Home · Register · Search · View Winners · Software · Hosting · Software · Join Upload & Sell

Moderated by: Fred Miranda
Username   Password

  New fredmiranda.com Mobile Site
  New Feature: SMS Notification alert
  New Feature: Buy & Sell Watchlist
  

FM Forums | Canon Forum | Join Upload & Sell

  

Archive 2009 · Educate Me | Autofocus
  
 
Harry T
Offline
• • •
Upload & Sell: Off
p.1 #1 · p.1 #1 · Educate Me | Autofocus


Alright I've seen this alot lately especially with the emergence of the 7D and how it compares or doesnt compare to everything else. I read about "auto focus up to F8.0" and "high sensitivity center focus point up to F4".

Can anyone explain or provide a link in what the heck these terms mean and how they correlate with each other? My camera bodies are a MK IIn and a 5D and auto focus is achieved at any aperature or so i though. So what am i clearly not understanding?




Sep 09, 2009 at 11:24 AM
frank kayser
Offline
• •
Upload & Sell: Off
p.1 #2 · p.1 #2 · Educate Me | Autofocus


The 1-series, including the Mark IIn, were the only Canon cameras capable of focusing at f/8 - and then, only the center focus point. The 5D and the other xxd and xxxd DSLRs were limited to f/5.6.

The 5D has "invisible helper focus points" that are active (I believe) at f/2.8 or faster.

The specs you state above are similar to the 1DM3 and 1DsM3 cameras, but with fewer focus points.



Sep 09, 2009 at 12:12 PM
Pixel Perfect
Online
• • • • • •
Upload & Sell: On
p.1 #3 · p.1 #3 · Educate Me | Autofocus


Harry when a camera focuses the lens is actually wide open and it doesn't change to the aperture you've set until the shutter is fully depressed. Say you have a 135 f/2L attached and want to take a picture at f/16. The lens is always @ f/2 until you depress the shutter fully at which time it stops down to f/16.

When we say a camera can AF to f/8 we mean that the lens can be as slow as f/8 wide open. For example a 500 f/4 + 2x TC becomes a 1000 f/8, ie wide open this lens is f/8. You can of course take a photo at other apertures slower than f/8 if you like. Now on the 1 series cameras will actually allow this lens combo to AF. All the other cameras have a maximum allowable aperture of f/5.6 wide open for AF operation, even the 7D. In this case if you took a 500 f/4 + 1.4x TC = 700 f/5.6, yo will get AF on your 5D.

Also the centre AF sensor on cameras like the 20D/30D/40D/50D/7D/5D/5D II are what are called cross-type sensors and are sensitive to horizontal and vertical lines and further more when a fast lens fo aperture f/2.8 wide open or faster is used these sensors become high precision sensors and allow for a smaller depth of focus (NOT the same as depth of field) than with slower lenses. On the 1 series the centre point is high precision with lenses as slow as f/4. When you AF there is a small range over which the camera will achieve focus; each time you push the shutter half way the camera may actually focus at very slightly different points, but they are within an alowed tolerance. This tolerance is reduced by 2/3 for high precision mode.


Hopefully that's clearer than mud.



Sep 09, 2009 at 12:15 PM
BrianO
Offline
• • • • •
Upload & Sell: On
p.1 #4 · p.1 #4 · Educate Me | Autofocus


Pixel Perfect wrote:
Harry when a camera focuses the lens is actually wide open and it doesn't change to the aperture you've set until the shutter is fully depressed. Say you have a 135 f/2L attached and want to take a picture at f/16. The lens is always @ f/2 until you depress the shutter fully at which time it stops down to f/16.


Yep. That's why using the DOF-preview lever on many cameras causes the image to get darker in the viewfinder; the lens is stopping down from its maximum aperture to the selected shooting aperture.

With the cheapo kit lenses, the widest aperture will be f/5.6 when zoomed out. So not only is the viewfinder image darker for us, but there's less light for the AF sensor as well.

Fast glass is worth the price!



Sep 09, 2009 at 12:27 PM
cgardner
Offline
• • • • •
Upload & Sell: Off
p.1 #5 · p.1 #5 · Educate Me | Autofocus


The rationale for spending big bucks for fast lenses isn't just their ability to blur the background, but because they focus quicker. The f/number is the ratio of focal length and physical diameter of the opening so as lenses get longer the elements must get much larger to maintain low f/numbers. Less expensive lenses telephoto lenses will compromise by using smaller elements resulting in higher f/numbers wide open. The aperture number of less expensive multi-focal lenses will also change with focal length while more expensive ones will maintain f/2.8 over the entire zoom range. That optical wizardry takes more elements and costs more.

AF works by detecting contrasting edges of objects. A cross type sensor will be better able to detect the edge contrast over a wider variety of shapes than one which is only horizontal or vertical. The reason AF doesn't work well in low light is because there isn't as much contrast. AF also has problems locking on large similarly toned areas like a blank wall or empty sky.

It boils down to the fact that the more light the AF sensors see, the better they can do their job. With Canon the most precise sensors don't come into play unless the aperture of the lens, which as noted above focuses wide open, is f/2.8 or faster. There is another set that works up to f/5.8. Beyond f/5.8 AF becomes dicey.

Chuck




Sep 09, 2009 at 12:51 PM
fraga
Offline
• • • •
Upload & Sell: Off
p.1 #6 · p.1 #6 · Educate Me | Autofocus


I would also like to add that some telephoto zoom lenses are f/6.3 at the long end, and, therefore, theoretically should only focus on 1 series cameras.
However, they report to the camera as being 5.6, so they can AF on all cameras.



Sep 09, 2009 at 03:11 PM
orangefirefish
Offline
• • • •
Upload & Sell: Off
p.1 #7 · p.1 #7 · Educate Me | Autofocus


fraga wrote:
I would also like to add that some telephoto zoom lenses are f/6.3 at the long end, and, therefore, theoretically should only focus on 1 series cameras.
However, they report to the camera as being 5.6, so they can AF on all cameras.

These are usually third party lenses though. I think in practice as long as there's enough light for the AF sensor to function, it will.



Sep 09, 2009 at 03:49 PM
PetKal
Online
• • • • • • •
Upload & Sell: On
p.1 #8 · p.1 #8 · Educate Me | Autofocus


cgardner wrote:
The rationale for spending big bucks for fast lenses isn't just their ability to blur the background, but because they focus quicker.


Faster lenses do not neccessarily focus quicker. Instead, they shed more light on the AF sensors thereby allowing the camera to focus quicker.

In fact, if you wish, the fastest Canon lenses ever made are the slowest ones to focus. Both 85L MkII and 50 f/1.0 take about 0.9-1.0 sec to drive their focus group from infinity to MFD. By comparison, 300 f/4 IS as well as 400 f/5.6 AF drive 'speed' is in the vicinity of 0.4 sec.



Sep 09, 2009 at 04:45 PM
Glen_C
Offline
• •
Upload & Sell: Off
p.1 #9 · p.1 #9 · Educate Me | Autofocus


Harry T wrote:
My camera bodies are a MK IIn and a 5D and auto focus is achieved at any aperature or so i though. So what am i clearly not understanding?


You're not understanding DOF (if you're even being serious) or thinking beyond what you personally need in a camera.

1DmkIIn has 8MP hiding some flaws to pixel peeping & more importantly didn't have hardware defects... a great, great body obviously. only flaw is high ISO by today's standard a few too little MP for most.

5D however is a slug and also has major problems off-center AF, especially when close up and at open aperature or in dim light. Of course it's non center-point AF are not cross sensor. Even worse it's focus screen options are totally inadequate for MF in action situations.

not everyone shoots landscape or people at f/5.6 'ya know.



Sep 09, 2009 at 04:52 PM
Harry T
Offline
• • •
Upload & Sell: Off
p.1 #10 · p.1 #10 · Educate Me | Autofocus


Hot damn! A ton of information in less than a day. I love this place!!!

You guys rock and thanks for explaining this to me. I'm now more familiar with the terms and can better understand the tech behind our cameras.

I maybe will mention my current lens line up is: 24-105 F4 IS, 70-200 F4 IS, 50 1.4, 300 F4 IS

Thanks again FM.com crowd!



Sep 09, 2009 at 07:40 PM
 

Search in Used Dept. 



jcolwell
Offline
• • • • • • •
Upload & Sell: On
p.1 #11 · p.1 #11 · Educate Me | Autofocus


Here's a good reference on 1D-series AF: Getting the Most from your EOS 1-Series Camera http://www.usa.canon.com/content/Handling/EOS_Digital.pdf


Sep 09, 2009 at 08:46 PM
bpark42
Offline
• • •
Upload & Sell: On
p.1 #12 · p.1 #12 · Educate Me | Autofocus


Harry T wrote:
Hot damn! A ton of information in less than a day. I love this place!!!

You guys rock and thanks for explaining this to me. I'm now more familiar with the terms and can better understand the tech behind our cameras.

I maybe will mention my current lens line up is: 24-105 F4 IS, 70-200 F4 IS, 50 1.4, 300 F4 IS

Thanks again FM.com crowd!


To make examples out of your lenses....

All of the f4 lenses (the 24-205, the 70-200 and the 300) will autofocus on all current bodies (obviously). With a 1.4x teleconverter attached they will still autofocus on all bodies (f5.6 effective max aperture). With a 2x teleconverter the effective max aperture becomes f8. At that point, they will only autofocus on 1-series cameras.

Note that technically this is an artificial limitation. You can tape the pins to prevent the teleconverter from communicating the adjusted max aperture and the camera will attempt to autofocus. It may or may not work depending on the available light and the particular lens+TC+camera combo.



Sep 09, 2009 at 08:59 PM
bpark42
Offline
• • •
Upload & Sell: On
p.1 #13 · p.1 #13 · Educate Me | Autofocus


Glen_C wrote:
You're not understanding DOF (if you're even being serious) or thinking beyond what you personally need in a camera.

1DmkIIn has 8MP hiding some flaws to pixel peeping & more importantly didn't have hardware defects... a great, great body obviously. only flaw is high ISO by today's standard a few too little MP for most.

5D however is a slug and also has major problems off-center AF, especially when close up and at open aperature or in dim light. Of course it's non center-point AF are not cross sensor. Even worse it's focus screen options are totally inadequate for MF in action
...Show more

You completely missed the point of this thread.



Sep 09, 2009 at 09:00 PM
bpark42
Offline
• • •
Upload & Sell: On
p.1 #14 · p.1 #14 · Educate Me | Autofocus



I forgot to respond to this:
Harry T wrote:
...I read about ... "high sensitivity center focus point up to F4".


The high sensitivity generally means a particular focus point will be more sensitive at certain apertures (read: "if there is enough light reaching the AF sensor.") Basically it will be able to analyze multiple dimensions in the area of the autofocus point and so can respond more accurately to different types of conditions.

Check out this link for a nice explanation and diagrams covering the sensitivity at different apertures (specifically the section, "NUMBER & TYPE OF AUTOFOCUS POINTS"):
http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm





Sep 09, 2009 at 09:11 PM
BrianO
Offline
• • • • •
Upload & Sell: On
p.1 #15 · p.1 #15 · Educate Me | Autofocus


Glen_C wrote:
You're not understanding DOF (if you're even being serious) or thinking beyond what you personally need in a camera.


That is one of the most tangential responses I have read here at FM. What does DOF have to do with the OP's question? And what would lead you to even consider that the question was not a serious one?



Sep 09, 2009 at 10:12 PM
MarcyJillGood
Offline
• • •
Upload & Sell: Off
p.1 #16 · p.1 #16 · Educate Me | Autofocus


This is hugely helpful stuff. Maybe if I read this thread a dozen times over the next six months, I will slowly get it.


Sep 10, 2009 at 03:19 AM
dwweiche
Offline
• • •
Upload & Sell: Off
p.1 #17 · p.1 #17 · Educate Me | Autofocus


bpark42 wrote:
I forgot to respond to this:

The high sensitivity generally means a particular focus point will be more sensitive at certain apertures (read: "if there is enough light reaching the AF sensor.") Basically it will be able to analyze multiple dimensions in the area of the autofocus point and so can respond more accurately to different types of conditions.

Check out this link for a nice explanation and diagrams covering the sensitivity at different apertures (specifically the section, "NUMBER & TYPE OF AUTOFOCUS POINTS"):
http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm



I guess I still need to be educated as I thought I already had this all figured out. I think its this picture from the website you linked that confused me:






These are the images the site displays when you roll-over the f5.6 text. This picture tells me that you can only use center-point AF on the XTi (400D) and only a lower-precision one-dimensional AF sensor? Not all 9 points are active at f5.6? Or is this actually how AF works on the 400D? I'm also surprised that this picture says that at f5.6 the 1DIII does not have an active cross point sensor? Again, this goes against what my thoughts were. Can someone un-wind my confusion?

And while I'm at it, I thought that now there are actually THREE types of AF sensors. I have an XSi (450D) and my interpretation of the AF sensor is that it has:
- 8 one-dimension AF sensors, effective to f5.6
- 1 cross type AF sensor shaped like a '+' at the center, effective to f5.6.
- The center AF sensor is "additionally sensitive" with lenses f2.8 or faster (meaning it is a cross sensor shaped like a '+' in combination with a cross sensor placed like an 'x')

What did I mix up in my interpretation?



Sep 10, 2009 at 04:30 AM
bpark42
Offline
• • •
Upload & Sell: On
p.1 #18 · p.1 #18 · Educate Me | Autofocus


dwweiche wrote:
I guess I still need to be educated as I thought I already had this all figured out. ...

This picture tells me that you can only use center-point AF on the XTi (400D) and only a lower-precision one-dimensional AF sensor? Not all 9 points are active at f5.6? Or is this actually how AF works on the 400D? I'm also surprised that this picture says that at f5.6 the 1DIII does not have an active cross point sensor? Again, this goes against what my thoughts were. Can someone un-wind my confusion?

I should have paid closer attention to the diagrams when I found that link, I guess. I am pretty sure the "High End" diagrams are showing the AF setup for the 1D Mark II, not the Mark III. As for it showing the XTi as only being able to use the center point at f5.6 I am not sure. If nobody else responds before I get home, I can test that with my wife's XTi...

I sent that link for the concepts more than the pictures.

dwweiche wrote:
And while I'm at it, I thought that now there are actually THREE types of AF sensors. I have an XSi (450D) and my interpretation of the AF sensor is that it has:
- 8 one-dimension AF sensors, effective to f5.6
- 1 cross type AF sensor shaped like a '+' at the center, effective to f5.6.
- The center AF sensor is "additionally sensitive" with lenses f2.8 or faster (meaning it is a cross sensor shaped like a '+' in combination with a cross sensor placed like an 'x')

What did I mix up in my interpretation?

Yes, some newer cameras have AF points that have additional diagonal sensitivity with fast lenses.



Sep 10, 2009 at 04:43 PM
hpjfromdk
Offline

Upload & Sell: Off
p.1 #19 · p.1 #19 · Educate Me | Autofocus


For those interested in how phase type autofocus conceptually works, I suggest reading this: doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Once that is understood take a look at this picture:






This is the AF sensor chip of the 400D (and 30D)

Schematically the line sensors of the AF chip, their orientation and their sensitivity are shown here:






The orientation and sensitivity of the corresponding AF points shown here:






In summary:
9 AF points:
6 Vertical F5.6
2 Horizontal F5.6
1 vertical (zigzag) F5.6 and 1 horizontal F5.6 or F2.8 combined to yield a "+" type sensor


OK, let's look at the 450D's AF sensor chip:






which compared to the 400 AF chip has a very similar general AF line sensor layout (plus a couple of color sensitive areas). Hence the 400D and 450D have the same number of and the AF point configuration.

In order to get a F2.8 capable cross AF point, the (baseline)distance between the the two parts of the sensor pair needs to be about the double of a corresponding F5.6 point. To avoid having to increase the size of the AF sensor (read: keep cost low) the 2 pairs used for a F2.8 capable cross AF point, are instead rotated by 45 deg. which among other things yield a 1.4 x longer baseline. This design has until now been deployed in the 40D, the 50D and the upcoming 7D will use it as well, but has not yet been ported to the xxxD series.

The details for the 1D mk lll AF can be studied in the associated white papers (google for "Canon white paper")

Hope that helped

- hans -






Edited on Sep 10, 2009 at 08:28 PM · View previous versions



Sep 10, 2009 at 05:57 PM
dwweiche
Offline
• • •
Upload & Sell: Off
p.1 #20 · p.1 #20 · Educate Me | Autofocus


Now THAT's what I call an explanation


Sep 10, 2009 at 06:13 PM





FM Forums | Canon Forum | Join Upload & Sell

    
 

You are not logged in. Login or Register

Username   Password    Retrive password