Scott Peden wrote:
> I got it 'cheap' last month, less than a 20 dollar bill after shipping, I
> had only put a bid on it to watch it, and on occasion I get stuff that
> cheap.
>
> I didn't understand everything you mentioned.
>
Nor did I, only because my math differs from Piers'
> Using a 3.8 lens, that would make it about a 6.3 right?
>
No, going two stops simply requires doubling the f number, so two stops
from f3.8 is f7.6
> Using a f8 it would be a f10?
>
Nope.
> Actually I know I didn't understand, the confusion must be 'stops'.
>
> How many numbers is an F stop? Or is this that 1.4 times itself thing?
>
> I have a 2 X Vivitar too, rather nice considering it isn't a Zuiko, but it
> does lose some clarity, I didn't check it out across the room against the 3
> X, maybe I can do that during the thunderstorms tomorrow hen I don't want to
> be outside.
>
> Ahh.. I have a book and I understood something, it is the 1.4 thingy. SO I
> multiply 3.8 or 8 times 1.4, two times to get an idea of where my new F stop
> is likely to be.
>
Ahh, now you are getting there, except it is really about three stops,
so 1.4 x 1.4 x 1.4 x 3.8 = 10.4, but it's not just 1.4, but the square
root of 2, which is slightly more, and the result is f10.7 And
> f3.8 x 1.4 = 5.32 x 1.4 = 7.5... acceptable if the glass is any good.....
>
f11
> f8 x 1.4 = 11.2 x 1.4 = 15.7 that's going to be fun......
>
f22, not fun at all.
> And again, my shutter speed should be 1/the lens mm?
> So a 210 mm lens with 3 x teleconverter should have a shutter speed
> somewhere around 1/600?
>
No, sorry again. The 1/ focal length rule of thumb is only valid for 35
mm format. For 4/3 format, it becomes 1/2*fl, or, in this case, 1/1200
sec. And that rule of thumb has three caveats
First, is makes assumptions about how big the image is going to be when
viewed. I think the rule of thumb is based on the whole 35 mm frame be
viewed as an 8x10 print at 10 inches. And unstated, by someone with
20/20 vision. So my 20/10 visual acuity might make me see such a shot as
less than really sharp.
Further, when shooting distant critters and tiny thingies, we often find
ourselves cropping. As you say so often on your Flikr iamges, "cropped"
or "cropped to the central portion" When you do that , you are blowing
up the image further than assumed in coming up with the rule of thumb,
and a faster shutter speed is required for a sharp appearing image.
Second, the rule of thumb only takes into account camera shake from an
average individual hand holding the camera. So someone better than
average can use slower speeds and someone worse than average needs
faster speeds. And as the lens gets bigger and heavier, I think the
ability to hold lenses steady gets worse, especially as ones arms get
tired. True for me, anyway.
Third, the rule of thumb assumes an unmoving subject, which is not true
of most critters. And even if you shoot a wooden decoy at the top of a
tall tree, the tree is always moving, more or less, depending on weather
conditions.
Another unstated assumption is that the portion of the image being
judged for sharpness is properly focused in the first place. If you take
a careful look at P2205953crop, for example, there are three obvious
sources of unsharpness that have nothing to do with lens quality.
- The bird is not in focus. The focal plane is way in front of it on the
tree. And as the lens gets longer, the DOF gets shorter.
- There is motion blur in the blur of the bird from a much too long
shutter speed.
- There is motion blur in some parts of the tree. with one littel branch
in particular very blurred. So even the tree itself is moving too fast
for the shutter speed.
It is, in fact, impossible to tell anything about the quality of the
lens from this image.
I'm guessing that capturing generally sharp images of those hawks with
600 mm is going to require at least 1/2500 sec, and maybe more.
I'm not trying to discourage you, just give some realistic ideas. Long
tele work with quality results simply isn't easy.
The classic solution by the serious wildlife photographers whose images
you admire has been to get closer to the subject. Blinds, enticements
and vast amounts of patience are behind many of those great images.
Digital has changed the balance some. With good AF and low high iso
noise, a lot more can be done now than before, but that's mostly not the
route you are going .
Moose
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