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]

OK then. My apologies.

No worries.

I keep trying to find a the word for the relationship between Field and Focus. They both have different values because Field is much larger but they are both relative to each other and the values change in equally when the Field is change by the user.

Like I said way back, and you can see from the above diagram, it's just similar triangles. You choose an acceptable circle of confusion, and everything else follows. If you shift the image plane by 1/8 of the depth of focus (which is what MA does, as far as I know), then you shift the subject plane (at the "Subject distance) by 1/8 of the depth of field, 'cos it's all proportional.

Is that the word you're looking for?

That is it. I must have had every word I could think of go through my head expect that one. In the forest and I couldn't see the trees again - or it could be age.

Thanks

I was just going through the Dot Tune video. Dot Tune suggests 50X but like I said that will be my minimal base for long lenses. I'll try at the distance I shoot at but hope I don't run out of DOF.

I was reading snapsy's post #4. This is pretty old but interesting based on what this conversation is about. 30 pages and I'm not going through everything. Wonder if they figured it out. Distance seems to be a key factor but that is old and early in the post. I'm not bringing this up in there. I don't have that much blood to give

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www.fredmiranda.com

 

[Zeee]

a little more current. I went from the back and worked forward. Page 28, post #7 - Interesting.

http://www.fredmiranda.com/forum/topic/1187247/27#12796873

The previous post shows Focal's recommendations. Not a fixed number. I see the distance to target diminishes as the focal length increases. My guess is it just gets harder to work at greater distances.

 

[WilbaW]

Rudy Winston said, "Each step is... 1/8th of the depth-of-field..."

THIS WAS CHUCK'S WESTFALL'S DIRECT RESPONSE:

"[Each] step is... [1/8 of] the single-side depth of focus..."

Thanks! At last we get to see what the real problem is - Rudy and Chuck directly contradict each other about whether it's the whole or a single side (half) of both depths, so (at least) one of them is wrong. All we have to do is test it.

 

[Zeee]

Rudy Winston said, "Each step is... 1/8th of the depth-of-field..."

THIS WAS CHUCK'S WESTFALL'S DIRECT RESPONSE:

"[Each] step is... [1/8 of] the single-side depth of focus..."

Thanks! At last we get to see what the real problem is - Rudy and Chuck directly contradict each other about whether it's the whole or a single side (half) of both depths, so (at least) one of them is wrong. All we have to do is test it.

If so 1/16 would make the moves smaller and distance to target even more prone to error as you get closer - if indeed getting closer is more prone to error. I think it is.

 

[WilbaW]

a little more current. I went from the back and worked forward. Page 28, post #7 - Interesting.

http://www.fredmiranda.com/forum/topic/1187247/27#12796873

The previous post shows Focal's recommendations. Not a fixed number. I see the distance to target diminishes as the focal length increases. My guess is it just gets harder to work at greater distances.

And then snapsy says do it at "infinity" (well beyond 50f), so whichever way you look at it, in the absence of a fundamental technical reason, we shouldn't take 50f as a commandment.

 

[WilbaW]

Rudy Winston uses DO Field as an example because that is what the general public can relate to. They both work together. If the DO Field Focus changes then the DO Field does as well. The DO Focus math would produce a lot of zeros and not mean much to most people.

I'd say it this way... MA is a correction applied in the DO focus space (it is a number, like a measurement of focus error, Best Focus Correction Values, and Circle of Confusion, that relates to what's happening at the image sensor and is used in calculations to manipulate that), and it has proportional consequences in the DO field space. The engineers who built EOS in the '80s were working in focus space, and the photographers who use it are working in field space.

 

[guy schlacter]

^^^^ YUP!!! ^^^^^^^^^^^^

with each MFA = 1/16 of the total Depth of Focus ~~ 1/16 total Depth of Field

(at distance<< hyperfocal)

 

[Zeee]

a little more current. I went from the back and worked forward. Page 28, post #7 - Interesting.

http://www.fredmiranda.com/forum/topic/1187247/27#12796873

The previous post shows Focal's recommendations. Not a fixed number. I see the distance to target diminishes as the focal length increases. My guess is it just gets harder to work at greater distances.

And then snapsy says do it at "infinity" (well beyond 50f), so whichever way you look at it, in the absence of a fundamental technical reason, we shouldn't take 50f as a commandment.

I asked him about it and he had not figured out why yet. I decided to present my findings with little repsonse. I'm still goiing to MFA at at least 50X or at distnace I plan to shoot at. I'll try infinity just to see. I still think the closer you the more critical get.

 

[Zeee]

So I finally got around trying this out. All systems IMO are go based on real life shooting but I was not too sure about my 100-400 MK2 @ 400mm with my 1.4 MKII. I'm not sure about infinity but I figure my target was abpout 100 ft away.

It would still blink and beep quite happliy at +20 and the best responce at the low end was +5. I was not too surpirsed as I have read some people had to MFA with a final result of about +12 at POTN. I took it off the TC and got about a +3 which I'm not going to worry about because my real world shots look great.

I know there is a process for this if you hit 20 but I could not watch the video. I'll do that at home and try this one more time. It appears that I could not even use Focal if I wanted to but there may be a work around. I have not looked at it for about 6 months.

 

[elfroggio]

Looks like we have reaching the 149 limit