Can I get some clarification on DOF and Hyperfocal Distance (HFD)?

jwbryson1

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I have been reading a lot about these lately and feel like I have a pretty good grasp on them, so I am either having an "off day" or I need to keep thinking about how they relate to one another.

With my cell phone app, I have calculated a sample DOF and HFD based on the following parameters: f/11, subject distance 75 feet, focal length 90mm. The app tells me the DOF near limit is 46.3 feet, and the DOF far limit is 196.6 feet, for a total DOF of 150.2 feet. It also tells me that the DOF starts at 28.7 feet in front of the subject (or 46.3 feet from me) and the DOF ends at 121.6 feet beyond the subject.

It also tells me that my HFD is 121.1 feet. So, based on an HFD of 121.1, my DOF should extend from 1/2 of the HFD to infinity, or from 60.55 feet (121.1 / 2) to infinity. :scratch:

So my question is as follows: If the DOF based on HFD starts at 1/2 of the HFD, or here at 60.55 feet, why does the DOF calculation say that the DOF starts at 46.3 feet in front of me instead of at 60.55 feet? :confused:


EDIT: To add to this discussion, if the portion of the photo that is in focus is within the proper DOF, in my example above would an object 51 feet in front of me be in focus or would it need to be more than 60.55 feet from me?


[Sits quietly and waits for KmH to come knock him around a bit...:mrgreen:]
 
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Back in my day, lenses had DOF scales right there on the lens!! People just don't value sharpness like they used to.
 
Not sure why you got this '1/2' thing stuck in your head. Maybe you just stumbled on a lens/focus point/aperture combination that has 150 ft as total DOF, and the focus point is 75...... which just happens that 75 is half of 150.

Drop your focus point down to 50 feet and see what happens.
 
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As I understand it...

The DOF information (starting / ending distance etc.) is based on your input focus distance (to subject).

The hyper focal distance is based on what it would take to get your subject distance to infinity, in focus. So the DOF numbers that you get when you actually use 75ft as your distance, don't really work backward to jive with the HFD. They are separate things.

But, having used your 75 feet to get the HFD, you could then change the input data into the calculator to use the HFD as your focus distance, then you would get the DOF numbers that would be accurate for when you take the shot (assuming you use the HFD).
 
As I understand it...

The DOF information (starting / ending distance etc.) is based on your input focus distance (to subject).

The hyper focal distance is based on what it would take to get your subject distance to infinity, in focus. So the DOF numbers that you get when you actually use 75ft as your distance, don't really work backward to jive with the HFD. They are separate things.

But, having used your 75 feet to get the HFD, you could then change the input data into the calculator to use the HFD as your focus distance, then you would get the DOF numbers that would be accurate for when you take the shot (assuming you use the HFD).


This makes sense. I think you could also just make sure that you close focus starting point is within the DOF calculations so you are good either way.
 
Yes, that's sort of how do it. I decide what I need my close focus distance to be and then use the calculator to figure out what aperture I'll need to get it and out to infinity (if I need to). Before, I would just dial it to F22 and know that I had enough DOF.

But knowing how to use HFD, I can figure out that I might only need F8 or F11, and thus get much better image quality.
 
To push this discussion a bit further using my numbers above as an example, if the calculations say that the DOF runs from 46.3 feet to 196.6 feet for a total DOF of 150.3 feet (196.6 - 46.3 = 50.3 feet), but the image is in focus from 1/2 of the HFD (say 60.55 feet) to infinity, doesn't that mean that the DOF is really much bigger than 150.3 feet? Doesn't it mean that the "true" DOF starts at 46.3 feet and runs to infinity? WTH? :(

Now I'm totally confused. Dammit, Jim! :banghead:
 
STOP THE PRESSES!! I THINK I HAVE THE ANSWER!! :lol:

If you focus at 75 feet (the distance to the subject per my example), the DOF will run from 46.3 feet to 196.6 feet, so 46.3 feet will be the close focus point and 196.6 feet will be the far focus point.

Alternatively, if you focus at the HFD of 121.6 feet, the DOF will extend from 1/2 the HFD (60.55 feet) to infinity.

So if you want a focus to infinity and not just to 196.6 feet, then you just have to make sure that the object is at least 1/2 the distance away from you to your HFD point and you are golden. In my case, because the subject distance is 75 feet (which is beyond 1/2 my HFD), I could focus at the HFD of 121.6 feet and have the subject in focus and also have the photo in focus to infinity.

I think this is correct. Yay me! :mrgreen:
 

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