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Old Jul 3, 2006, 1:04 AM   #11
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Nagasaki,

Can you tell me what goes oninside the cam once I set the aperture to F7 or above since the lens is only rated up to 6.3? When the lens is set to 6.3 which is the max, the lens is pretty much closed and only allows for very little light. Does a setting of F7 or more just adjust shutter speed? Basically, if the lens is only rated at such an aperture why is the D50 allowing more, and what goes on internally. Not sure if my question is clear enough?
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Old Jul 3, 2006, 7:50 AM   #12
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Actually, the lens stops down to f22. The aperature range you're referring to is the max aperature at minimum and maximum zoom. So at full zoom, the largest aperature you could select is f6.3. The lens thus is not pretty much closed, but is as wide open as possible. Most lenses are sharpest around f/8-f/11.

Remember, aperature is an inverse relationship. SMaller fstop numbers (f2.8, etc) allow in more light. Larger fstop numbers are smaller openings and allow less light in.
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Old Jul 3, 2006, 8:24 AM   #13
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I am only a hobbyist so my opinions should be taken with a grain of salt. But wouldn't it be better to be taking portrait pictures with a different lens? One that does not have such a long zoom range and, hence, lets in more light?

I can imagine that if I was travelling and wanted to take as little as possible, I would try to do everything with the same lens, for example an 18-200mm. But if I was not travelling and just going someplace local to take group photos of friends, I probably would take along the extra lens just for that.

I could only imagine that your overall results would be better with a different lens.
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Old Jul 3, 2006, 12:24 PM   #14
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So If I understand this correct, the range f3.5-6.3 means that when my 18-200 is at 18mm or so, then around 3.5 it is wide open and at around 200, f6.3 is wide open?

If this is true, I guess I was very confused on how the aperture ratings worked, but I will be veryglad that I know now. Yesteday I was shooting and was concerned that my aperture was at about F8, thinking it had exceeded the lens operating capacity, but apparently I was wrong. Oh, by the way most pics did turn out pretty sharp.

RJseeney,
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Most lenses are sharpest around f/8-f/11.
Do you own an 18-200 and have verifed this or is this just a general rule of thumb with zoom lenses




sti_sti
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I could only imagine that your overall results would be better with a different lens.
I also use a 50mm F/1.8 when I do portraits indoors, but yesterday I was shooting at the beach, so decided just to leave the 18-200 on. For the time being, I don't see myself purchasing more glass, since I believe there would be much overlap unless I was willing to spend big money for faster glass. I just do this for fun I am not willing to spend thousands on lens.


Great info guys, Thanks

Eddie



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Old Jul 3, 2006, 3:07 PM   #15
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No, I do not own an 18-200. Optically, these lenses are typically pretty bad at the extremes and wide open. I do some portrait and stock work and the distortions are noticeable.

Nearly every lens is sharpest a few stops down from the max opening.
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Old Jul 3, 2006, 3:15 PM   #16
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Edrod13

Apertuire when expressed as f/stop is a ratio between the focal length of the lens and the area of the iris opening diameter.

With a prime (non zoom) lens, you will see one aperture listed.

With a zoom lens, you usually see two apertures listed (the largest available aperture at wide angle zoom setting, and the largest available aperture at the full telephoto zoom position). When in between the widest and longest focal length of the lens, the largest available aperture will fall somewhere in between the apertures shown.

Some higher quality zoom lenses can maintain a constant aperture throughout their zoom range (with f/2.8 being the most common).

Lenses are rated by their largest available apertures (smallest f/stop numbers). But, most lenses can be set to use apertures of f/22 or smaller.

When you vary the apeture, you're controlling the iris in the lens (which like a pupil in your eye, can be opened up to let in more light or closed down to let less light in). So, this impacts the shutter speeds you'll need for proper exposure (since more or less light is getting through to the sensor).

The aperture scale in one stop increments (with larger than f/1 apertures theoritically available) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22... With each one stop move to a smaller aperture (represented by higher f/stop numbers), you will need shutter speeds twice as long for proper exposure for the same lighting and ISO speed.

Here is a handy online expoure calculator that you can use to get anidea of the shutter speeds required for any EV and Aperture. But, make sure to use your camera's metering, as lighting can vary. This is only to give you an idea of how the relationship between light levels, aperture, ISO speed (shown as film speed in the calculator) and shutter speed works.

http://www.robert-barrett.com/photo/...alculator.html

Aperture also impacts Depth of Field. Thelarger the aperture (represented by smallerf/stop numbers), and the closer you are to your subject (focus distance), and the longer your focal length (amount of zoom used), the less depth of field you will have (less of the scene in focus, as you get further away from your focus point).

Here is an online depth of field calculator. Plug ina camera model, then change focal length, aperture and focus distance to see what impact aperture has.

http://www.dofmaster.com/dofjs.html

As for lenses being sharper stopped down some (aperture not wide open), that's true for most lenses.

Most are sharpest about 2 or 3 stops down from wide open.

One good source of lens ratings based on MTF charts is http://www.photodo.com (but a lot of newer lenses are not included).

You'll notice looking at the specifics for a given lens that they tend to sharpen up when stopped down some (set to smaller apertures compared to wide open).

Another source of lens ratings is at http://www.photozone.de

Here is a test of the Tamron 18-200mm you're discussing:

http://www.photozone.de/8Reviews/len...3563/index.htm

If you look at the MTF Bar Graphs, you'll notice the quality going up as the aperture is stopped down some (especially in the corners).

But, if you stop one down too much, you start getting softer photos from diffraction. So, it's best to avoid aperture extremes (either wide open or fully closed) whenever possible.


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Old Jul 3, 2006, 5:13 PM   #17
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Quote:
Some higher quality zoom lenses can maintain a constant aperture throughout their zoom range (with f/2.8 being the most common).
What would be the most affordable lens that falls into this category and has similar coverage as the 18-200.


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Old Jul 3, 2006, 5:29 PM   #18
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There isn't one.

It would be very large and very heavy in order to accomodate aperture openings that large with acceptable quality throughout the focal range.

Basically, it would not be practical to produce an 18-200mm lens that could maintain f/2.8 throughout it's focal range for a DSLR with an APS-C size sensor.

Size and Weight would be too much for most users (not to mention the cost of producing one), and the types of users that would typically buy an f/2.8 lens, probably wouldn't accept the optical compromises a lens that has a focal range that varies that much from wide to long would give them.

Look at some of the 70-200mm f/2.8 lenses as examples. One of the least expensive lenses in this focal range is the Sigma 70-200mm f/2.8 EX DG (a bit over $800 discounted). It's a very large and heavy lens compared to your 18-200mm, and it's only covering the 70-200mm range.

http://www.sigmaphoto.com/lenses/len...mp;navigator=3

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Old Jul 4, 2006, 2:50 AM   #19
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Edrod13 wrote:
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I also use a 50mm F/1.8 when I do portraits indoors, but yesterday I was shooting at the beach, so decided just to leave the 18-200 on. For the time being, I don't see myself purchasing more glass, since I believe there would be much overlap unless I was willing to spend big money for faster glass. I just do this for fun I am not willing to spend thousands on lens.
Oh, ok! A beach is probably not the best place to start changing lenses, and you probably wouldn't want to lug around more than one lens to a beach anyway. So I understand!

Steve, Denmark
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