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Thread started 10 Feb 2007 (Saturday) 14:27

# F-question?

Feb 10, 2007 14:27 |  #1

I am reading Bryan Peterson's Understanding Exposures. Great book, and I also found some answers about F numbers, that I have been wondering about for a while.

He says:

The diamteter of the aperture is the focal lenght divided by the F number.

50mm F2 = 25mm aperture diameter. This Sounds resonable.

How about 16mm F22? Aperture diameter of 0.72mm!!!

Can this be true?

Espen

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Feb 10, 2007 15:28 |  #2

Why shouldn't it?

--Alex Editorial Portfolio
|| Elan 7ne+BG ||5D mk. II ||1D mk. II N || EF 17-40 F4L ||EF 24-70 F2.8L||EF 35 1.4L || EF 85 1.2L ||EF 70-200 2.8L|| EF 300 4L IS[on loan]| |Speedlite 580EX || Nikon Coolscan IV ED||

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Feb 10, 2007 15:38 |  #3

Pretty simple and straight forward once it sinks in.

Not sure why, but call me JJ.
I used to hate math but then I realised decimals have a point.
.
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Feb 10, 2007 15:44 |  #4

EspenW wrote in post #2686338
I am reading Bryan Peterson's Understanding Exposures. Great book, and I also found some answers about F numbers, that I have been wondering about for a while.

He says:

The diamteter of the aperture is the focal lenght divided by the F number.

50mm F2 = 25mm aperture diameter. This Sounds resonable.

How about 16mm F22? Aperture diameter of 0.72mm!!!

Can this be true?

Espen

Yep! (unless maths changed since I left school many years ago )

-- K e v i n --

Nikon D700, 17-35mm, 28-105mm, 70-200mmVR, 50mm f/1.4
Canon EOS 3, 24-105L, 135L

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Feb 10, 2007 15:45 |  #5

http://www.uscoles.com​/fstop.htm

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Feb 10, 2007 16:30 |  #6

EspenW wrote in post #2686338
I am reading Bryan Peterson's Understanding Exposures. Great book, and I also found some answers about F numbers, that I have been wondering about for a while.

He says:

The diamteter of the aperture is the focal lenght divided by the F number.

50mm F2 = 25mm aperture diameter. This Sounds resonable.

How about 16mm F22? Aperture diameter of 0.72mm!!!

Can this be true?

Espen

And now you understand where the topic of aperture-induced diffraction- limited lens performace has its roots!

You need to give me OK to edit your image and repost! Keep POTN alive and well with member support https://photography-on-the.net/forum/donate.p​hp
Canon dSLR system, Olympus OM 35mm system, Bronica ETRSi 645 system, Horseman LS 4x5 system, Metz flashes, Dynalite studio lighting, and too many accessories to mention

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Feb 10, 2007 16:53 |  #7

Thanks everybody.
I am a newbe, and I am learning new things every day.

It's true, of cource.
Like mentioned here, I just needed some time to let it sink in. The uscoles-document is good.
I understand it now, but I still think it's impressive.

This forum is a dream!

e

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Feb 10, 2007 17:10 |  #8

EspenW wrote in post #2686909
This forum is a dream!

e

Until you start buying glass. Then this forum is the Devil (to your wallet)

-----------
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...and some more stuff:
Current Gear List (and yet still hungry for more Toys!:o)
https://photography-on-the.net …p?p=2666482&pos​tcount=446

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Feb 10, 2007 17:26 |  #9

Are you yoking me man?

Bough my first SLR a few moths back.

Ohlala....I want so many lenses and other equipment already.

Espen

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Feb 10, 2007 17:33 |  #10

Go to your local camera store and play around with the 16-35L (or similar). Mount the lens on your camera and select an aperture of f/22. Point the camera straight at you, look squarely into the lens opening. The lens will be stopped at 2.8 by default. Push and hold the depth of field preview button, and witness something very cool.

Now it should sink in.

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Feb 11, 2007 01:23 |  #11

I just did that with my 24-105.
Now I can see with my own eyes, what a 1.09mm aperture looks like.

Cool!

E

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Feb 11, 2007 01:33 |  #12

EspenW wrote in post #2688941
I just did that with my 24-105.
Now I can see with my own eyes, what a 1.09mm aperture looks like.

Cool!

E

Keep in mind what a 'f-stop' means in terms of practical photography. Each full f-stop changes the amount of light let through the lens by a factor of two. In other words, double or half.

f/2.8 lets in half as much light as f/2.
f/4 lets in half as much light as f/2.8, and hence a quarter as much as f/2.

Going further, we note that f/22 lets in 128 times less light than f/2! Does it still surprise you about the aperture size at f/22?

Col | Flickr

Sony A7 + Leica 50 Lux ASPH, Oly E-M5 + 12/2
Canon 5D3, 16-35L, 50L, 85L, 135L

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Feb 11, 2007 05:37 |  #13

No, I understand this now. It's only a ratio. Smaller hole, longer exposure time, and vice verca.

What suprices me, is that a huge wide angle picture, full of colors and details can be exposed threw a 0.72mm pinhole.

e

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Feb 11, 2007 07:07 |  #14

The diamteter of the aperture is the focal lenght divided by the F number.

50mm F2 = 25mm aperture diameter. This Sounds resonable.

How about 16mm F22? Aperture diameter of 0.72mm!!!

Can this be true?

Almost true, but not quite. The f/# is the ratio of the inlet pupil to the focal length. The actual aperture diaphragm inside the lens is not necessarily the same diameter as the inlet pupil, all of which is governed by optics. A 50mm f/1.0 L does not necessarily have a 50mm maximum aperture diaphragm. It has a big enough aperture diagphragm to form a 50mm inlet pupil.

Small technicality.

My personal stuff:http://www.flickr.com/​photos/jngirbach/sets/
I use a Canon 5DIII and a Sony A7rIII

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Feb 11, 2007 09:27 |  #15

EspenW wrote in post #2689464
What suprices me, is that a huge wide angle picture, full of colors and details can be exposed threw a 0.72mm pinhole.
e

Time to have some fun. Build yourself a pinhole camera. not quite the same thing as whats being discussed here, but you can get real nice landscape pictures through a little tiny pinhole with no glass at all.

Not sure why, but call me JJ.
I used to hate math but then I realised decimals have a point.
.
::Flickr::
::Gear::

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F-question?
AAA
 x 1600 y 1600

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