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Author Topic: Understanding Lens specifications  (Read 342 times)

Dannzzigg

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Understanding Lens specifications
« on: August 05, 2018, 09:36:06 AM »

I've only recently begun exploring the world of DSLRs and photography, and I'm confused by one thing.

I have a T6i and the 18-55mm kit lens. The description of the lens says it's f/3.5-5.6

Does this mean that is the range of aperture choices I have when shooting with this lens? I don't think that's correct, as the camera allows me to select apertures all the way to f/22, so clearly there's something here I dont understand.

What is the f/3.5-5.6 referring to?

Thanks!
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Eric Myrvaagnes

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Re: Understanding Lens specifications
« Reply #1 on: August 05, 2018, 09:47:17 AM »

I believe the f/3.5 is the maximum aperture at the near end (18mm) of the lens 's range and f/5.6 is at the far end (55mm.) At in-between focal lengths the maximum aperture will be somewhere between f/3.5 and f/5.6.
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Dannzzigg

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Re: Understanding Lens specifications
« Reply #2 on: August 05, 2018, 10:59:52 AM »

Makes perfect sense. Thanks!
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Bitcohen

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Re: Understanding Lens specifications
« Reply #3 on: September 05, 2018, 09:13:27 AM »

I assume that you have used binoculars, and are familiar with the magnification they provide? When you are trying to photograph a subject that is distant, and you want it to fill more of the viewfinder, then you need a longer focal length lens.

Similarly, if you are trying to fit a subject into the viewfinder, but cannot fit it all in and cannot walk further back, then you need a shorter focal length.

When it comes to photographing subjects very close, so that small detail is visible, you need a lens with "macro" capability. You will find specialized macro lenses at a variety of focal lengths (50mm, 100mm) and also zooms with macro capability. Typically dedicated macro lenses will focus closer than zooms with macro capability.
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Two23

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Re: Understanding Lens specifications
« Reply #4 on: September 05, 2018, 09:41:00 AM »

The f3.5-f5.6 has to do with a ratio.  Let's break it down to it's simplest concepts.  Think of a lens as a tube with glass elements in each end.  The wider the diameter of the tube the more light goes through.  It's more than that though.  The length of the tube also determines how much light goes through.  Think of a piece of 2 inch diameter PVC pipe that's 4 inches long, and a second piece that's 2 feet long.  The shorter piece will be brighter to look through.  That's because of the RATIO of length to diameter.  To calculate how bright it is, you divide the length by the diameter.  Doing that with the short tube you get 4/2 = 2.  The ratio is 2.  For the long pipe it's 24 inches divided by 2 in., 24/2 = 12.  The ratio is 12.  The "f" on your lens that says "f=3.5"  is the the same concept.  Here, "f" means "ratio".  So with my pipe example, the first is f2, the second is f12.  Your lens is the same concept.  When it's shorter, it's f3.5.  You you zoom it out and the tube gets longer, it's f5.6.  You can't make the widest point in your lens wider, but you can constrict the light going through the tube with your aperture, which is what you're doing when dialing down to f22.  You are making the hole the light goes through smaller.  And finally, I don't recommend going any smaller than f16, and personally don't go tighter than f11.  That gets into diffraction which is a whole 'nother concept.


Kent in SD
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EricV

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Re: Understanding Lens specifications
« Reply #5 on: September 05, 2018, 05:03:49 PM »

What a horrible analogy :(  The diameter of a lens does determine how much light is collected, but the focal length of a lens does not determine how much light goes through, rather it determines how wide an area the collected light spreads across.

Everything should be made as simple as possible, but not simpler.  -- Albert Einstein
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JeanMichel

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Re: Understanding Lens specifications
« Reply #6 on: September 06, 2018, 11:59:53 AM »

Re f/x ratio
f = focal length
x - divider number
so:  focal length divided by x (1, 1,4, 2, etc.)

For a 50 mm focal length (regardless if it is a prime or a zoom setting), the diameter of the lens opening is 50 divided by x.
At f/1 the diameter is therefore 50 mm, at f/2 the diameter is 25 mm.
Now, basic geometry will inform you that dividing the diameter of a circle by 2 results in an area 4 times smaller; to only halve the resulting area you need to divide the diameter by the square root of 2, approximately 1.4.
An f/2 opening area is 4 times smaller than the area of the f/1 opening. To get an opening area that is only 2 times smaller (one stop in our parlance) than the f/1 opening area you have to divide f by the square root of 2 (1.4). So the one stop system we typically use goes: f/1, f/1.4. f/2, f/2,8, f/4, f/5.6 and so on.

The amount of light that enters through at a given f/x is the same for any focal length. The resulting diameter of f/x varies with the focal length:
At f/2
the diameter of a 50 mm lens is 25 mm
the diameter of a 70 mm lens is 35 mm
and so on

I imagine that it is easier to construct a zoom lens with a single aperture diameter for all the zoom settings. Since the diameter of the aperture remains the same, the value of x varies depending on the zoom setting.

To construct a zoom lens with a constant f/x, there must be a way to vary the diameter of the lens opening as the lens is zoomed in or out. The adds to the complexity of the manufacturing.
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Slobodan Blagojevic

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Re: Understanding Lens specifications
« Reply #7 on: September 06, 2018, 12:21:16 PM »

Why one simple answer when we can provide a dozen complicated ones? ;)

nirpat89

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Re: Understanding Lens specifications
« Reply #8 on: September 06, 2018, 08:13:05 PM »

Why one simple answer when we can provide a dozen complicated ones? ;)

+1

 :)
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NancyP

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Re: Understanding Lens specifications
« Reply #9 on: September 14, 2018, 04:36:33 PM »

There is no such thing as a de-geekification process.   ;D
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FranciscoDisilvestro

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Re: Understanding Lens specifications
« Reply #10 on: September 14, 2018, 05:39:44 PM »

Why one simple answer when we can provide a dozen complicated ones? ;)

 ;D ;D ... and we haven't even started to talk about entrance pupil and apparent lens diameter
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