# Where is infinity?

Started 4 months ago | Questions thread
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 Re: Where is infinity? In reply to sybersitizen, 3 months ago

sybersitizen wrote:

EinsteinsGhost wrote:

sybersitizen wrote:

EinsteinsGhost wrote:

sybersitizen wrote:

EinsteinsGhost wrote:
... If you focused past HFD (10m, for example), you’d not see any difference (3.60m to infinity will be in focus).

I assume you mean actually turning the focus ring to the 10m mark. If so, you were on solid ground until here, but that part is not right. DOF would now extend from (roughly) 4 or 5 meters to infinity - and actually 'beyond' infinity, meaning some of the distant DOF is being 'wasted'.

No. I don't really use the distance scale on the lens. By 10m, I meant the actual distance (measured or approximated).

The point is: If you refocused the lens from 7m to 10m (with or without markings), what I said above is true.

Are you instead saying that you left the lens alone and only moved the subject from 7m to 10m away from the camera? Of course that will change nothing regarding DOF covered by the lens.

No, I'm simply changing focus to a different distance, to make a point about how DoF will change, until you hit HFD when it goes to infinity. I'm not concerned about focusing at a subject, just the distance to focus.

As in my illustration, 24mm, f/4 on A580 will have an HFD of 7.2m, regardless of where you are focusing. However, if you focus at a point 7.2m away (meeting the HFD criteria), the DoF increases to infinity (and an additional 3.6m of it, from that point towards the camera).

But what you said here: "If you focused past HFD (10m, for example), you’d not see any difference (3.60m to infinity will be in focus)."... is wrong.

And what I said here: "DOF would now extend from (roughly) 4 or 5 meters to infinity - and actually 'beyond' infinity, meaning some of the distant DOF is being 'wasted'."... is right.

Surely you see that? You cannot really think that no matter where you focus the lens beyond 7.2m, the DOF is always going to range from 3.6m to somewhere in the distance. This can be disproven mathematically and through experimentation.

But... what can be said is that DOF is always going to range from 7.2m to somewhere in the distance.

Try it. Set up your test and focus the lens at 10m or 250m or 1000m or infinity and watch the objects at 3.6m go progressively blurrier.

And try it mathematically here:

http://www.dofmaster.com/dofjs.html

Now I see what you're arguing against. Yes, the "near limit" will indeed move farther away if we keep focusing farther past the HFD. It is why we can't assume that we can achieve HFD by simply focusing at those faraway mountains instead try to keep it as close to HFD (and just past it) to maximize DoF.

However, this increase is much less significant (near limit in the above setting will increase from 3.6m at 7.2m to 7.2m at 1000m.

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