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Thread started 19 Sep 2010 (Sunday) 17:12
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AF point with 50D!! Even More Confused!!

 
JonK
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Sep 19, 2010 22:34 |  #16

http://vimeo.com/15113​215 (external link)

Should explain it


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ankitj
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Sep 19, 2010 22:41 |  #17

jase1125 wrote in post #10940007 (external link)
If you need a greater portion of the frame to be in focus you will need to increase your depth of field. You should get use to single point focus points for more consistent results.


Thank you. One question I have is, let's say two people are posing together, standing shoulder to shoulder and a gap between their heads. What would you focus the center point on?


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JonK
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Sep 19, 2010 22:43 |  #18

ankitj wrote in post #10940091 (external link)
Thank you. One question I have is, let's say two people are posing together, standing shoulder to shoulder and a gap between their heads. What would you focus the center point on?

In the video I mention recomposing. I would choose an aperture that will allow for some variation in their positioning if they're not exactly inline... but what I'd do is (with my 5DII), place the center point on one of their faces, press my AF-ON (I focus with this button, not the shutter button) and then recompose the image.


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ankitj
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Sep 19, 2010 22:46 |  #19

JonK wrote in post #10940061 (external link)
http://vimeo.com/15113​215 (external link)

Should explain it

Thanks Jon. Pretty cool what you did. The 7D looks awesome.


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Snydremark
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Sep 19, 2010 23:54 |  #20

ankitj wrote in post #10939879 (external link)
Oh never mind. I get it now. When all the focus points are lit, the camera is going to automatically select the best point to focus on. By manual focus, you mean turn the dial and manually select which spot I want to camera to focus on.

Exactly.

ankitj wrote in post #10939879 (external link)
What I want to focus on multiple spots?

You cannot have more than one point of focus in an image. You can select a smaller aperture (f/8, f/11, etc) in order to have greater depth of field so that more of the image appears in focus from front to back; but there is only one actual point of focus in any given image.


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amfoto1
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Sep 20, 2010 00:49 |  #21

Careful not to mix up manually selected AF point, manual focus and manual exposure control. They are all quite different things!

Basically if you select all AF points (press the AF button and turn the dial until all points are lit in the viewfinder or shown on the LCD) you are leaving it up to the camera to decide what's the best point of focus. Often it will simply choose whatever is closest and covered by one of the AF points, particularly when using One Shot. If you have the camera set to AI Servo, it might try to track any moving subject and focus on that, but if you are panning with the subject, then as far as the camera is concerned everything is moving so most likely it will again revert to whatever is closest. (Never use AI Focus... that mode is supposed to decide whether or not the subject is moving, then, if it is use AI Servo, if it's not use One Shot. But it slows the process down, and adds another layer of the camera trying to make decisions for you... i.e. more potential for mistakes.)

Essentially, whenever you use multiple AF points, you are giving up control of the point of focus to the camera... so it's anyone's guess if it will choose the same as you would.

I leave my cameras with just the center point selected most of the time. That's my "default" setting. I also leave them in AI Servo, since it can be used with either moving or non-moving subjects. There are times to multiple points, or to use expansion points (which the 50D doesn't have, but some other models do) or zone focus (again, not on the 50D)... but as a rule if you want to control what the point of focus will be, choose the AF point yourself. Don't leave it up to the camera.

I also have all my cameras set to use "thumb focusing". On some it's with the AF On button, on other models it's the * button (older models and some vertical grips don't have an AF On button). This separates the focus function from the shutter release function, and again puts me in full control of the point where the camera focuses. If you don't separate the two funstions, choose just the center point (or any other single point), focus on your subject and then recompose, the AF system will refocus on whatever the AF point is now covering. What else can it do?

Alternatively, don't recompose. The downside to that is if, for example, you are always using the center AF point (perhaps because it's the most sensitive so should be the fastest and most accurate), then your subject always ends up centered. You can go back and recrop the image later in post processing, if you allow enough room for that.

Another option, of course, is to use one of the outer AF points. But it takes time to decide which one to use and then to set it on the camera. You can get pretty quick at this with practice, but it will always take a few seconds that may or may not be possible depending upon what you are shooting.

I agree that it helps a lot of the time to stop the lens down a little to increase depth of field. That gives you more margin for focus error, more likely the main subject will be within acceptibly sharp focus. Of course, this is not something you want to do when trying to get shallow DOF and/or want to really throw the background out of focus. But, also realize that crop sensor camera viewfinders exaggerate depth of field. In part it's because they are smaller that it looks like you have about one stops more DOF than you really do. Shoot a lot with large aperture telephotos and you soon realize this! I allow about one extra stop, over what I see in the viewfinder (as compared to using a full frame camera). If shallow f2.8 looks right to me in my 50D or 7D viewfinder, I'll shoot at f4. When shooting with 5DII, if f2.8 looks right, I'll use f2.8.

Because the aperture plays such a big roll, my cameras are set to Av usually, if I'm not shooting full Manual exposure. I occasionally use Tv when shutter speed is important. And I occasionally use P when I'm switching to shooting in drastically different light and just don't have time to stop and think about what settings to use.

Best is to just keep practicing. Part of the problem with today's cameras is that there is just so much automation between you and taking photos, and it's tempting to use it. Until a camera can actually think exactly like you do, the more control you take over exposure, focus and all aspects of the camera, the more likely you will be able to get the results you want. At the same time, AF is more accurate and faster than I ever was, focusing manually. It let's me nail shot after shot of fast moving subjects, something manually focused cameras were simply never as capable of. In=camera metering systems are the best they have ever been, allowing me to get a high percentage of accurate exposures in rapidly changing lighting situations. Still when there's little changing or it's only very gradual, I feel I can make more accurate exposure with fully manual settings.

So pick and choose carefully, how much automation to use and when to use it. Plus, nothing is 100%, so take several shots instead of just a single one. And use the LCD to check important ones, both zoomed way in on the image for focus and the histogram for exposure accuracy.


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David ­ Ransley
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Sep 20, 2010 01:49 |  #22

I know my 40D does this and I am sure the 50D will do the same. It is also explained in the manual, but i will need to locate the page. If you select only one AF point it instructs the camera to start there to look for contrast. If the levels are within tolerance levels and focus can be achieved, then it focuses on that point. Now two problems:

1. The focus are is larger than the little block. The camera may therefore select the basket.
2. The other points are also used and if the values for the selected one is "out of range" then an alternative point is used.

It is a pity that the camera doesn't indicate the move by blinking the second point. At least you would have realised that the centre isn't good enough.


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itzcryptic
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Sep 20, 2010 06:36 |  #23

David Ransley wrote in post #10940951 (external link)
It is a pity that the camera doesn't indicate the move by blinking the second point. At least you would have realised that the centre isn't good enough.

I don't think this is correct. If you choose a focus point, it either focuses or it doesn't. It does not focus on another random point in the scene, that I have seen.




  
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egordon99
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Sep 20, 2010 07:03 |  #24

ankitj wrote in post #10939750 (external link)
I meant Manual mode on the dial. I think I understand what you are saying. I chose full AF points with the center point focused between the dogs eyes. I haven't been able to recreate the "AF float". At this point I am chalking that phenomenon up to user error.

This is basically telling the camera "I don't care where you focus, as it not important to me that the subject be in focus. If you want to focus on the background, that is fine. Whatever you decide, I won't complain later"

But seriously, the AF sensors aren't smart enough to say "Hey, that's a dog you're pointing at, you probably want the eyes of the dog in focus, so I'll focus on the eyes" The sensors ONLY see contrast (phase) differences, they do not know about dogs or cats or birds or even (gasp) ducks! :D

Your manual should tell you how to explicitly select ONE focus point.




  
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egordon99
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Sep 20, 2010 07:05 |  #25

David Ransley wrote in post #10940951 (external link)
1. The focus are is larger than the little block. The camera may therefore select the basket.
2. The other points are also used and if the values for the selected one is "out of range" then an alternative point is used.

Point one is absolutely correct (and the source of many "My camera can't focus" problems)

Point two is absolutely incorrect.




  
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ankitj
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Sep 20, 2010 07:06 |  #26

Thank everyone for the insight. My understanding is the DOF is to increase the depth of the image, not the width. Let's say I use my 17-50mm to take a picture of two people sitting on a couch. What/Who would I center focus on? Person to the left or right? What if there were three people? What is the focus range of the individual focus points?


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egordon99
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Sep 20, 2010 07:06 |  #27

Oh, and your Rebel behaved the SAME way. You shouldn't have been using "All Focus Points" on that body, but rather select the ONE focus point that covers your intended subject.




  
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ankitj
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Sep 20, 2010 07:08 |  #28

egordon99 wrote in post #10941795 (external link)
Oh, and your Rebel behaved the SAME way. You shouldn't have been using "All Focus Points" on that body, but rather select the ONE focus point that covers your intended subject.

Yes, I misunderstood the point of selecting "All Focus Points". It's been a very frustrating night after coming to the realization that I have been using something the wrong way for 3 years.


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egordon99
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Sep 20, 2010 07:10 |  #29

ankitj wrote in post #10941805 (external link)
Yes, I misunderstood the point of selecting "All Focus Points". It's been a very frustrating night after coming to the realization that I have been using something the wrong way for 3 years.

Glad you got it figured out, I went through a Pentax *ist DS, K10D, and finally my 40D (over the course of ~3 years too!) before I started selecting the focus point myself.




  
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Sep 20, 2010 07:14 |  #30

Couple quick tips with the 50D. You can assign focus points to the joy stick so that you can quickly move around and select each point (check your manual for this). Also, to quickly return to the center focus point just press straight down on the joy stick. I will post up a thread on the 50D where folks posted their settings, which should help you. I will search for it; it's been awhile.

Found it. Here's the thread: https://photography-on-the.net/forum/showthre​ad.php?t=767875


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AF point with 50D!! Even More Confused!!
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