Influence from dust, scratch& haze
Most old lenses have some dust and/or scratches but if not serious, better to keep and use as-is instead of cleaning up. Dirt and scratches of the lens are not good to look, I mean ugly, and will impact to re-selling price definitely but not as much effect on the image quality as you might worry.
Think about the big iron plate (aperture blade) which block the lens diameter almost entirely at the center of the lens. The plate does not show up on the image, nor degrade image quality, only darken the screen since it block the some part of light. Do you ever wonder why the aperture blade is not visible on the image?
Just as the human eye cannot see something inside eyeball, lens cannot see aperture blade inside the lens itself. Not only inside but surface of the lens, cannot see as image. Then why nervous about tiny dust inside the lens? Of course, it is cleaner the better as an optical tool. Key point in here is if a lens performs well even though some internal dust and scratches exist, just use as it is because cleaning may not bring any improvement or even risk to misaligned lens elements with your own cost.
Haze, on the other hand, has bigger impact on picture quality. Haze is a phenomenon, thin layer of humidity or oil vapor build up on the surface of the internal elements. Unlike dust (point) and scratch (line), haze build up on area thus easily affect to image quality. When the haze is weak & transparent, picture contrast drop slightly which is manageable in post processing, but when the haze become thicker or area wider, photo quality degraded dramatically to call the time to go to repair shop.
The best countermeasure against haze is prevention. When storing the lens for a long time, put it in a dehumidifier or a plastic box with a dehumidifier.
A simple way to check for spherical aberration, ‘blooming test’
Blooming refers to the symptom of light scattering around the outline which indicates the lens performance indirect but practical way. It is easy to check the performance of an old manual lens as blooming refers lens aberration, especially spherical aberration.
Just take a picture with a high contrast (=/=, not reflection on metallic surface) and see if blooming appears in the periphery of the outline. Best target is white letters in black background or black letters on a white background. The blooming test is visual & subjective, so it looks unreliable compared to ‘Imatest’. But the ‘visual and subjective’ method can be more relevant and practical for photographers, because with MTF chart and table obtained from scientific test, hard to get a sense, how it comes out of the photograph at the end.
*click photo for large size view
I took my bookshelves with 2 different aperture setting. The focus is on the letter ‘Rollei’, and put two pictures together. Note the right photo is obviously milkier due to veiling flare.
Zoom in center, ‘Rollei’ written in yellow on a black background. Left photo shows clear outline of the letters, while right photo is not only fuzzy overall, but has a yellowish shaded contour around the letters.
Example photo of blooming in real world
The lens tested here, Sigma XQ 135mm f1.8 shows heavy veiling flare (overall color cast) and blooming at maximum opening, but after 1.5 stop down (f1.8 –> f2.8), both disappeared thus concluded the lens has excessive spherical aberration at f1.8 but no noticeable haze.
This blooming test, also as spherical aberration test has done only in the center of image. It is because at the center, only spherical aberration and chromatic aberration occur. On the other hand, the periphery area suffers astigmatism, field curvature, coma, and distortion as well as spherical and chromatic aberration. In general, it is getting worse as distance increase from the center. Therefore, even if the image blurred at corners of frame, hard to say whether it is due to spherical aberration or others.
Chromatic aberration is literally ‘color flange’ around outlines, e.g. a green or purple band appears on the contour of a subject. Spherical aberration shows no specific color shift, just cloudiness or fuzziness formed around outlines, so can be distinguished by the naked eye.
Most important thing, 90 degree alignment in the brick wall test
In fact, the real difference of lens quality comes from the periphery, not center and it varies greatly depending on the lenses. Most common whole image quality examination is brick wall test, as you can find a lot during internet search. It is simple but rational to make good sense about the lens as long as good alignment has done for the test.
To test the overall picture quality of the lens, especially corner’s image quality, you should align the camera with the test surface to 90 degrees as precise as you can.
It is very important to set the camera exactly 90 degrees toward target plane no matter test chart or brick wall. If the angle not aligned, for example, shoot the test surface at 85 degree angle, both ends of the frame (corners) will fade away from the depth of field. Then reviewer will judge that the lens’ peripheral image quality as poor and blurred. But in fact, it is because out of focus, not because of lens image quality itself.
If the shooting angle and the subject plane do not maintain 90 degrees as shown in this illustration, periphery image quality (A & B points) will be more affected by the depth of field than the lens performance itself.
(to be continue)