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Old Jun 15, 2008, 12:29 PM   #1
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Hi all,

I am a novice to DSLR and own a Nikon D50 with an 18-55mm kit lens, and also a 55-200 VR Nikkor lens.

Although I understand the princilple of an F Stop number, I am struggling to understand how for example, my 55-200mm Nikkor VR which is F4-5.6 at each end, but the D50 will allow me to get as far as F20 at 200mm when selecting one of the manual modes.

What is actually happening here? Is the lens still at F5.6 at 200mm even though my camera is showing F20?

I appreciate this (should be) obvious to many, but that is why I am here to learn.

Any replies that help explain appreciated...

Chris
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Old Jun 15, 2008, 4:51 PM   #2
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That's why the question and answer forums are here - none of us started out knowing everything, and most of us find that there is always something more to learn.

The lens specifications give the f/number for the largest lens opening. With most lenses, you are able to 'stop down' the lens to a smaller aperture, depending on the light available, and your creative choice regarding depth of field vs shutter speed. F/20 would give you a large depth of field, which can be appropriate for landscapes, or pictures in which both foreground and background action need to be in focus. The F number is normally written as f/#, which is the focal length of the lens divided by the aperture, so the larger number results in a smaller lens opening.

brian
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Old Jun 17, 2008, 8:50 AM   #3
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Chrisbenwalker wrote:
Quote:
... my 55-200mm Nikkor VR which is F4-5.6 at each end, ...
No, the note on the lens means the maximum aperature is f/4 at the short end (50mm) and 5.6 at the long end (200mm).
Quote:
...
What is actually happening here? Is the lens still at F5.6 at 200mm even though my camera is showing F20?
...
The lens is open to f/5.6 *UNTIL* you press the shutter release, then it stops down to f/20. That allows more light through for metering, viewing, and focusing. To see that happen, set your camera to manual mode, aperature to f/20 and shutter speed to something like 1 second. Then look into the lens when you press the shutter release - taking a badly exposed and badly focused picture of your eye. You should see the aperature stop down while the shutter is open.
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Old Jun 17, 2008, 3:13 PM   #4
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Thanks both for your informative replies...

I have a much better understanding of this now, and in just a few words from you both have saved me LOTS of reading in the hope of a simplified explaination.

So thanks again....

Best regards,

Chris
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Old Jun 17, 2008, 3:49 PM   #5
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BTW Bill, just tried that looking down the lens thing in manual mode, and see exactly what you mean.....why didn't I think of doing that?!!

And yes, I did get an over exposed, very blurred picture of my left eye....


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Old Jun 17, 2008, 9:42 PM   #6
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Chrisbenwalker wrote:
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....why didn't I think of doing that?!!...
Not possible to think of everything - but it is worth trying to figure out how to do your own tests for simple stuff. Though the comments/answers here are almost always good, you should do your own tests if they can be done easily. Noise is one of the best examples - how important it is depends on a whole bunch of things such as the size of the final image, subject, viewing distance, phase of the moon, and the eye of the beholder.

VR/antishake/whateveritiscalled is another example of doing your own testing. Shoot something with a lot of detail (page of a newspaper) at various focal lengths and shutter speeds. It will make you better able to figure out what works for you. Doesn't matter if someone else can handhold at 500mm with a ten second exposure, it is what you can do that matters to you.

What books say, and what folks here say, are worth listening to. But you should look at your own photos - if for no other reason than to make sure you understand what is being said. There really is no shortcut from that.
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