
Dirty lenses (2016) - pmoriarty
http://kurtmunger.com/dirty_lens_articleid35.html
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seaish
This article is missing a huge factor: aperture. If you go higher than f/11
the debris will start to get much more visible. All of the pictures in here
are at f/5.6 or less.

Each pixel basically takes a weighted average of all the light that travels
from a pixel-sized area in the world to the lens. As the picture with the dark
area shows, this makes the debris form a dark area over a large part of the
image. As the f number increases, the weighting gets stronger until 100% of it
is a single line from scene to lens to pixel. At this theoretical f/infinity,
you'll see all the lens's imperfections in perfect detail since the camera is
essentially a pinhole.

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Laforet
You'd run into diffraction limit for any reasonable pixel size long before the
pinhole diameter gets smaller than the wavelength of visible light. The rest
of your comment I fully agree with.

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Nition
Also fairly rare to be shooting at higher than F/11 anyway, isn't it?

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petepete
In normal situations yes. On occasions where you want to maximise DoF and
focus stacking isn't an option (say macro shots of moving insects) it's a
useful approach.

The DoF when shooting macro can be fractions of a millimetre.

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anon1253
This is true for lower apertures, but at higher apertures they become much
more noticeable especially at high magnification. This is especially annoying
for telescopes (it tends to look like this
[https://farm9.staticflickr.com/8679/16584213212_5b33222d6e_c...](https://farm9.staticflickr.com/8679/16584213212_5b33222d6e_c.jpg)),
which why astrophotographers use a trick called flat calibration to remove any
artifacts from dust or smudges in the optical train. The trick is surprisingly
simple: take a picture of an evenly illuminated surface (usually a flat box or
a clear sky) and make sure your ADU is in the linear phase of the sensor
(ergo, don't clip the highlights or darks). Then divide your original image by
the flat ($T * mean(flat) / flat). This is of course easier with a monochrome
sensor and works best with RAW data, but it can definitely be done with a
color CMOS as well. In case you ever find yourself with hard to correct dust
on the sensor: this trick might save an otherwise poor image.

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GlenTheMachine
Huh.

Optics are modeled by differential equations. They describe a massively
parallel phenomenon, light waves passing through and being modified by layers
of media that, essentially, performs calculations using them. Warps and scales
and so on. Clearly these particular differential equations are highly robust
to architectural changes.

There's a parallel there to something else I've read about on Hacker News...
what are those things called? Oh yeah, deep networks.

Are there any deep nets out there that attempt to mimic optics? or is anyone
applying ideas from robust differential equations to deep nets?

~~~
romwell
The words you are looking for are "Fourier transform".

Effectively, the lens computes a Fourier Transform for you[1]

One can probably compute a FT with NN, but we already have FFT :)

[1][http://web.mit.edu/2.710/Fall06/2.710-wk10-a-sl.pdf](http://web.mit.edu/2.710/Fall06/2.710-wk10-a-sl.pdf)

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holografix
I took some photos of Mt Fuji from a distance with a 28-70 at prob something
like 60mm f8. To my horror when I got home and fired up Lightroom there were
several, very visible, dark blotches against the bright clouds around the
summit.

I learned my lesson and will often use an air spray squishy thing to get rid
of dust from the lens.

~~~
ansgri
This was probably dust on the sensor, or the back element of the lens at
least.

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acomjean
I had dust inside a push pull zoom. I never noticed anything. I think the main
issue with dirt is if sunlight hits the dust on the lens it can do some
additionaly flaring.

I don't use lens caps either, but put a clear filter on the lens. Resale value
is hurt with scratches...

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azhenley
It is really, really hard to scratch a [modern] lens. Tony Northrop
demonstrated this by stabbing the lens with a knife repeatedly and it never
left a scratch.

UV filters though degrade image quality. Why buy nice glass just to put a
“protective” filter on it that gets in the way of the image?!

The real concern is decentering the lens by dropping it, but no filter will
protect it from that.

~~~
rangibaby
I think filters are useful as dust / junk collectors, especially modern ones
with repellant coatings. The less you clean your real lens the better. The
front glass is tough but not airtight so when you clean or dust it there is a
real chance of introducing dust or water into the rest of the lens.

Clear filters can’t hurt IQ other than introducing flare or ghosts in
backlighting. It’s possible to crack or scratch your front element with
minimal to no effect on your picture; a clear piece of glass isn’t going to do
anything.

~~~
breischl
>Clear filters can’t hurt IQ other than introducing flare or ghosts in
backlighting.

Or causing fringing, or reducing sharpness, or reducing contrast...

The absolute _best_ that a filter can do to IQ is nothing. But that's
basically impossible. Every air/glass interface causes reflections, and no
piece of glass is optically perfect. So it's going to hurt your IQ, the
question is merely how much, and whether it's worthwhile.

I used to use them. Then I decided I was probably never going to get much of
anything reselling my lenses, so I might as well just use them instead of
trying to baby them. Haven't had a problem with that decision, but YMMV.

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gomox
On the other hand, a finger smudge on your cell phone camera lens will destroy
contrast and the resulting picture, especially is there is any degree of
backlight in the image.

Make sure you wipe the lens with your t-shirt every once in a while.

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mertd
Nitpicking here. The element you can touch is not the lens. It is a protective
cover.

~~~
justtopost
Nitpicking more, the outer clear protective layer, still passes the image, and
is therefore also a lens, dispite not playing a large part in focus.

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markdown
I live in the tropics, and have fungi growing _inside_ my lenses. They leave
visible spots all the time, but photoshop takes care of that.

~~~
qmr
Oof. Use a drybox dude.

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Topolomancer
What I find fascinating about articles like this is that they demonstrate the
high quality of common objects (well, maybe not too common for those that do
not like to take photos with DSLRs, but anyway).

For some reason, I have almost been 'conditioned' to adopt a 'Everything that
we manufactured is really crappy' philosophy---maybe this comes from working
too much with software---so it is really nice when you learn about these
things.

Also, as an owner of a DSLR, this is news to me, making me less anxious about
now having the best equipment for cleaning everything with me all the time.

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tgsovlerkhgsel
I wonder how oily/fatty smudges behave. Because on eyeglasses, they are either
very noticeable, or, if you try to wipe them off but only end up smearing them
all over the place, not really noticeable but extremely tiring (probably from
subtle degradation and dispersing light coming from the side across what you
see).

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Jack000
dust on the sensor shows up a lot more than lens scratches

~~~
Laforet
The closer they are to the focal plane the more they show on the final image.
Hence having a scratched rear element is going to affect image quality.

