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09-11-2013, 06:27 PM   #1
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Lens numbers.

I'm new and I'm sure some will get a laugh out of this.

I'm just wondering what the numbers mean on the lens. If it says Sigma 28-200mm F3.5-5.6 I understand what the 28-200 means. What does the F3.5-5.6 mean. Do the numbers mean a closer picture. How do I buy a lens when I don't know what those numbers mean.

Thanks for your help


09-11-2013, 07:02 PM   #2
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That's the f-stop a.k.a. aperture range. The 3.5 means that the maximum aperture at 28mm (the wide end) is F3.5, and the 5.6 means that the maximum aperture at 300mm (the long end) is F5.6.

The larger the f-stop number, the less light passes through the lens, and the deeper the depth of field. So smaller numbers like F2.8 or F4 are usually better; that's what you'll see on pro zooms.

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09-11-2013, 11:08 PM   #3
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One thing to add to what Adam has posted. The f-stop listed on the lens represents only the widest the lens can be opened up at that particular focal length. In other words, if you want to let in the maximum amount of light or create the shallowest depth of field possible, you would set the aperture to the widest (smallest number) possible for the focal length. That is the number on the barrel of the lens. You are not limited to that f-stop setting. You can always set the f-stop to a higher number - f/8, f/11, f/16, etc. You just can't go wider (smaller than f/3.5, etc) than what the lens is capable of.
09-12-2013, 07:13 AM   #4
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And to add a bit more to confusticate you still more, most lenses get their sharpest performance at around the f8 to f16 range.

09-12-2013, 10:11 AM   #5
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When it comes to buying a lens, it's easy:

f3.5 to f6.3 - something I could possibly afford
f2.8 - whoa!
f1.2 - must be a misprint.

"Understanding Exposure" by Bryan Peterson does a good job explaining how shutter speed, aperture and ISO affect your photos, and especially when it doesn't matter what aperture number to use.
09-12-2013, 11:38 AM   #6
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QuoteOriginally posted by Just1MoreDave Quote
f1.2 - must be a misprint.
Especially if it's under $300 - unless damaged in someway.
09-12-2013, 12:30 PM   #7
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Unfortunately, optics and exposure, have a highly mathematical & scientific basis.

While some people, on the other hand have no mathematical or scientific aptitude.

Photography is one example where we tend to try and impose one type upon the other
09-12-2013, 01:35 PM   #8
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Try This!

Hello Photogurl, Welcome to the Forum!
All the previous posts should be helpful, f/stops are difficult to understand at first. But it's important to realize that the 'triangle' or trinity of exposure (f/stop, shutter speed, ISO) is the basis of making photographs, as opposed to taking snapshots. It depends upon how serious you are about photography.
Some people learn more easily by visual means or 'hands on' than by reading or diagrams. If that's true for you, here's a tip;
Take the lens off your camera.
Take the lens cap off.
Look through the lens, either from the front or rear.
While looking through the lens, gently move the small lever that sticks out of the rear of the lens. It will open and close the diaphragm or 'iris' of the lens. You will notice that the movement has 'steps' or increments of movement, if you move it slowly. Each one of those steps is one f/stop.
The smallest opening is the highest number (like f/22).
The widest opening is the smallest number (like f/2.8 or f/3.5 or f/5.6, depending upon the lens). I know this sounds backwards, but there's a (lengthy) mathematical reason for it. Let's skip that part!
For each step (f/stop) you OPEN the lens, twice as much light comes in.
For each f/stop you CLOSE the lens, half as much light comes in.
It might be handy to know the f/stops. Starting with f/1.4 (a VERY wide opening), f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11.0, f/16.0, f/22.0, f/32.0. Most lenses also have a 'half-stop' between each of the numbers, we'll skip that part for now, also! It's enough to know the 'full' stops, the numbers already listed.
Hope this helps!

09-17-2013, 07:15 AM   #9
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Wow! A lot to take in. I do understand a little better. I found a guy at the photoshop that is really good. He's one of the few salespersons that enjoys his job and likes helping people. I'll take what I've learned her and have a talk with him.

Thanks again


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