Photo Questions

[Jeff_Roush]

It is always a slow process to start a new group. Here's an idea to
get some posting started from the membership. How about everyone in
the group post "one" photograpic question? It can be as simple as a
photographic term you don't understand / or a question about your
camera / or a general photograpic question.
Let's see where this takes us.

Jeff

[Steve Parrott]

Ok, here is something I have wondered about. F stop numbers. I "think"
I know why the smaller number is a larger opening. If I am not
mistaken, the number relates not to the open circle, but the area that
is closed. It seems I once saw a diagram of a circle, with another
circle inside, (representing the aperture opening). The distance
between the open circle and the edge of the outer circle was the f stop
number... so the larger the inside opening is, the smaller the
"outside" number will be. Hope that makes sense. What I have never
understood is just WHAT does 2.8, 5.8, 8, etc. represent... where did
those numbers come from? 2.8 what.... centimeters? I don't think this
can be an actual measurement between aperture opening and the lens
barrel, or it would, seems to me, be a different number between
different lenses.

So maybe my thinking is all wrong on this. I know 2.8 on one lens has
to allow the same amount of light to enter the body as 2.8 on any other
lens. I just don't know what that number literally means.

So there ya go Jeff... clue me in on this one! :-)

Steve

[Jeff_Roush]

GOOOOOOOOOOOOOOOD question!!! Geesh, you're really going to test me
now huh (wondering if I'm sorry I asked for questions!)
Well, here's the scoop - to answer Steve's question -
Lenses today use a standard f-stop scale and range. This is an
approximate geometric sequence of numbers that correspond to the
sequence of the powers of - √2 (1.414) - or - f/1, f/1.4, f/2, f/
2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128,
etc. These values are rounded off to these particular numbers to make
them easier to remember. If you do the actual math you'll see the
numbers are a little different than actual f/stop numbers.

The slash (/) actually indicates division, just as it does in most
scientific application. For example, f/16 means that the iris/f-stop
in the lens is equal to the focal length of the lens divided by 16
(sixteen). If the camera has an 80 mm lens all the light that
reaches the film or sensor passes through this iris known as the
"entrance pupil" (correct technical photo term) that in this case is 5
mm (80 mm/16) in diameter. Make sense? If not, don't feel bad. This
type of information is over a lot of our heads, including mine. I
have to refer to my own notes and reference materials to explain
this... it's not something we think about often enough to remember.
But, that's how it works!

> > Jeff- Hide quoted text -
>
> - Show quoted text -

[Steve Parrott]

Ok... thanks Jeff, that actually does make sense. It answers my
question as to how f16 on one physical size lens could be the same as
f16 on a larger / smaller size lens. I suppose an easy way to express
it is that the f stop number is a relationship number.

Thanks for helping clear that up!

Steve

[Steve Parrott]

Here is another one. This may be an opinion matter rather than a
specific answer, but what is your feeling on using a UV filter on
lenses? Personally, I do not use them. To me it is just one more piece
of glass to shoot through, and unless you have a VERY expensive, very
high quality filter, why put a cheap piece of glass in front of a
thousand dollar lens? I always use a lens hood. I feel this gives me
lens "protection" as well as a filter, plus the added benefit of
stopping stray light from the side from causing flare. I have always
felt a cheap UV filter can do more harm than good, and I have never
been convinced a GOOD UV filter is really worth the money.

What about film vs. digital? Is a UV more of a help with film than
digital? (I still don't use one on my film camera though).

Any thoughts anyone?

Steve

[Jeff_Roush]

UV filters. I, like you have never used them much. Their purpose is
filter out ultaviolet light which, in some cases, can reduce haze in
the atmosphere. In some cases they will actually "warm" a "cool"
photo up some - like a portrait taken in open shade. I don't think
they are as popular in Digital photography being we can do so much
with PhotoShop now. It would be intersting to do a "side by side"
comparison so see if there is a true difference in digital "with" and
"without" a UV filter.
You statements about shooting through another peice of glass is the
reason I don't and never have used them. Unless it it optical quality
glass you are going to get some flare / some refraction / and some
loss.
Some Digital forums state that the purple fringing / glow that we
sometimes capture in highlights can be reduced some, although, I have
never experienced this personally.
As far as using a filter as lens protection - I think there are times
when that is advisable. If I were hiking through the amazon I would
put an optical / clear filter on the lens meerly for protection.
However, if I came upon an award winning photograph I'd more than
likely take it off ... :)
That's my take ... any other comments?
Jeff