What is causing the cyan outline in the building in the background of this image?

Question

I took this picture the other day. It's a JPEG from the Fujifilm X-Pro1. The sky is overexposed as was necessary to light Anya brightly — and because I was playing with shooting wide open, and that plus the high base ISO plus max shutter of ¹⁄₄₀₀₀th meant this went beyond what the metering really wanted to do. Maybe a bit much, as we've lost the top corner of her head.

But, never mind that. What I'm interested in here is the cyan blur outlining the Prudential building in the background. A one-for-one pixel-level crop is below the full image....

_{Anya on the Esplanade; photo by me and CC-BY-SA 3.0 in this size only.}

What is causing that blue outline? Is it just a matter of the out-of-focus blur interacting with the overexposed sky? Is chromatic aberration to blame? Or, is it because of the larger-than-normal color matrix used by this camera's unique sensor, and basically an artifact of RAW decoding?

Camera make  : FUJIFILM
Camera model : X-Pro1
Focal length : 35.0mm  (35mm equivalent: 53mm)
Exposure time: 0.0003 s  (1/4000)
Aperture     : f/1.4
ISO equiv.   : 200
Whitebalance : Auto
Metering Mode: pattern
Exposure     : aperture priority (semi-auto)

Asked by mattdm

Answer

The sky on a sunny day is cyan (primarily green and blue with some red). Where the intake of light is really bright, all three color components (red, green, blue) on the sensor chip are fully saturated (at their maximum counter values), which results in overexposure, or pure white in this case.

What you're seeing as a cyan outline is actually the result of that section of the image not being pegged to pure white, because the darkness of the gray building is interfering with (blurring) the ligthness of the sky, and thus preventing the sensor counters from attaining their maximum (e.g, overexposed) values in that region.

If you were to shoot the image again with the same exposure values, but this time with the building in sharp focus instead of the subject, then you would see a sharp line where the building is solid gray-ish and the sky is overexposed to pure white, but no more cyan fringing.

This effect is very similar to the laser bolts in the some of the space battles in Star Wars — where light green or light red charged-particle beams are a white line in the center, surrounded by a beautiful pure color fading out to black. The pure white in the center is simply an artifact of over-exposure; there is no actual white light there.

Note that if you had a light source that was pure red, pure blue, or pure green (such that it perfectly matched the receptor frequencies of your sensor), then you would not see over-exposure toward white, but instead over-exposure to a single color component only.

Answered by Todd Lehman

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How do I avoid white color in the sky near my subject with HDR photos?

Question

I have created a HDR image from a single raw file. The way I have created it was

1. Open the RAW file in Photoshop camera raw.
2. Do some adjustments like enhancing the color, cropping etc.
3. Exporting the first image as TIFF.
4. Change exposure to +2 and exporting next one.
5. Change exposure to -2 and exporting third one.
6. Adding all three TIFF files to Photomatrix Pro and making the final HDR.

This works well, but I am not getting realistic sky. Sky parts which are closer to the subject have a white color.

• The processed HDR is available here - http://500px.com/photo/5934871. You can see a color difference to the sky near the dome of that building.
• RAW file is available here - http://dl.dropbox.com/u/52296034/IMG_2248.CR2

Why does this happen and how can I fix it?

Asked by Appu

Answer

I both agree and disagree with Matt. I agree it is just standard HDR halos, but I disagree on how to fix it.

I do a lot of HDR work, but I concentrate on photorealistic HDR, not the Stuck in Customs style. Here one of my shots:

Notice that there are not any halos. (And in the hi-rez version you can see that the moon is properly exposed!)

Smooth Highlights is what you want to use if you ace using Tone Mapping, turn it to the right. It will brighten the sky and remove the halo around the building. And, when you are done, take it back in Photoshop and adjust your brightness and contrast. I find the best way to get photorealistic HDR is to not worry too much about B&C in Photomatix but rather fix it afterwards. Get the look you want in Photomatix, get the tonality in Photoshop (or Lightroom).

I just reprocessed the shot and turned Smooth Highlights all the way to the left. (Yes, in addition to saving your RAWs, you need to save your Photomatix XMP files so you can go back.)

This shot has that typical dark HDR sky with an obvious halo around the building. Yes, the sky was darker slightly near the top, but notice how the brightness tracks the house's outline! That certainly wasn't there in the scene. (Note, I didn't straighten or crop this as the original so you can easily see the halo.)

One trick to see halos when they aren't obvious, but still there, is to zoom way back. Here's the same image downsampled to 200 pixels. The halo jumps out at you now.

And I do agree with Matt, you probably don't need HDR for that shot at all, unless of course, you want the Stuck in Customs style, in which case the halo is probably fine. Not my cup of tea but the Internet seems to not mind.

Oh, and one more thing, Photomatix has a million sliders, there are certainly other ways to get what you want, just sit and play with them. But be warned, they all seem to interact with each other!

Answered by Paul Cezanne

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How do I eliminate lens circles in my pictures?

Question

I have taken a few pictures that show these circles, and I'm getting very annoyed at this problem. Seems most common on sunny days. This particular picture is not my own, but it is a good example of what I'm talking about. I can understand certain situations where I could use this to my advantage, but I am wondering right now, is there any way to reverse or prevent this effect?

P.S. As I said before, I have realized that I can sometimes use this to my advantage. In that respect, does anyone know how to cause it?

Asked by J. Walker

Answer

This is lens flare, where reflections within the lens end up showing on your photos.

General guidance to minimise it includes:

• Avoid getting the sun in shot (and ideally, avoid having it just-out-of-frame too)
• Use a lens hood to shade the front element
• Try to use lenses that have anti-reflective coatings
• Keep the front element clean, but follow the lens manufacturers advice about use of wet cleaners (generally, don't; only use a microfibre cloth)

Answered by Rowland Shaw

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Why does the left windmill have too many blades?

Question

Ok, check this shot out:

These are the 3 windmills in Marshfield Massachusetts. It was shot from Provincetown, MA about 35 miles away. (I wasn't out to shoot them, I was shooting Right Whales... I know, life is tough...

The shot is a pixel peeped full crop. Canon 40D body, the 24-105mm F4 lens, shot at 1/640sec at f4.0 using aperture priority. ISO 100.

What is going on with the left most windmill? It the blade at around 8pm is doubled. How did that happen?

Asked by Paul Cezanne

Answer

You're seeing the effect of a focal plane shutter (also see Wikipedia on the topic). At ¹⁄₆₄₀th of a second, the whole frame isn't exposed at once, but with a moving slit between the first and trailing shutter curtain. This catches the moving windmill in different places. Helicopter blades are the typical example here, because they often show up distorted in this way.

J. Andrzej Wrotniak has a nice detailed explanation of another possibility: slow readout of the CMOS sensor. I'm pretty sure it's the focal plane shutter in this case, though, as the given explanation there doesn't make sense in this case.

(And there's another nice example in this answer on Stack Exchange, which I mention with the caveat that the person who posted it went slightly insane with unexplained vitriol for no apparent reason and started trolling the site shortly afterwards.)

Answered by mattdm

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How to avoid jpeg compression artefacts when saving photos in web resolution?

Question

When I save a web-resolution image (for example, 1920×1024px), flat areas tints often have damaged shades of color. I'm saving in Photoshop CS4 with quality 2 in standard mode, because I need the files to be less than 300k. How can I make my JPEG images this small without the compression artefacts?

An example: http://www.screencast.com/users/capsize/folders/Jing/media/f858e229-1f05-431c-98ce-5334918aa1f3

Answer

If you save a JPEG image with an extremely low quality level, you WILL get compression artifacts. Its just a simple fact of JPEG lossy compression. If you wish to avoid compression artifacts, use a higher quality setting than 2. You won't need to save at maximum quality, as most images can be saved with a fairly low quality setting without noticeable loss in detail or visible artifacts.

One thing to note is that smooth gradients and JPEG do not mix well. JPEG compresses small blocks of the image, and when it comes to smooth gradients, it tends to make all the pixels in a block the same or very similar colors. This creates distinct banding (posterization) that can be quite visible. The only way around this is to use a higher quality setting, however that will obviously increase your file size.

An excellent article that might be of help can be found here:

• An Analysis of Lightroom JPEG Export Quality Settings

The above is for Lightroom, however the fundamental concept still applies to Photoshop. The important point from the article above is that the lower range of quality settings for JPEG tend to have a minor effect on image size, and it is only when you get to the upper few brackets that image size starts to climb by any significant degree. I would suspect that you could change your quality setting from 2 to 5 and only incur a small hit to your image size.

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What is the cause of these round spots on my photograph?

Question

Is this a finger print on the lens? I don't know.
What can be the cause of the round spot on this photograph?
I have many times cleaned the lens with my fingers, never saw anything like this before.

Canon PowerShot SX210 IS

Answer

Spots like this are due to something on or very, very near the sensor. See Dust-like speck visible every few pictures — is it dust, or worse? for another example. Dust in or on the lens can't cause this problem, because, like lens scratches, they'll be so far out of focus that the effect is undetectable (in the same way a window screen becomes invisible when focused far away through it).

On an SLR, you can access the glass protecting the sensor, and it's not much of a worry, since you can clean this. And automatic cleaning technology has gotten better. With my first couple of DSLRs, this was a constant annoyance. The first one had no real self-cleaning mechanism, and the second did, but it was ineffectual. With my current camera, which I got in 2009, I've never once had a problem with dust, and I change my lenses a whole lot more than I did five years ago.

With a compact camera, though, the whole thing will need to be disassembled, and this isn't something you can easily do at home — they're not made to be user serviceable. A camera service center will be able to do it, and if they're feeling charitable may in fact clean the sensor under warranty for free. Out of warranty, or if they decide the dust is due to environmental conditions you should have been more careful about, it's generally unreasonably expensive. (I once paid $100 to have a Fujifilm F31fd cleaned in this way.)

So, in this lovely world of disposable consumer gadgets, most people consider this time to get a new camera. If you get a new camera which doesn't have either built-in sensor cleaning or a way to access the sensor for manual cleaning, consider one with environmental sealing (waterproof is also dust-proof).

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Dust-like speck visible every few pictures — is it dust, or worse?

Question

I seem to have to dust-like speck on my Sony Cybershot S950's lens, but it appears randomly every few pictures. Could something be going bad internally?

Answer

That sure looks like sensor dust, for which I'd recommend seeing the answers in the Best way to clean a DSLR sensor thread. The spot may appear "randomly" because it only appears when you're using a narrow aperture.

That said, it seems you don't have a dSLR, thus no interchangeable lenses which are what usually causes the situation where dust can get inside the camera body. I suppose it's possible that dust got inside the camera through some other way although I think that's a fairly rare occurrence.

If the camera is still under warranty, it's worth a call to the manufacturer.

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What causes strange spiral pattern on my star line?

Question

Ok, so I created this test image. The camera was behind the window, I just tested long exposure photography on the single star I saw. It was about 20 minutes of expo, star was out of focus (I really just tested the principles of this kind of photography)

As you can see there is a darker spiral line on the bright line of the star. I would like to know what caused that spiral line?

Answer

The spiral line is caused by the pattern on curtain fabric that reflects from the window glass. You can see the darker and lighter lines continue past the start trail (crank up brightness/contrast if it's not visible otherwise). Where white fabric reflects, it adds to the intensity of the star line.

For better results, take your photo outside, so there will be no reflecting glass between your lens and the star.

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What is causing this white spot in every photo with my D90?

Question

Recently I noticed that the photographs from my D90 has an odd white dot in it. The dot is present in every image. The dot becomes clearly visible as I zoom into the image. I have attached a photo taken with dark background and marked the white spot.

What could be causing this white spot? Has any had such issues? Sample image with marked white dot http://imgur.com/trYbG

Answer

This is most likely a Defective Pixel. There's not much you can do about it aside from cloning it out in Photoshop when it is obnoxious. Are you doing a lot of long exposures? Back when I was doing astrophotography I'd see a bunch of dead pixels in my camera, but never saw them in regular use. So it may be related to sensor time.

You can always contact the manufacturer and see if they have a defective pixel policy. You may get it fixed!

---

Hmmm, I just fully read the wikipedia article. By their definition, the pixel in the camera sensor would always be off, not always be on! But I tend to doubt that, on or off, they are still defective. This article over at Ken Rockwell's site calls them hot pixels. Same thing in my book. Hot, defective, whatever...

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Is it normal for the Canon kit lens to exhibit lens flare even without a UV filter?

Question

I recently bought a Canon 550D and have been experimenting with this for a week. I have been pretty satisfied with the quality, until I faced this lens flare problem. Notice the unwanted blue light near the red board with yellow star:

Attached are two pics

1. Shot with Canon 550D - 18-55 kit lens with UV filter on:

2. Shot with Canon 550D - 18-55 kit lens without UV filter on:

Is this level of lens flare normal? Or are there any quality issues with my camera?

P.S.: Pics are little blurred — I have quite shaky hands.

Answer

Yes, this is normal. It's important to realize that this is almost entirely to do with the lens and not the camera body. See What causes lens flare? for more detailed background, but basically, flare is the result of stray light bouncing around, and is common with lower-cost lenses and with wide-angle lenses — and the 18-55mm kit lens is both.

You can see that the problem is worse with the UV filter. A large, flat, and probably cheaply-coated glass element is ideal for bouncing light (but not so ideal for avoiding artifacts like this). A higher-quality filter will contribute less to the problem, but won't improve the lens itself. A higher-quality lens will have a design intended to protect more against random bouncing light.

A cheap thing you can do to help, though, is to shade the lens from out-of-scene light. You can use your hand or a piece of cardboard in a pinch, but really this is what a lens hood is for. It screws on the front of the lens and keeps some of the "extra" light away. (For your Canon kit lens, the item you want is the EW-60C lens hood.)

Of course, that won't help flare caused by lights in the scene (as in your image, where the blue ghost light appears to be caused by the bright white lamp on the building). For that, the best option is to compose differently — move so that light source is obscured, or just hits the lens in a less-obtrusive way. If that's too limiting — sometimes, that's the picture you want to take! — a lens which better controls flare (which is to say, a more expensive lens) will help get this kind of shot.

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Why do I get horizontal grey bar with flash at shutter speeds slower than max sync speed?

Question

I'm a total newbie to flash photography. And I just trying to do some test shooting with Canon Speedlite 580 EXII using my 7D.

I saw on the user manual that the maximum sync speed is 1/250 sec.

However I got the horizontal grey bar on shutter speed 1/200 and even 1/160 !

I would like to know what I did wrong... PLEASE HELP ME OUT !

See the images here:

Thank you!

/////////////////////////////////////////////////////////

I have this issue with Cactus V5, as well as pocket wizard plus II, With Pocketwizard Mini TT1 and Flex TT5 I dont have that issue maybe because of the high speed sync support ?

Thank you for all the helps !!!

Answer

The 580EX should be fired in perfect sync with the 7D. Definitely when below sync speed. You seem to have a lag in your system here so the flash fires a fraction of a second after the 2nd curtain starts to close.

UPDATE - just noticed that the flash is actually mounted on the light stand and shoots through the umbrella - and it looks like you are using a non TTL wireless trigger to fire the flash. In this case it is possible that the radio trigger introduces that lag.

UPDATE 2: You can try setting the trigger to 1st curtain if it is set to 2nd curtain and it may help overcoming this lag with the slower shutter speed (say, 1/160).

I wonder why you don't use the built in remote flash controller in the 7D? The 580EXII+7D are a (almost*) perfect match in this sense.

(*) almost, b/c it does not support high-speed sync (HSS) mode in wireless mode.

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Are these ghost light spots and vertical grain indication that my scratched polarizer is ruined?

Question

Earlier, I unfortunately dropped my polarizer. There doesn't seem to be any damage save for a light scratch which I've uncovered only after a very thorough inspection. Nonetheless, I'm worried that there is damage worse than that.

The following feature all the same subject, a Christmas Tree in front of my department's building. This is my first time shooting suchlike subjects so I'm unsure if my polarizer is damaged or if the effects are something I should expect given my subject.

Set-up: Sony SLT-A35 with 55mm lens. Only thing on my lens is a Kenko polarizer.

First look at those "ghost colors" at the top left part of the tree. No matter how I turn the polarizer, they just won't go. But farther away from the subject and it's fine; they don't appear. And yes, I tried removing the polarizer while in this frame and distance and the ghost colors did disappear.

For example: http://chadestioco.deviantart.com/art/Christmas-Tree-Scrap-270316470

Should be pretty obvious what I'm talking about.

They respond to the polarizer the way glare does; that is they decrease/increase in intensity but never go away, at least not at that particular frame.

I have a close-up shot where the ghost colors didn't appear. See "Christmas Lights Scrap 2" in my DA scrapbook.

And then there's another thing that bothers me: some grain in my other shots. I'm not sure what/how they look like to you but it doesn't look like anything ISO-related to me.

Full picture at http://chadestioco.deviantart.com/gallery/#/d4gxt9o . Enlarge/view full image, around lower left of the tree, running from the grass lawn to the sidewalk ledge. Hope you can get what I'm talking about.

This is the most distinct I can find. As far as I've inspected my other shots, they all run vertical.

Also, I have some non-dark shots with my dropped polarizer and I don't see anything troubling with them. See "Front Facing Facade" in my DA gallery.

So, is my polarizer dead? Suggestions as to what I do with it?

Answer

1) extra images are just reflections from the polarizer. How can you tell? When you rotate it, they don't move. Therefore the angle doesn't matter, nor does the scratch matter. When you remove it, they go away. Therefore it must be causing it, but it is not related to the scratch or the angle being polarized.

2) The noise is just electronic noise. The higher you go in ISO the worse the noise gets. You can use noise reduction in post processing to remove it, but this has a price, you'll also loose fine detail and the image can start looking plasticky. You shot at 1600 ISO. Don't do that unless there isn't any other way to get the shot. Try again at ISO 100. Yeah, the shutter speed won't be 1/13, it will be much longer, so use a tripod.

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Why does color reflection appear in photographs of sun or fire?

Question

What is the new color that forms in the image when photographing a sun or similar things (see attachment with a rose glare)?

Why and how it forms?

Is it good for a image?

How to increase or decrease it?

Answer

You can get all sorts of interesting shapes and colours when shooting directly into a lightsource like that. All pieces of glass reflect a certain amount of light and transmit a certain amount, so you actually get flare from everything in your scene every time you shoot, only it's usually much dimmer than the rest of the picture so you don't see it.

When you shoot into a lightsource like that, what shows up white (the flame in this case) is actually way way brighter than white, it's just the camera sensor only records up to a certain brightness. So because the object emitting light is very bright, the internal reflections it generates are much brighter and so they show up as flare.

As different colours of light are bent different amounts by the lens optics they are also reflected different amounts so it's not unusual to have strongly coloured flare. Whether it's good or not is up to you, like many things in photography some people will like it and some wont.

You won't eliminate flare when shooting directly into a bright lightsource, but it will change position as you move, also different lenses show different types of flare and to different degrees, so you just have to experiment as it's hard to predict. Filters on the front of the lens can make flare worse as there's an extra glass surface to reflect light.

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Why do I have a red dot in every photo?

Question

Every picture that I took today have a red dot on the same place in every photo. It looks like a monitor "pixel error". Is just the lens dirty or did something bad happen to my cam?

Here some samples. Best mode to detect the dot is download the picture in original size (click the (i) then download)

Answer

You're exactly right — it's the same thing as a monitor pixel error, but on your camera's sensor rather than on an LCD screen.

You can either fix it in post-processing (automatically, with many RAW-processing packages) or have it mapped out with the camera's firmware.

If you're lucky, your camera model includes a built-in feature to do that yourself in the field; failing that, it's almost always covered under warranty service.

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What is causing strange horizontal “flare” on all my photos regardless of lens and/or POV?

Question

Suddenly on (almost) all photos I have a strange light band in the middle, regardless of lens and or angle on which I shoot the photos. The camera is an old Canon 1Ds Mk II and AFIAS the line of light appears when I shoot at shutter speeds faster than approx. 1/60 secs.

Here are some test photos:

It is bad? Can I fix it?

TIA for your feedback.

Answer

Looks like a shutter malfunction (which fits with it only appearing at certain shutter speeds). The shutter is made of a series of metal blades, it looks like one of these blades is misaligned, which is exposing that part of the sensor for longer than it should be, hence the bright area.

You can have the shutter replaced at a Canon service centre.

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How can I avoid soft dull “spots” in my pictures?

Question

Recently, I've been doing a lot more photography, particularly with my 100-400mm telephoto and 16-35mm wide. I am skilled enough that I can capture the shot mostly as I want with just the camera gear, requiring minimal post processing. I'm a huge fan of ETTR now, so most of my shots are overexposed to support more shadow DR and improve the noise performance of my el-chepo 450D body.

I've noticed quite a bit lately during post processing in lightroom that my sky's or any other lighter areas of my shots tend have spots on them. They are soft and dull to one degree or another (seems to depend on the zoom factor), so probably something either on the sensor or the lens. I'm having a hard time figuring out exactly what they are, as I try to use my lens pen brush before any shoot, yet I still have them. I am having to make quite a few healing corrections post-process to get rid of them.

Is there an easy way to identify where the spots are on my lenses? Once identified, if a quick brush of the outer lens element with a lens pen (or soft camel hair brush) doesn't do the trick, does that mean the spots are on my sensor? Is it possible there is particulate inside the lens body (all of my gear is less than a year old outside of my 450D body, and its all Canon L-Series gear, so I would home that I don't already have detrimental particulates floating around inside my lenses messing up my shots. :|)

Here are a couple examples. The spots are harder to see on screen, and I first noticed them when I printed a few of my more recent shots. There are quite a few, and the shot the examples below were taken from, where the sky took up about 30% of the top edge of the photo, there were probably 10-12 different spots, some of them darker, many of them lighter. The darker ones are the ones that are very visible in prints, where as the softer ones are not as visible, but still apparent enough to ruin an expensive print.

Answer

Some dust spots on the sensor will absolutely not shift with air-pressure (blowing) alone. To shift them you need to wet clean the sensor.

I was nervous the first time I did this as I can understand most people would be. But it is not that difficult or risky, basically it involves wiping the sensor assembly with a soft rubber "wand" with a cleaning fluid dampened wipe attached to it. At this point it's worth stressing:

You are not cleaning the sensor microchip itself you are cleaning a toughened piece of glass (the low pass filter) that sits on top of the sensor.

The only damage you can do (when using the correct fluid) would be to the delicate shutter curtains, so make sure the shutter is open and will remain open for long enough to clean the sensor. This usually means ensuring the camera's power source remains uninterrupted.

Eclipse fluid has been tested on all sensors and determined to be safe. It's basically just alcohol and will evaporate without residue. See:

http://www.photosol.com/guarantee.htm

Likewise PEC PADs are highly recommended to go with it. I suggest you get a kit that includes the basic rubber wand, fluid and pec pads, then replace the fluid and pads as they run out. This is a very cost effective solution compared to all in one wet cleaning solutions. All you have to do is wrap the pads round the wand and tape them each time.

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What is causing strange horizontal “flare” on all my photos regardless of lens and/or POV?

Question

Suddenly on (almost) all photos I have a strange light band in the middle, regardless of lens and or angle on which I shoot the photos. The camera is an old Canon 1Ds Mk II and AFIAS the line of light appears when I shoot at shutter speeds faster than approx. 1/60 secs.

Here are some test photos:

It is bad? Can I fix it?

TIA for your feedback.

Answer

Looks like a shutter malfunction (which fits with it only appearing at certain shutter speeds). The shutter is made of a series of metal blades, it looks like one of these blades is misaligned, which is exposing that part of the sensor for longer than it should be, hence the bright area.

You can have the shutter replaced at a Canon service centre.

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What is causing increased noise in my recent photos?

Question

In the last two shoots I've taken I have noticed much more noise in the resulting photos. In the first instance, I put this down to my fault, as I was shooting at high ISO levels, but in the second case I overcompensated and went to a very low ISO level.

Now, this could be a software thing — I've only just started using Adobe Lightroom and Photoshop, having been using Gimp for several years — but that doesn't sound likely. I've tried the noise reduction tool in LR but could not get a satisfactory result.

Equally unlikely, to me at least, is that the CF card has changed. I was running low on space during my days out with the camera and so changed my 1GB CF to a 4GB number. Could this be the culprit?

My camera is an EOS 30D and the lens in both cases was a Sigma 18-125mm 3.5-5.6

What could be causing this to suddenly start happening? Is there anything I can do to rescue the photos?

It might not be noise I'm seeing but something else. The background is certainly speckled which I've always assumed was noise but more worryingly is the loss of detail I'm seeing in certain areas — it's as if a low quality JPEG has been rendered from the image. The same region, when viewed in the camera's LCD, has plenty of detail in place.

The image is in RAW format and the lower ISO mentioned above was compensated for by using a flash.

Answer

High ISOs don't cause noise, lack of light causes noise. If you decrease the ISO without doing something to address the level of incoming light (note if you shoot in one of the auto modes your camera will do this for you) your signal to noise ratio will get worse.

There are a few other contributory factors, for example if the sensor is left on for a long period of time and becomes hot e.g. using live view then noise can increase.

Your post processing regime also contributes, if you pull the shadows up a lot then you will see a lot more noise.

But the major contributory factors in noise are "shot noise" due to the random emission of photons from a light source and not collecting enough photons to even out the randomness, and electrical noise, from the

Both of these get better with more light (by capturing more photons the signal produced is stronger with respect to the electrical noise). The fixes are to use a longer shutter time (and tripod if necessary), open the aperture more, or use an external source of light such as a flash.

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Why are those green spots on my photo?

Question

Took a pic with my 35mm Nikon lens last night, and got this light artifact. Is this something normal? Reflection? Is it dirt on my lens? How do I prevent this from happening? I think it only happened when there is backlighting.

UPDATE: new screenshot.

Answer

Looks like lens flare to me. Odds are there's a similarly-shaped pattern of lights in front of the camera (possibly behind/above the subject since you noted backlighting) and what you're seeing is a result of those lights being bounced around inside the lens. update: Now that you've posted an updated photo, one can see the chandelier which is being reflected either by the lens itself or possibly by a filter on the lens.

For more information, see the lengthy answers to What causes lens flare?

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What is this light artifact on my pic?

Question

Took a pic with my 35mm Nikon lens last night, and got this light artifact. Is this something normal? Reflection? Is it dirt on my lens? How do I prevent this from happening? I think it only happened when there is backlighting.

UPDATE: new screenshot.

Answer

Looks like lens flare to me. Odds are there's a similarly-shaped pattern of lights in front of the camera (possibly behind/above the subject since you noted backlighting) and what you're seeing is a result of those lights being bounced around inside the lens. update: Now that you've posted an updated photo, one can see the chandelier which is being reflected either by the lens itself or possibly by a filter on the lens.

For more information, see the lengthy answers to What causes lens flare?

Check more discussion of this question.