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!

 

[elfroggio]

Each +1 or -1 is 1/8th of the DOF.

@55mm f/5.6 10ft: the DOF is 2.15ft ==> -1 == 3 inches off

@250mm f/5.6 10ft: the DOF is 1.2 inch ==> -1 == 0.15 inches off

It does make the difference between a sharp photo and an almost sharp photo (to me it does)

 

[ErikH]

Ah, good point. Thanks!

 

[walkaround]

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!

 

[ErikH]

Lol, yes back focusing the scale is what most people do anyway, right.

(back to photography, focal plane)--I'm sure it doesn't matter. However, for the novice (who may not know to focus into a scene, etc.) the idea of more distance "behind" is probably beneficial.

Not a big deal. As long as it is sharper after the adjustment than before.

Thanks for all of the replies so far!

 

[Andrew53]

I've found that focus tends to be more consistent if you stay in the middle of the range. I'm using FoCal for my MFA but the DOT tune method gives similar results.

 

[WilbaW]

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).

That rule of thumb only applies as an approximation to hyperfocal shooting. With a typical arrangement for AF MA, DOF will be symmetrical around the focal plane.

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).

No, don't worry about it.

 

[Zeee]

Each +1 or -1 is 1/8th of the DOF.

@55mm f/5.6 10ft: the DOF is 2.15ft ==> -1 == 3 inches off

@250mm f/5.6 10ft: the DOF is 1.2 inch ==> -1 == 0.15 inches off

It does make the difference between a sharp photo and an almost sharp photo (to me it does)

--
Thanks
http://www.sritch.com
The Dogs of Vancouver, BC

I'm not a big MFA fan but I'm always interested in all photo subjects. I'm not not trying to start an is MFA good or bad conversation.

First off where did the ⅛ of the DOF come from?

I'm just curios about the numbers. I obviously must be really off with this because I'm not that well versed.

If you follow Canon's recommendation for distance 55mm X 50 = 9.02 ft (10 is close enough) to your target which the DOF calculator says (like you did) there is a total of 2.15ft. Front .96ft and rear 1.19 feet. We split the difference when we MFA so that is 1.075'

@ +-1 you are 3" inches off but you have about 12" in the front and 12" in the back of DOF so you have another 9 inches on either side or so before it would make any difference. Looks like you'd have to almost +-4 to see a difference.

I see 10ft when you talk about 250mm. Canon says 250mm X 50 = 41ft. @ f 5.6. The online calculator is telling me a total of 45 ft. 12.6 in front and 32.5 in the back. Again we split the difference so that is makes it 22.5' front and back.

Total of 45 divided by 8 = 5.625'. Again seems like we would need almost +- of 4 before you see a difference.

I must have missed a key step or misunderstood something along the way. I would appreciate any help as I'm considering taking a look at my 100-400 with my 1.4 and I'd like to better understand this. I'm not sure if it needs it but I'd like to take a look at it.

 

[Zeee]

Each +1 or -1 is 1/8th of the DOF.

@55mm f/5.6 10ft: the DOF is 2.15ft ==> -1 == 3 inches off

@250mm f/5.6 10ft: the DOF is 1.2 inch ==> -1 == 0.15 inches off

It does make the difference between a sharp photo and an almost sharp photo (to me it does)

--
Thanks
http://www.sritch.com
The Dogs of Vancouver, BC

I'm not a big MFA fan but I'm always interested in all photo subjects. I'm not not trying to start an is MFA good or bad conversation.

First off where did the ⅛ of the DOF come from?

I'm just curios about the numbers. I obviously must be really off with this because I'm not that well versed.

If you follow Canon's recommendation for distance 55mm X 50 = 9.02 ft (10 is close enough) to your target which the DOF calculator says (like you did) there is a total of 2.15ft. Front .96ft and rear 1.19 feet. We split the difference when we MFA so that is 1.075'

@ +-1 you are 3" inches off but you have about 12" in the front and 12" in the back of DOF so you have another 9 inches on either side or so before it would make any difference. Looks like you'd have to almost +-4 to see a difference.

I see 10ft when you talk about 250mm. Canon says 250mm X 50 = 41ft. @ f 5.6. The online calculator is telling me a total of 45 ft. 12.6 in front and 32.5 in the back. Again we split the difference so that is makes it 22.5' front and back.

Total of 45 divided by 8 = 5.625'. Again seems like we would need almost +- of 4 before you see a difference.

I must have missed a key step or misunderstood something along the way. I would appreciate any help as I'm considering taking a look at my 100-400 with my 1.4 and I'd like to better understand this. I'm not sure if it needs it but I'd like to take a look at it.

I messed up the 250mm calculations. It was late and entered 55mm in the focal length.

250 X 50 = 41 feet. Total DOF is 1.73 ft which equals 20.76 inches. 10 inches front and back.

20" total divide by 8 equals 2.595 inches. I'm still getting you would need +-4 to see a difference. It is all relative.

I decided to try 9 ft (instead of 10) for the 55mm just for fun to match canons 50X suggestion. 55 X 50 = 9ft. total DOF is 1.74 feet so it is exactly the same as above.

I tried one at 25X for those who use it. 250 X25 = a total DOF 0.63 feet which equals 7.56 inches. 3 ¾ inches front and back.

7.56 divide by 8 = 0.945 inches. 0.945 inches X 4 = 3.78 inches. I'm still seeing it needs about +- 4 to see a difference.

I have seen the term ⅛ of DOF but never really payed attention to it but was always curious about what it meant and how it is applied.

What does "1/8th of the DOF" actually mean? What is the DOF value? What I'm using in my calculations or something else?

 

[elfroggio]

I'm not a big MFA fan but I'm always interested in all photo subjects. I'm not not trying to start an is MFA good or bad conversation.

I do it because my cost is $0 and I see a difference. If you don't see a difference, then no point in doing it.

First off where did the ⅛ of the DOF come from?

Many people mentioned that including somebody explaining the MFA on canon-europe.com from the 7d times

If you follow Canon's recommendation for distance 55mm X 50 = 9.02 ft (10 is close enough) to your target which the DOF calculator says (like you did) there is a total of 2.15ft. Front .96ft and rear 1.19 feet. We split the difference when we MFA so that is 1.075'

I do not, I follow Lenscal 25x because it's more typical of what I take photos of.

 

[Zeee]

I'm not a big MFA fan but I'm always interested in all photo subjects. I'm not not trying to start an is MFA good or bad conversation.

I do it because my cost is $0 and I see a difference. If you don't see a difference, then no point in doing it.

First off where did the ⅛ of the DOF come from?

Many people mentioned that including somebody explaining the MFA on canon-europe.com from the 7d times

If you follow Canon's recommendation for distance 55mm X 50 = 9.02 ft (10 is close enough) to your target which the DOF calculator says (like you did) there is a total of 2.15ft. Front .96ft and rear 1.19 feet. We split the difference when we MFA so that is 1.075'

I do not, I follow Lenscal 25x because it's more typical of what I take photos of.

--
Thanks
http://www.sritch.com
The Dogs of Vancouver, BC

I'm not saying you shouldn't use it. If we assign a number like ⅛ of the DOF then there has to be a DOF value to work out the math. It can't be the lens aperture because that is not a value but an F stop and the total DOF changes when you change the distance to your target or subject.

Until I get the answer from someone I'm going to have to assume it is the total DOF that is within focus based on the DOF guide.

50X or 25X for any focal length gives me the same outcome.

I did the numbers for f1.2

50 X 50mm = 8.2 feet

8.2 = 0.36 ft total DOF or 4.32 inches

4.32 divided by 8 = 0.541 or ½ inch to round it out.

Split front and back = 2.15 inches

I still get +-4 to see a difference from a perfectly calibrated camera and lens. Now you are talking about ½ inch which will really show up if out a bit.

When you read the lens rentals document it talks about balancing a mismatched lens and camera. If a camera is -3 and a lens is +3 then it balances out which makes sense. If both are -3 or +3 I’m not sure if that adds up to 6? So if a camera is +3 and the lens is +5 are you out +5 or + 8? I’d be curious about that one.

So every lens and camera is going to have normal deviation and if they are opposites, within reason, they balance out. It is when both trend to one side or the lens or camera itself really swings to one side then you will see very poor focus. Even if your desired spot is in focus the DOF range really changes on an f 1.2 lens which will yield unexpected results, especially for a pro that depends in his work to make a living.

 

[Zeee]

I tried f 1.2 @ 25 ft

25 X 50mm = 4.1 feet

4.1 = 0.09 ft or 1.08 inches total DOF

1.08 divided by 8 = .135 inches

Split front and back = .54 inches or just over ½ inch

.135 X 4 = 5.4 inches. Still a shift of +- 4.

It appears the greater the distance to your target the more forgiving the process is. Liquidstone once posted he MFA’d at minimal focusing distance. I could never figure that out but you can’t argue with success. I’m going to do the math on that.

 

[Zeee]

My 300 F4 – minimal focusing distance is 4.9 ft

50 X 300 = 0.01 feet

0.01 ft or 0.12 inches total DOF

0.12 divided by 8 = 0.015 inches

0.12 Split front and back = 0.06 inches

0.015 X 4 = 0.06 . Still a shift of +- 4

 

[Zeee]

Unless there is another answer to the DOF value the calculations based on a perfectly calibrated lens and body, no deviation from perfect 0, which is impossible, you would need to shift + or - 4 to see any difference.

Of course as I mentioned as the DOF of a lens get very shallow total DOF range changes drastically and OOF will be much more obvious.

 

[ErikH]

You bring up a good point and I share your curiosity in this. By the math, it doesn't seem like it would make a difference.

As a note, when doing this procedure, I used the crop distance in my calculation (i.e. 55mm=88mm ect.)

That doesn't change anything, just thought I'd mention it.
--
ErikH

 

[Mr Ed M]

My 300 F4 – minimal focusing distance is 4.9 ft

50 X 300 = 0.01 feet

???

0.01 ft or 0.12 inches total DOF

0.12 divided by 8 = 0.015 inches

0.12 Split front and back = 0.06 inches

0.015 X 4 = 0.06 . Still a shift of +- 4

 

[walkaround]

Unless there is another answer to the DOF value the calculations based on a perfectly calibrated lens and body, no deviation from perfect 0, which is impossible, you would need to shift + or - 4 to see any difference.

The entire depth of field is not in equally sharp focus. There is still a focus point within the DOF.

 

[Zeee]

My 300 F4 – minimal focusing distance is 4.9 ft

50 X 300 = 0.01 feet

???

0.01 ft or 0.12 inches total DOF

0.12 divided by 8 = 0.015 inches

0.12 Split front and back = 0.06 inches

0.015 X 4 = 0.06 . Still a shift of +- 4

 

[Zeee]

Unless there is another answer to the DOF value the calculations based on a perfectly calibrated lens and body, no deviation from perfect 0, which is impossible, you would need to shift + or - 4 to see any difference.

The entire depth of field is not in equally sharp focus. There is still a focus point within the DOF.

That is interesting. You can see that on a ruler test but you can't change the focus point independently of the DOF area.

I bet a lot of the DOF is equally in focus here.

 

[Mr Ed M]

My 300 F4 – minimal focusing distance is 4.9 ft

50 X 300 = 0.01 feet

???

0.01 ft or 0.12 inches total DOF

0.12 divided by 8 = 0.015 inches

0.12 Split front and back = 0.06 inches

0.015 X 4 = 0.06 . Still a shift of +- 4

I get that, but, what does that have to do with 300X50?