What dictates minimum focus distance?

[IanAshworth]

Hi guys,

Just waiting for a new lens to turn up but while I was researching I found something and wondered if anyone could explain it for me.

I used to have a Canon 50mm 1.8 and the minimum distance for focus was 45cm away.

I've now bought a Nikon 35mm 1.8 and the minimum focussing distance is 30cm.

What dictates this distance - does anyone know?

 

[woof woof]

I could be very wrong... but as far as I know it's the design and physical positioning and movement of the whole or individual elements within the lens. Lots of distance or lots of travel and optical elements designed to focus the light at that distance or throughout the range of physical movement may enable closer focusing, I think. Increasing the distance of the whole assembly to the film or sensor with extension tubes within a range may allow closer focusing but may lose infinity but by having a mechanism which can move some eliments further away and then return them may keep the ability to return to infinity.

I bet there's a full and accurate explanation on the net...

 

[Canon Bob]

Alan has if pretty well covered. Lenses tend to be constructed using one of two methods.....Retro-focus or internal linear extension. The latter is not unlike having a set of built in extension tubes and will likely give an improved MFD. The other catch is that the focal length of a lens will reduce as the focal plane is brought in from infinity towards MFD....Canon's 100mm macro lens is actually 78mm when focussed close up.

A point worthy of note is that MFD is measured from the subject to the sensor plane and working distance is from the subject to the front element of the lens.

Bob

 

[woof woof]

I had a thought...

You can check what the effect is of moving things close or further by grabbing a magnifying class and a newspaper

Hold mag glass... move closer / further away from newspaper... move head about... note what happens and what makes the newsprint look bigger

Best make sure no one is watching first though