
Dirty lens article - pmoriarty
http://kurtmunger.com/dirty_lens_articleid35.html
======
_paulc
Long time since I did any optics but this basically just convolution. The
impact at the output plane of dust/imperfections on the lens is
(approximately) a 2D fourier transform of the dust/imperfections. If you have
a single dust particle (essentially an impulse) on the lens this will be
convolved into a dc response at the focal plane.

[1]
[http://en.wikipedia.org/wiki/Fourier_optics#Fourier_transfor...](http://en.wikipedia.org/wiki/Fourier_optics#Fourier_transforming_property_of_lenses)

~~~
darkmighty
It's approx. convolution and the impulse response varies with the distance to
focal plane: at focal plane it's impulsive but near the lens it's quite flat
(almost DC -- it's simply a "zoomed in" aperture, which is a big circle
usually).

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nakedrobot2
The wider the lens, the more dust on the front element affects the picture.
Especially specular light (bright light sources, especially the sun).

If you have a very wide-angle or fisheye lens (e.g. gopro) then you will most
certainly have lots of nastiness in your photo from having a dirty lens.

You'll notice that in this person's test shots, he has no specular light or
sunlight in the images. If he did, the test would have had completely
different results.

If you have a gopro, try it yourself. Have a dusty or greasy lens and take a
picture with/without the sun in it and the photo will certainly be worse.

Same with my mobile phone - if the front glass is greasy, well, of course the
photo looks much, much worse.

I'm a professional photographer specializing in 360º imagery shot with fisheye
lenses, so I know this stuff very well from experience.

~~~
Nav_Panel
Indeed, I ran into this today. Took the following photograph with a 28mm (wide
angle) lens on a Pentax K-x DSLR (crop sensor, so 42mm equivalent) + a
polarizing filter, stopped down to f/22:
[http://i.imgur.com/Wu7HTpo.jpg](http://i.imgur.com/Wu7HTpo.jpg)

If you look at the bottom right, you'll notice a lot of crap. It's all just
dust from the filter. It's fixable in Lightroom but it's a huge pain, as I
need to manually remove each speck. These dust specks did not appear when the
lens was stopped to f/8-ish.

Regardless, in response to OP, I've seen some really nice photos taken with
really beat-up lenses. But if you push the bounds at all and start to
photograph more challenging material exposure-wise, I imagine it'll get bad
real fast.

~~~
dietrichepp
The reason why this is happening is because your entrance pupil is small
(1.3mm) and close to the dust.

The crop factor is what makes things bad here: higher crop factors means that
for a given field of view, the dust is closer (because the lens is thinner)
and the aperture is smaller relative to the dust.

Paradoxically, the dust is actually less problematic for more challenging
exposures.

You may wish to use a wider aperture, on a 28mm lens and with modern sensors,
f/22 is likely to show blurring from diffraction through the small aperture.
The Airy disk will have a diameter of 30 µm, but the pixel pitch on a K-x is
around 5.5 µm... which means that you have a ~6 pixel wide blur applied to
your image before you even press the shutter!

But the good news is that the depth of field on a camera with a crop factor is
deeper for the same field of view and aperture.

~~~
Nav_Panel
>But the good news is that the depth of field on a camera with a crop factor
is deeper for the same field of view and aperture.

To be completely honest, that's the bad news and the reason why I miss my film
SLR so much after I switched over to digital. I absolutely adored my 50mm
normal lens's nice thin depth of field, and I miss having that on the smaller
format sensor.

>f/22 is likely to show blurring from diffraction through the small aperture

I'm gonna have to learn about these concepts -- I'd heard that diffraction
causes decreased sharpness when stopped down but I didn't know it was a
significant issue in modern lenses. I do know that I get significant chromatic
aberration on my 28mm, but I always figured it was due to its retrofocal
design. Do you have any reading recommendations?

~~~
dietrichepp
> Do you have any reading recommendations?

Not really. I remember using a table which gave you the optimum aperture for
maximum sharpness, based on the distance that the lens moved when focusing
between the front and back of the subject. That kind of thing is only really
useful for MF and larger, and I only used it for things like landscapes where
sharper was always better. The problem with small formats is that it's hard to
know how far the lens is moving, so you can't really use this technique.

In the most general terms, opening your lens all the way gives you
aberrations, closing it all the way gives you diffraction, so you'll typically
shoot in the middle unless you have a reason to do otherwise. For a rule of
thumb, take the apertures you use for 35mm and adjust them by the crop factor.
So if you liked to shoot everything at f/8 with 35mm (a common choice), then
lean towards f/5.6 with your 1.5x crop digital. Conversely, if you start
lugging around a 6x7cm camera, you'll use f/16 as your default.

Yes, for really small cameras, this means really big apertures. The iPhone 4S
always shoots at f/2.4, for example.

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beat
I love this! As a hobbyist in a couple of different gear-heavy fields
(photography and guitar/recording), I've seen from experience how much
gearheads obsess over exactly the wrong things. They'll worry about second or
even third-order issues, while ignoring basic, obvious issues.

The internet is full of detailed discussions of camera lenses - barrel and
pincushion distortion, lateral fringing, all sorts of issues you can only see
when you're photographing test charts on a wall, with plenty of learned
comments from other guys who shoot test charts on walls. This stuff panics
people who try to buy cameras, thinking their ability to take good photos will
be governed by these abstract numbers.

Feh.

~~~
jchrome
Maybe I don't see a lot of good photos because I'm not looking hard enough.
But the problem with "GAS" is that it puts all the focus on the gear and takes
attention away from technique/lighting/composition etc. You can make great
photos with a pinhole box camera if you wanted to (and I'd probably want to
see them more than some rich shmoe with a brand new DSLR with 400 mgpx's
photos).

Spend your money on books, classes and prints. Not on cameras/lenses.

~~~
beat
I always recommend two books to anyone asking about buying a camera -
"Understanding Exposure" (Bryan Peterson), and "The Photographer's Eye"
(Michael Freeman). These two books cover all the basics of exposure and
composition in a pretty accessible way.

Those two books will get you farther toward taking good photos than any piece
of gear will.

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jrapdx3
Not a whole lot new in this article, though its "experimental" results confirm
well-known phenomena regarding camera lens performance. In particular, small
front-element blemishes do not in general significantly degrade optical
performance.

That said, dirt or scratches become more important when there is greater depth
of field, for example, shooting with ultra-wide angle lenses stopped down to
small aperture. Naturally, how much image quality degradation is acceptable
depends on the circumstances, there's no single rule that can be applied.

I wouldn't try to get into the laws of physics/optics governing these
considerations, there are plenty of sites on the web for that. However I
recommend taking a look at the _LensRentals_ site for its wonderfully
practical, entertaining lens information (including info re: user-level lens
testing): [http://www.lensrentals.com](http://www.lensrentals.com)

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calinet6
True for very large lenses where the dust and front element is sufficiently
far and disproportionate to the aperture size!

But for smaller cameras, _especially_ cell phone cameras, that dust and smudge
is magnified immensely and has a much much greater effect. The epitome of this
effect is having dust directly on top of the sensor, which obviously is
visible directly. Imagine how small your cell phone camera is: that dust is
only a very short distance from the sensor, and it's not much better than
being right on top of it. Dust on a small lens is also proportionately larger
compared to the lens and the aperture. All in all, much greater effect. This
article might as well not even apply to small cameras.

Moral of the story: you should _always_ clean your small camera lenses, and
especially cell phone lenses! For larger lenses, the dust and smudge has less
of an effect.

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desdiv
There's this attachment called an "Anti-Reflection Device"[0] that goes in
front of rifle scopes. It's basically a honeycomb sunshade that, like the name
says, minimizes reflections from the lens to make the shooter harder to spot.
The cool and unintuitive thing about this gadget is that the honeycomb pattern
doesn't actually appear when viewed through the scope. It's the exact same
principle at play here.

[0] [http://www.amazon.com/Tennebrex-Killflash-Scope-Anti-
Reflect...](http://www.amazon.com/Tennebrex-Killflash-Scope-Anti-Reflection-
Device/dp/B001UCBSNU)

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andrewstuart2
While obviously the results show that scratches and smudges _can_ be
practically unnoticeable, this article is a little misleading.

For one, dust, smudges and scratches _will_ affect the contrast of an image,
and more noticeably with a deeper depth of field. I'm sure you've noticed this
on cell phone shots with oil or makeup on the lens, as it's much more
noticeable with deeper depths of field. This is simply because incident light
will diffuse into all areas of the sensor/film, causing dark areas to get
lighter than they should be.

Additionally, the results here are incomplete. The author keeps his lens
aperture wide open, which will keep depth of field pretty shallow. The deeper
the depth of field (from smaller apertures, wider lenses, or smaller sensors)
the more noticeable the scratches and dust will be, since it will localize the
effects of the diffusion on the sensor/film.

Deeper depths of field are often preferred by landscape photographers, to keep
fore-, mid-, and background elements all in focus, which will make
smudges/scratches more obvious. Additionally, many lenses are much sharper
when stopped down (smaller apertures) from wide open. Same problem.

I think it's a bit of a shame to see a perfectly good lens ("optically and
mechanically fine"), which could have been donated or still sold at full cost
(no photographer worth their salt cares about body scratches), be completely
destroyed because the author wanted to prove a flawed point.

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olla
It proves again the basic rules for handling photo gear in hostile
environment. Pick a lens that covers the needed focal distances and never
change or clean it on the field, as you will risk damaging camera or
decreasing the aftermarket value of your gear. If the dust on front element
becomes visible on pictures, just open up the apperture.

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perlpimp
rule of thumb - the longer the focal length the less are the effects of
scratches on the front lens. ultrawide likes of 35mm and smaller are very
sensitive to material on the front glass element. this translates into some
really nice savings if you want to get a lens in 90mm+ range for example if
there is a scratch on the front lens.

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cornellwright
Given all the comments, I'm surprised to see no one mention flat field images
and flat field correction. This would be the proper way to measure and even
correct for blemishes and dust like this.

Basically a flat field image is an image of a completely uniform illumination
pattern (i.e. given a perfect sensor and lens with no vignetting all pixels
would have the same value). This lets you measure all sorts of things, but
comparing flat field images of before and after dust/tape were applied would
actually highlight the exact effect.

Using flat field correction, you can even correct for these issues to a great
extent by scaling the pixels as appropriate to basically account for their
being light paths between the object in the scene and the image projected onto
the sensor.

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jscheel
If only this was true for sensor dust as well.

~~~
patcheudor
Sensor dust is the worst. I switched from shooting on Canon DSLRs several
years ago and now shoot on Sony mirrorless - the NEX-7 and a7R. Because I
exclusively use primes I tend to swap lenses quite a bit and as a result my
sensor gets dusty fairly quickly without a mirror to shelter it. Even with a
blower bulb it tends to show up. This isn't much of an issue because I also
like shooting wide open including down to f/0.95 and sensor dust is never an
issue when shooting fast, but then I'll decide I need to go to f/11 or slower
for a landscape shot and boom, I've got smudges in the photo thanks to the
dust on the sensor.

~~~
jscheel
Yep, I'm on a Fuji x-pro1 mirrorless. Ended up getting a boulder of dust on my
sensor when switching lenses at the top of Castillo San Cristóbal in San Juan.

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markrages
When the camera is stopped down (like for a bright desert scene) isn't just
the middle of the lens being used?

~~~
barrkel
No. Don't forget, the in-camera image is inverted. When a camera is stopped
down, the edges of the image are mapped by the opposite edge of the lens, more
like a pinhole camera.

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bitL
Placing stuff on your lens usually affects bokeh - try f1.4 in the night with
distant lights in the background with something stuck on your lens - you'd see
the effect everywhere ;-)

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donniefitz2
So weird. I've been reading Kurt Munger's reviews for years and now to see one
of his articles on HN. Strange crossing of paths.

