# Why do images appear sharper than they should?

Discussion in '35mm Cameras' started by Robert Feinman, Dec 22, 2003.

1. ### Robert FeinmanGuest

There is a never ending discussion of resolution vs
print size and capture media.
The mathematics and empirical testing usually
show that the usual expected resolution is in the range of
40-60 lines per mm. A good print should have 6 to 8 lpm.
So simple logic means that the maximum enlargement should
be 5 to 8x. Thus the best size print one could expect from
35mm would be on the order of 8x12 inches with correspondingly
smaller sizes from digital cameras.
In spite of this people still get prints that are "sharp" with
much larger magnifications.
Personally, I've been scanning 35mm color negative film with the
new Minolta 5400 lately and can print out inkjets that look "sharp"
all the way up to the 18x maximum scanning resolution.
I'm not one of those who doesn't know what a "sharp" print looks like
either, since I use formats all the way up to 4x5.

So what's going on?
My conjecture (a theory in progress):

For people pictures shot at normal distances we are used to seeing
detail only in limited areas of the face such as the eyes (lashes
and reflections in the pupil) and perhaps loose strands of hair.
For landscapes and the like, we can't see all that much detail in
distant leaves and grass, but we do see bare branches, telephone wires
and the like as sharp.
For buildings and other man made structures the detail is seen in the
building edges and things like window frames.

In all cases the "sharp" things are not those with a lot of fine detail,
but rather those with good edge contrast. In other words acutance.
Most digital processing involves a certain degree of sharpening. This
doesn't do much for real detail, but does increase acutance. This makes
those features that we search for in real life appear "sharper" so we
read the image as being sharp. We don't expect to see much fine detail
so we are not surprised when it is lacking as long as those sharpness
indicators have good edge definition.
There are categories of images where the detail is important such
as scanning electron microscope images and we always comment on
how much detail we see in them when viewed. This shows that we don't
normally expect to see the fine structures in an image.
So I'm guessing that since the images conform to our expectations from
viewing such scenes in real life we accept them as sharp even though
the resolution figures would indicate that they are not really that
detailed.

As I said, a theory in progress, comments welcome..

Robert Feinman, Dec 22, 2003

2. ### Bart van der WolfGuest

In general I agree with your analysis, but allow me to add a few

Your SE5400 is capable of extracting somewhat finer detail out of a low
speed film. The combination of a good lens on a tripod mounted (or steady
handheld) camera with "slow" film, allows, combined with the scanners
optics and lightsource, to extract something like 70-80 lp/mm. And we're
grain).
The human visual system responds primarily to edge contrast, especially
horizontal and vertical edges (due to genetics or training combined with
gravity and horizontal movement).
The Unsharp Masking mainly affects high frequency detail, unless we
deliberately choose to enhance lower frequencies in a separate pass.
Shape recognition by the human visual system is a very fast but crude
process. However, as you know from large format images, there's a quality
to a good/life-like image that can be describesd as surface texture. This
allows us to discriminate between, say, a piece of leather and a piece of
cardbord. If the color and specular/diffuse reflection characteristics are
similar, it takes surface structure to discriminate. For this, the human
eye needs to resolve fine structures like fibers or dimples.

Bart

Bart van der Wolf, Dec 22, 2003

3. ### David RuetherGuest

"Sharpness" varies with the viewer, of course, and the viewing
distance (permitting lower unit resolution in larger prints to look
OK). As you note, supplying good detail in limited important areas
can suffice for some people (I'm always amazed by how bad
image edge/corner resolution can be, and some people call the
image sharp ;-). In digital, one can selectively sharpen (and
selectively soften) areas to increase the impression of good image
resolution and smoothness, if done carefully. For me, distant
subject info MUST be sharp in a "landscape" or an "architectural"
image - and the "almost-sharp" of stretched-DOF-covered images
looks bad. Standards of sharpness can vary considerably by
use, with direct-viewed images requiring greater resolution (I look
at these close-up, regardless of size...;-), and with reproduced
or screen images requiring less resolution to look good (I think
the sharp screen introduced by the dot pattern in reproductions
and pixels in monitors contributes to the look of sharpness that
isn't real [as can noticeable sharp-edged film grain] and/or the
expectations are lower...;-).
Interesting area, image resolution perception...

David Ruether, Dec 22, 2003
4. ### JeremyGuest

--
x-no-archive: yes
I remember seeing Carl Zeiss chart that showed the effect of diffraction at
various f-stops. Once you go beyond f/8, as I recall, the number of lpm
drops off significantly. So, stopping down for maximum DOF has a price, in
terms of reduced sharpness. Unfortunately, this trade-off is not apparent
just by viewing the scene in depth-of-field-preview.

On the other side, opening the lens up to max or near maximum aperture often
results in softening, especially at the edges. So there is a relatively
narrow range of f-stops that will yield an image that is both sharp and does
not sacrifice lpm resolution on the film. Maybe 2 stops, if that????

Decisions, decisions . . .

Jeremy, Dec 22, 2003
5. ### Joseph MeehanGuest

I suspect you are at least mostly correct. Sharpness can't really be
measured as it is perception. LLP can measure resolution but not sharpness.
A quick read of your message indicates you have a good grasp of this.

Please note that this author is not the same Joseph Meehan who is a
professional author of Photograph materials.

Joseph Meehan, Dec 22, 2003
6. ### JerryGuest

Robert,

I think you're onto something there mate ... I agree with your theory.
People's perception is not always reality - a very good point.

Jerry, Dec 23, 2003
7. ### David RuetherGuest

Yes, though limiting apertures to f8 minimum is more limiting than necessary,
since diffraction effects are not very bad (or observable in prints) for 35mm
until somewhat past f11. While with a good lens of 50-90mm or so, the
resolution in the center may peak around f4-5.6 (and at the edges maybe a
stop smaller), the losses due to diffraction do not generally soften the image
appreciably even at f16 (though I avoid f22-32, except for high-magnification
macro work). With landscape work, it is necessary for the fine distant detail
to be sharply-rendered (this is more important than having the larger-scaled
nearer material technically equally sharp in setting up the focus distance for
best DOF) - nothing looks worse to me than soft landscape photos (unless
intended) resulting from use of too small a stop, or from poor choice of
focus distance (using DOF scales and tables results in this, unless the guides
for focus distance and aperyture are modified...).
Top-class non-zoom lenses in the 50-90mm range for 35mm are sharp
from wide-open to about f16, with the peak near the middle; lesser lenses,
ones significantly different in FL (especially super-wides), or zooms may
have much more restricted ranges of stops for good performance.
Good super-wides may perform really well only at f11-16; many good
zooms that include the WA range may perform well only at f8-16;
lesser-quality lenses away from the 35-135mm range may have no
really good stops at all. While it is fun to use the best lenses at only their
best stops, often in reasonable-sized prints the visual differences in the
prints between this stop and one with 1/4 or so less resolution may be
minimal (and would preclude using the lenses wide-open, which can
result in wonderful images...). Given the above, though, it is still amazing
how much better the image will look if shot with a significantly larger
format camera - the advantages in tonality can show easily, even if the
unit resolution may not be much different...

David Ruether, Dec 23, 2003
8. ### jjsGuest

And so what? Of course edge contrast enhances the perceived sharpness.
That's been known for hundreds of years, and in fact the effect was
intentionally used and intentionally _avoided_ for specific esthetic
reasons before we got techno-esthetic and obsessed by things that should
only concern a recon maven. I hope you aren't suggesting we impose some
kind of new metric on a photograph to discover whether, regardless of how
appealing it might be to the eye, it must stand against some rez chart.

jjs, Dec 23, 2003
9. ### Malcolm StewartGuest

But what hasn't been mentioned is how easily some of these advantages get lost
in publication - even in publications devoted to photography.
For some time, the British weekly "Amateur Photographer" ran a series of
landscape shots taken by one of the current masters of the art. For me, most of
the offerings were simply a waste of space having blocked shadows etc. and being
little better than my own attempts (35mm & 6MP DSLR) but also a lot more
contrived, and after a short time I began to be annoyed by their inclusion.
However, when I saw the originals at an exhibition I was completely bowled over
by just how good the 30" x 40" and larger prints were, and realised then that
the phrase "a good big un is better than a good little un" definitely applied to
them. I think they were mostly shot on 5x4 or may be larger.

Malcolm Stewart, Dec 23, 2003
10. ### David RuetherGuest

Uh, I believe I touched on this in my first post in this thread with,
"Standards of sharpness can vary considerably by use, with
direct-viewed images requiring greater resolution (I look at these
close-up, regardless of size...;-), and with reproduced
or screen images requiring less resolution to look good (I think
the sharp screen introduced by the dot pattern in reproductions
and pixels in monitors contributes to the look of sharpness that
isn't real [as can noticeable sharp-edged film grain] and/or the
expectations are lower...;-).
--
David Ruether

http://www.ferrario.com/ruether

David Ruether, Dec 23, 2003
11. ### David RuetherGuest

Um, but a person's perception (at the time) *is* their reality...;-)

David Ruether, Dec 23, 2003