Question: I've been experimenting with some photography for a few months now and I noticed that this happens on pictures with lower shutter speeds:

[![Night sky](https://i.stack.imgur.com/fdepu.jpg)](https://i.stack.imgur.com/fdepu.jpg)
[![City by night](https://i.stack.imgur.com/SJP6S.jpg)](https://i.stack.imgur.com/SJP6S.jpg)

What is this, why does it happen and what can I do to prevent it/fix it? It has affected some pictures I think are pretty good otherwise. Pictures were taken with a Nikon D3300 and a Tamron 18-200mm f/3.5-6.3 Di II VC on a tripod. Shutter speeds of about 30 seconds.

Answer: 
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> If a zoom lens has a focal distance scale on it, does that mean that the lens is parfocal?
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**No,** many zoom lenses are [*varifocal*](https://en.wikipedia.org/wiki/Varifocal_lens). As it's used in photography, the term [*parfocal*](https://en.wikipedia.org/wiki/Parfocal_lens) means that the lens should remain focused at the same distance as the focal length changes. Every zoom lens that I can think of has a distance scale, but most change focus slightly as the lens is zoomed.

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> Or does it just mean that the distance scale is inaccurate?
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I wouldn't say that they're inaccurate; it's more that they're just not that precise. In other words, they don't tell you very precisely where the lens is focused even if you haven't changed the zoom setting. The distance scales usually have relatively few markings; for example, the scale on a Canon EF 70-200 f/4 L ranges from 1.2m to infinity over a distance of a little more than an inch and with only about 5 intermediate markings.
I have never heard the term “parfocal” applied to a zoom lens, however I understand your question. Parfocal in the jargon of microscopes and telescopes pertains to an interchangeable objective and/or eyepiece. Microscopes often sport a turret that holds an array of objective lenses of various powers. One switches objectives by rotating the turret causing a different objective lens to become optically aligned. The same arrangement is available for telescope eyepieces. Various eyepieces are organized on a rotating turret. Parfocal objectives and eyepieces are adjusted as to their barrel lengths. Such an arrangement maintains focus as the adjustments are made. 

If a zoom lens has a distance scale, it likely is accurate throughout the zoom range. It will be easy for you to check. Identify several objects of known distance, now zoom; note the scale readings and the degree of sharpness observed.