Hyperfocal distance and DOF experiment

[Alan]

I tried an experiment using the HFD. Please give me some additional advice, because I'm not sure I'm doing this correctly.


The camera and lens: 5D Mk 2 and 24-105 L. IS was off. Camera was tripod mounted.


I disengaged the shutter button, using #3 in CFn IV. I used a remote cord to release the shutter.


Lens was at f/4, 24 mm, on aperture priority.


With the Mk 2, the Circle of confusion is 0.030 mm, so for f/4 at 24 mm, it's supposed to be 16 ft for the hyperfocal distance.


With a tape measure, I set the camera back 16 feet from a flat surface and that it was approx. 16 feet to the plane of the sensor. I then used the AF-ON button to focus.


I swung the camera around, then fired the shot with the remote switch. There was no wind (sorry about the exposure)


I don't see the distance in sharp focus at all (at least, not on my monitor).


It's approximately 200 feet to that chain link fence, behind that second tree.


The second picture is 100%, cropped.


I must be doing something wrong, since "infinity" is not sharp.


Your advice is appreciated. Thanks.


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.25.34/_5F00_MG_5F00_8869res.jpg[/img]


[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.25.34/_5F00_MG_5F00_8869crop.jpg[/img]

[KeVancity]

did you try moving the camera forward 1 foot, then try 2 feet. There are alot of variables at play. Could be your lens is back focusing.

[Alan]

Are you referring to when I initially focus?


If it's after I swing the camera around, why would a back focus affect the "infinity" end to this degree?


I'll try another lens or two, to see what happens.

[wickerprints]

Assuming your measurement of distances and shooting methodology is correct, one must conclude that either (1) the aperture is not actually f/4, (2) the focal length is not truly 24mm, or (3) the circle of confusion is not 0.03mm.


The precise hyperfocal distance with those three parameters is 4.824m or about 15 feet 10 inches.


Here's what I think is probably happening. The 21MP sensor of the 5DmkII has a resolving ability that exceeds the traditional 35mm film and therefore when viewed at 100% crop you are no longer dealing with a CoC = 0.03mm. In fact, I believe that if you are going to use the hyperfocal technique to achieve sharp focus at infinity at 100% crop, you will need to use a much smaller CoC, resulting in a longer hyperfocal distance.


I would explain this in more detail but I think the Wikipedia article is a better reference: http://en.wikipedia.org/wiki/Circle_of_confusion

[Alan]

wickerprints, it is my understanding that "precise" distance measurement from the focus point is not critical, since some claim that you can use the focal plane of the sensor, or the end of the lens. In this case, it falls within the 15 feet 10 inches, since the lens length is beyond the 2 inch difference.


I don't understand the CoC not being 0.03, when the 5D Mk 2 is listed as such: http://www.dofmaster.com/digital_coc.html


How then can I ever use a graph or equation, if I can't presume the CoC is as listed? I'd never be able to find the hyperfocal distance if I can't start with what is known.


And, as for the f/stop and focal length, if I can't rely on those (especially with a lens that shows that I did, indeed, have the correct f/stop and focal length in the EXIF data), then combined with the "unknown" CoC, I might as well throw the graphs and technique in the trash bin.


There must be more to it than this.

[wickerprints]

First, I only included the precise measurement for the sake of illustration.


Second, you have to understand that a CoC is not a fixed property of the imaging system. It is a number that is chosen based on a semi-subjective analysis of the desired viewing sharpness. If some site just gives you a value, you can't just arbitrarily take it for granted because it does not explain the context in which the CoC was calculated. What is the desired viewing resolution? The image enlargement from the format size? The viewing distance? All of this is explained in the Wikipedia article.


The CoC is not like f-number or focal length or shutter speed or focusing distance. You don't look it up in a table somewhere because it's not fixed. You need to estimate it empirically based on viewing conditions. What you are doing--looking at 100% crops on a screen--is not the same as making an 8x10" print and looking at it from 25cm away, which is what the 0.030mm figure is roughly based upon.


Third, I did not say that the f-number or focal length were reported incorrect by the camera or the lens. I merely stated that if your methodology was correct, the only way to explain the discrepancy was if one of those three parameters were not as claimed. From this I deduced the likely parameter that was not correct is the CoC--implying that it is highly unlikely that the other two are incorrect to the magnitude that would affect the calculation of the hyperfocal distance.


Finally, let me give you an example of how I would go about empirically determining a CoC. First, I do a calculation based on my viewing conditions to give me a ballpark estimate:


If I view the image at 100% crop, then what is the enlargement factor? A 5DmkII has 5616 pixels per 36mm sensor width. This is equivalent to (5616 px)/(36 mm) * (25.4 mm)/(1 inch) = 3962.4 ppi. On my computer screen, I have 1440 pixels in 13 inches of horizontal screen width, for a resolution of 110.8 ppi. Therefore, my enlargement factor is 3962.4 / 110.8 = 35.8 if I view the image at 100% crop, as you have done.


Now assume I view my screen from a distance of 50cm.


Next, what is the maximum resolution of my screen in line pairs per mm? 110.8 ppi = 4.36 p/mm, which is a maximum of about 2.2 lp/mm. I can't really distinguish more than that, even if I got super close to my screen. I would say my eyes could resolve the full 2.2 lp/mm at a viewing distance of 35cm.


Okay now that we've got all these numbers, we calculate the CoC = 50 / 35 / 2.2 / 35.8 = 0.018mm. This is significantly smaller than the 0.03mm figure you used, and would affect the hyperfocal calculation accordingly:


Hyperfocal distance @ f/4 @ 24mm @ 0.018mm CoC = 7.963m = 26.125 feet.


Is that going to work? The problem is, I don't know. You need to verify it empirically by then going out on a clear day and taking photographs and determining at what focusing distance you are getting acceptably sharp images at infinity. Once you do this, you will be able to establish a CoC figure that should then be independent of focal length or aperture, but is suited for your viewing conditions. You will need to do other calculations for CoC if you decide to print those images. Do you see how this works now?

[Alan]

wickerprints, yes, I do. Thanks.

[Daniel Browning]

As others mentioned, 0.03 is not small enough for a 100% crop of the 5D2 (which is like looking at a 6 foot wide print up close).


But more importantly, why are you bothering with a tape measure and charts? At their very best they will still be less precise than 10X liveview, and are much slower besides.

[Alan]

Daniel, I'll switch to that for the Mk 2, but I also still have my original 5D and the 40D, so I'd still need a tape measure for those if I'm out in the field.


The experiment was to see how well the HFD thing worked.

[Daniel Browning]

Daniel, I'll switch to that for the Mk 2, but I also still have my original 5D and the 40D, so I'd still need a tape measure for those if I'm out in the field.


The experiment was to see how well the HFD thing worked.
<div style="clear: both;"]</div>

Of course; that makes perfect sense.