5D in House.

wrote:
Paul Furman wrote:
wrote:
Paul Furman wrote:
Chris Malcolm wrote:
In rec.photo.digital.slr-systems
wrote:
David J. Littleboy wrote:
"Paul Furman" wrote:

Also note that for macros, the effective aperture is smaller at
1:1 than the aperture ring reads, if it has an aperture ring.
The 105/2.8 VR shows corrected values and has no ring. At 1:1,
it's not /2.8 but f/4.8 and f/57 stopped down fully, which
becomes f/32 at infinity.
This is a good point: one sees people recommending f/22 for
macros for DoF, but that seems a bad idea.

It sounds like he is including the exposure compensation for the
'bellows' extension in this 'f stop' calculation, which doesn't
induce more diffraction. "seems a bad idea" sitting at a desk
doing math calculations doesn't = real world experience.

Does that mean that macro lenses adjust their iris aperture openings
for close work in order to correct for the longer effective lens-image
distance involved in close focusing?

More like the aperture readout does the math for the given
magnification. Focused to infinity at f/2.8 - now focus to 1:1 & the
aperture readout changes to f/4.8.

But it's NOT the actual aperture it's showing if that is what it's
doing, the aperture size hasn't changed and THAT is what creates
diffraction.

It's never given as aperture diameter, but f/stop:

f/stop = focal length/aperture diameter

for macro work, a simplified formula is:

effective f/stop = f/stop (magnification + 1)
(magnification: 1:1 use "1" - like 1x, 2x, 0.5x, etc)

A simple, symmetrical 105mm f/2.8 lens at 1:1 would be:

2.8(1+1) = f/5.6

...but this one changes focal length with internal elements... so it
comes out f/4.8 and some shorter focal length... also there are pupil
magnification factors, etc.

But "effective f/stop" doesn't create diffraction was my whole point.

Yes it does.
A better term might be 'working aperture'.


I've never used lenses that electronically calculate effective f stops
and display that. Guess I'm "old school" and used to mechanical lenses
when doing close up work.

So you should be doing the math manually if diffraction is a concern.


--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

Art Warner wrote:
wrote:
Paul Furman wrote:
wrote:

What you have "explained" is the camera showing you this compensation
as the macro lens is being extended, not that the actual aperture has
changed size. So this "aperture reading" you discussed has ZERO effect
on diffraction. I would have expected you guys to understand this
given the TONE of your previous posts.
Sounds like you've got it wrong.

So you're going to claim that lengthening the bellows to focus closer
induces diffraction??

Yes.
The given 'aperture' (f/stop) is only accurate at infinity focus.
Turning the focus ring changes extension and changes the f/stop. It's
effectively the same as changing the focal length... at least it should
be apparent that the field of view changes with magnification. Cropping
does the same thing.

Focal length as we know it is calibrated to infinity focus. Even
microscope objectives are specified with a numerical aperture calculated
for a theoretical infinity focal length (although the specifics are over
my head).


No, but it does increase its effect. Diffraction spreads as a direct
function of distance from the edge which is creating it. Combined with
lower light levels from a reduced aperture making it even more visible.
Less light is devoted to the image itself making the diffraction more
apparent.

Sort of.


This is why shorter focal-length lenses are not bothered as much
by diffraction artifacts.

The reason short FL lenses are useful for macro is they need less
extension to gain magnification. The (simplified) formula I gave doesn't
even include focal length. Working distance can become a problem though.

However, modern lenses do all kinds of tricks so the simplified formulas
and rules of thumb may not work. All sorts of strange things happen at
extreme closeup distances like reversed perspective where objects taper
as they get closer to the lens and splay out in the distance. I think
that occurs when the entrance pupil ends up behind the subject... not
sure... weird stuff. Mostly not field relevant for 'normal' photography.


--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

"Paul Furman" wrote in message
-


I wasn't doing huge prints or using fancy film so it didn't matter in the
field. Mmmm, OK I do remember doing some cropped enlargements and how
grainy things got.

I have been playing with grain reduction using PS3. My technique is after
severe cropping, to incrementally increase the resolution as high as 2400
pixels/inch. I then increase the size of the image in inches and then reduce
the pixels to 350. I have seen a small reduction in grain and increase in
sharpness using this technique. I wonder if anyone has tried this. I am
looking for a sweet spot and would appreciate input form others willing to
experiment.

--
Peter