Here I will describe some moments to which it is worth to pay attention in case of a lens choice which was in use.
In the following article I will write about photocameras, but for now…

The choice of a manual (not autofocal) lens

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Abrazions
Old lenses were most often made of metal. Sometimes the case is from metal, and a casing of lenses  are from plastic. But in general there were a lot of metal and it allows us to define a level of the demand of this lens, because it is easy to torn off the color from metal  by other same lenses which were in the bag together or when it was knocked by the camera.
I mean such abrazions

Lens mount
Next we will look at its lens mount (in this case M42 thread)

Lens mount

Yes. Here are too many scratches too. The lens in the active use.  It is good and bad at the same time. Plus that this lens suited previous owner. And at minus is that it is necessary to look attentively at the existence of scratches on lenses and amount of dust inside. If the lens was unused and almost new, it was stored in a tube and there is any dust.

Lenses
Here already in a picture F is lens mount, from the double converter. Not all defects can be on one lens at the same time.

lens

In this case, we see spots and if to look narrowly, it seems like the lens was scraped. Most likely it was wiped with something that does not fit to this purpose. The effect was gained as from an emery paper. The lens is in scratches. And then if you will look at it more attentively, you will see a darkish spot.
By the way, when you will go to take the lens, take a brush with you, something like Lenspen. And if the seller claims that it simply dust or fat, wipe it and look, would this defect disappear or not.

In the same time, smal defects on a back of the lens are much more unpleasant, than on the front.
If you want to check the dust in a lens, just open a diaphragm and direct a lens on white object.
jupiter-37-aSet where is dust. If it is on the front lens outside — brush it away with Lenspen. If it is directly behind the front lens and it is not so much dust — it isn’t so bad. It is won’t be visible in pictures. If there is a lot of dust and it is on the front and on the back of the lens, it is bad. Especially if it is in the center. Depending on value of a diaphragm and focal distance of a lens such dust can be visible in pictures.
And if you are looking inside the lens, inspect the diaphragm. It is desirable that it was nielloed but if it is not, it is not so bad. Silver one can give flares or sometimes not.

Don’t forget to examine a diaphragm from both sides. It must have any lubrication.  Flares won’t arise, but nevertheless it is desirable that it wasn’t.

Construct
Now it is very important point: whether to take it apart or not ?
If it is the Soviet lens, this probability is high. In Soviet period every engineer simply considered for his duty to climb in any technique, even without understanding its device. There were exceptions, but in general… You shouldn’t take lenses which were demount.
Unless the owner will tell  you that the lens were demounted in a special laboratory and only once. It will be visible on screw, whether it was demounted or not.

jupiter-21mjupiter-37a

Well. Why we are talking about screws… I hope everybody know how looks the screw which was turned off and twirled and how looks the “virgin” screw.
If we successfully passed all previous stages, now we will test lens mechanic.
For a start we will open and close a diaphragm.
The diaphragm can walk smoothly or with clicks.
Does it works? Is it good?
Now try to rearrange a diaphragm on each division. It shouldn’t jump any divisions if it goes with clicks stoppers. If it goes smoothly, it must go slightly. Not to dangle and not to stick because of the lubrication which is inside or dirt.
I describe only fix lenses here, and zoom lenses must be checked on backlash.
Also we check a focusing ring. It shall walk smoothly, with easy effort not to stick and not to dangle.

Autofocal lenses

Here it is partly simpler. We will check contacts at first.

left: new lens, right: second-hand lens

left: new lens, right: second-hand lens

Screws also must be checked like on manual lenses.

You can check it in the manual mode. At the same time you will check the minimum focusing distance if you don’t know it yet :)
But the focusing ring in autofocal lenses most likely will dangle a little bit. It because they are not for the manual mode.
Check an autofocus on the diaphragm. Better on completely open for this lens.
But it is better to take a laptop with you (it if you buy lenses in the shop), to connect the camera to the laptop or after trial pictures to load them and to check the sharpness.
It shall be mandatory sharply at the center and a little less sharply at the edges. If it is visa versa, for example,it is a production defect.

photo

Also pay attention that the lens will be less sharp even at the center of the frame, than on clamped to F4 and more. On opened lenses from F1.4 to F 2.8 most of lenses have not really good rates in respect of sharpness on the edge of a frame. In case of diaphragm gripping to F8, all lenses have identical rates.
Also the autofocus is set up only on one diaphragm (the front-back).
I recommend to set up it for the most open diaphragm due to the camera setup or on the adjustment of a lens. Yes, on more closed diaphragms it will be fuzzy.
I hope, that I showed all in this article. Next time we will investigate the old film camera.