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11-09-2009, 04:52 PM   #1
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Glass is glass right??

I have heard of some lenses referred to as "high contrast" and such, which totally eludes me. I don't understand how one lens can yield a high contrast photo and the other can yield a low contrast photo.


Thanks!

11-09-2009, 04:59 PM   #2
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Mostly has to do with the number & types of coatings.
11-09-2009, 05:06 PM   #3
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There's glass and then there's other glass!

QuoteOriginally posted by Raptorman Quote
I have heard of some lenses referred to as "high contrast" and such, which totally eludes me. I don't understand how one lens can yield a high contrast photo and the other can yield a low contrast photo.
Thanks!
There's many kinds of glass: glass is not all created equal.

Then there's how it's put together.

In some lenses, not all the elements are made of glass (e.g. Fluorite. quartz).

Some lenses only focus some of the wavelengths to a single plane, others more (apochromatic).

Lenses aren't created equal.

Last edited by Banjo; 11-09-2009 at 05:36 PM.
11-09-2009, 05:12 PM   #4
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also regarding the "high contrast" quality low contrast lenses are those prone to flare. i.e. if you look at a bright light source such as a window or a light through the lens it'll seem to illuminate the whole field of view. this may be particularly noticeable with a bright light but less noticeable lights will also illuminate improperly the field of view. this leads to low contrast. in film it was a bitch to fix. in digital it can be done easily but STILL means you lose quality.

11-09-2009, 05:53 PM   #5
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As noted above, glass is an extremely variable substance. The Wikipedia article is a good place to start:
Glass - Wikipedia, the free encyclopedia
The first section on production has a lot of good stuff related to how the physical and optical characteristics are affected by production methods and composition. In general, it is enough to say that the way that light is bent by a lens at different wavelengths is partially due to the formulation of the glass.

Beyond the basics, there are certain subtle factors like color rendition that are related to formulation as well.

Steve
11-09-2009, 07:06 PM   #6
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An uncoated flint glass surface reflects more than 4% the light incident on it. Therefore an uncoated lens in air reflects at least 8%. Many lenses have three groups so that's a total transmission of about (.92)^3=.78, so at least 22% of the light coming in would be refelcted. This reflected light goes every which way and decreases the lens' effective contrast...it adds a constant light background to the image, thereby decreasing contrast.

An individual multi-coated lens' surface might have 1/10 the reflection leading to less than 1/1000 the total reflection for a lens of three groups.

Dave in Iowa
11-09-2009, 07:23 PM   #7
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QuoteOriginally posted by Raptorman Quote
I have heard of some lenses referred to as "high contrast" and such, which totally eludes me. I don't understand how one lens can yield a high contrast photo and the other can yield a low contrast photo.


Thanks!
It refers to microcontrast. Compositional contrast has nothing to do with what lens you use.

Understanding Lens Contrast
11-10-2009, 01:24 PM   #8
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Look through clear window. You can see dark dark darks and light lights - large amount of contrast. Now look through a pair of sunglasses or other piece of darker colored glass. Everything is darker overall, for one thing, but even if you took a picture through it and lightened it, you'd find there just isn't as much difference between the lights and the darks when shooting through the wine bottle. That's because the glass used in sunglasses has much less contrast. And this is true at both the "global" level and the "micro" level.

That's proof the effect is real and easily verifiable. Of course, no one makes lenses out of the glass used in sunglasses (ok, polarizing filters excepted), but still, there is difference in the glass itself. And since lenses are made by stacking different pieces of glass together, that can also lead to differences.

11-11-2009, 09:30 AM   #9
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Even window glass has some differences. A few years ago I was building a passive solar heated, porch/sunroom on our old house and found out the differences in window glass. Most window glass has a high iron content and can block 30-40% of the light coming through, good for a normal house, not so good for solar heating. I got those figures from the guys trying to sell me the low iron glass windows so they may be exaggerated some but there is a difference in the quality of glass. Lens making is a precision craft. I saw that video on lenses on Discovery Channel's "How it's Made" and after watching it I'm surrprised our lenses don't cost even more.
11-11-2009, 10:20 AM   #10
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before we go too far debating glass is glass, I do want to point out that some elements might actually be plastic, or perhaps maybe this is not the best way to express non glass elements.

Also remember that glass (like in your windows) is not actually solid, It flows over time.
11-11-2009, 10:35 AM   #11
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+1's all around...

Equally, color saturation seems to be similar to lens contrast - some have it more than others.

A lot also has to do with lens design, materials used etc. The designer may tweak those transmission characteristics that the eye sees as contributing to contrast and/or saturation.

Finally, two lenses, using the same exposure of the same scene, will produce different histograms. One lens may have more to each end of the graph, another doesn't reach as far. And they may have different shapes, and different shapes yet to the R/G/B graphs.

One time I shot the same scene with the 16-45 and the 43. When I look at the picture, at first the 16-45 seems contrastier and more saturated. Yet, looking carefully, and looking at the histograms, the zoom is compressing at the extremes, shadows especially, and the 43 in fact has a greater range of tone.
11-11-2009, 11:20 AM   #12
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I'll add two comments. First, the comment about lens elements being made of plastic is true but also hybridized elements of partially finished glass molded into plastic to complete the lement have greatly improved the cheap end of the zoom market. It's true that glass to air surfaces have a big impact on the amount of light allowed to pass correctly thru a lens so bonding elements together with plastic can greatly improve things.

And secondly the quality of an image produced by a lens is affected not only by the type of glass used in the elements but how they are ground and finished. Many of the classic lens designs require curved element surfaces that are short-cutted by the manufacturers as flat surfaces. Many of the cemented surfaces should be curved but ground as flat. And the degree of surface finish applied to the glass elements has a big impact on quality of image. All of the glass elements are machine finished but the degree of finish varies widely.

All of the above explain a $100 kit zoom, a $1k Zeiss ZK prime and a $4k Leica prime.
11-11-2009, 11:29 AM   #13
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QuoteOriginally posted by Nesster Quote

...Finally, two lenses, using the same exposure of the same scene, will produce different histograms. One lens may have more to each end of the graph, another doesn't reach as far. And they may have different shapes, and different shapes yet to the R/G/B graphs...
Wow! These three short sentences say a ton! Many times people poo-poo the notion of different rendering from different lenses, but the histograms tell all.

Steve
11-11-2009, 12:00 PM   #14
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QuoteOriginally posted by stevebrot Quote
Wow! These three short sentences say a ton! Many times people poo-poo the notion of different rendering from different lenses, but the histograms tell all.

Steve
this is all about loss of contrast due to internal reflections.

most specifically a measure of the quality of the coatings.

for many years pentax was regarded as having absolutely the best coatings.
11-11-2009, 01:38 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
before we go too far debating glass is glass, I do want to point out that some elements might actually be plastic, or perhaps maybe this is not the best way to express non glass elements.

Also remember that glass (like in your windows) is not actually solid, It flows over time.
if you're referring the old stained glass windows on cathedrals it's been proven that they are thicker at the bottom due to the manufacturing process. they just put the thicker part on the bottom since putting it on the top obviously would make the window more fragile. glass IS considered solid nowadays - an "amorphous solid" since it doesn't form crystalline structures.

also, I think glass elements can only be found on the cheapest of compacts. If they could make good quality glass elements it'd be a breakthrough and the end of heavy lenses.
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