I thought I would offer my own hierarchy of lens faults and their probable impact.

1. Dust
   Dust in a lens can be from internal or external sources.
   • Internal:
     When a glass element is mounted, its circumferance is painted flat black before mounting to minimize edge reflections by maximizing light absorption. If this black paint from the lens edges or mounting falls off, you get black dust inside the lens. In small amounts, it has minimal impact on the image. Being black, it doesn't reflect or scatter much light. If there is a lot of dust in the lens, its effects may start adding up, but not badly.
   • External:
     External dust won't be uniformly or mostly black in color. Some lenses have lens mount openings for various couplings that make it possible for dust to get into the lens. Being more reflective than flat black paint dust, this dust can be more of a problem. It also suggests problems in dust and grit in the mount to be wary about.

2. Spots on the lens
   Spots on the lens may occur through some droplet evaporating on the lens, leaving a residue and mark. Microfiber cloths carefully used can remove some marks, but others may bond to and discolor the lens coating. Spots on a lens are often a minor problem, hardly if ever noticeable on film.
   

3. Marks on the lens
   Marks aren't much of a problem if small, usually reducing contrast slightly. Marks near the edge of the lens may have even less impact unless shooting nearly wide open. Some marks increase the tendency to flare due to the coating flaw, but again this is often very minor and subjective opinion.
   
   

4. Rub Spots
   Most rub spots are minor, often relating to an area of the lens coating being worn away. Be sure it isn't impacting the optical glass curvature underneath. Be aware that rub spots which go so far as to flatten an area of the lens curvature can introduce image impacting optical defects. Only lens element replacement or very expensive lens element refiguring can repair a flat spot worn on the lens element. Large areas with rub spots and fine scratches may have worse optical effects than a large scratch.
   
   

5. Fingerprints on the lens
   Fingerprints are generally big marks, which if not removed can actually etch the surface of the lens permanently. See note below. This problem is worse than a scratch in my book if large and near center of lens.

   Shoptalk - N. Goldberg Oct. 1974 p.72 Popular Photography
   Speaking of grease, a single greasy fingerprint on a lens can knock down the contrast level of the image it forms by as much as 20 percent. Some fingerprints are more watery than oily, and in many cases, the acids in the watery fingerprint can etch the glass surface if allowed to remain there.

   
   

6. Scratch
   A small scratch will increase tendency to flare and reduce contrast by scattering light to where it shouldn't be on the film. One simple solution advocated by some photographers is to carefully fill the scratch with dark black india ink. The idea is that it is better and less impact if you reduce the light scattered by the scratch, rather than just leave it.
   
   

7. Gouges and chips
   A gouge is just a deep scratch. Flare tendency increases a good bit, contrast is reduced more, and lens price should reduce likewise. Again, I would probably carefully fill in the gouge with black ink as above.
   

   A chip is a spawled piece of glass, usually near the lens edge. Stopping down may minimize the impact and reduce flare from lens edge defects. Chips may hide fractures which penetrate farther into the lens than the visible damage lines. Lens stresses during use may expand the size of these hidden defects and their on-film impact. Cycles of heating and contracting from cold may also cause further cracking and deterioration.
   

   Speaking of impact, both lens chips and gouges are direct evidence of abuse and possible lens mis-alignment clues. Carefully test, and expect a large price discount!

   
   

8. Fine Scratches
   Surprise. Many folks think fine cleaning scratches from polishing a lens with dirty tee-shirt or contaminated lens cleaning cloths is much worse than a deep scratch. The impact on the film can be worse, as the fine scratches act to diffuse and scatter light all over the film, reducing contrast substantially. Repair is impossible, since the lens coating is usually ruined. Recoating is so expensive that replacing the lens is almost always cheaper.
   
   

9. Deep scratches
   Bad news, especially if many scratches, or nearer the center of the lens. Simply worse than many fine scratches, with more scratches being worse.
   
   

10. fungus among-us
    [Fungus on Lens Views]
    
    Fungus is very bad news. Fungus can grow on the lens element surfaces, especially if your lens is stored in a garage or other dark place. Fungus grows in a very distinctive and delicate tracery pattern on the lens.
    

    Early on, the fungus can be removed before it does major damage by disassembly and cleaning by a camera repairperson. Direct exposure to ultraviolet light is reported to kill fungus too.
    

    After a while, the fungus actually etches a pattern into the lens surface. It is like a fingerprint, only potentially even worse in its adverse effects on contrast. Usually, the cost to fix the lens once it has become etched by fungus is so high as to be uneconomic.

    
    

    Mostly Older Lens Faults

11. Cloudy
    Lens develop cloudy elements for a variety of reasons. The most common problem is oil or lubricants evaporating from the lens mount or camera body onto the lens surfaces as a cloudy deposit. Cloudy lenses do major bad things to your lens contrast, usually causing flat prints and other problems.
    
    

12. Separation
    Lens separation is rare in most 35mm lenses, but more often seen on older large format lenses cemented by Canadian balsam or other early adhesive. Moderate amounts of separation at the edge of a lens may not impact its use seriously, as the edges are rarely used stopped down on view cameras. Recementing lenses is not trivial work, as proper centering and alignment requires careful optical work.
    
    

13. Bubbles
    Bubbles are usually only found in older larger format lenses. Unless large, bubbles reportedly do surprisingly little to impact image formation. In older lenses, bubbles were even used as an indication of higher quality, hard to make glass present in a quality lens. Bubbles in more recent lenses are usually unintended defects and should raise questions.
    
    

14. Discoloration
    Discoloration can obviously cause problems, acting like a color filter on the lens. As they age and oxidize, some adhesives may discolor, usually towards a yellowish color. Again, this problem is mostly seen in older lenses using early adhesives. Another source of yellow coloration can be really poor multicoating (as from failed recoating efforts). 
    
    

15. Radioactive Glass
    Many early glass lenses have a surprising amount of radioactivity, often due to radioactive impurities in rare-earth enriched glasses. The effect on the lens is minimal, usually microscopic particle tracks accumulating in the glass over time. But the impact on nearby sensitive or fast films can be significant, especially if close to your older lens or stored over a longer time period. [See Radioactive Glass and "Hot Lenses" pages for suprising lists of "hot" radioactive lenses - is yours one of these?]
    

    
    

    Location, Location, Location

16. Front or Rear?
    The worse place for a lens flaw is at the rear of the lens, rather than at the front. A scratch at the rear will project contrast reducing light rays directly onto the film. A similar scratch at the front of the lens would have its impact reduced by the subsequent lens elements.
    
    

17. Center or side position?
    Obviously, a scratch or mark at the center of the lens is worse than one at the edges, since the center of the lens is always used. A scratch on the far edge of the lens may not have much impact if you often shoot closed down.
    
    

18. Flare
    Flare can be somewhat controlled by selection of aperture, positioning of subject and camera position with respect to bright light sources, and by using a lens hood to block off-axis light. A lens hood is a very good investment if you shoot in bright light conditions (e.g., outdoors).

Non-Problems and Myths


Some problems are not really problems, but many people think they are.


1. Uncoated lens
   An uncoated lens is simply an early lens that lacks the few molecules thick optical coatings of calcium or other elements deposited on lens to reduce internal reflections. These single coating techniques were developed just prior to the second world war.
   

   Uncoated lenses can give good performance, especially if they are carefully controlled in flare prone situations (see hints above).

   
   

2. Single Coated vs. Multicoated lenses
   Transmission rates:
   • uncoated - 60 per cent
   • single coated - 95 per cent
   • multi-coated - 98 per cent
   • super multi-coated - 98 per cent (its marketing hype)
   
   

   Some modest improvements in flare control and contrast result from using multi-coated versus the older single coated lens designs. In one Popular Photography comparison of similar speed and focal length lenses from Pentax, the multi-coated lenses reduced flare from circa 1.6 per cent on the single coated lens to circa 0.9 per cent on the multi-coated lens. An uncoated lens had flare in the 3 to 5 per cent range.
   

   A zoom lens has more surfaces by far than most older lenses, so a typical zoom really does need multicoated lens surfaces to perform well in flare producing situations.
   

3. Older lenses aren't color corrected
   Even if you are shooting just black and white film, you still want all colors of light to focus on the same plane. Even early twentieth century lenses were designed to do so, and therefore also worked well with the subsequently invented color films without changes.
   

   So most 20th century lenses will do a decent job with color film, but there are exceptions. Some lenses are highly corrected for bringing all colors to the same film plane, called apochromats. By contrast, some achromatic lenses ignored some colors (red) in their formulations, being designed for non-panchromatic films.

   
   

4. Older lenses are often better
   Many older lenses may be nearly as good as current lenses, but few if any are truly better than the best current lens technology can achieve. Careful measurement and comparisons nearly always show that today's lenses are usually the equal and frequently superior to earlier lens formulations.
   

   On the other hand, many older lenses perform remarkably well, and yield results very nearly as good as their much more expensive current versions. Some lens designs haven't changed in decades (e.g., nikkor 105mm f2.5 lens). Studies of comparative ratings and tests can often identify particular lenses which are just as effective and far less costly than the latest lenses for the intended usage.
   

5. Faster lenses are better
   Faster lenses are necessarily harder to design than slower lenses, often a lot harder. Some faster lenses are also optimized for wide open performance. If you rarely shoot wide open, you may be paying a lot more for a lens that performs no better (and maybe worse) than its slower and cheaper cousin.
   
   

6. Coma, Spherical Aberrations, Pincushion and Barrel distortion etc.
   These lens faults are inherent in the lens design and represent a series of design and cost tradeoffs. Many of these effects in budget lenses can be minimized by stopping down several stops from wide open aperture. On the other hand, you may find your smallest aperture is limited by diffraction effects. A mid-range aperture is often best for minimizing lens distortion effects.
   

7. Deforming Glass Myth
   See postings below, and Is glass liquid or solid? by Philip Gibbs for interesting review.

   
   

8. Water Absorbing Glass Semi-myth
   See postings below, but unstable optical glass is very rarely encountered by users, and almost always in older glasses only.
   
   

   Various lens rating services online provide reviews of these built-in design tradeoffs and optical limitations. The more you pay, the more you hope these optical defects are minimized and quality control is maintained. When buying used lenses, you should check the individual lens out with film (as described in camera and lens testing article). Even the most costly lens could have been knocked out of alignment, so don't rely on brand name to obviate the need for testing the lens carefully.
   

   see Top Ten Myths of Photography
   

   Hopefully, the ideas and opinions shared here may suggest some opportunities while warning of some problems to be considered in your selection and choice of lenses. Many good performing lenses with minor but visible defects are sold at huge discounts. The techniques and tests described in this article (and related camera and lens testing article) will help you locate some good buys and avoid problems in your photo purchases. Good Luck!

Lens Test Definitions

Aberrations
Lens design is a tradeoff, so you can't eliminate all aberrations entirely, and costs go up rapidly the more you improve optical qualities.

Electronic Bench Test
Photonics are used to compare contrast levels electronically, using fine and coarse slits, in a relatively objective measure of lens contrast

Manufacturing Defects
Defects such as mis-centered elements will reduce lens performance, perhaps significantly. Amateur lens 'repairs' can induce similar problems.

Star Test
The image of a point of light is examined under a microscope, any deviation from ideal shows the nature and degree of aberration. As with most lens tests, this one depends on experience and subjective factors.

NOT IMPROVED BY STOPPING DOWN:

Curvature of Field
Inability to bring all points on a flat object into focus on a flat image or film plane; not improved by stopping down.

Longitudinal Chromatic Aberration
As the color of light changes, the focus shifts; this aberration is not improved by stopping down.

Distortion
Image lines of a square bows out (barrel type) or in (pincushion type); doesn't influence sharpness; isn't improved by stopping down.

Lateral Chromatic Aberration
The degree of magnification varys as the color of the light varys; this aberration is not improved by stopping down.

Spherical Aberration
Spherical aberrations cause a shift in focus as you stop the lens down or open up.

IMPROVED BY STOPPING DOWN:

Astigmatism
Lines radial to the optical axis focus in a different plane than lines perpendicular to the radial lines. Astigmatism is improved by stopping down.

Coma
Off-axis points show as tear-drop shapes instead of round points; improved by stopping down.

Flare
Reduces contrast, may be improved by stopping down, depending on source.

Optimium Aperture
Point beyond which no further improvement in image contrast can be seen, e.g. when stopping down.

Vignetting
Corners of film are under-exposed; improved by stopping down.

Adapted from Anonymous, Lens Test Glossary, p. 24, May 1973, Pop. Photography.

See also Focus shift posting below.

Notes:

MP april 1966 p. 16 Keppler on the SLR From Modern Photography, April, 1966, H. Keppler, SLR column, p. 16:

Inexpensive lenses are surprisingly well color-corrected. Their faults are generally not along these lines.

First of all, there seems to be an erroneous assumption that cheap - er - inexpensive lenses are not properly color corrected and may do well for black and white but will certainly fall down when it comes to shooting color. Actually, this just isn't so.

From Modern Photography, June 1965, Bennett Sherman, Techniques Tomorrow, p.31:

What about the difference between the popularly priced lens and the very expensive one? First of all, there is not a very great difference between the optical performance. Most lenses are very nearly the same optical designs, such as the familiar Biotar types. In the expensive lens, an extra effort is made to keep the focal length of the manufactured lens very close to the design value. In the less expensive types, the focal length may vary a bit more. There can also be a small variation in the correction qualities for close ups, and the less expensive lens might show a bit more variation of sharpness at various apertures. You'll probably never notice it in everyday shooting, but careful testing including resolution charts, can show up these slight differences. Because of close tolerances in manufacturing and testing, the more expensive optics show a greater uniformity of performance, lens to lens. [italics in original] In any case, careful testing can tell you what to expect from your lens, and quickly identify a clunker.

Related Postings:

From: jbh@magicnet.net (John Hicks)
Newsgroups: rec.photo.equipment.large-format
Subject: Re: Perceived lens problem
Date: 3 Feb 1998

Darrell Messenger dmess@iu.net wrote:

Yesterday I bought a f8/90mm Super-Angulon ... (with dust..)

Don't worry about it; the dust won't cause any problems unless there's really lots of it.

jbh

rec.photo.equipment.large-format
From: dickburk@ix.netcom.com (Richard Knoppow)
[1] Re: Advice needed on scratched lens
Date: Fri Feb 06

Sven Sampson sven@slip.net wrote:

I just received a used 16.5" Goerz APO RD Artar in barrel that I was planning to send off to Steve Grimes for mounting. I air dusted it, brushed it with a camel hair brush and then used Kodak lens cleaning paper and a photographic lens cleaning solution to gently clean up the lens surfaces.

The front and back of the rear element are very clean. The rear of the front element is very clean. The front of front element has a very fine, but pervasive, circular network of scratches on it with a heavier concentration near the center of the lens.

My question is:

1) How much should this affect the lens performance ( I was planning on using it as a long lens on 4x5)?

I am trying to decide if I should return the lens and start looking for another, or if this will work well as is.

Thanks in advance for your thoughts.

Sven

Its impossible to know for sure without seeing the lens. However, although others may disagree with me I think this kind of scratching acts a lot like a diffusion filter on the lens and lowers its contrast. If its bad enough it will but halos around highlights. A couple or three shallow scratches aren't a big deal but this sort of thing is likely to be. Really the only way to tell how much degradation there is is to compare it with an undamaged lens.

If you are going to spend the money to have a lens mounted in a shutter I would insist on an undamaged one. I suggest you return it and look for another. There are plenty of Red Dots around just now at reasonable prices.

---
Richard Knoppow
Los Angeles, Ca.
dickburk@ix.netcom.com

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