Minimum Focal Distance

deeky

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So here's the question - will zooming in or out change the minimal focal distance of a lens? Or is it the changing of the dof that makes it appear that the minimum focal distance is changing? Or is it just my skewed perspective?

I'm particularly thinking of my Sigma 70-300 as it has the greatest range of focal length.
 

Gavjenks

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It is not supposed to change the minimum focus distance, no. In reality, it may "drift" a very tiny bit with some lenses when zooming to different lengths, but we're talking at most a difference of a few % (similar to how focal length drifts a bit when you zoom, but not enough to worry about).

Don't worry about it, unless you're doing a photoshoot where you absolutely MUST stand exactly in a certain place, and absolutely MUST be able to focus on something exactly 3.1 feet away at multiple zoom levels, and your lens happens to be rated for 3.05 feet, or something ridiculous like that. (even then, a very short extension tube would solve your problem anyway)
 

bratkinson

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Gavjenks hit the nail on the head. The minimum focal distance marked on the lens shouldn't change regardless of what focal length one is zoomed in at. But, in reality, there may be a very slight drift. But what DOES happen as the minimum focal distance is focused on, say an ant on the sidewalk, is that the DOF gets very small, probably the thinnest possible DOF the lens can produce.

As I am not a macro photographer, when just trying to 'find out' how close I could focus with one of my zooms, I moved in to about 2-3 feet from a flower and then moved back until the autofocus could lock in. I then zoomed in enough that the flower filled the viewfinder image and took the shot. I was satisfied with that as my macro photography 'attempt'.

On the other hand, if I was interested in 'trying' macro photography, I'd get a couple of extension tubes (no glass, just air) that moves the lens further away and thus 'enlarges' the image as the light rays are diverging as they exit the rear of the lens. But to get 'real' macro images, ie, 'actual size' on the sensor, it typically requires a true macro lens that can be very close to the subject (inches), but then then depth of field gets extremely small.
 

Helen B

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It's not uncommon for fixed, focus-by-wire, varifocal zooms to be able to focus quite a bit more closely at the wide end than the long end. It is not common for interchangeable zooms designed to be parfocal or close to it (parfocal - the point of focus does not vary when changing focal length). Very few zooms intended for still camera use are truly parfocal, so you can expect some change in focus point as they are zoomed.
 
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deeky

deeky

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Thanks for the comments! That explains well what I think I am seeing.
 

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