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Old Dec 27, 2006, 7:30 AM   #21
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Yes, the 30D has the same " joystick" function. I will check into that. I guess the eye opener for me was the wide open aperture/focus point connection. I know with an open aperture the focus plane is smaller, I just thought the center one would always work ...even in that siutation. donna
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Old Dec 27, 2006, 7:45 AM   #22
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D.Ann wrote:
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I guess the eye opener for me was the wide open aperture/focus point connection. I know with an open aperture the focus plane is smaller, I just thought the center one would always work ...even in that siutation. donna
Donna, it's an issue of shallow depth of field - not necessarily wide open aperture. While aperture is one of the contributing factors to depth of field - it isn't the only one. Say you were usinga 50mm 1.8 lens (if you had it) wide open at 50mm - wide open in that case is 1.8. Now say your subject is 50 feet away. Your depth of field is21.5 feet (8.5 feet in front and13 feet behind your focus point). So, you've got quite a bit of DOF there and yet the lens is WIDE OPEN.

Now, let's take a 400mm lens at f11 (quite a bit closed down from the 1.8 value above) and your subject is 50 feet away. Now your DOF is only 2 feet (approx 1 foot in front and 1 foot behind your focus point). So in this example, an aperture of f11 produced a shallower DOF and thus requires more accurate focusing. Why? Because aperture is only 1 part of the equation.

DOF is controlled by 4 factors: camera sensor, focal length, aperture, distance to subject. Since your camera is a constant, you are really concerned with the other 3 factors - all of which play a part. This online DOF calculator is pretty useful - you can play around with various values to see how DOF is affected.

http://www.dofmaster.com/dofjs.html





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Old Dec 27, 2006, 9:18 AM   #23
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Not trying to beat this subject to death,

okay I understand what you are saying...it will take a while for this to become second nature to me when setting up a photo...but when taking wildlife pictures when there isn't much time for thought, using a long lens, say 300mm, in daylight would center focus be the safer bet...trying to keep center point on the bird? Maybe I should ask some of the great shooters on the wildlife forum what they do??? Donna
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Old Dec 27, 2006, 1:23 PM   #24
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When you have the subject centre-frame then it absolutely makes sense to use the centre point only.

For tracking action it depends a lot on the type of subject. AI Servo depends on having all the focus points active so that it can track the subject as it moves across them, but for trying to follow a bird in flight for example, I have had better success using simply the centre AF point and one shot mode. For larger subjects you would be better with AI Servo and all AF points.

You will have to develop your own style, depending on the subject matter, because I don't think there is necessarily a "right" way of doing it.

JohnG uses AI Servo frequently, NHL and I use one-shot most of the time.
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Old Dec 27, 2006, 1:52 PM   #25
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peripatetic wrote:
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AI Servo depends on having all the focus points active so that it can track the subject as it moves across them,


Not true - ai servo works with single point OR with all points. All AI servo does is constantly re-focus. for instance, when shooting sports I typically have AI servo turned on and only the center focus point active. If you have all focus points active the camera will try to track the subject using all of them (you won't see any focus points actually light up) but the camera can get sidetracked if other objects are in the frame. If you use only a single focus point, the camera will constantly re-focus using that focus point only - so you have to keep that point over your subject.

Now, the difference between ai-servo and one-shot is that in one shot, the camera focuses ONCE - and will not allow the shutter to fire until it has a focus (of course there is no gaurantee the camera focused on what you wanted it to it just focused on something). If you fire a 5 shot burst, the camera focuses for shot 1 and uses that same focus setting for the 4 subsequent shots. This works great for many wildlife applications as you typically have a subject that is far away and moving perpendicular to your position - so it isn't moving in and out of the focal plane. If you want the camera to refocus you must release the shutter (or focus button if you are using the CF allowing focus via the * button) completely. So this is a poor choice when your DOF is shallow and subject is moving towards or away from you.

In ai-servo mode, the first shot in a burst will fire whether focus is achieved or not. Subsequent shots in the burst are focus priority - i.e. they won't fire until the camera focuses.In addition,as the camera tracks a subject in this mode, it will attempt to predict where the subject is going and thus 'cheat' the focusing. Again, no gaurantee the camera is focusing on what you want it to. This method works much better when the subject is moving towards or away from you which is typical in a sporting situation.

Take this example: your 30d with a 300mm 2.8 lens and a human 80' away running towards you. At those settings, DOF is only 2.3 feet - 1.13 feet in front of the image. Now, assume a 40 yard dash time of 6 seconds (fairly slow for a good athlete) - that's 6.6 feet per second. Now, if you're 30d has 5 frames per second and you fire a 5 shot burst. That means by the 3rd shot, your subject is out of the DOF from that initial focus point. So, if you were shooting in single shot, and your focus was dead on for the first shot, it's probably a little blurry by second shot and 3rd shot and beyond have zero chance of being in focus.

But, if you're tracking a bird at 60 yards at 5.6 - you have 23 feet of DOF to play with. If your subject is moving relatively perpendicular to your position, it may only change distance relative to you by 4 or 5 feet - still within the same focal plane of your initial shot - so single shot works well - versus the camera trying to re-focus and getting it wrong.

So, I disagree with Peripatetic - it isn't so much a matter of style as it is understanding the different modes and knowing which mode works best for a given situation - no different than knowing whether a focus-recompose method will work given the DOF of your subject or whether you need to select the exact focus point. Peripatetic and NHL's approach works because in their shooting situations the subject is not changing focal planes during their burst.

So, if you plan on taking a burst of shots and your subject is going to remain in the same focal plane during the burst then one-shot will likely yield better results. If your subject can change focal planes during the burst (which at least for sports is extremely likely) then servo focusing will serve you better.

Edit:

Sorry, I made a mistake - in my above example, a 40 yard dash time of 6 secondes equates to 20 feet per second (not the 6 feet I originally had indicated) - so at 5 fps that's 4 feet per frame - so the subject will be outside the focus plane by the 2nd shot.

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Old Dec 27, 2006, 2:41 PM   #26
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Good response John.

I hardly every shoot bursts. I try to time the shot and use one shot. On the few occasions where I have shot bursts the subject was not moving perpendicular to the focal plane.

Of course I hardly every shoot wildlife or sports. I don't do action stuff as a general rule, but your handy explanation there will certainly make life easier next time I do.


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Old Dec 27, 2006, 3:06 PM   #27
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If you check out my web site, you'll see that I almost exclusively shoot birds and I sell my work. I also teach wildlife photography and am lucky enough to hang out with some shooters who are much better than me (which has accelerated my learning by a lot over the years.) So I think I qualify as one of the "great" wildlife shooters around here (the quality in the wildlife section in generally is really quite high, if you haven't checked it out.)

I basically agree with all of what JohnG said. 99% of the time I use the center AF point an IA Servo. I have the * button on the back of the camera set to stop focusing when the subject is stationary.

Something that hasn't been brought up is this. On the Prosumer models (30D and DRebel family) the center AF point is the best. On the 1-series cameras, the 7 "middle" AF points are the best.

On the Prosumer bodies, the center AF point has a higher performance mode when used with an f2.8 lens where it will focus faster (definitely), in lower light (I believe), and with less hunting (I believe). (The less hunting is anecdotal from experience. Not "Fact". The lower light is speculation.) If you force the camera to only use the center AF point you might see the difference, especially when tracking moving wildlife. Note I don't mean you have to be shooting at f2.8, just that the max aperture of the lens is f2.8. This is because all focusing is done at the largest aperture, the instant you take the picture the lens is stopped down to the aperture you've selected.

Now the real reason to only use one AF point at a time is because of what JohnG said. The camera will pick whichever AF point has the closest thing underneath it. You might not want to focus on the distracting closer branch that is next to the hawk, you might want the red-tailed hawk in focus. With all the AF points selected, the camera will pick the branch 100% of the time unless you can force it to use the AF point that is pointed at the hawk. This is a HUGE problem when photographing birds/wildlife on water. Clearly, there will be some water that is closer to you than the animal. The camera will lock on that every time. Having only 1 AF point active prevents this.

And I agree with JohnG, focus and recompose isn't evil. It can cause problems in some situations, but it also works well in others. Don't inherently write it off.

This is why I'd love Canon to make a digital camera that tracks eye movement to pick AF point. They made a film camera that did that awhile back. Then I could pick the composition easily and still have it find the AF point I want without me having to do anything. And when you're in the field photographing wild animals, any time you can simplify what you do is a good thing (not working to change AF points, for example.)

I really should post more pictures in the wildlife forum. I stopped doing it awhile back and I don't really know why….. I guess it's because I concentrate on making prints now. Making good images for the web is considerably easier.

Eric

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Old Dec 27, 2006, 5:54 PM   #28
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and also the 30d has a third option, AI Focus AF which switches back and forth as it detects movement in the subject or lack there of...Donna
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Old Dec 27, 2006, 9:19 PM   #29
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JohnG wrote:
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In ai-servo mode, the first shot in a burst will fire whether focus is achieved or not. Subsequent shots in the burst are focus priority - i.e. they won't fire until the camera focuses...
JohnG - This is only partially true: subsequent shots in the burst are focus priority up until a pre-set time out (200ms for USM and 250ms for other lenses) period to maintain the camera's frame rate i.e. the camera will fire away even if can't focus (in low-light or the action is too fast for the AF example)

Try it: Put the lens cap on the lens in AI-servo, the camera will merely fire away every single shot even if nothing is focus in subsequent shots
In Single-shot AF the camera will not fire at all with the lens cap on because it can't focus!

-> Single-shot AF guarantees the focus has locked - The AI-servo won't (it will track if it can but will take the shot regardless)
Again try to shoot something closer than the minimum distance for example. The camera will not let you take the shot in single-shot AF, but in AI-servo you can take as many out of focus shots as you want...
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Old Dec 28, 2006, 6:39 AM   #30
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NHL wrote:
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-> Single-shot AF guarantees the focus has locked - The AI-servo won't (it will track if it can but will take the shot regardless)
focus lock is not the same as focus accuracy - you can still get oof shots with single-shot. Try as you might, you're not going to convince me or the thousands of other sports shooters (and quite a few wildlife shooters - Eric included) that single shot produces better results. I have yet to run across a competant sports shooter that uses single shot for non-stationary sports (i.e. single shot can work fine on a golfer, or on high-jump or hurdles if you're pre-focusing). And, I can't seem to get through to you that shooting a human is different than shooting a bird. Take someone running a sprint towards you - a burst allows you to select the best stride as well as facial expression - you don't have to worry so much about the facial expression of a bird in flight. Or take someone driving the lane in basketball. Should a shooter simply accept a single shot of that action as an imposed limitation? Or should they get the take-off the in-air shot and the dunk - not really possible if you have to release and re-obtain focus for each of the 3 shots - and dof is too narrow at 2.0 or so.

When you learn how to acquire and track in servo it produces great results on the first shot - with less lag because it already has focus.

It's great that you get good results with single-shot. But I think you mislead people into believing it's better than servo - it's not inherantly better or worse. Itj's just different. It's like arguing that by using shutter priority mode you'll always get better shots than aperture priority - no, you won't - you might in certain circumstances. Same here - in certain circumstances single-shot gives better results - in others it doesn't. You like to conveniently ignore cases presented where servo works better simply because it doesn't fit your argument. At least I try to provide a balanced argument indicating the pros and cons of both. There is a reason why both modes are there - and there's a reason why so many sports and wildlife photogs use both modes. Or is there something you know that the shooters for Getty, Sports Illustrated, ESPN and major sports pages around the nation don't know? I'm sure they'd be glad to hear they don't really know what they're doing
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