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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 05 Sep 2018 (Wednesday) 02:31
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EOS-R - It's out. Thoughts?

 
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TeamSpeed
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Oct 14, 2018 15:17 |  #1846

samueli wrote in post #18728741 (external link)
On my 50D, I bounced back and forth between good focus on the short end then the long end. It was several days of an hour or more each evening to reach a happy point across the lens' zoom range. I even bought focus charts. I did not enjoy the process.

You have to find a median back in the day when we couldn't MFA both ends. The alternative was to adjust where you spent most of your time. The only thing you had available beyond this is what we had to do before AFMA, send in gear for adjustments. If it was a 3rd party lens, you had to send equipment to multiple places. It was terrible.


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Oct 14, 2018 15:45 |  #1847

patrick j wrote in post #18728642 (external link)
If nothing else the idea of the focus always being on the nose gets rid of any uncertainty about a lens possibly being out of adjustment.

Is MFA for the lens or for the body? I thought MFA was just for the body. Due to manufacturing and adjustment tolerances, the length of the light path to the AF sensor could be different than the length to the image sensor. It's not an issue for mirrorless cameras because the image sensor *is* the AF sensor.

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Oct 14, 2018 15:53 |  #1848

mcoren wrote in post #18728758 (external link)
Is MFA for the lens or for the body? I thought MFA was just for the body. Due to manufacturing and adjustment tolerances, the length of the light path to the AF sensor could be different than the length to the image sensor. It's not an issue for mirrorless cameras because the image sensor *is* the AF sensor.

Mike

If the faults/tolerances were just camera-based, there wouldn't be support for a) different AFMA values for every lens you own up to 20 and b) adjustments for both the wide and telephoto ends of a zoom lens.


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Oct 14, 2018 16:10 |  #1849

TeamSpeed wrote in post #18728766 (external link)
If the faults/tolerances were just camera-based, there wouldn't be support for a) different AFMA values for every lens you own up to 20 and b) adjustments for both the wide and telephoto ends of a zoom lens.

Gotcha. I know in some of the bodies you can do one adjustment for all, instead of or in addition to adjustments for individual lenses. AF is pretty sophisticated these days, and I suppose part of the computations occur in the lens' processor. The lens isn't just responding to focus in/focus out commands from the body. The lenses themselves have firmware which occasionally need to be updated to improve AF or fix bugs with it (third party lenses mainly).

So having just written all of that, that would seem to say that you still need MFA for a mirrorless body. Why is that not true?

Mike (my world is crashing down on me!)


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Oct 15, 2018 01:31 |  #1850

mcoren wrote in post #18728786 (external link)
Gotcha. I know in some of the bodies you can do one adjustment for all, instead of or in addition to adjustments for individual lenses. AF is pretty sophisticated these days, and I suppose part of the computations occur in the lens' processor. The lens isn't just responding to focus in/focus out commands from the body. The lenses themselves have firmware which occasionally need to be updated to improve AF or fix bugs with it (third party lenses mainly).

So having just written all of that, that would seem to say that you still need MFA for a mirrorless body. Why is that not true?

Mike (my world is crashing down on me!)

The AF sensors in a DSLR are at a different distance from the rear of the lens compared to the sensor so there has to be a calculation to have the lens focus at a different position so the projected image is in focus on the sensor. If focus is done on the sensor, that does not have to be done.


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sploo
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Oct 15, 2018 03:50 as a reply to  @ Choderboy's post |  #1851

umphotography wrote in post #18728562 (external link)
exactly this

IBIS is not a deal breaker for me. However non dual cards is absolutly a deal breaker

Back in the mists of time (before SSDs were widely available) I was working for a company that was doing a lot of storage tech. At the time, solid state (flash) memory was small (in capacity) and slow by modern standards. We had an in-house designed chip with RAID capabilities, so put a bunch of flash units together with one of these RAID chips, into a box the size of a 2.5" HDD. The end result was a laptop style "hard disk" with either the speed & capacity of a modern SSD, or mirroring capabilities (so all data was stored twice).

Anyway... the point of my waffle is that I'm surprised no one has made a RAID1 SD card - e.g. a 128GB card that actually has 256GB of storage; and everything is written twice.

The card would appear like a normal 128GB SD in any device (for reading and writing). They'd likely need some proprietary software to access the mirror functionality - so you could pull data from just one "half" if the other were corrupt.

Not as good as having dual card slots, but would help for some types of failure. Probably the market isn't big enough to justify.


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sploo
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Oct 15, 2018 04:01 |  #1852

Tom Reichner wrote in post #18728611 (external link)
..
I must be an exception. . "No need to MFA" means nothing to me at all. . Heck, I only attempted to MFA a lens once, and it ended up not needing any adjustment at all. . No lens I've used needed any MFA. . They're all exactly "on", without exception. . And I am a pixel-peeper extraordinaire.
.

It's purely luck of the draw with the lenses and bodies you have. Manufacturing tolerances mean that any pair of body + lens could be spot on (either individually or collectively), or quite some way off.

Choderboy wrote in post #18728656 (external link)
Some Sony users (that I consider to be credible) say they do have a higher keeper rate and attribute it to the accuracy of on sensor AF. If it achieves focus, it's spot on.
Compared to a DSLR which tends to have a small variation.

Years ago there was a table showing the AF inconsistency of various Canon DSLRs. It was when 1D2 and 7D were current.
A higher score meant higher inconsistency.
From memory the 7D scored about 40, the 1D2 about 20. (Cheaper bodies scored more than 40)

It makes me wonder how mirrorless would score. 0? 5? Would a Sony A9 be more consistent than an A73?
Would the EOS-R beat a 1DX2?

The two main issues are precision and accuracy. If you repeatedly defocus a lens and AF on a target, does it hit the same focus distance each time (precision) or is there a spread of "locks" slightly in front or behind the median focus distance.

In theory, contrast AF should be pretty much spot on every time - assuming no hacks in the camera to try to speed things up.

Accuracy is how close to the right focus plane the camera is hitting. A high quality camera and lens combo (using phase AF, that needs some MFA), could be very precise, but not accurate (i.e. all "locks" are almost at the same focus distance, but that focus distance is consistently front or back of the desired plane. MFA should be able to fix that.

I'd assume that phase AF on the sensor (e.g. Dual Pixel) should always be accurate (correct focus plane) but may be less precise than contrast AF. Albeit the Dual Pixel phase AF will be much faster than contrast AF - as phase gives direction information; i.e. in which direction are we out of focus.


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John ­ Sheehy
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Oct 15, 2018 12:00 |  #1853

sploo wrote in post #18728229 (external link)
I still have my 7D, and it gets used occasionally - mostly for astro stuff.

I moved from it as my main body to a 5D3, then to a 5D4. If the EOS R has essentially the same sensor as the 5D4 then you're going to have a ball vs what comes off the 7D sensor ;-)a

The full benefit requires bigger lenses, with shallower DOF, for the same FOV. That is often forgotten.




  
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Oct 15, 2018 12:28 |  #1854

Choderboy wrote in post #18729085 (external link)
The AF sensors in a DSLR are at a different distance from the rear of the lens compared to the sensor so there has to be a calculation to have the lens focus at a different position so the projected image is in focus on the sensor. If focus is done on the sensor, that does not have to be done.

That was my understanding too, and that’s pretty much what I said in post 1847. But TeamSpeed’s post that I quoted (#1848) says that there are lens-related faults/tolerances that AFMA corrects too. If that’s true, they don’t simply disappear when the lens is on a mirrorless body.

Mike


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Oct 15, 2018 12:39 |  #1855

.

mcoren wrote in post #18729442 (external link)
TeamSpeed’s post that I quoted (#1848) says that there are lens-related faults/tolerances that AFMA corrects too. If that’s true, they don’t simply disappear when the lens is on a mirrorless body.

Mike

.
Yes, Mike, the focus-related effects of those faults do simply disappear when the lens is put on a mirrorless body, because the focusing is done on the sensor itself, thereby automatically accounting for and correcting all of the focus-related "faults" that may lie between the subject and the sensor.

People in this thread keep explaining this to you over and over again, yet you seem to be impervious to understanding it. . It's like, somehow, your brain just isn't grabbing ahold of the concept of focusing being done on the sensor, and all of the implications that go along with that.


.


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Post edited 5 months ago by Choderboy.
     
Oct 15, 2018 13:34 |  #1856

mcoren wrote in post #18729442 (external link)
That was my understanding too, and that’s pretty much what I said in post 1847. But TeamSpeed’s post that I quoted (#1848) says that there are lens-related faults/tolerances that AFMA corrects too. If that’s true, they don’t simply disappear when the lens is on a mirrorless body.

Mike

Solve lens problem on DSLR:
Using values 0-100, 0 being minimum focus and 100 being infinity focus:
The AF sensor has the lens move to 50 which is good focus, at the AF sensor. The imaging sensor is at a different distance.
So the camera tells the lens to move to 45, the value it calculates should result in the image being in focus on the image sensor.
But it isn't, in fact the lens needs to be at 44.

By using an MFA value of -5, when the camera tells the lens to move to 45, it moves to 44. Problem solved.
(The AF sensor does not use the MFA value, it does not need to. It is an AF sensor and it can sense when the image is in focus, at the AF sensor.)

The same lens on a mirrorless, only the first step, done by the AF sensor in the DSLR is required but is done by the imaging sensor so no MFA required to solve that problem.


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Oct 15, 2018 14:01 |  #1857

Tom Reichner wrote in post #18729455 (external link)
.

.
Yes, Mike, the focus-related effects of those faults do simply disappear when the lens is put on a mirrorless body, because the focusing is done on the sensor itself, thereby automatically accounting for and correcting all of the focus-related "faults" that may lie between the subject and the sensor.

People in this thread keep explaining this to you over and over again, yet you seem to be impervious to understanding it. . It's like, somehow, your brain just isn't grabbing ahold of the concept of focusing being done on the sensor, and all of the implications that go along with that.

.

I apologize if you find my questions to be frustrating, Tom. Feel free to pull up my profile and hit the "Ignore" link at any time.

I get that in a DSLR, the AF sensor and the image sensor are physically different components in different places. The camera manufacturers do their best to equalize the optical path distances to both of these, but tolerances are extremely tight to achieve critical focus. AFMA is provided to allow that difference to be fine-tuned by the end user in order to account for mechanical tolerances and other factors.

That much I get, and I believe I have now stated that at least three times in the past 24 hours.

The "concept" which my brain "just isn't grabbing ahold of" is that this explanation involves elements which are entirely within the camera body, so I don't understand how the lens affects this. If the two optical paths in the camera body are mismatched, wouldn't they have the same mismatch regardless of which lens was on the camera? So why do I have to set different AFMA values for each lens?

Obviously, the lens does affect the optical path length difference between the two sensors, otherwise Canon wouldn't provide AFMA settings for different lenses or different focal lengths. (or maybe it's a marketing gimmick that we've all swallowed hook, line, and sinker).

A mirrorless camera only has one optical path, and the image sensor is the AF sensor, so when it sees focus, it's in focus. I get that. My real question is how does the lens affect the differential optical path length difference within the body of a DSLR, making it require different AFMA settings for different lenses?

I admit that I may not have asked the question clearly. It's less about why mirrorless cameras don't need AFMA (which I understand), and more about why DSLRs need AFMA values for every lens and even for the wide/long ends of the same lens. And please don't answer by saying that the AF and image sensors are in different places. I understand that (that's four, BTW). Explain how the lens construction and focal length affects the path length difference within the camera body.

Mike


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Post edited 5 months ago by TeamSpeed. (2 edits in all)
     
Oct 15, 2018 14:09 |  #1858

mcoren wrote in post #18729528 (external link)
I apologize if you find my questions to be frustrating, Tom. Feel free to pull up my profile and hit the "Ignore" link at any time.

I get that in a DSLR, the AF sensor and the image sensor are physically different components in different places. The camera manufacturers do their best to equalize the optical path distances to both of these, but tolerances are extremely tight to achieve critical focus. AFMA is provided to allow that difference to be fine-tuned by the end user in order to account for mechanical tolerances and other factors.

That much I get, and I believe I have now stated that at least three times in the past 24 hours.

The "concept" which my brain "just isn't grabbing ahold of" is that this explanation involves elements which are entirely within the camera body, so I don't understand how the lens affects this. If the two optical paths in the camera body are mismatched, wouldn't they have the same mismatch regardless of which lens was on the camera? So why do I have to set different AFMA values for each lens?

Obviously, the lens does affect the optical path length difference between the two sensors, otherwise Canon wouldn't provide AFMA settings for different lenses or different focal lengths. (or maybe it's a marketing gimmick that we've all swallowed hook, line, and sinker).

A mirrorless camera only has one optical path, and the image sensor is the AF sensor, so when it sees focus, it's in focus. I get that. My real question is how does the lens affect the differential optical path length difference within the body of a DSLR, making it require different AFMA settings for different lenses?

I admit that I may not have asked the question clearly. It's less about why mirrorless cameras don't need AFMA (which I understand), and more about why DSLRs need AFMA values for every lens and even for the wide/long ends of the same lens. And please don't answer by saying that the AF and image sensors are in different places. I understand that (that's four, BTW). Explain how the lens construction and focal length affects the path length difference within the camera body.

Mike

Perhaps this will clear things up.

Here are the factors involved in focusing when there is a mirror in place:
- the lens focusing elements and mount error tolerances
- the optical path to the mirror and then onto the phase detect AF sensor
- the lens focusing to an exact preset location as told by the body from the phase detect AF
- the optical path to the sensor with mirror out of the way for recording the image (different than the mirror->phase AF)

A mirrorless body won't have a separate AF system or mirror, so this eliminates points 1,2, and 4. Regarding point 3, you still do have lenses that may not quite AF to the exact location as told by the body, however since the sensor is doing both the AF and recording, the body can compensate for point #3 for us. This leaves no real need for AFMA on a mirrorless, at least these days with such low error tolerances as it is.


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Oct 15, 2018 14:16 |  #1859

Choderboy wrote in post #18729508 (external link)
Solve lens problem on DSLR:
Using values 0-100, 0 being minimum focus and 100 being infinity focus:
The AF sensor has the lens move to 50 which is good focus, at the AF sensor. The imaging sensor is at a different distance.
So the camera tells the lens to move to 45, the value it calculates should result in the image being in focus on the image sensor.
But it isn't, in fact the lens needs to be at 44.

By using an MFA value of -5, when the camera tells the lens to move to 45, it moves to 44. Problem solved.
(The AF sensor does not use the MFA value, it does not need to. It is an AF sensor and it can sense when the image is in focus, at the AF sensor.)

The same lens on a mirrorless, only the first step, done by the AF sensor in the DSLR is required but is done by the imaging sensor so no MFA required to solve that problem.

Thank you, Dave. I understand how AFMA works, really. My question is more about why it differs for each lens. See my post #1857.
Mike


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Post edited 5 months ago by TeamSpeed. (3 edits in all)
     
Oct 15, 2018 14:18 |  #1860

mcoren wrote in post #18729540 (external link)
Thank you, Dave. I understand how AFMA works, really. My question is more about why it differs for each lens. See my post #1857.
Mike

Just 2 reasons I can think of:

- Because one lens might be off a few microns in the mount depth over another lens
- Because one lens may not quite AF to the exact amount as told by the body, consistently

The first reason is why a lens has a different AFMA slot than that lens + a TC. They will register as different lenses so you can put in different values.

Sigma goes a step further and allows you to set up to 16 different AFMA values, across focal lengths and focusing distances. So this means that Sigma has identified errors in their lens groups that could cause issues in IQ in different configurations, and gives you the ability to change all of these. Again, I see no need for this though with mirrorless?


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