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Thread started 15 Jul 2009 (Wednesday) 08:41
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Canon 5DMKII: AF Questions, Thanks

 
MarkV
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Jul 15, 2009 12:28 |  #16

Trey T wrote in post #8284644 (external link)
is it the same distance?

No, but the distance doesn't change just because you use a different AF point. In your example, it's impossible to have all people in the group perfectly in focus. You have to pick one area of the group that will be in focus and accept the out of focus aspect of every other part of the frame. The only difference in using an outer AF point vs. using the center AF point and recomposing is that you have a greater chance at focus accuracy using the more accurate center AF point.


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tonylong
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Jul 15, 2009 12:30 |  #17

I'll toss in a couple thoughts:

1) In regards to the AF system If I am using a f/2.8 or faster lens is it optimal to use the center focus points and the invisible assist point directly on top and below the center point all within the center focus circle? Also if I am shooting vertical with my grip I would use the center focus point and then the invisible AF assist point to the left and right of the center focus point?

3) When using AI Servo mode do you Canon users use the Center focus point to hit your subject with and then while they move just keep your subject while moving somehow within that center circle hit by the center point and the 6 invisible assist points? Or do yall use all the AF points in AI Servo?

In regards to the AF assist points, a lot depends on your subject. For small subjects, AF can get thrown off by a busy background, and the assist points can make things worse. In practice I never use the assist points, but they could come in handy in some cases. Let's say, for instance, you are shooting a car race, where there may not be a lot of good contrasty detail right at the center point. Turning on the assist points gives you a bigger area to grab something with, assuming your subject fills the entire center area.

When shooting AI Servo, by the way, I always use the Center Point to attain focus. Occasionally I will have all points active for Servo handoff, but more often I'm using center point only.

When shooting still subjects in good light, I use off-center points a lot -- better composition.

4) On my D700 and pretty much all the Nikon's, there is an AF assist illuminator next to the lens mount. On the 5DMK2 there is not one. I have the 580exII speedlight which has the AF assist beam. Is there a way to use the AF assist without firing the flash in low light situations?


You can turn the flash off while still using the AF Assist beam, but you have to do it on the flash. The 5D2 does not have that function (1D bodies do, though). You can also get a Canon wireless flash controller that is smaller and has an AF Assist Beam.


Tony
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MarkV
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Jul 15, 2009 12:31 |  #18

CyberDyneSystems wrote in post #8284677 (external link)
I like to focus on the eyes as opposed to the belly button :)

Hence, recomposition! :P

I was shooting a wedding reception a few weeks ago with a 17-40 and some of the people commented that it looked like I was taking photos of their belly instead of their face (after I recomposed the shot, of course!). :)


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tonylong
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Jul 15, 2009 12:38 |  #19

MarkV wrote in post #8285179 (external link)
Hence, recomposition! :P

I was shooting a wedding reception a few weeks ago with a 17-40 and some of the people commented that it looked like I was taking photos of their belly instead of their face (after I recomposed the shot, of course!). :)

That's fine right up until the point where it's not. Haven't you seen shots where part of a subject is in focus (let's say that belly) but the part of needed critical sharpness (let's say the eyes) is not? That's what happens when you focus in the wrong spot with a narrow depth of field.

To me, a rule of thumb is to use the best point for the job as long as the light will allow it.


Tony
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MarkV
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Jul 15, 2009 12:39 |  #20

CyberDyneSystems wrote in post #8284710 (external link)
Because "Focus using center and Recompose" certainly can be a problem.

Yeah, my statement was meant to be if recomposure wasn't a problem. I do realize it can be for some types of photography, but keep in mind, my original contribution to this thread specifically stated that center AF worked better with my style of photography. YMMV.

...but IMHO the odds are far better in your favor to select the AF point that is over the area you want in subject and NOT recompose.

But again, that depends on the style of the photography/photograph​er. Selecting a particular feature of a frame to focus on inherently means that all other aspects of the frame at different distances from the camera will be OOF to a degree dictated by the DOF. If I'm photographing a bride at a wedding reception, but I also want a relatively narrow DOF, then I'm going to focus on her eyes using the most accurate center AF point, and then recompose the shot to put her face at the top of the frame.


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MarkV
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Jul 15, 2009 12:43 |  #21

tonylong wrote in post #8285223 (external link)
That's fine right up until the point where it's not. Haven't you seen shots where part of a subject is in focus (let's say that belly) but the part of needed critical sharpness (let's say the eyes) is not?

Absolutely. That's why, in my limited experience, it's much better to control the camera's AF point selection by using the center point, focus on the eyes, and recompose the shot. This way, it's impossible NOT to have the eyes in focus.

Again, I'm not saying this works for every style of photography, but for the situation you described, I find it to be the most consistent method.


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TexRex
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Jul 15, 2009 12:49 |  #22

Thanks for the tips guys!


Canon stuff....

  
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apersson850
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Jul 15, 2009 13:10 |  #23

MarkV wrote in post #8285161 (external link)
No, but the distance doesn't change just because you use a different AF point. In your example, it's impossible to have all people in the group perfectly in focus.

Not at all. Line them up in a nice line, focus on the one in center and they'll all be in focus.

But, you say, those out at the ends are farther away from the camera!

Yes, they are, but you made the same mistake in your thoughts as I did the first time I heard about this. You also forgot that good lenses are calibrated to make a plane, parallel to the sensor/film sharp, not a spherical bowl.

So that's the reason for why focus/recompose doesn't work well, if the distance change exceeds the DoF too much.

Apart from that, if you use evaluative metering, then the camera will bias the exposure on which focus points are active. But it does lock exposure (provided you use One Shot AF!) automatically, so chance are reasonable that you'll get the correct value anyway. Or you can meter with a different method and/or set the exposure manually.

The 40D/50D cameras are the only Canon cameras with cross-type AF sensors only. They also support cross-type center high-precision AF with lenses with f/2.8 or better.
The 5D/5D Mark II had linear focus points, except that the center point becomes a cross-type point when using lenses with f/2.8 or better. On the other hand the 5D has a more dense array of focus points in the center, with the six assist points, but unlike the 1D Mark III (19 linear/high-precision cross-type main points and 26 assist points), the assist points in the 5D Mark II only works when in Servo AF mode.

I've not used any 5D/5D Mark II more than a few images, so I'll not compare these to the 40D/50D. But I have used the 400D and 40D a lot, so I can compare these two. The 400D has an AF module that's identical to the one in the 30D, which in turn is about as in the 5D, but lacking the assist points. When using One Shot AF they should be reasonably equal.

The 40D is noticeably better than the 400D when it comes to autofocus. But, again, if extrapolating that observation into the 50D/5D Mark II comparison has any relevance, I don't know.
With the 40D, I often use one point when using One Shot AF. Since they are all cross type, it doesn't matter so much. When using Servo AF, I sometimes use all points, and hope the camera will be able to transfer focus tracking from the center point to a peripheral point if needed. Sometimes it works, sometimes not.

It's possible to set the external flash (580 EX II) to not fire the flash, only the AF assist beam, if you want it to.


Anders

  
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MarkV
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Jul 15, 2009 13:32 |  #24

apersson850 wrote in post #8285417 (external link)
Yes, they are, but you made the same mistake in your thoughts as I did the first time I heard about this. You also forgot that good lenses are calibrated to make a plane, parallel to the sensor/film sharp, not a spherical bowl.

I suppose this is true. I don't guess I've yet run across a situation where the distance between a recomposed shot using center AF point focus on a feature near the edge of a frame moves the subject out of the DOF during recomposition. But, you are right - a focus plane is just that, a plane, not a sphere.

:o


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thatkatmat
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Jul 15, 2009 14:02 |  #25

Also, another focus-recompose problem can be the metering, but that can be remedied by locking exposer.


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MarkV
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Jul 15, 2009 14:17 |  #26

Not a problem if you're shooting manual... ;)


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thatkatmat
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Jul 15, 2009 14:19 |  #27

Doesn't matter what mode, if you focus and recompose at the beach or whatever your metering could be off unless you lock exposer


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nicksan
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Jul 15, 2009 14:27 |  #28

thatkatmat wrote in post #8285789 (external link)
Doesn't matter what mode, if you focus and recompose at the beach or whatever your metering could be off unless you lock exposer

But Matt, in Manual mode, there is no "Lock Exposure"...(If you mean hitting "*" when you are in Tv/Av mode for instance)

So you would meter on what you want to meter off of, recompose, then just shoot without worrying about what the exposure meter is doing as a result of the recompose, effectively "locking" exposure.




  
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nicksan
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Jul 15, 2009 14:28 |  #29

MarkV wrote in post #8285534 (external link)
I suppose this is true. I don't guess I've yet run across a situation where the distance between a recomposed shot using center AF point focus on a feature near the edge of a frame moves the subject out of the DOF during recomposition. But, you are right - a focus plane is just that, a plane, not a sphere.

:o

Try recomposing with your 50 1.4 wide open at close to MFD.;)




  
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thatkatmat
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Jul 15, 2009 14:35 |  #30

nicksan wrote in post #8285848 (external link)
But Matt, in Manual mode, there is no "Lock Exposure"...(If you mean hitting "*" when you are in Tv/Av mode for instance)

So you would meter on what you want to meter off of, recompose, then just shoot without worrying about what the exposure meter is doing as a result of the recompose, effectively "locking" exposure.

Ahhh, I stand corrected, I only use manual mode like 20% of the time (mostly landscapes where I have time) other than that, I'm AV and TV


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