Comments on MFA

[neuroanatomist]

By the way, depth of focus is different from depth of field.Depth of field deals with object space, such as the person that is 10 feet away with 5 inches DOF, while depth of focus deals with image space, such as the tiny fractions of a millimeter in front of and behind the sensor.

Yep - I knew this, but when I asked the question of Chuck I didn't think through to the fact that any specification related to the AF sensors would have to be based on depth of focus, not depth of field. Chuck corrected me as well.

Very interesting. Wouldn't be the first time he was wrong, but I'm surprised he's so specific and adamant about this.

Me, too. I asked the question because your explanation and excellent analogy seemed logical to me at the time.


Of course, this has major implications for very fast (e.g. f/1.4-1.2) lenses. With off-center AF points, the depth of f/5.6 focusing precision is going to be much wider than the depth of field at f/1.2, and even when using an f/2.8 high-precision center AF point with 1/3 depth of focus precision, that zone offocusing precision will still be wider than the actual depth of field (depending on subject distance).


@<span class="user-name"]<span>HDNitehawk, I think you've got it...

[HDNitehawk]

I wanted to resurrect this thread for an instance to see if I could get some feed back.


I have read how to do a mfa on the camera, many of them are long winded time consuming ways. I have done several of the methods, after giving it some thought I tried another way.


I was doing this with the telephoto on a tripod but not sure why it wouldn

[neuroanatomist]

I

[HDNitehawk]

I've thought about this method, but never tried it though it seems logical. One way you could test it, since the lens you tried seems to need nn adjustment, would be to apply increasing amounts of adjustment and see if/when you detect it as misadjusted. For reference, using the LensAlign I often find that 3-4 adjustment values look 'ok' (e.g. +1 to +3, so I just use the average); usually, a focal length/aperture combo with thinner DoF gives a tighter range. So, I'd think you'd want to see the effect by +3 or -3 for your lens (although more might not be bad as long as you can get a range and take the average - of course, you might get better accuracy with your method.

I think I will experiment using live viewwith the 24-70 lens tonight on the 5D Mark II. The lens has only been used once by my wife since I got it back from Canonafter I sent it in for service about 8 months ago. See if an average would work, then check it taking sets of pictures and do a comparison that way.


What got me thinking on this was that I wanted to do a mfa last week with the 7D in the field, because it seemed like it was back focusing on the 500mm F4L. ( I didn't have time to check the new camera before I left on my trip). I think in reality it was focusing just fine and the probelms I thought I was having were actualy my technique and learning the new AF system. It was dead on off the tripod at home.


It would seem logical to do it this way, in the field you could just set up on any point, do a focus and then a 10x check just to make sure you were hitting your target.

[neuroanatomist]

It would seem logical to do it this way, in the field you could just set up on any point, do a focus and then a 10x check just to make sure you were hitting your target.

Upon further reflection during an otherwise relatively boring flight home to Boston after a day-trip to New Jersey (business - I wouldn't go there by choice...), I think I see a flaw in the logic.


I guess the limitation on the Live View method is the resolution of the LCD on the camera. With this method, you are in large part judging best focus by the sharpness of the 10x magnified image on the camera display. How many times does something look nice and sharp even when zoomed all the way in on the on-board LCD, but when you transfer the image to your computer you find that it's a little off. That's why the 'unofficial but Chuck Westfall-recommended so as official as unofficial gets' procedure for AFMA involves viewing images at 100% on your computer to judge sharpness.


I think that's an advantage to a tool like the LensAlign - you're setting AFMA based on a DoF scale, so that the plane of focus is centered on anangled rulerthat'sprecision-aligned to the focus target. I find that my quick reviews on the LCD (to make sure I've got the optimal adjustment bracketed within +10 to -10) usually give an estimate that's close to the final chosen setting (but not always exact, usuallywithin 1 or 2 units). The ruler means you're partly judging by sharpness, but really by the location of the region of sharpness as it moves along the ruler from setting to setting, as opposed to within-shot sharpness. Of course, after selecting and applying an AFMA to a lens, I always re-check the sharpness of AF shots in a 'real-world' setting (often my daughters eyelashes) - but again, that's checking at 100% on the computer.


--John

[Jon Ruyle]

I guess the limitation on the Live View method is the resolution of the LCD on the camera. With this method, you are in large part judging best focus by the sharpness of the 10x magnified image on the camera display. How many times does something look nice and sharp even when zoomed all the way in on the on-board LCD, but when you transfer the image to your computer you find that it's a little off.

Is 10x 1-1? If so, why is it less sharp than a computer screen? If not, I wonder why they don't just make it display 1-1?


Then again, isn't the 5DII screen supposed to have 900,000 dots or something? So at 10x, it should be able to display a 90 megapixel image at 1-1. (I know not the whole screen is used, but more than 25% of it, surely). Is 10x magnified beyond 1-1? It doesn't look like it to me.


What am I missing?


(Sorry for going a bit off topic here... I seem to have that habit)

[HDNitehawk]

Is 10x 1-1? If so, why is it less sharp than a computer screen? If not, I wonder why they don't just make it display 1-1?


Then again, isn't the 5DII screen supposed to have 900,000 dots or something? So at 10x, it should be able to display a 90 megapixel image at 1-1. (I know not the whole screen is used, but more than 25% of it, surely). Is 10x magnified beyond 1-1? It doesn't look like it to me.


What am I missing?


(Sorry for going a bit off topic here... I seem to have that habit)

To follow up on my reply to John earlier, and comment on this one. I do notice that if you are viewing a picture you have taken, sometimes it can look very sharp on 10x on the camera and still when it gets on the computer its just not as sharp.


But when focusing in 10x live view I have found it to be very accurate.

[HDNitehawk]

[quote user="neuroanatomist"]I guess the limitation on the Live View method is the resolution of the LCD on the camera. With this method, you are in large part judging best focus by the sharpness of the 10x magnified image on the camera display. How many times does something look nice and sharp even when zoomed all the way in on the on-board LCD, but when you transfer the image to your computer you find that it

[neuroanatomist]

Some good food for thought here, gentlemen...I'll try to serve up some more. [8-|]
<div>

Then again, isn't the 5DII screen supposed to have 900,000 dots or something? Is 10x magnified beyond 1-1? It doesn't look like it to me

</div>


Yes, 10x is magnified beyond 1:1. The specs for the 5DII and 7D list LCDs with "Pixels:Approx. 920,000 dots (VGA)" although on their 5DII specs page, Canon leaves out the word 'dots' which makes it even more misleading than leaving it in. Nowhere do they tell you that dots &ne; pixels, although it's implied by 'VGA'. In fact, they count each red, blue, and green subpixel as a 'dot' so they are calculating display resolution as VGA x 3, i.e. 640 x 480 x 3 = 921,600 dots.


Bottom line is that the display is 640x480, and since the aspect ratio of the LCD on these cameras is not 3:2, the image fills the width but not the height. So with a 7D's 5184 pixel width viewed at 10x, those 518 pixels are interpolated up to 640 pixels, i.e. 1.23:1, and with the 5DII's 5616 pixel width, viewing at 10x means 1.14:1. I'm not sure what algorithms Canon uses for the interpolation, but regardless, upscaling is bad for sharpness.


I don't think that viewing on the computer is strictly necessarywhen doing the AFMA, since if you're applying stepwise adjustments, you're not looking for the bestabsolutesharpness (for which you'd need to use the computer, due to the upscaling mentioned above), but rather the bestrelativesharpness across the range of adjustments (and the relative best should not depend on the viewing platform, provided the display is sufficient to allow you to resolve the differences; but that might not be the case...).

when using the manual "point at something and use live-view at 10x" method to perform the AFMA, the target was an image containing very fine grids

I've read about using moire-inducing patterns displayed on a computer screen for this as well. I did try that this morning, and found that with 10x Live View of a moire-inducing pattern on my laptop, I could move the camera back and forth a substantial distance without much change to the moire patterns (or at least the changes were too subtle to discern on the camera's LCD).


The other issue with that method (and with focusing on any image as a target) is the alignment of the target with the camera's sensor. Ideally, you want the target to becompletely flat andperfectly parallel to the sensor. In his description of how he shoots the ISO 12233 crops, Bryan mentions that, "Thecamera/lens is multiple-laser-aligned to the target..." That's really the main function of the LensAlign tool (without lasers, of course) - the sighting gates that allow you to align the target to the camera. If not for that, I could just prop a ruler against a wall and have saved a few bucks.

I ended up at +12, I found I could set the mfa doing it the live view way but it was just as tedious because you could almost see an acceptable range over 10 settings.

Hmmmm...I've never seen a range that large. There are several possible reasons for that. It could be the difference in method (Live View + AF vs. just AF and shoot), or to the different resolution and smaller size of the camera LCD relative to a computer display, or it could just be that your 24-70mm is really bad. [:P]


Actually, I think it's the difference in method - not just the Live View issue but the fact mentioned above - I'm using an alignment tool with a DoF scale so I'm not relying primarily on overall sharpness to judge the range.

Actually when doing Macro work I find the 10x method to be the most accurate method of getting focus and it can be very accurate

I agree, it's the most accurate. But consistent with your response to Jon, it's important to note thatviewing an image at 10x on the LCD to judge sharpness is not the same thing as focusing using 10x Live View, at least not for me. When I focus using Live View, it's an active, iterative process with visual feedback (just like the camera's autofocus, only I'm a lot slower) - I move the ring back and forth a few times to make sure I'm centered on the region where I want critical focus. If you AF on a point then switch to 10x Live View and see if it's sharp, that's static viewing; if you then rack the focus to check if the AF hit the spot, how do you know you're ending up at the same point? I think the only way to know for sure is to actually take the shot with AF then take the shot with MF 10x Live View, and view them on a larger display. Not just one shot, either. AF accuracy should be normally distributed(in the statistical sense) around the point of best focus - but a normal distribution doesn't mean spot on that best point every time. So you still need to take multiple shots.


I think it would be a bad idea to take the shortcut - i.e., out in the field, do an AF, switch to10x Live View and look, see if it's ok, if not then apply some adjustment and do it again until it looks right. If you're going to rely on AF, you want it as accurate as possible and setting it based on n=1 and/or shooting some random feature in the field is risky. I think that's why Canon recommends doing it only if necessary, and warns that it may prevent correct focusing from being achieved (presumably a result of doing the adjustment incorrectly, although the instruction manual doesn't actually say how to select the correct adjustment).


Ultimately I think the best way to judge is not to compareAF vs. 10x Live View MFon a single shot (or without a shot at all), but rather to apply stepwise adjustments and compare the results over several shots. Once you factor in the need to do that multiple times at each adjustment setting, I'm not sure that using AF and 10x Live View MF offers any time savings for AFMA, compared to just shooting the target with AF at a range of adjustment settings. That's especially true if you want to view the images on the computer.

[Alan]

Several times in this string, I saw the word tedious.


I'll tell you what is tedious. Reading all this stuff. By the time the 3rd page (or, less) of "comments" roll around, I need a 60 horse power sorting machine to sift it all out.


A lot of these topics start off with good intentions, and by the time they get to the last page (which often times seem endless), the topic is so murky (if it is still ON topic), that the average schmuck like me doesn't know WHO or WHAT is right about the subject matter at hand.


Don't some of you guys work in corporations? Ever hear of bullet points?


How about this suggestion: after all the countless words and technical stuff, why can't someone PLEASE summarize the whole topic?


BULLET POINTS!


To quote Keith B: Sheesh!