Pixel Perfect wrote:
Steve_T90 wrote:
The only other thing I thought would make a difference is the CF control of AF/AE, so I tested out Lexvo's statement for myself. No matter what CF4 is set to, manual exposure calculation is never locked on my 20D. Just as well for Canon, as I can imagine that they would be less than thrilled if people started saying, "The 20D can't walk and chew gum at the same time..."

What do you mean by this. If it's a manual exposure then by definition it's a locked exposure. You've set aperture and shutter speed, and they are now fixed. Exposure lock only makes sense in the context of AE. The camera may indicate that the locked exposure is different to it's metered value as you swing the camera around, but it doen't affect your settings.

Bolded sentence you wrote is exactly what I mean -- the camera never stops metering (or "calculating exposure," to use my terms above) in M mode.

[What do you mean by this. If it's a manual exposure then by definition it's a locked exposure. You've set aperture and shutter speed, and they are now fixed. Exposure lock only makes sense in the context of AE. The camera may indicate that the locked exposure is different to it's metered value as you swing the camera around, but it doen't affect your settings.

Bolded sentence you wrote is exactly what I mean -- the camera never stops metering (or "calculating exposure," to use my terms above) in M mode.

Mere light level indication probably doesn't call for much in terms of calculation--remember that cameras used to do it with nothing more than a tiny galvometer. That's all that's happening with manual exposure measurement. Evaluative auto exposure mode does take some computer horsepower.

RDKirk wrote:
Mere light level indication probably doesn't call for much in terms of calculation--remember that cameras used to do it with nothing more than a tiny galvometer. That's all that's happening with manual exposure measurement. Evaluative auto exposure mode does take some computer horsepower.



Ah yes but you can use evaluative metering in M mode, just as you can use spot or partial or centreweighted. Just that it's recommendation is not necessarily what you think

RDKirk wrote:
Mere light level indication probably doesn't call for much in terms of calculation--remember that cameras used to do it with nothing more than a tiny galvometer. That's all that's happening with manual exposure measurement. Evaluative auto exposure mode does take some computer horsepower.


Pixel Perfect wrote:
Ah yes but you can use evaluative metering in M mode, just as you can use spot or partial or centreweighted. Just that it's recommendation is not necessarily what you think

Yes, it does, and to do so, it's running comparisons between the immediate scene and the thousands of scenes in its database. Very capable computer there. But that's still more horsepower than it's using just to indicate light level in other meter modes.

As for whether it's using the maximum aperture or the shooting aperture to determine the depth of focus, I'd still say it's using the maximum aperture. It's got to use some figure to determine the focusing solution, and the focusing solution does not appear to change if you change shooting aperture.

Thanks to the person who posted that piece of information regarding Canon lens focusing.It took time and concern. But, I do not know how to apply it. I do not interpret writings well where I can't define half the vocabulary used. All is want is a lens that works. I read on another forum (dpreview.com) many people have this backfocus problem with 70-200 f4L lens and others. I was planning to buy a 20D, 17-40L and the 70-200L this week. But now I am having second and third thoughts. I do not want to spend about $2500, not including filters, cards etc to have to constantly send it back. If anyone can help me understand how to apply that technical posting or finding an alternative lens from Sigma to the 70-200L I would appreciate it. If this is a lot to ask, that's ok. I just might go Nikon, or film, seems to be less lens problems there. (a 30 yr canon user)
thanks
frank

frankjay wrote:
Thanks to the person who posted that piece of information regarding Canon lens focusing.It took time and concern. But, I do not know how to apply it. I do not interpret writings well where I can't define half the vocabulary used. All is want is a lens that works. I read on another forum (dpreview.com) many people have this backfocus problem with 70-200 f4L lens and others. I was planning to buy a 20D, 17-40L and the 70-200L this week. But now I am having second and third thoughts. I do not want to spend about $2500, not including filters, cards etc to have to constantly send it back. If anyone can help me understand how to apply that technical posting or finding an alternative lens from Sigma to the 70-200L I would appreciate it. If this is a lot to ask, that's ok. I just might go Nikon, or film, seems to be less lens problems there. (a 30 yr canon user)
thanks
frank

There are nowhere near as many problems in practical usage as the complaints in the forums would have you think.

Proportionately, I don't see any fewer problems in the Nikon forums. If you'll notice, the Canon forums always have two or more times more traffic than Nikon forums--which means you'll see two to three times more complaints in absolute numbers.

Moreover, the Internet "lens effect" makes everything seem bigger than it really is. Happy people are taking pictures; unhappy people are complaining. And they tend to complain repeatedly, so you see the same single complaint repeated in many topics and over several forums.

One of the problems Canon faces with being the marketing leader is that they also have a lot more inexperienced people (for instance, Canon sells cameras in Wal-Mart--Nikon doesn't) so there are a lot more people who will have problems. A lot of those people also have computers.

Much of the complaints are because people are "pixel peeping" instead of printing. The focusing system is designed to produce a good 6x9 print viewed at 10 inches. Most people who complain are looking at their images enlarged to 40x60 and viewing it at 10 inches. Of COURSE they will see problems--the system is not designed to be that accurate. But print out a picture to your normal size and see if you have a problem. People still shooting film aren't having all these problems because they judge the system based on the prints they make.

Then there is the final factor: Automatic focusing actually takes some skill to use. Not much skill, but what is necessary is necessary. People have to do some practicing and some exercises to find out just how the system works and how to make it do what they want it it to do.

10DFT wrote:
Stoffer wrote:
Thanks for a really great post!

This got my attention big time:


""2. The actual focus sensor arrays are three times larger than the viewfinder marks. A user could put an intended subject in the mark, but if there is a strong detail just outside the mark (but within the sensor area), the camera would focus on that strong detail. This is a source of much of the complaints of the back- or front-focusing -- especially with the "ruler tests." Also, as far as the camera is concerned, a focus lock on anything within the sensor area is good, which sometimes covers more area than the photographer intended.""

Gotta remember this that next time I use my 135L at f/2!

I keep this handy image of the 10D sensor arrangement overlaid in red on top of the sensor indicators in the viewfinder to illustrate this very point:

I've done that testing and found the diagram to be accurate. Actually, my testing had mapped out the diagram before I ever saw the diagram.

To test, try making some bold black marks on small cards, one mark to a card: Horizontal, vertical, diagonal in both directions.

Tape the cards to a blank wall with a good amount of distance between them so that you won't have any problem getting only one into a focusing area.

Use a tripod so that you can move a focus point toward any one card in very small increments.

If you run tests moving each direction of line toward each AF point from each of the primary directions (every 45 degrees in a 360 circle), you'll wind up mapping the diagram.

hi rDkirk

how many shots to do this mapping?

I did just a couple , not to map them but to evaluate
where the centre mark starts
ie background is very far away, object is 5ft away and i slowly
move the centre AF towards the object

AF focused on background right until it hits the object.
ie: object border is within the AF markings

There is plenty of contrast between object and background.

Could you try my method and see if you get something like this abnormality?

maxim_me wrote:
hi rDkirk

how many shots to do this mapping?

I did just a couple , not to map them but to evaluate
where the centre mark starts
ie background is very far away, object is 5ft away and i slowly
move the centre AF towards the object

AF focused on background right until it hits the object.
ie: object border is within the AF markings

There is plenty of contrast between object and background.

Could you try my method and see if you get something like this abnormality?

I put the marked cards on a wall--thus a 2D target that is mostly unfocusably blank with a few focusable marks arrayed on it.

That way, there is nothing the camera can focus on except a specific mark as I nudge the AF point closer and closer to it from one direction. I press the focus button with each nudge, until it gets close enough to grab the mark.

Then I move the AF point away from that mark and approach it or another mark with the AF point from a different direction.

maxim_me wrote:
10DFT wrote:

The "Circle of Confusion" is a place I visit often. Just about the time I think I understand something...

Dwain

RDKirk wrote:
damjr1 wrote:
Folks..., would someone in the know please clarify one thing in regards to the "f2.8 or faster lens" & "increased sensor capability" feature. I have read the Westfall article several times, btw...

When a lens with a maximum aperture capability of f2.8 or greater is attached:
a) the increased sensitivity is available while using "any" aperture on that lens.

b) the increased sensitivity is available when the aperture of the attached lens is set to f2.8 or greater.

Hope I have made myself clear with this wording.

Thanks,
Doug


When you attach a lens with a maxium aperture of f2.8 or greater, the camera turns on a second vertical line of phase detection pixels in the center sensor pixel array.

This is what Canon calls the "high precision" mode and supposedly will place the actual plane of focus within 1/3 of the depth of focus (the level of accuracy at the sensor plane).

It appears to me that switching on the additional sensor is a function of the identification information the lens provides to the camera ("My maximum aperture is f2.8") rather than the amount of light being transmitted by the lens. I believe it applies to any shooting aperture used while that lens is mounted--the camera always uses the maximum aperture depth of focus factor for all its focus solutions, not the shooting aperture.

I don't have a document stating this outright, but looking at the chain of events depicted in Canon's patent application, the camera calculates the focus solution for a shot before it calculates the exposure solution (this is definitely true when using evaluative mode autoexposure), so when it sends the movement command to the lens, it doesn't yet know what the shooting aperture will be.

Just to put a point on this - its worth spending the extra money for f/2, f/1.8, f/1.4, and even f/1.2 max aperture lenses for use with the 20D - not just for low light situations, but even when I shoot at, say, f/8.

Is that right?

Just to put a point on this - its worth spending the extra money for f/2, f/1.8, f/1.4, and even f/1.2 max aperture lenses for use with the 20D - not just for low light situations, but even when I shoot at, say, f/8.

Is that right?


Logically it would seem so.

However, perusing the complaints at DPReview, a possible problem has occured to me. A number of people are complaining that their 20D cameras focus perfectly well with lenses that have a maximum aperture of 2.8, but poorly with faster lenses.

Now, it could be possible that the 20D computer simply isn't good enough to keep within the tolerances, but some of these people have said that their 300D cameras do better.

I'm wondering if a possible firmware bug might involve a camera not "getting the message right" when it first interrogates the lens for maximum aperture. If the lens says "My maximum aperture is f1.4," but the camera erroneously fails to poke that info into the right memory location (thus, not overwriting the maximum aperture recorded from the previously mounted lens), then it's doing its calculations with the wrong number. But put an f2.8 lens on it, and it's fine...it would also work fine with a slower lens.

If the Canon techs are using an f2.8 tool lens (I don't know what kind of lens they use), then they won't see that problem.

RDKirk wrote:
Just to put a point on this - its worth spending the extra money for f/2, f/1.8, f/1.4, and even f/1.2 max aperture lenses for use with the 20D - not just for low light situations, but even when I shoot at, say, f/8.

Is that right?


Logically it would seem so.

However, perusing the complaints at DPReview, a possible problem has occured to me. A number of people are complaining that their 20D cameras focus perfectly well with lenses that have a maximum aperture of 2.8, but poorly with faster lenses.

Now, it could be possible that the 20D computer simply isn't good enough to keep within the tolerances, but some of these people have said that their 300D cameras do better.

I'm wondering if a possible firmware bug might involve a camera not "getting the message right" when it first interrogates the lens for maximum aperture. If the lens says "My maximum aperture is f1.4," but the camera erroneously fails to poke that info into the right memory location (thus, not overwriting the maximum aperture recorded from the previously mounted lens), then it's doing its calculations with the wrong number. But put an f2.8 lens on it, and it's fine...it would also work fine with a slower lens.

If the Canon techs are using an f2.8 tool lens (I don't know what kind of lens they use), then they won't see that problem.

You make a good point, but i highly doubt this is the case, because if the camera wasnt communicating the lenses maximum aperture correctly, then you could assume the camera also wouldnt let you dial the aperture down past 2.8, to say, 1.4, or whatever it would be.

I read somewhere that it was a good idea to turn the camera off before changing lenses so that the the CPU's could reset to the new data presented by the new lens. I'm not sure that makes any sense, but it seems like a good idea anyway.

Wonotch wrote:
RDKirk wrote:
Just to put a point on this - its worth spending the extra money for f/2, f/1.8, f/1.4, and even f/1.2 max aperture lenses for use with the 20D - not just for low light situations, but even when I shoot at, say, f/8.

Is that right?


Logically it would seem so.

However, perusing the complaints at DPReview, a possible problem has occured to me. A number of people are complaining that their 20D cameras focus perfectly well with lenses that have a maximum aperture of 2.8, but poorly with faster lenses.

Now, it could be possible that the 20D computer simply isn't good enough to keep within the tolerances, but some of these people have said that their 300D cameras do better.

I'm wondering if a possible firmware bug might involve a camera not "getting the message right" when it first interrogates the lens for maximum aperture. If the lens says "My maximum aperture is f1.4," but the camera erroneously fails to poke that info into the right memory location (thus, not overwriting the maximum aperture recorded from the previously mounted lens), then it's doing its calculations with the wrong number. But put an f2.8 lens on it, and it's fine...it would also work fine with a slower lens.

If the Canon techs are using an f2.8 tool lens (I don't know what kind of lens they use), then they won't see that problem.

You make a good point, but i highly doubt this is the case, because if the camera wasnt communicating the lenses maximum aperture correctly, then you could assume the camera also wouldnt let you dial the aperture down past 2.8, to say, 1.4, or whatever it would be.


You could be right, however the communication of lens characteristics does appear to continue beyond the initial interrogation, as noted by the fact that variable aperture zoom lenses appear to communicate their changing apertures to the camera (or they communicate the changing focal length and the camera calculates the changing aperture, which still amounts to continuing communication).

I'm swinging in the dark here--too many unknowns.

Wonotch wrote:
RDKirk wrote:
Just to put a point on this - its worth spending the extra money for f/2, f/1.8, f/1.4, and even f/1.2 max aperture lenses for use with the 20D - not just for low light situations, but even when I shoot at, say, f/8.

Is that right?


Logically it would seem so.

However, perusing the complaints at DPReview, a possible problem has occured to me. A number of people are complaining that their 20D cameras focus perfectly well with lenses that have a maximum aperture of 2.8, but poorly with faster lenses.

Now, it could be possible that the 20D computer simply isn't good enough to keep within the tolerances, but some of these people have said that their 300D cameras do better.

I'm wondering if a possible firmware bug might involve a camera not "getting the message right" when it first interrogates the lens for maximum aperture. If the lens says "My maximum aperture is f1.4," but the camera erroneously fails to poke that info into the right memory location (thus, not overwriting the maximum aperture recorded from the previously mounted lens), then it's doing its calculations with the wrong number. But put an f2.8 lens on it, and it's fine...it would also work fine with a slower lens.

If the Canon techs are using an f2.8 tool lens (I don't know what kind of lens they use), then they won't see that problem.

You make a good point, but i highly doubt this is the case, because if the camera wasnt communicating the lenses maximum aperture correctly, then you could assume the camera also wouldnt let you dial the aperture down past 2.8, to say, 1.4, or whatever it would be.


Wonotch,

RDKirk was referring to a firmware bug in the AF code section, and not necessarily to a camera to lens communication problem. His example [not overwriting the previous lens data, as it ought], is just one possible bug scenario for the AF code section not getting the message.

RDKirk,

My take on the perception of service techs to focus accuracy issues is that mechanical shot-to-shot variance in the USM system is well outside of 1/3 DOF. Since system is open loop, techs would not interpret small excursions past 1/3 DOF as camera calculation error, but lens shot-to-shot mechanical variance.

RDKirk,

My take on the perception of service techs to focus accuracy issues is that mechanical shot-to-shot variance in the USM system is well outside of 1/3 DOF. Since system is open loop, techs would not interpret small excursions past 1/3 DOF as camera calculation error, but lens shot-to-shot mechanical variance.


If I understand you correctly, I think there should be minimal shot-to-shot mechanical variance because the lens drive process is closed-loop to the extent that the lens monitors itself to ensure it followed the command from the camera and makes corrections as necessary.

That is also my understanding, that the lens focus group position is monitored after completion of the move command. I assume the tolerances of that system is what results in shot-to-shot focus variance. The only real info from Canon techs that I got was that they didn't expect 1/3 DOF due to mechanical variations. The rest is my inferrences.

jmaio wrote:
I read somewhere that it was a good idea to turn the camera off before changing lenses so that the the CPU's could reset to the new data presented by the new lens. I'm not sure that makes any sense, but it seems like a good idea anyway.

It's not necessary to turn the camera off (which would be intolerable to most professionals).

The camera interrogates the lens whenever a new lens is mounted. In fact, I believe a consistent problem I have with a Tokina f2.8 80-200 zoom is specifically because the 20D interrogates the lens even when it's "asleep"--apparently the Tokina is programmed to react first to some standing signal sent when the camera is awake--lacking that signal when the camera is asleep, the lens won't respond to interrogation.

i use the 1dmII,, and have often wondered and asked other users what mode they use as far as moving subjects,, i.e.,,, say a bear that is just moseying around, no particuluar speed, or an animal that is stil now, but could bolt or move off at any time,,i know my non moving focus is fine, but i seem to get a way more oof shots on small movements, as types of movement menttioned above,, the oof does not appear to be user movemtn, just softness,,while it appears to get caught between focus lock hunting movements,,

any suggestion or thought

You make a good point, but i highly doubt this is the case, because if the camera wasnt communicating the lenses maximum aperture correctly, then you could assume the camera also wouldnt let you dial the aperture down past 2.8, to say, 1.4, or whatever it would be.


Different memory registers. It stores the maximum aperture somewhere and uses that constantly for CoC calculations. As far as I can tell, the working aperture is only used for exposure calculation.

jmaio wrote:
I read somewhere that it was a good idea to turn the camera off before changing lenses so that the the CPU's could reset to the new data presented by the new lens. I'm not sure that makes any sense, but it seems like a good idea anyway.

I don't think that makes a difference with the 20D (and probably not with the 350XT, either). I have a Tokina 80-200 that is balky about communicating with the camera if I attach it when the camera is in sleep mode. Once it's attached, no manipulation of the on/off switch will re-interrogate the lens. I have to either remove and remount the lens or remove the battery from the camera. As far as I can tell, the camera interrogates the lens whether it's on or off, just as it reads a new CF card whether it's on or off.

maxim_me wrote:
10DFT wrote:
Stoffer wrote:
Thanks for a really great post!

This got my attention big time:


""2. The actual focus sensor arrays are three times larger than the viewfinder marks. A user could put an intended subject in the mark, but if there is a strong detail just outside the mark (but within the sensor area), the camera would focus on that strong detail. This is a source of much of the complaints of the back- or front-focusing -- especially with the "ruler tests." Also, as far as the camera is concerned, a focus lock on anything within the sensor area is good, which sometimes covers more area than the photographer intended.""

Gotta remember this that next time I use my 135L at f/2!

I keep this handy image of the 10D sensor arrangement overlaid in red on top of the sensor indicators in the viewfinder to illustrate this very point: