I think because the option for AF Assist lamp is on AUTO, you can force it ON or OFF if I'm not mistaken.

In very low light condition, it'll be very hard to focus especially if the AF Assist Lamp is not turned on. Harder if the colour is very similar.

What's the aperture and ISO you set previously?

Are you able to focus in Shutter Priority mode?

Did the AF assist lamp turned on when you half press the shutter button on other modes?

Are you very close to your subject? What focusing mode AF-A? AF-S? AF-C?

When ISO is high, you can use bigger f number (smaller aperture - to get more depth of field) and/or faster shutter speed (to compensate for blur).
Your camera should have a Depth of Field preview button, set your f number to smallest number then press and hold the Depth of Field preview button and look at the lens, see the aperture is widely open (more light can enter).
Then try to set the f number to bigger number and try again, you will see the aperture become smaller (less light can enter); It's the same concept as human eye.

Usually try to set lower ISO, but in situation where there is not enough light (low light situation) and you don't want to use flash, you can increase the ISO to use faster shutter speed (for moving objects to avoid blur or avoid shake), or when you need to use bigger f number to get more depth of field.
When you increase the ISO, you'll sacrifice on the quality/details when the noise become more apparent. Know your camera and the acceptable ISO/noise level.

For static object, you can use slower shutter speed, and if you have tripod even better so you don't have to compensate for hand shake.

35mm on DX body will give your focal range of 52.5mm, which is good for most situation.
50mm on DXbody will give your focal range of 75mm, which is quite tight for tight space. Best if you try it out before you buy and decide yourself.

Even if you don't mind manual focus, I still think having auto-focus is important; Think properly as regret later is not so nice feeling. With the 35mm f/1.8G, you can have both auto-focus and manual focus, in auto-focus mode, you can still manual focus without flipping any switch, which is another bonus.
But if you're more into portrait, the 50mm f/1.8D will give better result and the auto-focus on body with motor is faster than the 35mm f/1.8G.
Manual focus can be a pain if you didn't get the focus right and you might need to practice to get better result.

Suggest you to try out both the lens at the shop before deciding. The 35mm f/1.8G, you can get for around RM760++.

Thanks for the detailed information.
On Auto mode, the Focus mode is also using AF-S? I don't know auto use what mode or can we override, sorry

Probably like what celciuz mention about focusing on the side with single point. Did you focus on the side?

What shutter speed you're at and at what focal length (zoom).
Under low light, if the AF Assist lamp did not fire, then it's hard to get focus although possible (especially since you have candle light), you might see the focus working/trying to focus, you can keep try to half press the shutter to try to get focus. If you're zoom in and shake will prevent the focusing especially with single point focus.
I've tried focusing on very low light (dark room with very little light source (table lamp far away -, baby sleeping), shooting ISO up to 6400 (not using tripod) to get proper exposure but at high expense of detail and noise (not to mention tons of hot red pixels?); it's very hard to focus especially with AF assist lamp turned off and harder to differentiate colours at night, but still possible sometimes.

This post has been edited by Andy214: Dec 8 2010, 11:50 AM

If I'm not wrong, you can turn the AF Assist Lamp to ON (always ON). Have to double-check again, but I know you can turn it OFF (not sure if this applies to every model tough, but should be able to as I believe it's software controlled).

As for your case, if you're focus on the centre, perhaps the camera sense the lighting is adequate? (from the candle) thus it didn't fire the AF Assist lamp? You have to experiment with it.

Getting the prime lens (such as 25mm f/1.8G) definitely worth investing for indoor or low light, I would say a must have; well at least personally for me. I shoot mostly indoors with low light and I need fast shutter and I don't use flash. So, imagine at f/1.8 and shutter > 1/50 minimum, I need ISO around 1600 already to get proper exposure. If using kitlens with aperture f/3.5, gone case.... more if you zoom in where the aperture will be even smaller...

Definitely helpful when in low light, like birthday, house warming party, open-house, bachelor night, etc. But if you need to group photo, you might need to to use smaller aperture to get more depth of field.
Anyway, with the prime lens, you can get nice bokeh, good for portraits and also faster and accurate focus.
Since you already have a zoom lens, it's definitely worth getting a prime lens; You can zoom with your legs
Only problem is to get wider angle shots, which you can depend on your kit lens instead.

If your camera body have built-in motor, then can get the cheap 50mm f/1.8D which cost less than RM400.
Else, go for the 35mm f/1.8G for around RM760++.
Try out both at the shop and see the difference in distance; For indoor, tight space, 35mm is better, altough can still be a little tight since it's real length is 52.5mm on DX body.

The car looks sharp

Nah, just when I first saw the picture, I notice the car looks sharp, maybe the colour or reflection attracted my attention.

Maybe not his fault, camera Intelligent Focus kicks in

Imagine if the GF is there and question him immediately where he focus???
Just kiddin'

You can get 35mm f/1.8G which is just around RM760++, not more than RM800.

If you want to upgrade to D90, then you have option to use the cheaper 50mm f/1.8D. But the range is quite tight when space is a concern. Or if you pay a little more you can get the 50mm f/1.4D, below RM1K. I find the focus much faster than the G version, but it has only 7 diaphragm blades compare to the G which has 9 for more rounded bokeh.

I think it's best to decide yourself; Go try out the D90 and also understand more on your D3100. Can the D3100 meet your requirements and is it worth to change to D90. Also consider the loss you will suffer, unless maybe you get a very good deal on the D3100?

For 35mm and 50mm, also best if you try out both yourself, see the difference in distance. 50mm on DX body will be equivalent to 75mm, which is a medium telephoto. Try your current kitlens, zoom to 35mm, and zoom to 50mm and see the difference. You'll struggle if you're sitting on tables trying to take picture, but if you're taking portraits, it's great. The 35mm is more flexible for many situations, but if you can bear with the longer focal length of the 50mm, then you can consider it. Again, suggest you try out both lens as only your know best for yourself.

I think he planning to sell D90.

Not sure got Nikon brand sd card. The bag is standard from Nikon not sure about the sd card.
The shop either give you 4gb or 8gb, either way, it's very cheap and not much difference. They give class4 only as I know.

My advice is to survey as many shop as possible becuase they give different price and even same shop, the price is different the second time you ask. Some shop very steady, straight give you best offer, 3800 for body only. Some will try to charge you extra 3-5 percent for cc, while some no extra charge at all, all gimmick.Some shop even go to the extend of telling you the price you stated is their cost price which is ridiculous, moreover, another branch of theirs can offer it. Be wise when negotiating.

35mm f/1.8 you can get as low as RM760++ in Low Yat.

I think it's the highest or 2nd highest floor, near lift, there is a computer shop which also sells camera.

You can try other shops also as I find those shops sell camera usually change their price when ask by different person. Most shop would offer 35mm f/1.8 for around RM780++, the cheapest I've inquired before is RM760++; See if you manage to get cheaper.

16GB SDHC Extreme III (Class 6 - 30MB/s) is less than RM180.
16GB SDHC Extreme III (Class 10 - 30MB/s) is less than RM210 (I think).

Don't buy at Camera shop, they usually sell higher price.

Source:
http://forum.lowyat.net/topic/1402252

That's wierd. Me too, but he's still learning.

Most camera also can get this price, you just need to ask and bargain, some shop very good, straight offer you best price, some will pretend and tell you can't that is their cost price, bla bla bla.
The free gift is standard.

Just try every shop before you buy. My friend went lowyat and ask each floor, the price given for body only range from 3800 to 4200. So, buy from the shop which give you best offer and best service, friendly, etc. Some shop charge extra for cc, some don't, they tell you it's the same. Found a good shop, make sure the camera is new, some they give you those that been mess before by other or the staff themselves. The nice shop will let you test out other camera and lens while you're negotiating or buying process. My firiend tested the D300S on display and several lenses as well.

do you have adobe lightroom?
you can adjust the white balance or temperature there, then see the difference.

Its not about how many person, DOF is the Depth.

If 2 person standing, 1 is close to you, another far away behind, you will need really small paerture to capture both in focus.
If you have 5 person standing on same row, even big apertures you can get everyone in focus,
But if 5 person standing seprately in different rows, like line up, you'll need small apertures and focus on the middle person to get everyone in focus.

Ya, that's why I didn't specifically mention what aperture, it really depends on the depth of field. Even 1 person, depending how he stand, posture, at f1.4, not everything can be in focus as well