What to add to D90, lens or flash?

[firebox40dash5]

Originally Posted by monosodium

1) The D90 kit lens (18-105VR) is around f/4 at 35-50mm. Going to f/1.8 is basically a little over a stop.

You need to check your math. Aperture is not linear, and going from f/4 to f/1.8 is actually 2 1/3 stops. That means a shutter speed over 4 times as fast to keep the same exposure.

As for getting adequate DOF, that depends on the distance you're shooting at. A 35mm at f/1.8 and 8' from the subject has >1' of DOF, which is plenty as long as you can focus quickly.

Edit: The max aperture with the 18-105mm (or 16-85mm, and probably 18-200mm also) is f/4.5 @ 35mm, and f/5 @ 50mm. So the actual difference is 2 2/3 stops for the 35mm, and 3 stops for the 50mm, assuming you're comparing the f/1.8 versions.

[monosodium]

Originally Posted by firebox40dash5

You need to check your math. Aperture is not linear, and going from f/4 to f/1.8 is actually 2 1/3 stops. That means a shutter speed over 4 times as fast to keep the same exposure.

As for getting adequate DOF, that depends on the distance you're shooting at. A 35mm at f/1.8 and 8' from the subject has >1' of DOF, which is plenty as long as you can focus quickly.

Edit: The max aperture with the 18-105mm (or 16-85mm, and probably 18-200mm also) is f/4.5 @ 35mm, and f/5 @ 50mm. So the actual difference is 2 2/3 stops for the 35mm, and 3 stops for the 50mm, assuming you're comparing the f/1.8 versions.

The post still stands; from my experience with similar subjects and the kit the OP has I find the flash more valuable and the benefits of a prime negligible at best.

A prime gives you more light but at the cost of framing flexibillity and DOF, both of which you need in spades. Once you've stopped the prime down you're at the apertures you had in the zoom, only without the capability to zoom any more and no VR. Flash isn't without it's drawbacks (you have to learn to use it properly), but off-camera the speedlight is going to be a heap more value than the glass.

[sk66]

Originally Posted by firebox40dash5

As for getting adequate DOF, that depends on the distance you're shooting at. A 35mm at f/1.8 and 8' from the subject has >1' of DOF, which is plenty as long as you can focus quickly.

That's true and fine, if you want a 7 ft wide FOV captured. At 4' and 3ft FOV the DOF is about 4".

The 50mm is even less DOF, but narrower FOV to start with so it *almost* balances...

[lil red bird]

I would say get the flash. It will allow you more flexibility in the long run. IMHO the kit lens is fine for most work, it is a general purpose lens. Read strobist and then think again.
I was/am in the same situation as you and went with the flash so i could start to play with the light using the CLS system on the D90.

[mschwarz]

First off - congratulations on the baby!

I think you can't go wrong with either choice. However, I think the flash might be the way to go for now - only because I imagine you will be taking a ton of indoor pictures of the baby. While the lens can definitely help improve indoor shots (I have the 50mm f/1.4), I think the flash will give you more flexibility to catch shots you might not otherwise be able to get.

I've got the SB-600, and I've used it only as a bounce flash so far, but it's helped tremendously on just my D60 kit lens (18-55mm).

Either way - good luck, and post pictures of the baby!