Comment by nakedrobot2 | Hacker News Reader

Comment by nakedrobot2 | Hacker News Reader

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.

Actually, the dust is more pronounced on these examples you gave (GoPro, mobile phones) because of a (pretty complicated) relation between the size of the sensor, the aperture and the depth of field, not necessarily because of the wide angle, although it exacerbates the problem.

Greasy is something else, and affects all lenses pretty much the same way, by messing with the transmittance and refraction.

Two reasons those lenses are susceptible:

- Small surface implies dirt occupies a large angular portion.

- Very small aperture (pinhole-like) focuses both near and far objects, which includes dirt.

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

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.

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.

>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?

> 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.

I am getting similar specks from the dust on the sensor, not so much on the lens on a D7000.

No sunlight in the images? For starters, here is a n image of the sun: http://kurtmunger.com/sitebuildercontent/sitebuilderpictures...

Yes, you're right, sorry. There is one example, maybe it's cherry-picked, maybe it's just one example that happens not to have this issue. What I'm saying is, it absolutely is a real issue, which is contrary to the thesis of this article. What's more, it is easy to mitigate the issue. I've spent more time writing these replies than wiping dust off my lenses ;-)

He specifically said that he took multiple images of the sun and couldn't find any issues.

Adding to what's been said, I believe the worst situation (in terms of seeing dust on a lens) would be:

1) wide aperture

2) depth of field that (even partially) includes objects on the lens itself

In normal situations, these two conditions are somewhat mutually exclusive. That is, a wide aperture decreases depth of field, making it less likely for anything as close as the lens to be in (partial) focus.

The reason you may see the effects of a dirty lens more with fisheye or wide-angle lenses is that they often have a very short hyperfocal distance (bringing most everything into focus), so it's much more likely that condition #1 won't negate the effects of #2. These conditions would also be likely to occur in low-light macro photography.

When I see experimental evidence, and somebody arguing against it, I trust the former, especially if the latter feels the need to say 'trust me, I am a professional!'.

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