Lenses T-Stop / Uncoated Lenses.

[Moriturii]

1)So I've been thinking, how well does an incident light meter work or even an hand held reflective meter when a consumer lens is measured in F stops instead of T stops? How do we know that a Summicron 50mm f2 at f2 really is f2 and not more like f2.8 due to it having glass elements that such up a portion of the light and how different does this effect old uncoated lenses and the most modern lenses that are multi coated? How does this effect measuring and metering light in a scene regards to a hand held incident and reflective meter?

I was checking the metering on my Hexar RF and it was 1-2 stops under or overexposing depending on where I was pointing it and measuring light with the internal light meter (a even lit wall, wooden floors, a lit lamp etc) and then testing my Gossen Digisix and saw what it said. After being puzzled for a few days I took out the lens, put in front of my gossen digisix and measured again and lo' finally I was getting same readings as I did with Hexar RF's internal meter.

2)And also, modern lenses are so very contrasty, is that why people say "overexpose and under-develop in sunny scenes"? And will this rule be thrown away if one uses an uncoated lens such as, I don't know, Elmar 5cm f3.5 etc that doesn't give such extreme contrast in pictures with deeeeep shadows?

Just some random thoughts in my heads, anyone willing to shed some lights on these random points? Not sure how to formulated these ideas into a post that makes sense.

Discuss!

 

[Roger Hicks]

T-stop to f/stop differences are usually trivial, rarely more than 1/3 stop. Uncoated lenses are 'faster' in the sense that veiling flare lightens shadows, but 'slower' in that they lose more light reflected back out of the front. Meters vary widely, especially if their 'weighting' is different. It's all inside the range of normal experimental error in photography, the reason why some people give a little more exposure or development, while others give a little less. Don't look for more precision in a system than exists. Some people look for more precision than can exist!

Cheers,

R.

 

[Sparrow]

One of the few things I learned over the years is not to compare light meters ... they never agree

 

[rxmd]

Just look at your usual lens diagram and count the glass/air surfaces. Approximate loss is 4% for an uncoated glass/air transition, 2% for a single-coated one and 0.5% for a coated one.

Now remember that an f/stop translates to a 50% loss of light. For that you need about 15 uncoated glass/air transitions. So unless your uncoated lens has elements in more than eight groups, your transmission loss will be less than an f/stop. So no reason to be afraid that your f/2 lens is actually T2.8. In fact, most lenses don't have more than four groups, so if you have an entirely uncoated lens, the maximum loss is about 20%, or less than a third of a stop. In other words, don't worry.

 

[Moriturii]

Very interesting, hmm, thank you!

 

[menos]

Dante Stella has written a very interesting article for the uninitiated about this very topic.
I enjoyed reading it back then, but can't find it around.

 

[rxmd]

Dante Stella has written a very interesting article for the uninitiated about this very topic.
I enjoyed reading it back then, but can't find it around.

This one: http://www.dantestella.com/technical/fast.html?

 

[menos]

This one: http://www.dantestella.com/technical/fast.html?

Yes ;-)


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