Kit lens issues related to mechanical construction may prove very difficult to resolve, in short these include lens de-centering ( which really grates on my pixels), poor focus due to sloppy lens fit inside the lens barrel, poor cross frame clarity due to misaligned mounting faces and perhaps a few other mechanical oddities, like misshapen apertures.
All of the above will conspire to place the focus in unpredictable places that are always challenging and often impossible to fix in post production. Most of these mechanical problems are due to sample variation, so if you get a dud, see if you can get it swapped out for a good one.
Lets pull the problems apart, first off, de-centering. In practice all the lens elements of the lens should be perfectly aligned and in a well made lens they are. In other words the absolute centre of each of the lens elements are aligned perfectly and the lens will perform as designed. If one or more of the elements are misaligned you can get uneven clarity across the frame, with perhaps one side sharper than the other, soft spots, blurry corners and a sharp centre, flare spots or perhaps in extreme cases just mush all over the place.
Good quality lenses are constructed in such a way that the centering of the elements is adjustable in manufacture, but many cheap kit lens are just simply fixed in their construction.
High quality lenses can normally be realigned, generally kit lenses cannot.
I suspect that more often than not de-centering issues are caused by the lens being dropped, which gives you a hint as to why I won’t buy a second hand lens with a dented or chipped filter thread ring.
The good news is that kit lenses are not designed to be de-centered from the outset, if everything is put together correctly then it should work fine. The bad news, if yours is de-centered, it is not going to be economical to fix it, so unless covered by warranty, buy another copy.
And the other good news, another copy will probably cost very little anyway, as there are a huge number of “up graders” selling their cast off kitties on eBay for next to nothing. Hell it might be worth your while to buy a couple, keep the best one and sell the other....or perhaps keep one in case you accidentally sacrifice the other one to gravity. Mind you I have noticed that gravity seems to be far worse (sucks more) with more expensive lenses. There are also well known high gravity spots, these include Canyon edges, the tops of buildings, any area where concrete covers the ground, I also have it on good authority that the sides of cruise liners and other boats are subject to especially high gravitational fields.
Now moving on to more insidious mechanical nasties
I have actually used lenses where once you cranked the barrel out for close focus or telephoto settings the front of the lens gets lazy and droops. The inevitable result of such misbehavior is poor focus across the top or bottom of the image (dependent upon where you have focused). This effect usually gets worse as the lens ages and becomes sloppier in the barrel, such lenses are a total annoyance but you can often even things up by applying just a little upward pressure on the front of the lens. Nonetheless a lens like this is just frustrating to use and probably should be replaced with something better or newer or perhaps you could discipline yourself and not focus too close or zoom to far out. Or here is a thought, turn you lens into a fixed focal length one by gluing the zoom to a fixed optimal setting, which is likely somewhere in the mid range.
Most modern kit lens do not have smooth metal helicoils inside to give that tight but buttery smooth focus action of old, instead they use clever plastics which work OK when new but eventually develop slop and make precise manual focus difficult or impossible. Naturally this can’t be fixed but a lens will usually need a lot of use before it gets to this point. Sadly, a good number of lenses seem to come out of the factory in a rather loose state so it is something you should check before you take your new baby home.
Some kit lenses also have plastic mounting flanges, again these can wear in high use samples leading to slop and misalignment at the mounting faces. I must say the slop is pretty rare as a lens would need a lot of un-mounting and mounting to wear enough to become lose, so its just a little thing to consider with second hand kitties.
Finally and thankfully rare, you can occasionally get a lens that refuses to focus to infinity, this is a manufacturing defect for sure but if you are not aware of what is going you will likely think the lens is just not sharp. The fix, get it swapped out or buy a new one, old school kittys like the 35-70 Minoltas and Nikons can probably be sorted via a quick adjustment by a tech person.
A final issue is flare resistance and indeed many kit lenses are less than perfect in this regard, but I humbly suggest that for most instances, actually placing the lens hood on the lens and/or shooting a little more carefully would have negated the issue. Any lens can be made to flare if you shoot into the sun or other bright light sources, the issue is not “will it flare”...but “how bad will the flare effects be”.
Older lenses are typically much poorer than new lenses due to radical improvements in lens coatings over the last couple of decades. I have many older lenses that have flares like a seventies disco given the right circumstances but regardless are still terrific lenses 99% of the time. Despite my Sony 18-55 OSS being touted as a bit flare prone in tests, I really have never had an issue, so don't sweat it yours is probably fine too.
BIG TIP: More often than not in classes when someone says their lens is soft and flare prone the problem is dirty haze and mushy prints on the front element, so keep it clean and fingers off.
Back in the hood.....If you didn't get a hood with your lens, (and oddly some makers skimp on this essential piece of kit hoping you will pay through the nose for one as an accessory) you can pick one up on eBay for just a few dollars.
The kit lens hoods are a bit of a compromise, typically having a petal shape that prevents vignetting when you adjust the lens to the wide angle setting, but providing minimal protection at the longer focal lengths. If you like me you find yourself using your kit lens at predominately the one focal length you could purchase a far more efficient hood than the standard petal one.
As a tip I often use a foldable old style rubber lens hood that can be adjusted to suit the different focal lengths.......and there is a big bonus with it. Should you need to shoot through glass it can be held up against the glass and will both act as a shock absorber and cut out reflections of the glass.....oh and they are very cheap on eBay too, like less than $5.00!
To conclude todays blog entry, from my perspective a good kit lens is one that has the mechanical criteria well under control but may display some of the fixable optical issues we discussed in the previous blog entry.
Ultimately your technique and artistic flare and editing skills will prove far more important than the actual quality of your lens, with the next entry we will explore the positives of your kitty.