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12-15-2013, 10:13 AM   #1
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18-55mm F3.5-F5.6 question

Hi all.

So, I'm a newbie where it comes to DSLR technology, and this is probably a stupid question so if it offends you, please disregard.

I recently procured a Pentax K50 with a 18-55mm F3.5-F5.6 WR lens. Why can I set my camera to higher F-stops than the lens supports? The thumbwheel will allow me to set it up as high as F29.

Does it only stop down to F5.6 and slower settings on the body are irrelevant/ignored? Or does the camera somehow 'simulate' a slower aperature?

Thanks...

12-15-2013, 10:25 AM   #2
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F-stop is a mathematical formula derived from the physical characteristics of a lens. At any given focal length, the f/stop is focal length of the lens divided by the diameter of aperture. So when you look through your lens and move the stop down lever, you will see a hole open and close. The size of the hole is fixed, but the focal length of your lens changes when you zoom. The f/stop number becomes larger as the focal length becomes longer. This is shown on the front of your lens as F3.5-F5.6. At 18 mm, the largest hole is f/3.5. At 55mm the hole is only good for f/5.6. The effect of this is that at a given ISO setting (sensitivity, reaction to less/more light) the shutter speed will slow down as you zoom to allow the same total number of lumens to the sensor or film, and you should be able to see it in your viewfinder. It will get darker as you zoom into a scene.

Some lenses, such as my DA* 16-50 f/2.8 actually keep the same f/stop over the zoom range. The lens changes the size of the aperture to compensate when I zoom. The disadvantage to this is a much higher price and the lens weighs a ton. The advantage to this is that I don't "lose" any light as I zoom.

To completely confuse you, the f/stop is only what is shown at some ridiculously exact focal length, and is estimated and displayed as "close enough" for all the rest.

Last edited by Canada_Rockies; 12-15-2013 at 10:31 AM.
12-15-2013, 10:26 AM   #3
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f/29 IS a slower aperture. It's f divided by 29, which is less than f divided by 5.6.
12-15-2013, 10:47 AM   #4
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QuoteOriginally posted by JustDisGuy Quote
F3.5-F5.6
This means that the 'fastest' or biggest hole available on that lens is f/3.5 @ 18mm and f/5.6 @ 55mm. Because it is a variable aperture zoom it is not a fixed figure as it must change as the focal length changes as explained above. The lens also has a range of apertures at the 'slow' or small hole end. I don't know exactly what those are but you can test it by putting the lens at 18mm and scrolling to the smallest aperture and then setting the lens to 55mm and doing the same thing.

12-15-2013, 10:54 AM   #5
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QuoteOriginally posted by Canada_Rockies Quote
F-stop is a mathematical formula derived from the physical characteristics of a lens. At any given focal length, the f/stop is focal length of the lens divided by the diameter of aperture. So when you look through your lens and move the stop down lever, you will see a hole open and close. The size of the hole is fixed, but the focal length of your lens changes when you zoom. The f/stop number becomes larger as the focal length becomes longer. This is shown on the front of your lens as F3.5-F5.6. At 18 mm, the largest hole is f/3.5. At 55mm the hole is only good for f/5.6. The effect of this is that at a given ISO setting (sensitivity, reaction to less/more light) the shutter speed will slow down as you zoom to allow the same total number of lumens to the sensor or film, and you should be able to see it in your viewfinder. It will get darker as you zoom into a scene.

Some lenses, such as my DA* 16-50 f/2.8 actually keep the same f/stop over the zoom range. The lens changes the size of the aperture to compensate when I zoom. The disadvantage to this is a much higher price and the lens weighs a ton. The advantage to this is that I don't "lose" any light as I zoom.
Wow. Thank you for an incredibly thorough explanation. I'm going to have to keep re-reading it until I get it.

QuoteOriginally posted by Canada_Rockies Quote
To completely confuse you, the f/stop is only what is shown at some ridiculously exact focal length, and is estimated and displayed as "close enough" for all the rest.
Mission accomplished!

I think I understand the gist of what you're saying here, but I'm not entirely sure how what you've said actually relates to my initial question. It seems like you're saying that aperture and f-stop are two different things. Until now, I figured f-stop settings were directly correlated to aperture diameter.

So, when my camera is set in aperture priority mode, if I set my K50 body to "f29" with the kit 18-55 f3.5-5.6, what is actually happening? My kit lens doesn't have an aperture control ring, it's all managed in software. For argument's sake, let's say the lens is at 35mm focal length.

And, I'm sorry if I'm being obtuse. I really do appreciate your insights.
12-15-2013, 10:56 AM   #6
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QuoteOriginally posted by jatrax Quote
This means that the 'fastest' or biggest hole available on that lens is f/3.5 @ 18mm and f/5.6 @ 55mm. Because it is a variable aperture zoom it is not a fixed figure as it must change as the focal length changes as explained above. The lens also has a range of apertures at the 'slow' or small hole end. I don't know exactly what those are but you can test it by putting the lens at 18mm and scrolling to the smallest aperture and then setting the lens to 55mm and doing the same thing.
AH-HA! The light goes on!

The f3.5-5.6 is the *FAST* aperture at varying focal lengths, not the *range* of apertures available.

Thanks!
12-15-2013, 11:06 AM   #7
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QuoteOriginally posted by JustDisGuy Quote
The f3.5-5.6 is the *FAST* aperture at varying focal lengths, not the *range* of apertures available.
Yep, It is something like f/3.5 to f/22 @ 18mm and f/5.6 to f/29 @55mm. There are zoom lenses that have constant apertures through the focal length range. Like Canada_Rockies noted the DA*16-50 is f/2.8 at all focal lengths but making it do that requires a bigger and heavier lens.
12-15-2013, 11:17 AM   #8
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Ain't this forum great? Jatrax filled in the holes in my explanation, and you got it!

12-15-2013, 11:26 AM   #9
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QuoteOriginally posted by JustDisGuy Quote
AH-HA! The light goes on!

The f3.5-5.6 is the *FAST* aperture at varying focal lengths, not the *range* of apertures available.

Thanks!
It's a trap for new photographers. The original expression shows the aperture as a fraction, but with variable aperture zooms and some dropped characters, doesn't make sense as written and would be impossibly long otherwise. With experience you figure out what they mean.
12-15-2013, 11:30 AM   #10
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QuoteOriginally posted by Canada_Rockies Quote
Ain't this forum great? Jatrax filled in the holes in my explanation, and you got it!
You're right - this is an awesome forum. I have to point out though that the forum is only as good as its members!

Thanks guys!

Last edited by JustDisGuy; 12-15-2013 at 11:30 AM. Reason: spelling
12-15-2013, 11:48 AM   #11
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QuoteOriginally posted by Just1MoreDave Quote
It's a trap for new photographers. The original expression shows the aperture as a fraction, but with variable aperture zooms and some dropped characters, doesn't make sense as written and would be impossibly long otherwise. With experience you figure out what they mean.
It's even displayed as F 5.6 on most cameras, which doesn't help at all.
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