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 Post subject: nikon 17-55mm f/2.8
PostPosted: Wed Aug 15, 2007 11:05 pm 
what are the pros/cons of using a lens such as this w/ a fixed aperture?
http://www.nikonusa.com/template.php?ca ... uctNr=2147

i am enjoying my 18-200mm VR, but am curious as to the pros/cons of the above mentioned lens.

i know that the 17-55mm f/2.8 is a "fast" lens...but not sure what that relates to in practice..


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 Post subject:
PostPosted: Thu Aug 16, 2007 1:02 am 
Larger apertures = more light reaches the sensor, and therefore you can use a faster shutter speed than for the same exposure with a "slower" lens.
Faster shutter speed means less blur because of camera shake (when VR not present) and also a better chance to "stop the action" in low light (the VR cannot help with this). Larger apertures also mean shallower DOF.

Cons: larger apertures mean bigger, heavier and more expensive lens :wink:

Darrin


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 Post subject:
PostPosted: Thu Aug 16, 2007 1:26 am 
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If you're interested in some samples, we used the Nikkor 17-55 f2.8 to test the Nikon D200. Check out our D200 Gallery here:

http://www.cameralabs.com/reviews/NikonD200/page5.shtml

Gordon


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PostPosted: Thu Aug 16, 2007 1:12 pm 
Gordon you have me second guessing my decision to buy the D80 and 18-200mm VR....

the d200 is something else....is the metering system in the d80 nearly as good?

do you think the 17-55mm would be a better lens for shooting random material? how much wider is 17mm then 18mm? noticeably?


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 Post subject:
PostPosted: Thu Aug 16, 2007 8:22 pm 
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I certainly wouldn't complain about the D80's metering! It's very good!

As for 17 vs 18mm, technically speaking on a Nikon DSLR, that's 25.5mm versus 27mm, so certainly a little wider, but not by a great deal. You'd have to take photos with both zoomed-out though becuase some lenses end up wider or shorter than their specs imply - the Canon 17-85mm for example gets a tad wider than Canon's other zooms which start at 17mm. Sadly i don't have the 17-55mm f2.8 Nikkor around to do a comparison.

This lens is pricey though thanks to its bright aperture, so you'd only go for it if you really wanted f2.8 and a shorter range. It is a nice lens, but shame it doesn't have stabilisation like Canon's 17-55mm f2.8 IS.

Gordon


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PostPosted: Thu Aug 16, 2007 9:46 pm 
Gordon, what do you use personally?


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 Post subject:
PostPosted: Thu Aug 16, 2007 10:30 pm 
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I've bought a selection of 'benchmark' cameras for reference, but spend most of my time using the ones on test to maximise my familiarity of them.

Gordon


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PostPosted: Fri Aug 17, 2007 8:55 pm 
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At least we know that Gordon has the following gear:
Canon EOS 5D, 85mm f1.8, Speedlite 580 EX II
-------------
B.t.w., Gordon: You never showed us a review of your 85mm/1.8 :cry:

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 Post subject:
PostPosted: Fri Aug 17, 2007 9:58 pm 
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Thomas, at least you know Gordon has access to those parts!


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 Post subject:
PostPosted: Fri Aug 17, 2007 11:16 pm 
i was just outside playing around w/ my d40 and 18-200mm VR. i was tinkering around with aperture priority mode in attempts to get that "blurry behind the main object" thing going... do you need a REALLY fast lens (like a 2.8 ) to make this happen? i could get it a little bit but had to stay at the wide end of the barrel and keep the aperture as large as possible. does shutter speed play a large roll or is it primarily aperture?

i'm wondering if maybe the 18-200mm VR isn't for me as it seems somewhat "slow"?

remember -- i am a total newbie!

i wish there was a lens that did it all..... 18-200mm VR f/2.8

any speculation that the new products from nikon will include body VR?


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 Post subject:
PostPosted: Fri Aug 17, 2007 11:37 pm 
Not that I know of. Just speculation. Nikon and Canon dont spend R&D money on stuff like In body VR/IS.


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 Post subject:
PostPosted: Fri Aug 17, 2007 11:53 pm 
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Man, there's nothing to it. Look, here's a brief run through. Listen up...

DOF limits

A symmetrical lens is illustrated at right. The subject at distance s is in focus at image distance v. Point objects at distances DF and DN would be in focus at image distances vF and vN, respectively; at image distance v, they are imaged as blur spots. The depth of field is controlled by the aperture stop diameter d; when the blur spot diameter is equal to the acceptable circle of confusion c, the near and far limits of DOF are at DN and DF. From similar triangles,

\frac {v_\mathrm N - v} {v_\mathrm N} = \frac c d

\frac {v- v_\mathrm F} {v_\mathrm F} = \frac c d

It usually is more convenient to work with the lens f-number than the aperture diameter; the f-number N is related to the lens focal length f and the aperture diameter d by

N = \frac f d\,;

substituting into the previous equations and rearranging gives

v_\mathrm N = \frac {fv} {f - Nc}
v_\mathrm F = \frac {fv} {f + Nc}

The image distance v is related to an object distance u by the thin-lens equation

\frac 1 u + \frac 1 v = \frac 1 f\,;

substituting into the two previous equations and rearranging gives the near and far limits of DOF:

D_{\mathrm N} = \frac {s f^2} {f^2 + N c ( s - f ) }

D_{\mathrm F} = \frac {s f^2} {f^2 - N c ( s - f ) }

[edit] Hyperfocal distance

Setting the far limit of DOF DF to infinity and solving for the focus distance s gives

s = H = \frac {f^2} {N c} + f,

where H is the hyperfocal distance. Setting the subject distance to the hyperfocal distance and solving for the near limit of DOF gives

D_{\mathrm N} = \frac {f^2 / ( N c ) + f} {2} = \frac {H}{2}

For any practical value of H, the focal length is negligible in comparison, so that

H \approx \frac {f^2} {N c}

Substituting the approximate expression for hyperfocal distance into the formulae for the near and far limits of DOF gives

D_{\mathrm N} = \frac {H s}{H + ( s - f )}

D_{\mathrm F} = \frac {H s}{H - ( s - f )}

Combining, the depth of field DF − DN is

\mathrm {DOF} = \frac {2 H s (s - f )} {H^2 - ( s - f )^2} \mbox{ for } s < H

[edit] Moderate-to-large distances

When the subject distance is large in comparison with the lens focal length,

D_{\mathrm N} \approx \frac {H s} {H + s}

D_{\mathrm F} \approx \frac {H s} {H - s} \mbox{ for } s < H

\mathrm {DOF} \approx \frac {2 H s^2} {H^2 - s^2} \mbox{ for } s < H

For s \ge H, the far limit of DOF is at infinity and the DOF is infinite; of course, only objects at or beyond the near limit of DOF will be recorded with acceptable sharpness.

[edit] Close-up

When the subject distance s approaches the lens focal length, the focal length no longer is negligible, and the approximate formulae above cannot be used without introducing significant error. At close distances, the hyperfocal distance has little applicability, and it usually is more convenient to express DOF in terms of magnification. Substituting

s = \frac {m + 1} {m} f

and

s - f = \frac {f} {m}

into the formula for DOF and rearranging gives

\mathrm {DOF} = \frac {2 f ( m + 1 ) / m } { ( f m ) / ( N c ) - ( N c ) / ( f m ) }

At the hyperfocal distance, the terms in the denominator are equal, and the DOF is infinite. As the subject distance decreases, so does the second term in the denominator; when s \ll H, the second term becomes small in comparison with the first, and

\mathrm {DOF} \approx 2 N c \left ( \frac {m + 1} {m^2} \right ),

so that for a given magnification, DOF is independent of focal length. Stated otherwise, for the same subject magnification, all focal lengths for a given image format give approximately the same DOF. This statement is true only when the subject distance is small in comparison with the hyperfocal distance, however. Multiplying the numerator and denominator of the exact formula by

\frac {N c m} {f}

gives

\mathrm {DOF} = \frac {2 N c \left ( m + 1 \right )} {m^2 - \left ( \frac {N c} {f} \right )^2}

Decreasing the focal length f increases the second term in the denominator, decreasing the denominator and increasing the value of the right-hand side, so that a shorter focal length gives greater DOF. The effect of focal length is greatest near the hyperfocal distance, and decreases as subject distance is decreased. However, the near/far perspective will differ for different focal lengths, so the difference in DOF may not be readily apparent. When the subject distance is small in comparison with the hyperfocal distance, the effect of focal length is negligible, and, as noted above, the DOF essentially is independent of focal length.

[edit] Near:far DOF ratio

From the “exact” equations for near and far limits of DOF, the DOF in front of the subject is

s - D_{\mathrm N} = \frac {Ncs(s - f)} {f^2 + Nc(s - f)}\,,

and the DOF beyond the subject is

D_{\mathrm F} - s = \frac {Ncs(s - f)} {f^2 - Nc(s - f)}

The near:far DOF ratio is

\frac {s - D_{\mathrm N}} {D_{\mathrm F} - s} = \frac {f^2 - Nc(s - f)} {f^2 + Nc(s - f)}

This ratio is always less than unity; at moderate-to-large subject distances, f \ll s, and

\frac {s - D_{\mathrm N}} {D_{\mathrm F} - s} \approx \frac {f^2 - Ncs} {f^2 + Ncs} = \frac {H - s} {H + s}

When the subject is at the hyperfocal distance or beyond, the far DOF is infinite, and the near:far ratio is zero. It's commonly stated that approximately 1/3 of the DOF is in front of the subject and approximately 2/3 is beyond; however, this is true only when s \approx H/3.

At closer subject distances, it's often more convenient to express the DOF ratio in terms of the magnification

m = \frac f {s - f}

Substitution into the “exact” equation for DOF ratio gives

\frac {s - D_{\mathrm N}} {D_{\mathrm F} - s} = \frac {m - Nc/f} {m + Nc/f}

As magnification increases, the near:far ratio approaches a limiting value of unity.

[edit] Focus and f-number

Not all images require that sharpness extend to infinity; the equations for the DOF limits can be combined to eliminate Nc and solve for the subject distance. For given near and far DOF limits DN and DF, the subject distance is

s = \frac {2 D_{\mathrm N} D_{\mathrm F} } {D_{\mathrm N} + D_{\mathrm F} }

The equations for DOF limits also can be combined to eliminate s and solve for the required f-number, giving

N = \frac {f^2} {c} \frac {D_{\mathrm F} - D_{\mathrm N} } {D_{\mathrm F} ( D_{\mathrm N} - f ) + D_{\mathrm N} ( D_{\mathrm F} - f ) }

When the subject distance is large in comparison with the lens focal length, this simplifies to

N \approx \frac {f^2} {c} \frac {D_{\mathrm F} - D_{\mathrm N} } {2 D_{\mathrm N} D_{\mathrm F} }

Most discussions of DOF concentrate on the object side of the lens, but the formulae are simpler and the measurements usually easier to make on the image side. If vN and vF are the image distances that correspond to the near and far limits of DOF, the required f-number is minimum when the image distance v is

v = \frac {2 v_{\mathrm N} v_{\mathrm F} } {v_{\mathrm N} + v_{\mathrm F} }

The required f-number is

N = \frac {f^2} {c} \frac { v_{\mathrm N} - v_{\mathrm F} } {v_{\mathrm N} + v_{\mathrm F} }

The image distances are measured from the camera's image plane to the lens's image nodal plane, which is not always easy to locate. In most cases, focus and f-number can be determined with sufficient accuracy using the approximate formulae

v \approx \frac { v_{\mathrm N} + v_{\mathrm F} } {2} = v_{\mathrm F} + \frac { v_{\mathrm N} - v_{\mathrm F} } {2}

N \approx \frac { v_{\mathrm N} - v_{\mathrm F} } { 2 c },

which require only the difference between the near and far image distances; focus is simply set to halfway between the near and far distances. View camera users often refer to the difference vN − vF as the focus spread; it usually is measured on the bed or focusing rail. On manual-focus small- and medium-format lenses, the focus and f-number usually are determined using the lens DOF scales, which often are based on the two equations above.

For close-up photography, the f-number is more accurately determined using

N \approx \frac {1} { 1 + m } \frac { v_{\mathrm N} - v_{\mathrm F} } { 2 c },

where m is the magnification.
Defocus blur for background object at B.
Defocus blur for background object at B.

[edit] Foreground and background blur

If the equation for the far limit of DOF is solved for c, and the far distance replaced by an arbitrary distance D, the blur disk diameter b at that distance is

b = \frac {fm_\mathrm s} {N} \frac { D - s } { D }

When the background is at the far limit of DOF, the blur disk diameter is equal to the circle of confusion c, and the blur is just imperceptible. The diameter of the background blur disk increases with the distance to the background. A similar relationship holds for the foreground; the general expression for a defocused object at distance D is

b = \frac {fm_\mathrm s} {N} \frac { \left| D - s \right | } { D }

For a given scene, the distance between the subject and a foreground or background object is usually fixed; let that distance be represented by

x_\mathrm d = \left | D - s \right | ;

then

b = \frac {fm_\mathrm s} {N} \frac { x_\mathrm d } { D }

or, in terms of subject distance,

b = \frac {fm_\mathrm s} {N} \frac { x_\mathrm d } { s \pm x_\mathrm d } ,

with the minus sign used for foreground objects and the plus sign used for background objects. For a relatively distant background object,

b \approx \frac {fm_\mathrm s} N

In terms of subject magnification, the subject distance is

s = \frac { m_\mathrm s + 1 } { m_\mathrm s } f ,

so that, for a given f-number and subject magnification,

b = \frac {fm_\mathrm s} {N} \frac { x_\mathrm d } { \frac { m_\mathrm s + 1} {m_\mathrm s} f \pm x_\mathrm d } = \frac {fm_\mathrm s ^2} {N} \frac { x_\mathrm d } { \left ( m_\mathrm s + 1 \right ) f \pm m_\mathrm s x_\mathrm d }

Differentiating b with respect to f gives

\frac {\mathrm d b} {\mathrm d f} = \frac {\pm m_\mathrm s ^3 x_\mathrm d ^2} {N \left [ \left ( m_\mathrm s + 1 \right ) f \pm m_\mathrm s x_\mathrm d \right ]^2 }

With the plus sign, the derivative is everywhere positive, so that for a background object, the blur disk size increases with focal length. With the minus sign, the derivative is everywhere negative, so that for a foreground object, the blur disk size decreases with focal length.

The magnification of the defocused object also varies with focal length; the magnification of the defocused object is

m_\mathrm d = \frac {v_\mathrm s} {D} = \frac { \left ( m_\mathrm s + 1 \right ) f } { D },

where vs is the image distance of the subject. For a defocused object with some characteristic dimension y, the imaged size of that object is

m_\mathrm d y = \frac { \left ( m_\mathrm s + 1 \right ) f y } { D }

The ratio of the blur disk size to the imaged size of that object then is

\frac b { m_\mathrm d y } = \frac {m_\mathrm s} { m_\mathrm s + 1 } \frac {x_\mathrm d } { Ny },

so for a given defocused object, the ratio of the blur disk diameter to object size is independent of focal length, and depends only on the object size and its distance from the subject.

The effect of focal length on background blur is illustrated in van Walree's article on Depth of field.

[edit] Asymmetrical lenses

The discussion thus far has assumed a symmetrical lens for which the entrance and exit pupils coincide with the object and image nodal planes, and for which the pupil magnification is unity. Although this assumption usually is reasonable for large-format lenses, it often is invalid for medium- and small-format lenses.

For an asymmetrical lens, the DOF ahead of the subject distance and the DOF beyond the subject distance are given by[6]

\mathrm {DOF_N} = \frac {N c (1 + m/P)} {m^2 [ 1 + (N c ) / ( f m ) ] }

\mathrm {DOF_F} = \frac {N c (1 + m/P)} {m^2 [ 1 - (N c )/ ( f m ) ] },

where P is the pupil magnification.

Combining gives the total DOF:

\mathrm {DOF} = \frac {2 f ( 1/m + 1/P ) } { ( f m ) / ( N c ) - ( N c ) / ( f m ) }

When s \ll H, the second term in the denominator becomes small in comparison with the first, and

\mathrm {DOF} \approx \frac {2 N c (1 + m/P)}{m^2}

When the pupil magnification is unity, the equations for asymmetrical lenses reduce to those given earlier for symmetrical lenses.

[edit] Effect of lens asymmetry

Except for close-up and macro photography, the effect of lens asymmetry is minimal. A slight rearrangement of the last equation gives

\mathrm {DOF} \approx \frac {2 N c} {m} \left ( \frac 1 m + \frac 1 P \right )

As magnification decreases, the 1 / P term becomes smaller in comparison with the 1 / m term, and eventually the effect of pupil magnification becomes negligible.

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PostPosted: Fri Aug 17, 2007 11:54 pm 
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Alternatively...

http://www.cambridgeincolour.com/tutori ... -field.htm

Zorro 8)

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Various lenses, SB800 & Manfrotto 190 with 460MG head


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 Post subject:
PostPosted: Sat Aug 18, 2007 12:28 am 
mwahlert wrote:
i was just outside playing around w/ my d40 and 18-200mm VR. i was tinkering around with aperture priority mode in attempts to get that "blurry behind the main object" thing going... do you need a REALLY fast lens (like a 2.8) to make this happen? i could get it a little bit but had to stay at the wide end of the barrel and keep the aperture as large as possible. does shutter speed play a large roll or is it primarily aperture?


The only setting that influences depth of field is aperture.

The VR 18-200 will allow you f/4.8 at 50mm, which will give you a nice soft blur on the background (but you can still see the shapes). This is if you use a typical portrait distance.

The background blur varies with your distance to the subject. At 1 or 2 metres away, you get more that at 5 metres.

Also, at a focal length of 200 mm you get more blur than at 18 mm (as you have already discovered).

You can try this experience: place a bottle and a newspaper on a table, separated about 1 metre of each other. Then focus on the bottle label and use various focal lengths and apertures. Repeat, moving yourself away from the table. For each photo, the letters on the newspaper will seem different; on some, you will almost be able to read the big titles, while on others, you won't even be able to see any letters at all. This will give you a good idea of how your camera will perform when you intend to take a portrait of someone.

Do the same kind of thing with a line of trees and with a set of flowers and you will get an idea of how the camera performs, under the various settings, for landscape and macro photography.

In a word: experiment. :wink:

If you want to have a lens that is both affordable and good for portraits (and that will give you a very shallow depth of field when you want it), try the AF Nikkor 50mm f/1.8D. This lens will not auto-focus with your current D40; only with your soon to arrive D80.


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 Post subject:
PostPosted: Sat Aug 18, 2007 1:20 am 
i'm not so much concerned with cost as long as it does what i need....

i'm actually not even sure if i'm sticking with the d80... waiting it out to the end of the month to see if nikon has anything up their sleeve...otherwise i'm probably gonna go for broke with the d200...


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