There is no difference in the effect of small apertures between digital
and film. The difference has to do with diffraction and the size of the
image. Your E-3's image is dramatically smaller than your 6x17cm film.
The size of the image is important since it's intimately tied up with
how much you will enlarge the image for viewing or printing.
As the aperture of any lens closes down the smallest size "spot" the
lens can create (called the Airy disc) grows larger. For example, in
green-yellow light the Airy disc is 7.5 microns in diameter at f/5.6,
14.8 microns at f/11 and 29.5 microns at f/22. If it weren't for the
rounding here we'd see that the Airy disc is doubling in diameter every
time we stop-down by 2 stops. Since I specified the color of the light
you can also surmise that the size of the Airy disc varies by frequency.
The more energetic the light (blue vs. red) the smaller the Airy disc
and the more detail the lens can resolve at that color. Yellow-green is
in the middle of the visible spectrum and what our eyes are most
sensitive to.
It's important to realize that this diffraction effect is related only
to focal ratio. It doesn't matter what's the focal length of the lens,
or the size of the film or sensor. The only other thing that matters is
the quality of the lens. The numbers above are for a "perfect" lens.
To the degree that the lens is not perfect the Airy disc will be even
larger.
Since the size of the Airy disc is constant at a given aperture you can
see that it ultimately controls maximum resolution. If the size of the
Airy disc at a given aperture is larger than the pixel pitch of your
digital camera or larger than the grain size of your film then an
individual pixel or grain can not be resolved. For the E-3 with its
12MP 4/3 sensor the full resolution of the sensor cannot be achieved at
apertures smaller than about f/5.6. The same number of pixels spread
over the larger area of a full-frame sensor would allow the use of f/11.
Film is more difficult to figure since grain size is not constant and
grains are randomly distributed but each film type has a stated maximum
resolution value. Obviously, if the Airy disc is larger than the width
of a line to be resolved then the full resolution value of the film will
not be achieved.
But the next question is: Does it matter? If you make a small print
from the E-3 you're not using the full resolution of the sensor anyhow.
If you make a larger print you might be. But you could make that same
larger print from the 6x17 negative with only a small amount of
magnification. For more see:
<http://www.luminous-landscape.com/tutorials/resolution.shtml>
Chuck Norcutt
On 9/11/2011 4:04 AM, Brian Swale wrote:
> A couple of weeks ago we were discussing the effects of small apertures on
> digital image quality, compared with the same for film.
>
> The message was that smaller than maybe f/8, the effect was not good.
>
> Two days ago I was using my E-3 as an exposure meter for a 6x17 film
> camera. Probably not a good thing to do, and I should know in a couple of
> weeks or so after I send a few rolls off, then get them back from processing.
>
> However. I had the E-3 at f/22 for the film camera, and forgot (for a few
> exposures) to return the f stop setting to my customary f/8.
>
> Having looked at these f/22 images on my computer, there is no doubt in my
> mind that the images are significantly softer. I took all the digital photos
> that
> day hand held, and took about the same degree of care over all of them.
>
> Brian Swale.
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