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10-30-2012, 03:10 PM   #1
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Aperture compared to glass size

A friend and I were having a discussion on aperture and we couldn't seem to reach an agreement.

He says that a fast lens with a larger aperture just means that it has more glass, and I argued that a fast lens means it has a larger hole created by the blades, which does have a relationship to the glass size, eg. my fa50 1.4 doesn't have enough glass to open up to 1.2.

But he said that different lenses could have the same aperture with different size glass, eg his 1.4 could be bigger than mine, and therefore faster...

I think he's wrong but he brought up a good point - an iPhone has a 2.4 aperture but its much smaller than 2.4 on my DSLR... How is that possible?

Thanks,
Daniel

10-30-2012, 03:28 PM   #2
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f/1.4 on one lens is theoretically just as fast as any other f/1.4 lens, one wouldn't be faster because it has more glass. Aperture is related to focal length. The f in f/x is for focal length. I'm a little fuzzy on the actual math but it's something along the lines of, focal length, divided by f number (say 1.4), eguals aperture size.
A 200mm f/2.8 lens will therefore have an aperture 4x larger than a 50mm f/2.8 lens.
The iphone lens has a tiny sensor, therefore a very, very short focal length, therefore a very tiny aperture.
10-30-2012, 03:33 PM   #3
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The f-number is a fraction (focal length/iris opening diameter). That is why it is always written as f/<some number>. In theory, for a given focal length a smaller f-number should translate to larger glass. Unfortunately, that is not quite true. Consider two lenses that I own. My Vivitar 28/2.5 (Kino) has a huge front element that requires a 67mm filter. Compare that to my Tamron 28/2.5 which has a tiny front element and takes 49mm filters. Same focal length, same registration, same maximum aperture, different optical designs.


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10-30-2012, 03:52 PM   #4
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I am lazy, here. F-number - Wikipedia, the free encyclopedia

10-30-2012, 03:58 PM   #5
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Just a comparison, the iPhone has around a 4mm lens, it only needs a 1.7mm aperture opening to achieve f2.4. While the Pentax DA 35mm f2.4 need an aperture opening of around 15mm.
10-30-2012, 07:27 PM   #6
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And for a given focal length, each whole f number (ex, 1, 1.4, 2, etc.) will represent the area of a circle, decreasing in size-

(FL/fn x 0.5 (to get the radius) squared) x Pi = radius of f number circle. For a 50mm lens, F1 has an area of about 1963.5 sq mm, f1.4 has an area of about 1001.75 sq mm and f2 an area of about 491 sq mm. Each time the area is cut in about half, and so is the light coming through the opening. This is just the aperture opening. As was stated, the lens design will determine how big the lens elements need to be.

And we shall see if glass is still an important part of "lens" making in the not too distant future. Scientists have now developed a silicone overlayed with gold lens that is escentially FLAT! Makes it easier to get rid of those nasty distortions! The future is coming, hang on tight!
10-30-2012, 07:38 PM   #7
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So the aperture relates to the size of the opening created by the blades, not by the size of the glass?

And a "fast lens" is called a fast lens because it has a larger maximum aperture opening (smallest possible f number), and doesn't have anything to do with the amount of glass, the quality of the glass, or the lens design?

This is how I understood it but my friend thinks that a "fast lens" is fast because it has more glass.
10-30-2012, 08:44 PM   #8
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QuoteOriginally posted by djmundy Quote

This is how I understood it but my friend thinks that a "fast lens" is fast because it has more glass.
You are right but ..... faster lenses are generally more expensive, often use better glass elements and usually need bigger (front)elements and this is because they have bigger apetures.

I guess your friend will be the one buying the drinks tonight then.

10-30-2012, 09:55 PM   #9
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A 2X tele-converter on a 2.8/50 is equivalent to 4/100. After remove the TC from 4/100, the lens is 2.8/50; 2 stops faster, but less glass. So we see lens speed also depends on what is behind the front element (front element size itself does not determine lens speed). Zooms are like that compared to primes. Generally, however, a faster lens has bigger elements, and therefore more glass.

Lens 'speed' (smallest f/#) is measured with the aperture blades completely retracted, no?

So, technically, both of you are both correct and incorrect, about equally afaik.
10-30-2012, 10:01 PM   #10
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Is it possible for one 50mm f2.8 to be faster than another 50mm f2.8 because the size of the front glass increases the intensity of light delivered to the sensor?
10-30-2012, 10:53 PM   #11
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QuoteOriginally posted by djmundy Quote
Is it possible for one 50mm f2.8 to be faster than another 50mm f2.8 because the size of the front glass increases the intensity of light delivered to the sensor?
Two f/2.8 lenses both deliver light of equal intensity to the sensor.
10-30-2012, 11:05 PM   #12
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QuoteOriginally posted by rhodopsin Quote
Two f/2.8 lenses both deliver light of equal intensity to the sensor.
In a perfect world they would be identical, but the numbers are not exact, there is usually rounding involved. My Pentax M 35mm f2.8 and my Tokina made Bushnell 35mm f2.8 aren't identical, my Pentax is slightly wider, and about 1/3 stop faster than the Bushnell.
10-31-2012, 04:46 AM   #13
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QuoteOriginally posted by elliott Quote
In a perfect world they would be identical, but the numbers are not exact, there is usually rounding involved. My Pentax M 35mm f2.8 and my Tokina made Bushnell 35mm f2.8 aren't identical, my Pentax is slightly wider, and about 1/3 stop faster than the Bushnell.
Makes sense to me. Yes the f number is a ratio between focal length and aperture, but optics do also play a part in how much light reaches the sensor - as well as several other factors (otherwise Pentax couldn't charge so much for putting a splat * on certain lenses). Fortunately the light lost to the optics is reasonably static, so the f values still do have a relationship to each other within a particular lens.
10-31-2012, 06:09 AM   #14
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Which brings up another related subject, "T" stops. These are "calibrated" f stops so that an f2.8 lens and another f2.8 lens will pass the same quantity of light. I believe you need to send your lenses in to have the calibration done, or at least you are provided with the exact f values for each f stop position. Most of us live quite easily withthe f numbers provided on the lens.

T stops are used, or were used, quite a bit in the motion picture industry where it was important to make certain the light levels on the film, scene to scene, were the same. Now with post production and digital it becomes less important.

And the argument of wider opening or more glass is like the chicken and the egg question. Did the lens get bigger glass because the aperture was bigger or did the aperture get bigger because there was bigger glass? But pragmatically, what company, to keep profits up, would have a big piece of glass up front if they only went to f4? It would be a waste of time and energy. Getting larger lenses ground properly is more expensive 'cause if you screw it up it has to get recast, reground, etc. A lot of waste compared to a smaller element lens. You can cast more small element lenses in a single casting cycle than you can the larger lens elements.It just becomes a matter of real estate in the casting oven!

Regards,
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