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Thread started 30 Nov 2011 (Wednesday) 03:53
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micro adjustment questions

 
NicuB
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Nov 30, 2011 03:53 |  #1

Hi all,
I'm not sure if this was discussed before, I tried to check all the threads before opening this, but if I missed something please excuse me.
When I had my film camera ( something without AF ) I had none of this problems. With my first digital I noticed that there are some shots not that crisp as I would expect them to be. I considered at that time my fault and lived with it. My first DSLR camera was 1000D and then 30D and now I made the big jump to 5D2. 5D2 comes with a nice feature MAF which I wanted to test.
So after this big introduction let me tell you my questions:
1. Canon and others recommend 50X FL for testing the focus. But at this distance the DOF for a 35mm at f/2 is in my opinion 'huge' for the level of focus that I want to achieve. Any other opinions on this? I can live with this no problem.
2. I saw no big difference between +10 -10 +20 -20 0 on the test that I did yesterday. The best was at + 13 but still I think it could be my errors that prevent me for having this nailed down. The question here is : for a given FL is there any way to say that going from +1 to +2 will affect the focus plane with this much at a certain working distance ( focus distance)?
3. Is there anything that I should take into consideration except for : tripod, MLU, delayed shutter, lowest iso possible, enough light, try to stay as much as you can parallel with the focusing chart, make sure that you only focus on one point ( separate the grey scale from the black focus point), move the focus to infinite before taking each shot ( or to MFD )?
Thank you all for your inputs!
Cheers,
Nic




  
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FlyingPhotog
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Nov 30, 2011 04:00 |  #2

Are you pro-actively adjusting or are you trying to null out an error that's been identified and is repeatable?

If its the former and you're doing this simply because you can, may I respectfully suggest you reset everything to zero and just go shoot. Shoot early, shoot late, shoot shallow DOF and deep DOF.

In fairly short order, if there's an error that MFA can fix, you'll see it and have a better idea which way to adjust. Otherwise, you're trying to fix something that may not need fixing.


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NicuB
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Nov 30, 2011 04:06 as a reply to  @ FlyingPhotog's post |  #3

FlyingPhotog thanks for your fast answer. I would call this more pro-actively adjusting than a real error. In some of my portraits I saw a little bit of front-focusing ( about 3 mm using a 85mm at f/2.0 with working distance of about 1 meter), but it could be that I was moving ( I do not feel very comfortable using this working distance).
And I totally agree that I should shoot more than post on forum. But at least I can post on forums during my working hours.
Thanks again,
Nic




  
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FlyingPhotog
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Nov 30, 2011 04:10 |  #4

Cheers Nic...

You don't say which 85 but any of the popular choices by either Canon or Sigma yield mighty thin DOF at that large an aperture so I'd definitely test on as solid a setup as possible to rule out sway by either shooter, subject or both simultaneously.

Good Luck...


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NicuB
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Nov 30, 2011 04:15 as a reply to  @ FlyingPhotog's post |  #5

it's 85f/1.8 from Canon.
Still I think that the questions that I raised are adequate for someone that will try in the future to adjust.
I'm trying to get as much info as I can and also maybe help others when they will face the same situation.
Cheers,
Nic




  
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FlyingPhotog
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Nov 30, 2011 04:17 |  #6

Oh no question, you nailed the pertinent details for proper testing but I was simply trying to keep you from going on an unnecessary Snipe Hunt. ;)


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NicuB
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Nov 30, 2011 04:30 as a reply to  @ FlyingPhotog's post |  #7

I will not do more than necessary for this. I'm pretty happy with my setup and I will continue to take pictures with or without focus-adjustment. In old times nailing the focus was not even necessary, it was more a discussion about ideas and light, but nowadays we must keep up with the technology ;) . Since I do not shoot more than 200mm be sure I will not become a Sniper.
If there any opinions on the above mentioned questions, everybody please feel free to say them, if someone is wrong we will not start a flame but try to correct the 'not so sharp' view.
Cheers,
Nic




  
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tzalman
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Nov 30, 2011 05:08 |  #8

A. I strongly advise that you read this article on focus accuracy and consistency. It restores a bit of sanity to a maddening pursuit:
http://www.dpreview.co​m …ation-facts-and-fallacies (external link)
B. Although the DoF at 50(FL) is around six inches, three inches front and back, that does not mean it is uniformly sharp throughout the range. From the maximum obtainable sharpness at the focus point it tails off to "sharpness considered acceptable by the average viewer of an 8x10 print," a definition that I think you will agree is maddeningly broad, subjective and unquantifiable. At any rate, any respectable pixel-peeper will enlarge his image well beyond a mere 8x10, which will shrink the DoF and make discerning differences between test images easier.
C. "In old times nailing the focus was not even necessary," is not the way I remember it (and believe me, when I remember "old times" they are nearly pre-historic.)


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wimg
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Nov 30, 2011 05:20 |  #9

NicuB wrote in post #13472789 (external link)
FlyingPhotog thanks for your fast answer. I would call this more pro-actively adjusting than a real error. In some of my portraits I saw a little bit of front-focusing ( about 3 mm using a 85mm at f/2.0 with working distance of about 1 meter), but it could be that I was moving ( I do not feel very comfortable using this working distance).
And I totally agree that I should shoot more than post on forum. But at least I can post on forums during my working hours.
Thanks again,
Nic

Just in case this was handheld: at 1m, 85 mm FL, F/2, DoF is only 13.9 mm, and a human body sways easily +/- 25 mm backward and forward just to stay in balance, automatically, continually, without anyone even noticing. Human body balance is a dynamic thing, which is why we don't notice it - and if you try, you may find you can easily sway about 10 cm / 4" backward and forward, without loosing your balance or breaking into a sweat :D.

Furthermore, focusing on digital works slightly differently from film, in that from my experiments it seems that with digital focus is often times slightly more towards the front than with film. I have an explanation for this, but that goes a little too far here. Also, AF is a statistical thing: every time focusing is done, the focused point changes slightly, although it will always be somewhere around the point aimed at, and within Canon specs for the AF system (1/3 of an f-stop of DoF for fast lenses, aperture larger than or equal to F/2.8, and 1 f-stop of DoF for lenses with a maximum aperture smaller than F/2.8). This is one of the reasons why many use the *-button for focusing, i.e. independently of the shutter button, so that it is impossible to inadvertently change focus when taking a photograph.

Anyway, human body movement, variation in AF achieved, the thin DoF as quoted for the parameters you indicated, and potential and accidental refocus may all easily cause a slight misfocus in the resulting picture. And slight here means a potential OOF picture :D.

Also note that DoF here is calculated for a CoC (Circle of Confusion, in effect the largest size for an individual point or thickness of a line) of 0.3 mm, being the CoC for FF sensors and 35 mm film, and which means a 6 lp/mm (line pairs, IOW 12 pixels) resolution in a 30 cm X 20 cm print (12"X 8") viewed at ~30-35 cm (12"- 14") distance. This is the general or industry standard reference used for DoF calculations, based on the acuity of the human eye, and a standard of viewing which on average is twice as close as is comfortable for looking at a picture. Pixel peeping at 100%, IOW, doesn't help here either. :D

HTH, kind regards, Wim


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NicuB
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Nov 30, 2011 06:03 as a reply to  @ wimg's post |  #10

@tzalman good reference - i was aware of this, but i think it's contributing to the thread.
When I was referring to 'nailing the focus' I had in my mind : the printing size that normal (not pro) people were printing, and also the fact that nobody was pixel-peeping that hard so if you were let's say pretty close to that point you were good. Today most of the people do not print. I print mostly A4 (~12"x8") and with this size I cannot tell a difference for something that is slightly front-focus or back-focus. Another example would be : Robert Capa :) .
@wimg: this is the main thing that I thought about: maybe I was moving. That picture where I observed was hand-held, the tests from yesterday - tripod. Very good information from your post. I'm not aware of what is different between my EOS3 and my digital when it comes to focus. Can you please elaborate on that?
Thank you all for contributing.
Nic




  
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Nov 30, 2011 07:32 |  #11

Click on my signature's link for the focus genie, and under that sale ad, there is a link to the free DIY version "arts and crafts" edition of that chart. Give it a go. :)

Stand back as far as you can where you fill the center AF with the focus word, level to that word, in great light, as wide open as you can get, at the lowest ISO you can go without going too low on the shutter.


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wimg
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Nov 30, 2011 08:02 |  #12

NicuB wrote in post #13473003 (external link)
@tzalman good reference - i was aware of this, but i think it's contributing to the thread.
When I was referring to 'nailing the focus' I had in my mind : the printing size that normal (not pro) people were printing, and also the fact that nobody was pixel-peeping that hard so if you were let's say pretty close to that point you were good. Today most of the people do not print. I print mostly A4 (~12"x8") and with this size I cannot tell a difference for something that is slightly front-focus or back-focus. Another example would be : Robert Capa :) .
@wimg: this is the main thing that I thought about: maybe I was moving. That picture where I observed was hand-held, the tests from yesterday - tripod. Very good information from your post. I'm not aware of what is different between my EOS3 and my digital when it comes to focus. Can you please elaborate on that?
Thank you all for contributing.
Nic

Hi Nic,

Regarding the difference between digital and film AF, film emulsion has a certain thickness which a sensor doesn't have, and this thickness is quite large compared to the CoC, namely 0.2 mm. That gives one two things: first of all DoF will be deeper than anticipated, and secondly the DoF fall-off will be much more gradual.

Now imagine an AF sensor, which does either contrast or phase detection. It has a rather sharp cut-off point for in focus or out of focus, and because an AF sensor essentially is a scaled down version of an imaging sensor, it will also have a zero thickness focusing plane. This implies that the curve for being in focus is steep, where that for film is more gradual.

I found that AF on digital is generally adjusted, certainly with bodies up to and including the 5D II, at a point just before reaching maximum sharpness. In film this is on the shoulder of the DoF curve, within DoF, but also not right at the sharpest point. However, DoF transitions are more fluid with film than with sensors, because, as mentioned, the sharpness/DoF curve with film is gentler.

Furthermore, especially with the lower MP sensors, there is a sharpness plateau, caused by maximum resolution of the sensor and the cut-off point of the AA-filter, something film obviously doesn't have.

The above causes the DoF curve in film to look like a nice, normal distribution or sinusoid type curve, while the DoF curve in digital looks more like a slightly rounded block wave type of curve.

If you look, f.e. :D, at pictures of brick walls with a lot of detail, taken at 45 degrees to the wall, using larger apertures, in digital you are likely to see 5 distinct sharpness zones as a result :
1. Very unsharp/smeared, OOF in front of the focused zone
2. Getting a little sharper, on the shoulder of the DoF curve in front of the focused zone, some details are almost visible
3. Sharp, the DoF zone
4. Getting less sharp, on the shoulder of the DoF curve behind the focused zone, some details are almost visible, but tapering off rapidly
5. Very unsharp/smeared, OOF behind the focused zone

Looking carefully, you might even be able to see the transitions, or beding points, between these zones, they seem quite distinct too, which is exactly what seems to point more towards a block wave type of DoF zone.

Now, since the AF sensor and AF system make the lens focus towards the top of the slope leading towards the actual DoF zone, this will look slightly less sharp with digital, especially when pixel peeping, whereas with film, due to the gentler slope, and much more gradual transition, this isn't a problem.

Note that the AF system works this way in order to maximize subject DoF.

The difference btween film and sensor DoF is also why I recommend focusing 1/3 into a subject, or important part of a subject, in order to get the entire subject as sharp as possible. Yes, DoF distribution is supposed to be 50% / 50%, but focusing that way will cause more in front of the focusing zone to be unsharp with digital, as opposed to film :D.

A few years ago I created a little diagram to demonstrate this, with a slightly exaggerated set of curves for film and digital, with the focusing point drawn in. I have attached this drawing below. In yellow the sensor DoF distribution and curve, in green the one for film, and a combined set, to see the difference more easily, and one of each with and without Dof zones delimited, so 6 little drawings in total. The curves do not reach the bottom of the graphs due to noise and/or spurious resolution, and/or minimum film density :D. Neither do they reach the top, but that is because I am imaging system rather than medium (film or sensor) or lens resolution. Top row depicts film system resolution, middle row depicts digital system resolution, and bottom row is the combined set.

Kind regards, Wim


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NicuB
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Nov 30, 2011 09:44 as a reply to  @ wimg's post |  #13

@wimg Thank you for the detailed answer. Now that you explained it made more sense.
What you described seems more like a PhD thesis, but still it makes a lot of sense in real life.




  
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Nov 30, 2011 12:14 |  #14

NicuB wrote in post #13473724 (external link)
@wimg Thank you for the detailed answer. Now that you explained it made more sense.
What you described seems more like a PhD thesis, but still it makes a lot of sense in real life.

ROFL

Now you know why I was a little hesitant to post this :D.

Kind regards, Wim


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kory
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Nov 30, 2011 13:36 |  #15

wimg wrote in post #13474451 (external link)
ROFL

Now you know why I was a little hesitant to post this :D.

Kind regards, Wim

Hey Wim really cool post. Do you do MA to move the focal plane back about 1/3 standard, or is it just done manually while taking pics.




  
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