How much additional light does an f2.8 really let in?

Question

I currently have the Nikon 18-55mm kit lens that came with my D3100, but I am looking for something that will let more light in among the entire focal range. I have been eyeing the Tamron 17-50mm f2.8 as a potential option, but I am wanting to clear up just a few concerns:

1) I have heard the Nikon kit lenses are actually pretty decent in terms of sharpness, contrast, optical distortion, etc. How does this compare to the Tamron lens in question? I am buying the lens to get a higher f-stop but I don't want to do that at the expense of quality.

2) Since I have only shot with kit lenses, it is difficult for me to quantify just how much extra light will be let in with a f2.8 (compared with f3.5 on my kit lens). My goal is to take sharper indoor/low-light photos without the aid of flash or too much ISO. What is the real world differences between these f-stops?

Answer 1

The Tamron is known to be optically very good and sharp wide open across the frame. I know semi pro Nikon users who use that one on a crop camera for e.g. wedding shots. The F number on your kit number is only F3.5 on the widest and if you go into Av and keep an eye on it, you see it drops very quickly to F5-5.6. The range 2.8 - 4 is a stop (double the shutter time - F3.5 vs 2.8 is a factor 1.56) and 4-5.6 is another stop (quadruple the shutter time).

Now the thing is that the longer the focal length the shorter the shutter needs to be so it is working against itself. at 18 you can do fine with 1/27s, maybe 1/40s to be sure, while at 50mm you need 1/75s minimum or 1/125 to be sure.

Another problem is that if you get serious and try to control the composition including depth of field, ambient/flash light ratio, exposure manually, you have to worry about zooming just a bit as it might change your balance.

Also if you have a filter on that makes everything darker, a Fx-5.6 lens will go darker than F5.6 for most part of your zoom range, disabling the AF which doesnt work under F5.6.

So this is all the trouble you are relieved of in a fixed aperture lens.

If your most important objective is sharp indoor photos without flash, you might need to sacrifice the convenience of zooming, and go straight for F1.4 or F1.8 primes. That's 1-2 stops faster yet from the F2.8. A F1.4 50mm is 16 times faster than your kit lens. When I shoot indoor in normal living room light I already have to use Iso 800 or 1600 depending on the light and whether I use 28mm, 50mm or 85mm. I do stop it down a bit to F2.2 to improve sharpness a bit and get a bit more focus in DOF and that gives a fast enough shutter (1/50s - 1/150s).

Answer 2

1-stop is twice, or half, the light depending on the direction of change. So from F/4 to F/2.8 you could shoot with a shutter speed that is twice as fast and get the same exposure. To achieve the same exposure with the same shutter speed you would have to adjust the ISO from, for example, 100 to 200.

So, your real world calculation would be almost twice as much light. And I believe 5.6 is the next full stop so the difference between 2.8 and 5.6 would be 4x (2.8 -> 4 = 2x + 4 -> 5.6 = 2x for a total of 4x).

Also, even though you said you don't want to use flash, if you ever did the wider aperture makes your flashes more powerful because aperture is pretty much what controls the amount of flash-light that gets to the sensor (shutter speed doesn't matter).

I think the most common reason for the wider apertures, however, is that they produce a much better bokeh, Dof, etc. Depending on what you're shooting and how much control you have it may not be an issue, but a lot of people like the 2.8 for the background blur.