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09/10/2007 11:40:35 AM · #1 |
Okay, I understand that the stop is the ratio of focal length to actual aperture diameter.
Hence, F/4 on a 100mm lens means the opening is actually 25mm in diameter.
Fine. But this raises two questions for me:
1) My 70-200 has a constant max. aperture of F/2.8. But when I look through into it from the open end toward the camera, it's all the way open at both 70 and 200mm: no aperture blades are visible at either end. Thus, the aperture is the same diameter. So how can the f-stop remain constant?
2) Why aren't superwide angles ridiculously large for max. aperture? I mean, my Sigma 10-20 has a max. aperture of only F/4, or 2.5mm actual aperture size. With that small a focal length, it seems it should be easy to have a much larger aperture. For example, my Tamron 17-50 is about the same physical size (the lens itself), and it has a max. of F/2.8. At 50mm, that's 18mm, which on the other lens at 10mm would yield an f-stop below 1.0.
I'm sure I'm just ignorant of something related to lens building.
Can anyone enlighten me on these two questions?
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09/10/2007 11:42:50 AM · #2 |
| It may have to do with the distance the light has to travel from the glass to the shutter. I would say that a shorter lens, doesn't need to open as much to let as much light in, because it is such a short distance? I dont know though. Good questions. |
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09/10/2007 11:50:30 AM · #3 |
Originally posted by levyj413:
Okay, I understand that the stop is the ratio of focal length to actual aperture diameter.
Hence, F/4 on a 100mm lens means the opening is actually 25mm in diameter.
Fine. But this raises two questions for me:
1) My 70-200 has a constant max. aperture of F/2.8. But when I look through into it from the open end toward the camera, it's all the way open at both 70 and 200mm: no aperture blades are visible at either end. Thus, the aperture is the same diameter. So how can the f-stop remain constant?
2) Why aren't superwide angles ridiculously large for max. aperture? I mean, my Sigma 10-20 has a max. aperture of only F/4, or 2.5mm actual aperture size. With that small a focal length, it seems it should be easy to have a much larger aperture. For example, my Tamron 17-50 is about the same physical size (the lens itself), and it has a max. of F/2.8. At 50mm, that's 18mm, which on the other lens at 10mm would yield an f-stop below 1.0.
I'm sure I'm just ignorant of something related to lens building.
Can anyone enlighten me on these two questions? |
The aperture would be 25mm in AREA, not diameter.
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09/10/2007 11:55:19 AM · #4 |
Originally posted by HawkeyeLonewolf:
Originally posted by levyj413:
Okay, I understand that the stop is the ratio of focal length to actual aperture diameter.
Hence, F/4 on a 100mm lens means the opening is actually 25mm in diameter.
Fine. But this raises two questions for me:
1) My 70-200 has a constant max. aperture of F/2.8. But when I look through into it from the open end toward the camera, it's all the way open at both 70 and 200mm: no aperture blades are visible at either end. Thus, the aperture is the same diameter. So how can the f-stop remain constant?
2) Why aren't superwide angles ridiculously large for max. aperture? I mean, my Sigma 10-20 has a max. aperture of only F/4, or 2.5mm actual aperture size. With that small a focal length, it seems it should be easy to have a much larger aperture. For example, my Tamron 17-50 is about the same physical size (the lens itself), and it has a max. of F/2.8. At 50mm, that's 18mm, which on the other lens at 10mm would yield an f-stop below 1.0.
I'm sure I'm just ignorant of something related to lens building.
Can anyone enlighten me on these two questions? |
The aperture would be 25mm in AREA, not diameter. |
No I believe it is diameter //www.uscoles.com/fstop.htm |
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09/10/2007 12:03:00 PM · #5 |
Right - it's diameter, not area.
But getting back to my original questions ...
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09/10/2007 12:03:30 PM · #6 |
| From what I understand it's the size of the aperture as seen by the rear element of the lens that counts and not the absolute size of the hole in the aperture blades. |
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09/10/2007 12:04:48 PM · #7 |
| This thread should be interesting. Optics is not an easy subject. I researched it briefly when I was finding out about telescopes and eyepieces. Figuring out one or two lenses was pretty tough... but when you're talking about camera lenses with 20 internal elements, things get tricky... :) |
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09/10/2007 12:06:18 PM · #8 |
I bet the answer lies in this paragraph from Erick's link...
I took my lens apart. The aperture is nowhere near as big as the calculation shows. What's up?
You're right. I had an email from a guy who had taken apart a Rokkor 300mm f/4.5 (for other reasons, not to check my measurements) and he said the diameter of the f/stop blades was way smaller than the calculation would indicate. The calculations above would be accurate if the aperture blades were mounted right in front of the front element. In fact, they're buried in the lens somewhere and, on the Nikkor 300mm f/4.5 IF-ED I used to own, were actually located behind all the lens elements. They still have the same relationship but the manufacturer can make the aperture blades way smaller in the light path partway back. However, the relationship is the same between each of the adjacent stops.
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09/10/2007 12:09:08 PM · #9 |
Originally posted by Raziel:
From what I understand it's the size of the aperture as seen by the rear element of the lens that counts and not the absolute size of the hole in the aperture blades. |
I stand correct -- err... reminded. I was thinking of the area of the aperture created which is what halves/doubles.
Back to our show...
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09/10/2007 12:27:18 PM · #10 |
err, i completely misread
Message edited by author 2007-09-10 12:27:54. |
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09/10/2007 01:07:02 PM · #11 |
Originally posted by DrAchoo:
I bet the answer lies in this paragraph from Erick's link... |
Okay, that explains that the actual aperture size can change depending on where the blades are. Maybe that's related to why, even at 10mm, the max. aperture is only F/4.
But it still doesn't address how a lens can go from 70-200 and keep an aperture of F/2.8. I mean, the aperture should be getting bigger by a factor of 20/7 over the course of that zoom to keep the f-stop the same.
And here's an even weirder thing. My Sigma has a max. of F/4 at 10mm and 5.6 at 20mm. At 10mm, F/4 yields an aperture of 2.5mm (or something scaled depending on location within the lens). but at 20mm, F/5.6 (the max. aperture at that focal length) yields an aperture of 3.6mm, so the physical hole is actually getting BIGGER (in fact, nearly doubling in area) even as the f-number is saying there's 1/2 the light.
Curiouser and curiouser.
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09/10/2007 01:38:00 PM · #12 |
Originally posted by levyj413:
But it still doesn't address how a lens can go from 70-200 and keep an aperture of F/2.8. I mean, the aperture should be getting bigger by a factor of 20/7 over the course of that zoom to keep the f-stop the same. |
It *is* the apparent aperture size, which means the size of the aperture as "seen" by the camera. The intervening optical elements may make the "apparent aperture" appear either larger or smaller than the physical opening. As a lens zooms, the relationship of the physical opening to the optical elements can change, further complicating things. |
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09/10/2007 01:51:12 PM · #13 |
Take 4 toilet paper tubes. Tape 3 together end to end. Look through the single one. Look through the combined one. See a difference? But the width of the opening is the same on each...
Simplified, I know, but the same basic principle, isn't it? |
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09/10/2007 02:14:53 PM · #14 |
Originally posted by BeeCee:
Take 4 toilet paper tubes. Tape 3 together end to end. Look through the single one. Look through the combined one. See a difference? But the width of the opening is the same on each...
Simplified, I know, but the same basic principle, isn't it? |
That's the best explanation I've seen yet for why the hole size isn't as relevant as the ratio of focal length to hole size. Thanks!
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09/10/2007 02:49:16 PM · #15 |
Originally posted by levyj413:
And here's an even weirder thing. My Sigma has a max. of F/4 at 10mm and 5.6 at 20mm. At 10mm, F/4 yields an aperture of 2.5mm (or something scaled depending on location within the lens). but at 20mm, F/5.6 (the max. aperture at that focal length) yields an aperture of 3.6mm, so the physical hole is actually getting BIGGER (in fact, nearly doubling in area) even as the f-number is saying there's 1/2 the light.
Curiouser and curiouser. |
Shorter range lense has a wider view angle and thus more light comes in.
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