DotTune MFA question...

[ErikH]

Hi - I just MFA my 70D using the "DotTune" method outside on a bright day with 2 of my lenses. Here are my results and my questions below:

55-250 STM

(W)ide -8 +5 = -1 (or -2)

(T)ele -5 +3 = -1

18-135 STM

W -6 +5 = 0

T -7 + 5 = -1

1) Because my adjustments are so minimal, is it even worth making the change? The reason I'm asking, is (if I'm understanding correctly) this method balances the focal plane evenly whereas typically the focal plane is thought to be 1/3 in front & 2/3 behind (speaking generally).

Since my camera has a (slight) tendency toward the minus side, could that be part of the correction DotTune is trying to make to even "things" out? (hopefully that makes sense).

Any comments appreciated!

 

[Zeee]

MFA values from Canon:

this is clarifying what Chuck said about Field of Focus either in front or behind the focal plane.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments.

So based in this what is the answer? I know there will be an impact if I make one MFA move on an f1.2 lens at close distance. I will see a significant impact. If whatever MFA device I use tells me I need one move will I see it on an f5.6 lens at 100 feet? I'm trying to prove mathematically you won't see a difference.

If I were Canon I would have never gave a value of ⅛ and just said fine increments. There is still no real value for what ⅛ is.

Zee,

Each "click" of MFA is equal to 1/16 of the whole DOF from DOF calculator (calculated for the specific lens focal length, wide open aperture, the subject distance) for mfa. So, the 1/16 is a number that changes based on subject distance for a fixed focal length lens at its widest aperture.

Each "click" can be represented as (1/16) x Total DOF*

*BUT NOTE: this is for the lens calculated DOF at wide open aperture. Any stopping down will incur the value from the wide open aperture calculated.

Again, small numbers are small changes, but MFA does help AF place/center the focal plane during auto focus. Said another way, it shifts the actual focal plan and DOF relative to the desired focus plane. This is useful when your DOF just "fits" your subject or other reasons necessitating alignment of the whole DOF around the desired af plane.

Example: Shooting a face wanting eyes, nose and ears to be in focus when focusing on the eyes. IF no MFA done, the AF might create sharp eyes at the front of the DOF so the nose is OOF. Thus negative MFA would be needed.

OK so it is the whole DOF area divided by a value. That makes this very interesting especially when you apply different focal lengths, distance to target and aperture. The DOF changes each time so does the value. I realize this is not perfect as it does start to blur out near the edges but there is an area that is not effected and is in focus. This of course changes with different apertures.

This explains why some suggest 50X and others MFA successfully at MFD. At member POTN said that some company in Sweden (I think) MFA's 500-800 lenses at 21 ft or something like that.

I think the closer the target the less forgiving the process resulting in possibly more errors. Maybe that is why Canon suggests 50X or depending on what Canon document you read you should MFA at the shooting location My tests have shown different results at different distances. Not much though and maybe it does not matter that much. Others suggest 25X. I have a few beefs about MFA and this was one of them. This may be a logical answer.

I'm happy with all my combos but I'm not sure about my 100 - 400 with a 1.4 TC. I'm not going to go MFD but closer to the target, see what it tells me and then go out and shot. I'll try at the MFA it tells me and 0 and compare.

Good input from everyone.

 

[WilbaW]

There is actually very complex math for dof calculation with differing formula for what is considered near field and far field.

The formulae are complicated, but the physics is just similar triangles.

However, by definition and related to the circle of confusion, the entire dof is in focus, and the single point is rendered as a single point, not oblong.

No, by definition a point in the scene appears as a circle of confusion on the sensor (assuming a perfect lens), and only points in the plane of focus appear as points on the sensor (COC = 0). We arbitrarily define an "acceptable" limit to the COC in the hope that viewers of an image don't notice that elements in front of or behind the plane of focus are actually out of focus.

I agree with your better explanation about circle of confusion, and you are really saying the same thing, that within DOF, we deem things look sharp.

However, in the first point, I believe you are confusing Field of View which is simple triangle math, with DOF which is much more complex.

a source: http://en.wikipedia.org/wiki/Depth_of_field

No, check out the diagram under the heading "Derivation of the DOF formulae".

 

[WilbaW]

Let's not forget, the inch of total DOF is not "in focus". It is, in theory, not noticeably out of focus to someone with ordinary eyesight viewing an image under the conditions assumed in the calculation. Those assumptions don't apply for instance if you're peering at the image at 100% on a nice big monitor.

I did say there is a little room for error for that.

Not sure which bit you're referring to or whether you're being serious.

As usual thanks for your help. Always good info.

No worries, thanks.

 

[guy schlacter]

MFA values from Canon:

this is clarifying what Chuck said about Field of Focus either in front or behind the focal plane.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments.

So based in this what is the answer? I know there will be an impact if I make one MFA move on an f1.2 lens at close distance. I will see a significant impact. If whatever MFA device I use tells me I need one move will I see it on an f5.6 lens at 100 feet? I'm trying to prove mathematically you won't see a difference.

If I were Canon I would have never gave a value of ⅛ and just said fine increments. There is still no real value for what ⅛ is.

Zee,

Each "click" of MFA is equal to 1/16 of the whole DOF from DOF calculator (calculated for the specific lens focal length, wide open aperture, the subject distance) for mfa. So, the 1/16 is a number that changes based on subject distance for a fixed focal length lens at its widest aperture.

Each "click" can be represented as (1/16) x Total DOF*

*BUT NOTE: this is for the lens calculated DOF at wide open aperture. Any stopping down will incur the value from the wide open aperture calculated.

Again, small numbers are small changes, but MFA does help AF place/center the focal plane during auto focus. Said another way, it shifts the actual focal plan and DOF relative to the desired focus plane. This is useful when your DOF just "fits" your subject or other reasons necessitating alignment of the whole DOF around the desired af plane.

Example: Shooting a face wanting eyes, nose and ears to be in focus when focusing on the eyes. IF no MFA done, the AF might create sharp eyes at the front of the DOF so the nose is OOF. Thus negative MFA would be needed.

OK so it is the whole DOF area divided by a value. That makes this very interesting especially when you apply different focal lengths, distance to target and aperture. The DOF changes each time so does the value. I realize this is not perfect as it does start to blur out near the edges but there is an area that is not effected and is in focus. This of course changes with different apertures.

This explains why some suggest 50X and others MFA successfully at MFD. At member POTN said that some company in Sweden (I think) MFA's 500-800 lenses at 21 ft or something like that.

I think the closer the target the less forgiving the process resulting in possibly more errors. Maybe that is why Canon suggests 50X or depending on what Canon document you read you should MFA at the shooting location My tests have shown different results at different distances. Not much though and maybe it does not matter that much. Others suggest 25X. I have a few beefs about MFA and this was one of them. This may be a logical answer.

I'm happy with all my combos but I'm not sure about my 100 - 400 with a 1.4 TC. I'm not going to go MFD but closer to the target, see what it tells me and then go out and shot. I'll try at the MFA it tells me and 0 and compare.

Good input from everyone.

Zee,

the best and most consistently accurate results are when you MFA under similar lighting (sun vs incandescent vs flourescent), distance, and focal length that you anticipate being closest most often to actual use. All 3 factors do affect the AF.

 

[WilbaW]

Each "click" can be represented as (1/16) x Total DOF*

Quote from Canon:

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open.

Each step = 1 click. How does an 1/8th becomes a 1/16th?

this is clarifying what Chuck said about Field of Focus either in front or behind the focal plane.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments.

Are you saying that "moving forward (toward the camera) or back (toward the background)" makes "the depth-of-field you'd have with the current lens wide-open" actually mean the single-sided DOF (which is 1/2 the total DOF), rather than the total DOF?

 

[Zeee]

MFA values from Canon:

this is clarifying what Chuck said about Field of Focus either in front or behind the focal plane.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments.

So based in this what is the answer? I know there will be an impact if I make one MFA move on an f1.2 lens at close distance. I will see a significant impact. If whatever MFA device I use tells me I need one move will I see it on an f5.6 lens at 100 feet? I'm trying to prove mathematically you won't see a difference.

If I were Canon I would have never gave a value of ⅛ and just said fine increments. There is still no real value for what ⅛ is.

Zee,

Each "click" of MFA is equal to 1/16 of the whole DOF from DOF calculator (calculated for the specific lens focal length, wide open aperture, the subject distance) for mfa. So, the 1/16 is a number that changes based on subject distance for a fixed focal length lens at its widest aperture.

Each "click" can be represented as (1/16) x Total DOF*

*BUT NOTE: this is for the lens calculated DOF at wide open aperture. Any stopping down will incur the value from the wide open aperture calculated.

Again, small numbers are small changes, but MFA does help AF place/center the focal plane during auto focus. Said another way, it shifts the actual focal plan and DOF relative to the desired focus plane. This is useful when your DOF just "fits" your subject or other reasons necessitating alignment of the whole DOF around the desired af plane.

Example: Shooting a face wanting eyes, nose and ears to be in focus when focusing on the eyes. IF no MFA done, the AF might create sharp eyes at the front of the DOF so the nose is OOF. Thus negative MFA would be needed.

OK so it is the whole DOF area divided by a value. That makes this very interesting especially when you apply different focal lengths, distance to target and aperture. The DOF changes each time so does the value. I realize this is not perfect as it does start to blur out near the edges but there is an area that is not effected and is in focus. This of course changes with different apertures.

This explains why some suggest 50X and others MFA successfully at MFD. At member POTN said that some company in Sweden (I think) MFA's 500-800 lenses at 21 ft or something like that.

I think the closer the target the less forgiving the process resulting in possibly more errors. Maybe that is why Canon suggests 50X or depending on what Canon document you read you should MFA at the shooting location My tests have shown different results at different distances. Not much though and maybe it does not matter that much. Others suggest 25X. I have a few beefs about MFA and this was one of them. This may be a logical answer.

I'm happy with all my combos but I'm not sure about my 100 - 400 with a 1.4 TC. I'm not going to go MFD but closer to the target, see what it tells me and then go out and shot. I'll try at the MFA it tells me and 0 and compare.

Good input from everyone.

Zee,

the best and most consistently accurate results are when you MFA under similar lighting (sun vs incandescent vs flourescent), distance, and focal length that you anticipate being closest most often to actual use. All 3 factors do affect the AF.

Well that is what the manual says amongst other tidbits of MFA info which I'm a big supporter of.

 

[Zeee]

Each "click" can be represented as (1/16) x Total DOF*

Quote from Canon:

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open.

Each step = 1 click. How does an 1/8th becomes a 1/16th?

this is clarifying what Chuck said about Field of Focus either in front or behind the focal plane.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

When setting the Microadjustment, you'll see a scale on the camera's LCD monitor with up to + or - twenty steps. Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open. And that 1/8th of the depth of field is only moving forward (toward the camera) or back (toward the background) from the sharpest plane of focus. The main thing to remember here is that these are very fine increments.

Are you saying that "moving forward (toward the camera) or back (toward the background)" makes "the depth-of-field you'd have with the current lens wide-open" actually mean the single-sided DOF (which is 1/2 the total DOF), rather than the total DOF?

I'm curious as well.

 

[Zeee]

Let's not forget, the inch of total DOF is not "in focus". It is, in theory, not noticeably out of focus to someone with ordinary eyesight viewing an image under the conditions assumed in the calculation. Those assumptions don't apply for instance if you're peering at the image at 100% on a nice big monitor.

I did say there is a little room for error for that.

Not sure which bit you're referring to or whether you're being serious.

I was referring to you have a total DOF but all the 8 increments may not be in perfect focus. For example 1 and 9 might start blurring.

As usual thanks for your help. Always good info.

No worries, thanks.

 

[Zeee]

Not sure if this has been posted.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here is something I found on page 1.

 

[RogerZoul]

Not sure if this has been posted.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here is something I found on page 1.

If one uses the often suggested 50 x lens focal length doesn't this make the adjustments be based on focal length? It could be a matter of degree, but often times I feel like the adjustments should be done at the minimum focus distance as that seems to be were the DOF is the smallest and where the errors, or lack of, will matter the most. Please point out where my logic is wrong. I have my MFA set to disable, btw, and I shoot mostly long FLs like 640mm or 960mm, so this is the context in which I'm most concerned about AF errors. Up close is where you get the best/sharpest feather details on a bird, for example, and that is when/where I want the absolute best from my lenses.

 

[WilbaW]

Let's not forget, the inch of total DOF is not "in focus". It is, in theory, not noticeably out of focus to someone with ordinary eyesight viewing an image under the conditions assumed in the calculation. Those assumptions don't apply for instance if you're peering at the image at 100% on a nice big monitor.

I did say there is a little room for error for that.

Not sure which bit you're referring to or whether you're being serious.

I was referring to you have a total DOF but all the 8 increments may not be in perfect focus. For example 1 and 9 might start blurring.

Okay, well there are a few things we can say about that. It's easier if we think about 0 at the plane of focus, and minus and plus from there. It's important to understand that any position away from the POF is actually out of focus, IOW, - 1, +2, - 3... are all OOF, but - 5 and +5 should be on the edge of where people with ordinary visual acuity might start to notice it, if they are looking at an image for which the COC was chosen. If they are looking at an image under different conditions, e.g. from closer than the image diagonal or with better than average visual acuity, - 3 to +3 might be perceived as OOF. Or if they are looking at an image from many times the image diagonal, - 12 to +12 for instance might appear to be in focus.

The crucial thing to understand is that DOF is entirely about viewer perception. It has no meaning other than that.

 

[guy schlacter]

Not sure if this has been posted.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here is something I found on page 1.

If one uses the often suggested 50 x lens focal length doesn't this make the adjustments be based on focal length? It could be a matter of degree, but often times I feel like the adjustments should be done at the minimum focus distance as that seems to be were the DOF is the smallest and where the errors, or lack of, will matter the most. Please point out where my logic is wrong. I have my MFA set to disable, btw, and I shoot mostly long FLs like 640mm or 960mm, so this is the context in which I'm most concerned about AF errors. Up close is where you get the best/sharpest feather details on a bird, for example, and that is when/where I want the absolute best from my lenses.

Roger,
The MFA click adjustment results are based on/relative to adjusting the focal plane relative to the dof, not by a fixed distance.

 

[Zeee]

Let's not forget, the inch of total DOF is not "in focus". It is, in theory, not noticeably out of focus to someone with ordinary eyesight viewing an image under the conditions assumed in the calculation. Those assumptions don't apply for instance if you're peering at the image at 100% on a nice big monitor.

I did say there is a little room for error for that.

Not sure which bit you're referring to or whether you're being serious.

I was referring to you have a total DOF but all the 8 increments may not be in perfect focus. For example 1 and 9 might start blurring.

Okay, well there are a few things we can say about that. It's easier if we think about 0 at the plane of focus, and minus and plus from there. It's important to understand that any position away from the POF is actually out of focus, IOW, - 1, +2, - 3... are all OOF, but - 5 and +5 should be on the edge of where people with ordinary visual acuity might start to notice it, if they are looking at an image for which the COC was chosen. If they are looking at an image under different conditions, e.g. from closer than the image diagonal or with better than average visual acuity, - 3 to +3 might be perceived as OOF. Or if they are looking at an image from many times the image diagonal, - 12 to +12 for instance might appear to be in focus.

The crucial thing to understand is that DOF is entirely about viewer perception. It has no meaning other than that.

It was late and I meant 1 and "8" (not 9) might start blurring but that was just an example. It could starting much sooner. It will vary depending on the f stop. The beginning of that paragraph in the link is interesting as well. The adjustments are no based on depth of field, not focal length which again confirms what we have concluded.

I'm going through this. This surprised me. This is exactly what I have adopted. I'm not suggesting anyone else do this. That is not why I'm doing this. Each to his own.

Top of page 2.



Top of page 6. This document, like the manual suggests to MFA at the distance you normally shoot at.





I like this overall approach and I do promote Rudy's videos. I was on another site that talked about MFA in general. I person that has a 600mm lens just uses street signs or neighbours mailbox numbers (I guess you have to know and like you neighbours) and sets up to the approximate average shooting distance. The only real issue is trying to insure the target is as parallel to your sensor as possible and that might be a challenge. As long as it relatively strait and level. I'll go to a local park and try this, MFD (at home) then compare to no MFA and see what the real world shows me.

 

[Zeee]

what came out if this? Everyone knows I have a few beefs about MFA but this conversation opened a few new paths. Easier to work on something when you understand how it works. It won't effect the final results. I don't know about anyone else but I found this very interesting. I had a bit or a rough time trying to explain myself as this was new to me.

I have concluded this document is my MFA gospel as it is very close to what the manual says and what I do.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here are two DOF's, only change is distance to target.





The math. As you can see you can MFA at MFD or longer distances. The ⅛ values change but as you get closer to the target they get smaller. It would me much easier to MFA at 50 feet. At MFD the ⅛ value is 0.015. Methods like WilbaW's may not even defocus and focus correctly. Focal may never be a able to deal with those small values.



Again my guess is that "some" Canon documents suggest 50X just to reduce the margin of error. The multiple distances suggested by several sources has always been one of my beefs as there was never an explanation. It never made sense to me. Why is there not in universal number or distance? I don't know if this is correct but it makes sense to me. Not sure how else to look at the DOF value.

I still think in a theoretically perfect world if both lens and camera are at perfect zero you won't see a focus change at +-3. You should see the DOF range shift with really shallow lenses. Of course this is not a perfect world and even lenses and cameras calibrated at canon aren't at perfect zero.

 

[RogerZoul]

Not sure if this has been posted.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here is something I found on page 1.

If one uses the often suggested 50 x lens focal length doesn't this make the adjustments be based on focal length? It could be a matter of degree, but often times I feel like the adjustments should be done at the minimum focus distance as that seems to be were the DOF is the smallest and where the errors, or lack of, will matter the most. Please point out where my logic is wrong. I have my MFA set to disable, btw, and I shoot mostly long FLs like 640mm or 960mm, so this is the context in which I'm most concerned about AF errors. Up close is where you get the best/sharpest feather details on a bird, for example, and that is when/where I want the absolute best from my lenses.

Roger,
The MFA click adjustment results are based on/relative to adjusting the focal plane relative to the dof, not by a fixed distance.

Yes, but they are obtained at a recommended distance and dof does depend on distance and focal length, no?

 

[Zeee]

When I said that Canon MFA document was my gospel now I meant the overall process. Once you get to the MFA stage choose whatever method you like best. WilbaW's method, Dot Tune, Focal, Lens Align, downloaded paper scale, ruler, etc. Whatever works for you.

 

[guy schlacter]

Not sure if this has been posted.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here is something I found on page 1.

If one uses the often suggested 50 x lens focal length doesn't this make the adjustments be based on focal length? It could be a matter of degree, but often times I feel like the adjustments should be done at the minimum focus distance as that seems to be were the DOF is the smallest and where the errors, or lack of, will matter the most. Please point out where my logic is wrong. I have my MFA set to disable, btw, and I shoot mostly long FLs like 640mm or 960mm, so this is the context in which I'm most concerned about AF errors. Up close is where you get the best/sharpest feather details on a bird, for example, and that is when/where I want the absolute best from my lenses.

Roger,
The MFA click adjustment results are based on/relative to adjusting the focal plane relative to the dof, not by a fixed distance.

Yes, but they are obtained at a recommended distance and dof does depend on distance and focal length, no?

Don't think of MFA as a fixed distance for all.shooting with th that lens. It is anreoative distance. Asna given time,it is dependent on distance.

 

[guy schlacter]

what came out if this? Everyone knows I have a few beefs about MFA but this conversation opened a few new paths. Easier to work on something when you understand how it works. It won't effect the final results. I don't know about anyone else but I found this very interesting. I had a bit or a rough time trying to explain myself as this was new to me.

I have concluded this document is my MFA gospel as it is very close to what the manual says and what I do.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here are two DOF's, only change is distance to target.





The math. As you can see you can MFA at MFD or longer distances. The ⅛ values change but as you get closer to the target they get smaller. It would me much easier to MFA at 50 feet. At MFD the ⅛ value is 0.015. Methods like WilbaW's may not even defocus and focus correctly. Focal may never be a able to deal with those small values.



Again my guess is that "some" Canon documents suggest 50X just to reduce the margin of error. The multiple distances suggested by several sources has always been one of my beefs as there was never an explanation. It never made sense to me. Why is there not in universal number or distance? I don't know if this is correct but it makes sense to me. Not sure how else to look at the DOF value.

I still think in a theoretically perfect world if both lens and camera are at perfect zero you won't see a focus change at +-3. You should see the DOF range shift with really shallow lenses. Of course this is not a perfect world and even lenses and cameras calibrated at canon aren't at perfect zero.

Why are you going back to 1/8 instead of the 1/16 of total dof? You even linked to he same Canon material stating 1/8 is for just the front, and 1/8 for rear behind the focal plane.

Am I missing something?

 

[Hokie Jack]

Why are you going back to 1/8 instead of the 1/16 of total dof? You even linked to he same Canon material stating 1/8 is for just the front, and 1/8 for rear behind the focal plane.

Am I missing something?

Where did the 1/16th come from? The 'Canon material' says: "Each step is a very fine increment, equal to 1/8th of the depth-of-field you'd have with the current lens wide-open".

 

[Zeee]

what came out if this? Everyone knows I have a few beefs about MFA but this conversation opened a few new paths. Easier to work on something when you understand how it works. It won't effect the final results. I don't know about anyone else but I found this very interesting. I had a bit or a rough time trying to explain myself as this was new to me.

I have concluded this document is my MFA gospel as it is very close to what the manual says and what I do.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Here are two DOF's, only change is distance to target.





The math. As you can see you can MFA at MFD or longer distances. The ⅛ values change but as you get closer to the target they get smaller. It would me much easier to MFA at 50 feet. At MFD the ⅛ value is 0.015. Methods like WilbaW's may not even defocus and focus correctly. Focal may never be a able to deal with those small values.



Again my guess is that "some" Canon documents suggest 50X just to reduce the margin of error. The multiple distances suggested by several sources has always been one of my beefs as there was never an explanation. It never made sense to me. Why is there not in universal number or distance? I don't know if this is correct but it makes sense to me. Not sure how else to look at the DOF value.

I still think in a theoretically perfect world if both lens and camera are at perfect zero you won't see a focus change at +-3. You should see the DOF range shift with really shallow lenses. Of course this is not a perfect world and even lenses and cameras calibrated at canon aren't at perfect zero.

Why are you going back to 1/8 instead of the 1/16 of total dof? You even linked to he same Canon material stating 1/8 is for just the front, and 1/8 for rear behind the focal plane.

Am I missing something?

No. It's just me. Maybe it is 1/16 but I read it as ⅛ in a forward movement and ⅛ in a rear movement. Maybe Rudy is saying that it is equal. Not ⅛ to the front and 1/16 to the rear or something like that.

I don't think the difference between ⅛ and 1/16 is critical at this point. If it actually is 1/16 then the steps are even finer which supports his statement you won't see a drastic change at -5. I was going with about +-3. Of course your lenses/camera factory calibrations, DOF and other factors make that variable. If someone feels they see a difference with 1 or 3 MFA ticks I won't dispute that.

What is most important is that after 5 years I finally have an answer to why there are different focal lengths instead of a universal one, which I consider doing MFA at distance you shoot most often should be. 25X and 50X are good starting points if you can't.

These recent revelations will put me more at ease with MFA. I still won't settle for mis-calibrated gear from Canon and I still believe can be overused because it is available but those are personal choices and I respect that. Also this probably answer some things about this thread.

http://www.dpreview.com/forums/post/55820385

The members DPreview will probably be happier because I won't whine as much because I finally got some answers.