Small lens test Porst Weitwinkel 1:3.5 35mm

[truls]

I try to test some lenses, the result may also interest others on the net. I want to improve the lens testing, any suggestions are most welcome. Here, a Porst Weitwinkel 35mm 3.5:



It is a big lens, bigger than my Canon FD 50 1.4... The focusing is butter smooth, very uncommon for this type of cheap lens, m42 mount, Olympus M43 camera, Iso 200.

Here is my main test setup, a German ad that came in the auction package:



100% crop from center

Left f 3.5 right f 5.6


Left f 8 right f 11


Left f 16 right f 22

It looks like the lens is best at f8 and f11, no surprise. Soft at f3.5, and gets softer at f16 and f22. I have not tested the lens at the corners, but sharpness is better when stopping down to f8 and 11.

So far I conclude this is not a very good lens. On the other hand, if making real images, those test charts do not count very much. A real picture consists of more elements, making a picture good or bad.

My wifes test flowers:


100% crop from center

Left f 3.5 right f 5.6


Left f 8 right f 11


Left f 16 right f 22

[Stephen]

Test gives pretty much what I would expect from the Porst lens, but bear in mind you are only testing the centre section of the 35mm image area on an Micro 4/3.

So the fall off in sharpness may be greater than the test shows if the full area was tested.
We usually greatly benefit by using a 35 mm lens on digital 4/3, as only the centre is used, effectively doubling the apparent focal length as an extra.

The Porst lens is probably a Tokina product, and it's behaviour in the shots confirms it is an average lens, but OK at F8 for definition, exactly the way most lenses behave.

The target is perfectly good, there is no need really for test figures in lines per mm etc., but also a good Graph Paper target can be used to show barrel and pincushion distortion.

However this is of minor interest to digital users as most good graphic programs can correct for both.

Stephen.

[philbirch]

The sensor on your camera is approximately half of the 35mm frame these lenses were designed for. The image is soft at f3.5 imagine what it would be like on full frame.

The great thing with these old lenses is that you can get reasonable results with crop cameras/

some nice photos btw

[Stephen]

The softness that comes when fully stopped down is a bit miss understood, it is usually described as diffraction, but it is down to the iris position and the thickness of the blades, which become proportionally bigger reflective surfaces at small apertures, in relation to the light travelling through them.
Also the position of the iris alters as the F stop alters in terms of the relation ship with the effective centre of focus within the lens.
To correct for this, on cheaper lens you put up with it, or on more expensive lens aspheric grinding can correct for it, or mechanical moving of the iris position, (very rare).
It is not true that all small stops destroy sharpness, a dedicated close up lens can have waterhouse stops in very thin blackened metal that work fine, and you can build a lens from 135 mm components that has a big enough waterhouse stop slot to adjust the pin hole back and forwards to get the best results.
I have an old Vivitar 135 with fixed stops to F130, it does not suffer from poor definition, and has a fantastic depth of focus!..just very long exposures! The stops are made from quarter thou stainless steel, blued with heat and smoke blacked before use.
It was designed to take realistic angle of view close ups of Model Railway subjects, where lack of depth of field is critical.
Again modern digital can do the trick with combination of several shots, giving fantastic depth, but a lot of work to do such shots.
Stephen.

[philbirch]

Hi Stephen, I noticed a little blur on the stopped down shots too. I didn't comment as it is common enough on cheap lenses, otherwise they wouldnt have a 'best aperture'.

When I learned photography I knew about the thickness of the blades etc and what it does. That is what I thought diffraction actually was. It would be interesting to learn the correct definition of diffraction and its effects.

People do use the wrong terms for things and this can confuse. I notice you use both depth of focus and depth of field interchangeably whereas they are two different things - please accept my apologies if you actually meant depth of focus.

[truls]

Depth of focus.. maybe too advanced for me. Could someone explain in a simple term? Thanks! Yes, it could have been some unwanted blur, assembling all those images took most attention.

[philbirch]

Depth of focus is the same as depth of field except it happens behind the lens rather than in front of it. Old camera manuals use the terms interchangeably.