Dirty lens article
kurtmunger.com
kurtmunger.com
[1] http://en.wikipedia.org/wiki/Fourier_optics#Fourier_transfor...
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.
Greasy is something else, and affects all lenses pretty much the same way, by messing with the transmittance and refraction.
- Small surface implies dirt occupies a large angular portion.
- Very small aperture (pinhole-like) focuses both near and far objects, which includes dirt.
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 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.
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?
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.
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.
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.
Spend your money on books, classes and prints. Not on cameras/lenses.
Those two books will get you farther toward taking good photos than any piece of gear will.
1) Buy the best possible camera - no good pictures ever
2) Buy the best possible lenses - barely any good pictures
3) Buy the best possible lights - now we are getting somewhere
4) Learn how to compose and capture the moment - Bingo!
Most people assume 1) and 2) are the most important, barely anyone does 3) and some do 4). Any digital camera since 2010 or so can produce breathtaking pictures with 4) and cheap zoom kit lenses, for studio work it's all about 3). Then to get to the ultimate pro level it's about Photoshop skills to finalize the look as desired.
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
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.
[0] http://www.amazon.com/Tennebrex-Killflash-Scope-Anti-Reflect...
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.
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.