How to check coating in a 14mm fisheye?

I think I'm pretty good spotting haze etc. in anything from a 21 to 50mm, but the fisheye is impossible for me.

 

How do you do it?

Best bet is to open up the diaphragm and shine a flashlight from the rear thru the lens while observing it from the front, then reversing the process.

I tried that and all I got was a tiny spot on the objective lens - I had the aperture wide open and the spot on the objective lens was maybe 1/4" in diameter.

 

I have a 21mm w/light haze and I could see that clearly by getting just the right reflection of light bouncing off the objective lens. That method was much better than shining light through it. A 1:1.4 50mm shining light is very effective.

 

Maybe using a light table would be more effective on the fisheye?

 

I have stereo microscopes - would there be a way to use them?

Why not just use the lens and look at the quality of results?

 

Internal haze easily makes itself known by lowering contrast and increasing the tendency to flare. Since it's quite difficult to keep light sources outside the FOV of a fisheye, the consequences of internal haze should be easy to spot.

 

As SCL suggests, shining a hard light source (a diffused lightbox is useless) through the lens should tell you all you need to know. You look at the light source directly through the lens, not obliquely at the spot of light hitting the lens surface.

As the coating is antireflection, coating defect reflect.

 

With a bright point source into the lens, you should see somewhere an unexpected reflection.

Or maybe with a bright diffused source.

 

I suppose the large depth of field might cause such defects to image into the film plane,

otherwise if you can't see them, I think I wouldn't worry about them.

 

You might also try taking picutres with a dark background, and single bright (close to)

point source in the image. As the coatings aren't perfect, you might find the usual

reflections from lens elements.

 

Unusual (out of place) reflections might indicate coating problems.

This would be especially true if the image of them doesn't move around as the light

source moves.

When I try to "look into the light," I am basically blinded by the white and can't see anything.

 

However, when aiming the lens to the very dark under my desk and then shining a LED flash light at oblique angles perhaps trying to get reflections of the lens walls. When I can see a horror show of pits/dust/dirt and haze that is hear the edge of the white light image - if I can see any of the white light, I'm blinded and can't see anything. This particular discovery on a Mamiya CS 50mm Macro and a 1:1.4 50mm

 

I'm also having difficult cleaning the outer lens surfaces. Using Eclipse and Pec Pads and when I lift the pad, the last island of alcohol leaves a smudge - not to mention dust. (In my house, the dust falls like snow) I have a camel hair brush blower and the brush appears to be contaminated - I don't know if a new brush/blower would work.

 

I just checked a NOS 135mm CS which I bought new. When I opened the box, I put a 1A filter on it to protect the lends. I put it on the camera a couple times and never used it since I had the zoom. That lens is failing the under desk light test. It is not unlike looking at a desert sandblasted windshield. There is also a possibility it is static sticky tiny dust specs that when I try to clean, I'm just moving them around.

 

I'm beginning to think that all Mamiya CS lenses which are at least 30 years old are suffering from issues.

 

I wonder if my Auto XTL ES lenses are suffering...

The nearest lens to me is a Tamron 18-270mm zoom.

 

Not quite fisheye, but at 18mm not so far off, either.

 

I have a bright white LED flash light which, from not so far from the

lens, covers the lens elements other than the outermost one.

 

Uncoated glass reflects about 4%, multicoated maybe 0.5% or

maybe down to 0.1%. (Different for different wavelengths and angles of incidence)

 

An LED flashlight is close to a point source, but not a narrow beam.

 

I see many spots that are reflections, and reflections of reflections off

the lens elements.

 

From: TAMRON | 18-270mm F/3.5-6.3 Di II VC PZD

it has 16 elements in 13 groups, so 29 reflecting surfaces.

It might be 29 spots, so maybe only single reflections.

 

If I move the light around, the spots move ... except for two.

 

It seems that there is a tiny piece of dust on one element,

and a small but not quite as tiny on another. I am not too worried

about those.

 

(There is also some dust and haze on the UV filter, on both sides.)

The UV filter has been on it close to when I got it, and I think not

removed, except possibly for cleaning.

 

I suspect like your fisheye, the image of lenses, and probably

the lenses themselves, get smaller as they go in. I didn't try looking

from the back.

 

In any case, I do see two specs of dust, and no coating flaws.

As I noted above, uncoated glass reflects about 4%, so should be

fairly visible if a big piece came off.

 

The flashlight I used is also a (red) laser pointer. I also tried

it with that, though I have to move it around to see all parts of the

lens surfaces. The previously noted dust is also visible this way.

 

This one might be about six years old.

When I try to "look into the light," I am basically blinded by the white and can't see anything.

 

- Use a dimmer flashlight!

 

There is no 'under the desk light test' to fail. Nearly all lenses will show specks of dust and little smudges. I think you're just being paranoid about nothing if you think all your lenses are 'faulty' in some way. Just use 'em, and only worry if the results are poor.

 

You're more likely to damage a coating or scratch a lens through over-cleaning, than by just leaving well alone. Especially if you're using a microfibre cloth and alcohol. A blower brush to remove loose dust/grit, followed by a gentle rub with a new lens tissue and a little huff of breath, is usually all it takes to clean a lens.

 

Don't touch the hairs of your blower-brush with fingers, it can leave grease on the brush. If the brush gets greasy it can be washed, like hair, using shampoo, a good rinse and blow dry.

 

Keep your lens cleaning materials - blower-brush, pack of lens tissues and cotton-buds, in a ziplock bag.

 

Incidentally, 1A skylight filters generally have a faint pink tint. That's not something I'd want permanently on my lenses. UV (0) or protector filters are neutral and don't add a colour tint.

Edited December 12, 2018 by rodeo_joe|1

After the previous test, I got out one of my least interesting (to me), and likely somewhat old, lenses.

 

It is a 28+35mm. Note that it is not 28-35mm (a zoom range), but only 28mm and 35mm.

 

Looking with the same flashlight, I did get somewhat blinded.

 

I think the skylight (1A) filter is single coated or not coated. The reflections off

lens elements aren't so bad, though. Maybe more than the Tamron above.

So I took the filter off.

 

Looking at this one from the front, I could see some small dust specks, but also

a faint fingerprint (part of one) which is on the back element. This reassures me

that I can see all the elements, and anything on (or off) of them.

 

Note that the effect of a fingerprint is often cancellation of the antireflection coating.

Adding grease a fraction of a wavelength thick will undo the cancellation that

gives it the antireflection property.

 

I am reasonably convinced that I can easily see things that make pretty much

no difference in image quality.

 

As above, a good coating should reflect about 0.5% or so.

That should not blind you with even. a pretty bright LED flashlight.

(Probably easier with a single LED, only one spot to follow.)

 

If the spots are brighter, it might be the 4% of an uncoated lens.

 

Yes, the 1A is slightly pink. In addition to absorbing UV, which causes a

blueness in many color films, they warm slightly. That was intentional.

The warming is small, and some people like it. I suspect it is less than

the step of the white balance knob on many DSLRs. It may have to do

with other properties of old color film sensitivity or dyes, but people tend

to like a little warming much more than a little cooling.

Update: finally had a chance to inspect the fisheye better and there is heavy "crud" on the internal filters. I'm going to pull the front element and clean. The filters are terrible looking. Best way to explain what I'm seeing: imagine if you had the lens near a stove where grease accumulates on everything. That's what it looks like. Very uniform, not smooth, and seemingly thick layer.

I find it most convenient to examine lens front elements under light from North facing window, rotating the lens and using a magnifier or reading glasses. Second choice is a fluorescent magnified desk / hobby light. Since I keep filters on all lenses, and use lens hoods as well, usually only the filters need cleaning, with possibly an occasional dust mote to be brushed off the actual front element. Haven't had to use lens tissue or cleaning solution (ex, rarely on the filters) for quite a few years.

Anti-reflection coatings depend on interference of reflections off layers 1/4 of the wavelength thick.

 

A grease or oil layer close to that thickness will negate the cancellation.

 

Normally inner lens surfaces stay clean, but there might be some sources of oil within the lens.

I had oil streak down the back of the front group on a 1:1.4 CS 35mm Mamiya. Their grease is awful - way, way too thick - in cold weather makes lens impossible to focus - opposite of Nikon F series.