 2002 5:36 pm

never to actually take the shot. But if I want to check in the distance to see the planes on final, it sometimes won't focus, so I use manual to focus those aircraft.

Wietse
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sudden
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RE: Auto Focus Tips

Sat Jul 27, 2002 4:23 pm

I also use the autofocus cause in "Sport" mode it adjusts to the object, if the object is moving towards/away from me.
And it goes rather fast for the cam to adjust as well.  Smile
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