Neocameras lens buying guide claims:
Lenses most often increase in value over time until a significant change in technology.
Is this accurate? Why would value increase?
Empirically, this seems to be false - at least for your "mortal" (as in: not costing 100 000 €) lenses. MSRPs usually don't change/increase through the life of a lens, but the internet has a tendency of making things cheaper the longer they are sold because of the competition.
There's a site called Geizhals (the German word for "skinflint") that compares the prices of some/all vendors - and even gives some statistics about the price over time. Please note that some peaks appear to be wrongly set prices of the vendors - and that sometimes, a product is listed in the list even before sales started.
I made some screenshots of the charts of some semi-random* lenses - The X-axis goes from the first appearance of the lens on the site until today (18-01-02). The Y-axis is in € and displays vendors from across the EU.
* As a Canon-user, I know their lenses better than those of the competition. That's why 3/4 lenses are from Canon. If you don't trust my statistics, you can look up all sorts of lenses on the website yourself ;-)
Canon EF 24-70mm 2.8 II L USM: Release Date: 2012-02-07; MSRP: 2 319 €; -43%
Canon EF 100-400mm 4.5-5.6 L IS II USM: Release Date: 2014-11-11; MSRP: 2 619 €; -17.4%
Canon EF 400mm 2.8 L IS II USM: Release Date: 2010-08-26; MSRP: 10 799(?) €; +1.25%
Leica Apo-Vario-Elmar-T 55-135mm 3.5-4.5 ASPH: Release Date: ???; MSRP: ???; -11.9%
Also, if you look at sites like eBay, the results say the same - you wouldn't find a used 100-400mm L IS II today for more than it cost a year ago. However, some "premium"-lenses (e.g. the L-series from Canon) hold their value quite well, even in used conditions - so they will depreciate less than, say, a Tamron 70-300mm.
As always, there will always be exceptions. The biggest exception will be "premium" lenses (so not your average 18-55 kit lens) that go out of production without successors - think of the Fisheye-Nikkor 6mm f/2.8 or the Canon 50mm f/1.0 L. Also, maybe you get a rare U2-signed 50mm f/1.8 that will cost 1 000 € after their next album release - or maybe you simply get lucky and buy a lens that was "mediocre" when sold, but with a new wave of Lomography or the likes of it, it will be one of the most looked-after lenses in the market? That, however, is speculation - so saying that
Lenses most often increase in value over time is like saying "play the lottery - most of the time, you will win!"
Lenses do not increase in value over time, they decrease.
In a few instances, lenses will become more valuable. This is the case when the lens has some special, coveted quality and is no longer being made by the manufacturer. For example, the legendary Leica 50mm f1.2 Noctilux has been superceded by lenses with "better" performance, but the image quality of this lens is unique and there are people who will pay big bucks for that special look.
In general the value of a used lens will decrease over time. Also in general, the value of digital camera bodies will decrease by a greater ratio over the same time.
Historically, the values of mid to higher end lenses were not as volatile as they have been in recent years. Perhaps some of that was due to higher profit margins on lenses sold by the likes of Canon, Nikon, Sony, and Pentax? With the recent release of higher quality lenses than their past offerings by such makers as Sigma and Tamron, the major camera makers have had to adjust and lower their markup on lenses with direct competition from the high quality third party lens makers.
One thing that can affect the short term volatility of lens prices, in either direction, are the fluctuations in currency exchange rates between countries where lenses are made and/or sold wholesale and the countries where those lenses are ultimately bought by the consumer.
Canon, for example, bases much of their pricing on the value of the currency in their home country: the Japanese yen. A couple of years ago the yen fell significantly compared to the U.S. dollar and other "benchmark" currencies. In some cases the price of new Canon lenses, in U.S. dollars, dropped lower than the prevailing price in the U.S. for used examples of the same models had been just a few months earlier. The used market made downward adjustments to reflect the new lower prices of brand new examples of the same lenses.
So where will lens prices go in the future?
Like your stock broker always tells you in the fine print somewhere, past performance is no guarantee of similar results in the future.
Most products depreciate in value over time. There are rare exceptions. We are talking about the law of supply and demand. If a product is praiseworthy and demand is high and availability is low, it likely will appreciate in price. However, most good but used lenses will depreciate in value. The value of a used lens only inflates if is a valued antique or, if modern, images magnificently with low availability. An exception, today’s lenses are mass produced. They are tested and substandard units are rejected. Those that pass the quality control tests are marketed. Because each is an entity, some that pass might be a cut above or otherwise perform exceptionally. Given this scenario, an exceptional unit can appreciate.
Why would they increase? I think your assumption is incorrect.
Some primes are in effect 'best of class' I expect them to drop a bit from new, then degrade depending on wear. So for this class of lens I expect them at best to hold their own, unless they are "collector's items".
Most lenses are subject to new glass formulas, new anti-reflection coatings, faster autofocus, smoother diaphrams. For these reasons I would generally expect this years f/2.8 400 mm lens to me more sought after than the 2012's version. Once again, older lenses will drop in value compared to newer ones.
Zoom lenses in particular have benefitted from progress. Lighter, smoother, studier. Zooms I would expect to drop even faster than average as they age.
Recently I read an article about someone who put an 80 Mpixel back on a view camera. He used original lenses. He was complaining about how soft the lenses.
If the value of the lens is estimated as it's selling price at the time, the claim is obviously wrong, as the others allready answered.
If you consider the value as [a money earned because of the lens] divided by [money invested in having it] the situation is completely different!
If you are not a pro, change the money earned by expirience gained and consider whether it was worth the money or not.