Why coating on the front element?

[Joe]

I'm confused. Lens coatings are designed to reduce reflections, right? They don't have any effect on the light that goes through the lens, they just cancel out reflected frequencies, reducing the amount of stray, unfocused light that bounces around and reduces contrast. So what is the point of having coating on the front of the lens? The light that is reflected off of the front element isn't going into the lens, so why does it need to be coated? Is it just so it looks pretty?

 

[fbf]

protection,filter out light source... i am purely guessing.

 

[antiquark]

AFAIK, the entire lens is coated in a furnace, so both the back and front surfaces would be coated at the same time.

Also, the front surface needs to be coated to avoid reflections from filters.

 

[raid]

Uncoated lenses are preferred for B&W photography to get more shadow details. Coated lenses seem to give better color saturation.

 

[jb17kx]

Coatings on the front element also help to cut down on flare and ghosting - in conjunction, naturally, with the rest of the internal coatings.

 

[sanmich]

I remember I had a conversation about this with a repairman.
The only logical reason for coating the front seems to be to cut out reflections from the filter.
of course, both sides of a lens are coated together so, that's the way it goes.
But this also means that bad coating has no affect on image as long as you don't put a filter on....

 

[Rob-F]

Uncoated lenses are preferred for B&W photography to get more shadow details. Coated lenses seem to give better color saturation.

I wouldn't go that far. There has been an example or two of single coated lenses being preferred to multicoated ones for B&W. But uncoated lenses? I think they are preferred only for "glow." Low contrast shots with veiling glare.

Raid, I have a feeling that's what you meant.

 

[Roger Hicks]

An uncoated lens typically reflects around 10% of the light falling on it. Why throw that light away? Sure, it's trivial -- but why throw any light away?

Cheers,

R.

 

[sanmich]

An uncoated lens typically reflects around 10% of the light falling on it. Why throw that light away? Sure, it's trivial -- but why throw any light away?

Cheers,

R.

Roger,
I think the issue is not about throwing away good light. If my calculations are right, 10% is just 20% of a full stop, which is neglectible for negative and any TTL camera compensates for it.
I'm not sure about the explanation but the issue is internal reflection. In any lens the light is bouncing on each glass surface, creating what can be describe as a white noise, a basic enlightment of the film, thus lowering contrast. If the lens is coated, much less light is bouncing on glass, and we get a higher contrast.

 

[Roger Hicks]

Roger,
I think the issue is not about throwing away good light. If my calculations are right, 10% is just 20% of a full stop, which is neglectible for negative and any TTL camera compensates for it.
I'm not sure about the explanation but the issue is internal reflection. In any lens the light is bouncing on each glass surface, creating what can be describe as a white noise, a basic enlightment of the film, thus lowering contrast. If the lens is coated, much less light is bouncing on glass, and we get a higher contrast.

Dear Michael,

Yes, I said it's trivial.

But if you're coating the back surface, it's just as easy to coat the front as well -- so why not do so?

Also, as others have pointed out, put a filter on, and the front glass becomes an internal element.

In other words, the question is, "Why NOT coat the front surface?" At the dawn of coating, some lenses were coated only internally, because the coatings were so fragile. But most modern coatings are as tough as, or tougher than, the glass itself.

Cheers,

R.

 

[sanmich]

Dear Michael,

Yes, I said it's trivial.

But if you're coating the back surface, it's just as easy to coat the front as well -- so why not do so?

Also, as others have pointed out, put a filter on, and the front glass becomes an internal element.

In other words, the question is, "Why NOT coat the front surface?" At the dawn of coating, some lenses were coated only internally, because the coatings were so fragile. But most modern coatings are as tough as, or tougher than, the glass itself.

Cheers,

R.

Well pretty obvious indeed. You're perfectly right.
So, let's hijack the thread and ask the only question that is not trivial on the topic:

Why are we so concerned when that not-so-usefull front coating is getting some marks?
My answer: because most of us belong to the AR club (i let you guess what is AR) and we can't bear the idea that our precious jewels can be less than perfectly, totally, absolutely, cosmically, perfect. 🙄

And I am president of the club :bang:

 

[J J Kapsberger]

Coating improves light transmission. It minimizes the amount of light that's reflected away or scattered. Coating therefore helps to maximize contrast, which improves sharpness. Each layer of coating might be trivial, as you reckon, but it does add to the cumulative improvement.

There's a great difference between glasses that have AR coating and those that don't. Such a difference on only two surfaces tells me that every coating in a multi-element arrangement, including the coating on the outer front element, is significant.

 

[hlockwood]

Dear Michael,

Yes, I said it's trivial.

But if you're coating the back surface, it's just as easy to coat the front as well -- so why not do so?

Also, as others have pointed out, put a filter on, and the front glass becomes an internal element.

In other words, the question is, "Why NOT coat the front surface?" At the dawn of coating, some lenses were coated only internally, because the coatings were so fragile. But most modern coatings are as tough as, or tougher than, the glass itself.

Cheers,

R.

Typical glass reflection coefficient is about 4%, not 10%. But an anti-reflection (AR) coating on the front surface works both ways. If there is any light reflected toward the front element from internal surfaces, it exits the lens, through the AR coated surface, without adding further reflections.

Also, most coatings are sputtered from a source that sees only one side of the lens at a time. So, I suspect that coating both sides consists of two separate steps, the extra step being considered worthwhile from both a mechanical and an optical perspective.

Harry

 

[sanmich]

Funny,
I meant A nal R etentive...Not Anti Reflection😉

 

[VinceC]

I read in several very old magazines that, before lens coating was common, many photographers preferred well-aged old lenses that had spent time in bars and the such because they had developed a natural coating.

There is a lot of reason to not have a shiney front lens. Along with more efficiency, there is the fact that light bouncing off the front of the lens will then be reflected onto the rim, onto the inside of the shade, onto all sorts of things.

I wear eye-glasses (a single lens if you don't count my bifocals) and i've had them coated since the early 1980s. I can attest that it makes a spectacular difference -- no flare, no unwanted reflections. You can try this yourself by looking through an uncoated piece of glass in different harsh light conditions, especially back-light.

 

[raid]

I wouldn't go that far. There has been an example or two of single coated lenses being preferred to multicoated ones for B&W. But uncoated lenses? I think they are preferred only for "glow." Low contrast shots with veiling glare.

Raid, I have a feeling that's what you meant.

Rob,

Yes, you correctly rephrased what I had on my mind.
Single coating versus multi-caoting.
Non-coated lenses are for a "look".

 

[Roger Hicks]

Typical glass reflection coefficient is about 4%, not 10%. But an anti-reflection (AR) coating on the front surface works both ways. If there is any light reflected toward the front element from internal surfaces, it exits the lens, through the AR coated surface, without adding further reflections.

Also, most coatings are sputtered from a source that sees only one side of the lens at a time. So, I suspect that coating both sides consists of two separate steps, the extra step being considered worthwhile from both a mechanical and an optical perspective.

Harry

Dear Harry,

Having checked the figures, rather than relying on increasingly fallible memory, I find you're absolutely right about the former as a minimum figure (reflection 4% at refractive index 1.50), though the higher the R.I. of the glass, the more the light loss; thus, R.I. 1.85, reflection loss (uncoated) 9.00%. And of course, the higher the R.I., the more effective a single, old-fashioned magnesium fluoride coating.

For the second, ('AR works both ways') your logic is equally indisputable.

For the third, I'm not sure that 'sputtered' is an adequate description, but I'm thinking of (a) vacuum coating chambers I've seen, and (b) electron micrographs of modern hard-coated lenses. I had the impression from the former that both sides were coated simultaneously, but I could well be wrong. Even if I am wrong, the extra cost of coating during manufacture is trivial.

In case I give the wrong impression, of dismissing your points, I'd add that this is the last thing I intended to do. You have corrected me, and I am more than happy to acknowledge this.

Cheers,

R.