When I typed 2.5, I meant 3.5, but it didn't matter, because the thrust of
what I wanted to say was "don't"!
I bow to your greater accuracy with the typing, Moose; your complete
explanation for the reason not to go this route; for the conclusion you
reach; and for your perfect use of an apostrophe to mark the possessive of
my name.
--
Piers
-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
Of Moose
Sent: 27 February 2007 10:52
To: olympus@xxxxxxxxxx
Subject: [OM] Re: spam: Re: Vivitar Automatic Tele Converter 3X-21
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
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|