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12-09-2009, 02:57 AM   #1
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Please help with F Stop vs MM

Hi guys, I have yet another question this world of photography is amazing too bad I had a late start but you know what they say, better late then never. okay my question is this

My K-x lens says 18-55mm 1:3.5-5.6

1 - why can't I get 3.5 @ 55mm? are those considered more expensive lenses?

2 - I thought as the "mm" or focal length increases, you get a narrower dof. So why doesn't the F Stop drop as well instead of going up? To me it seems it should say on my lens 18-55mm 1:5.6-3.5 NOT 18-55mm 1:3.5-5.6.

These questions been bugging me, any help would be appreciated, thanks!

also real quick - I am guessing, if you have like a 500mm telephoto lens, you don't really need a low F Stop number right? Because the zooming in by itself will give a nice blurry background, please correct me if i'm wrong


Last edited by boodiespost; 12-09-2009 at 03:07 AM.
12-09-2009, 03:07 AM   #2
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QuoteOriginally posted by boodiespost Quote
Hi guys, I have yet another question this world of photography is amazing too bad I had a late start but you know what they say, better late then never. okay my question is this

My K-x lens says 18-55mm 1:3.5-5.6

1 - why can't I get 3.5 @ 55mm? are those considered more expensive lenses?

2 - I thought as the "mm" or focal length increases, you get a narrower dof. So why doesn't the F Stop drop as well instead of going up? To me it seems it should say on my lens 18-55mm 1:5.6-3.5 NOT 18-55mm 1:3.5-5.6.

These questions been bugging me, any help would be appreciated, thanks!
1. The kit lens, along with most consumer-grade zooms, are variable aperture by design to simplify the lens design and make it cheaper to produce. The f-number is actually a fraction, IIRC it's the iris diameter divided by the focal length. So if the lens has to maintain a constant aperture then the iris diameter has to increase in size as you zoom in, making the lens more complicated and expensive. The DA* 16-50, Tamron 17-50 and Sigma 18-50 are examples of constant f/2.8 zooms in the similar focal length range.

2. As mentioned the f-number is based on the physical design of the lens. The DOF you get is a result of the aperture and focal length used, among other factors. I'm not familiar with the math, but I do think 55mm, f/5.6 has shallower DOF than 18mm, f/3.5.

Hope I got it right, and that it helps you out
12-09-2009, 05:06 AM   #3
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Couple of things to clarify. As the aperture number increases, the size decreases. For example, f/5.6 is a smaller opening than f/2.8. Depth of field increases (more in focus) as the number increases and the opening gets smaller. Depth of field decreases (less in focus) as the number decreases and the opening gets larger.

The actual amount that is in focus is dependent on the focal length used (mm), the aperture, and the distance to subject. Quick example, a 50mm lens with an aperture of f/22 at a distance to subject of 2" has less DOF than a 100mm lens with an aperture of f/2.8 at 20ft.

A handy calculator here Online Depth of Field Calculator. Also others out there.
12-09-2009, 10:26 AM   #4
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thank you all for the helpful information

12-09-2009, 10:27 AM   #5
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QuoteOriginally posted by boodiespost Quote
My K-x lens says 18-55mm 1:3.5-5.6

1 - why can't I get 3.5 @ 55mm? are those considered more expensive lenses?
Yes. When a zoom lens says 18-55 1:3.5-5.6, that means its maximum aperture is f/3.5 at the short end (18mm) and f/5.6 at the long end (55mm). In other words, the numbers are listed in the same order.

Zoom lenses capable of keeping a constant maximum aperture at all focal lengths are indeed more expensive - and one's that able to do so at f/2.8 (like the DA*16-50) are usually more expensive than ones that can do so at f/4 (like the DA17-70).

QuoteQuote:
2 - I thought as the "mm" or focal length increases, you get a narrower dof.
If you keep the aperture and distance to subject the same, yes. So the DOF at 55mm and f/5.6 would indeed be less than the DOF at 18mm and f/5.6 for the same shot.

QuoteQuote:
To me it seems it should say on my lens 18-55mm 1:5.6-3.5 NOT 18-55mm 1:3.5-5.6.
That would mean a lens that could do f/5.6 at the 18mm end and f/3.5 at the 55mm end. That basically physically impossible. Unless specially designed to be constant aperture, the natural order of things is for a zoom lens to have a smaller maximum aperture at the long end, because aperture are ratios of diameter to focal length. So as focal length goes up, aperture goes down if the diameter stays the same. Meaning constant aperture lenses really do perform some pretty special "magic" to make this not happen.

These questions been bugging me, any help would be appreciated, thanks!

QuoteQuote:
I am guessing, if you have like a 500mm telephoto lens, you don't really need a low F Stop number right? Because the zooming in by itself will give a nice blurry background, please correct me if i'm wrong
No, you don't need low f-number (large aperture) to get blurry background with a 500mm lens. Depth of field is going to be very shallow no matter what. However, you often want a large aperture to get fast enough shutter speeds to handhold.
12-09-2009, 12:06 PM   #6
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thank you for that detailed explanation, i am amazed by the helpful nature of the people in this forum, glad I'm part of it.
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