Zoom Lens- Close Focus Distance

Does the close focus distance quoted in the specification for a zoom lens apply across it's zoom range or only at the short end. I'm thinking in particular about the Pentax 17-70 f4 which states focus range is "0.28m to infinity"

For some lens, like the Tamron and Sigma 70-300, you can focus to the minimum distance only at the long end of the zoom. For other, it will work for any focal length.

This is slightly convoluted thinking and may display a misunderstanding of DOF, but...

Surely on a variable aperture zoom the minimum distance will be shorter at whichever end has the slower maximum aperture? On the basis that DOF extends in front of the subject as well as behind, anyway?

I'd imagine it to be in the order of millimetres but still, it must have some effect?

Are you thinking of lenses with a Macro capability? or does that apply to none macro as well

Why do you ask?..... Are you using it in a confined
space?..... Don't forget that the focal length changes
as you focus closer

Edit: You already have a 17-70mm... can't you just
measure it?

Those two have Macro.

With all the lenses i have the minimum focus is the same across the zoom range.

I think the manufacturer would quote the minium distance as being in the "middle" of the DOF.

When using the DA17-70 yesterday I noticed if I was close to the subject, with the lens @ 70mm, it would not lock on to focus. Without moving and setting the lens @ 17mm it would.

Probably better with MF at close distance.

The dof is wider at 17mm then at 70mm and the 17-70 is known not to lock on so well at 70mm if i'm correct.

Am I thinking about minimum focus distance as given in a lens Specification in the wrong way? I thought it was the smallest distance the lens would focus at, with depth of field either side. With your talk of DOF, you seem to be defining it as the smallest distance anything can be in focus at. i.e. focus distance minus DOF between lens and subject

I think you're mixing things together a bit.
Minimum focus distance is the minimum focus distanced measured from the sensor, nothing more nothing less.
Dof is another thing not directly related to this.

The Depth of field is more or less entwined with the magnification, the higher the magnification the narrow the DOF.
As for the distribution of the DOF that is also related more or less with the magnification, at 1:1 magnification the distribution of the field is more or less equal but when you focus at infinity more of the DOF will be behind the focus point.

That's the basic though there is more behind it but it's getting quite technical and you don't really have to know that, that's why i use the words "more or less" a lot.
Here is the full read if you're up to it.
http://toothwalker.org/optics/dof.html

That is what I said. The DOF does not come into the equation in order to answer my original question.

I didn't read that though because you said this
And that isn't true, DOF doesn't come into play when measuring the focus distance.
So your latest statement is correct

As for the original question, the scale is true for the whole zoom range but there are some variables and tolerances off course so it isn't 100% correct but close enough at least.

As for the Macro capability off some zooms, those mostly only work at the longer focal lengths like Yves said. Those lenses also have scale which go down at those lengths so the focus scale should give you always a fair idea.

We are on a language thing here Stephan. I believed I was saying the same thing in both statements.

Maybe but then i find it strange that you say the thing i quoted from you.
But ah well we're on the same wavelength now.

But just the be sure the distribution of the DOF in regards to the focuspoint is not always 1:1, it's only that when at 1:1 magnification and it goes up to 1:∞ when you focus on or past the hyperfocalfocuspoint.