A swallowtail butterfly and my Nikon Z 180-600 mm lens

Canadian tiger swallowtail butterfly, Leduc, Alberta.

Although I love good prime telephotos, the truth is that they can be a little limiting, literally.  The minimal focus point is often five or six feet away.  Not a problem with birds, as they rarely ever get that close to you, but it is a different story with my more diminutive subjects.  

I recently acquired a Nikon Z 180-600 mm zoom lens.  I have been shooting a lot with it lately, exploring its features and enjoying the slightly longer reach it has over my 500 prime.  It has a similar shortcoming (pun intended), in that its minimum focus point is almost eight feet away at 600 mm.  However, when zoomed in at 180 mm, the focus distance decreases to just over four feet.

According to Nikon, the reproduction ratio of the lens remains the same throughout its entire zoom range.  Now, I take exception to that fact because the distances and the zoom values don't align.  At 180 mm, the minimum focal distance should be just over 1/3 times closer than the 600 mm focal length.  It's not, it's less than half.  That means the reproduction ratio will reduce as you zoom in.

So, I did a simple experiment.  I used a ruler, minimum focus, and various focal lengths to see how much of a ruler could be captured across a full-frame sensor (3.6 mm across).  Dividing one by the other gives reproduction ratio.  My results are below.

Minimum focus distances at different zoom values.  The pinecone shows telephoto compression.

Clearly, reproduction ratios are not the same through all focal lengths.  That's not a problem for me, but it does give me a better understanding of what the lens can do.

The main fact here is that you can be physically closer to your subject (albeit with less magnification) by zooming out.  The working distance at 600 mm is very far, but gives the best magnification.  In the butterfly photo, I was right on top of the insect, and my working distance was maybe 4 feet.  The only way to get a focus lock was to zoom out.  In a situation like this, you can achieve the best reproduction ratio by going to minimum distance on manual focus, and set the lens to its minimum zoom (180 mm).  Then slowly zoom in until the subject becomes sharp.  This will give you the best reproduction ratio at the working distance.

If you set your lens to manual focus and look at the focus bar in the viewfinder, you may find a distance value for the focus point.  It's another way to determine distance from the focal plane to the subject.

I found this interesting.

Thanks for reading.

Eric Svendsen     www.ericspix.com

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