Explain this to my newbie ears..

People like to use 1.4, 1.8, 2.0 etc.. lens for low light correct?

However, if I were going to use a Sigma 30 1.4 (this is just an example) for interior church shots where flash is forbidden, wouldn't I simply be getting shots with a very narrow depth of field with this lens?

If I wanted to capture details with f8 (again, just using this for example) aren't I going to get the same photo from a Sigma 30 1.4 at f8 as I would any other lens (kit lens) at f8 too?

Or will the 1.4 lens be faster no matter what f-stop it is at as opposed to a lens that is 3.5-5.6?

No, you're correct a sigma 30 at f/8 will let in the same amount of light as the kit at f/8.

The reason you would shoot at 1.4 is to let in more light, but here's where it gets tricky. The dof is not set only by the aperture, it's also the distance to the subject.

So unless you're getting close-ups, you're not going to have a ridiculously shallow dof, (again, you might, but it depends on how far away the subject is) but you will have the extra light that 1.4 allows you.

ClickClick wrote in post #8120562
People like to use 1.4, 1.8, 2.0 etc.. lens for low light correct?

However, if I were going to use a Sigma 30 1.4 (this is just an example) for interior church shots where flash is forbidden, wouldn't I simply be getting shots with a very narrow depth of field with this lens?

If I wanted to capture details with f8 (again, just using this for example) aren't I going to get the same photo from a Sigma 30 1.4 at f8 as I would any other lens (kit lens) at f8 too?

Or will the 1.4 lens be faster no matter what f-stop it is at as opposed to a lens that is 3.5-5.6?

You're right about aperture and depth of field (though it also depends on your distance from the subject). The amount of light coming in the lens will be the same with both lenses at f/8. However, depending on the optics of your kit lens (the image stabilizes version is supposed to have better optics than the previous iteration), you may find that a nice prime lens (like the sigma 30mm f/1.4) will give you a sharper image.

Wider apertures are good to shoot indoor portraits, especially for moving subjects.

For indoor architecture, your best bet is a tripod. Primes may have a better IQ even at f/8, but a zoom with IS may work better than a prime if you cannot take a tripod (there is no wide prime with IS).

No, f8 is f8 no matter the speed of the lens. A faster lens will, or should, focus faster than a kit lens, the kit or slower lens may not even gain focus on low light. Remember a lens is wide open until the shutter is pressed. Try manually focusing with a f/1.4 lens and a slow f3.4 or f4 lens, you will see the difference.
What a fast f1.4 or f1.2 will give you in low light is a faster shutter speed, maybe but not necessarily allowing you to hand hold the camera. It will make the depth of field narrow therefore making focus precision critical

jack

If you're always shooting your Sigma 30/1.4 @ f/8, you wasted $400.

And f/8=f/8. That's why we use the "f" notation.

Bryan Peterson's "Understanding Exposure" is a good book to read that covers many basic concepts of photography. And at less than $20, it's ALOT less expensive than a Sigma 30mm f/1.4

Depth of Field (DoF) is directly related to aperture and distance to subject. If you are shooting a large church at f/1.4, most likely you will get a lot of the image in focus.

Let's use your example of the Sigma 30mm f/1.4....

If your subject is 20 ft. away from you and you are using the max aperture of f/1.4, then there will be approx. 3 ft in front and 4 ft behind your subject that will be in focus.

Now, if you backed up to 30 ft from your subject and still remained at f/1.4 then you would have 7 ft in front and 10 ft behind your subject that would be in focus.

I think you can see the point I'm trying to make. The closer you get to your subject with a larger aperture then the thinner the depth of field becomes.

For indoor architecture, where the shallow DoF of a large aperture prime becomes limiting, IS starts making a lot of sense. If you cannot bring a tripod, that is.

ClickClick wrote in post #8120562
People like to use 1.4, 1.8, 2.0 etc.. lens for low light correct?

However, if I were going to use a Sigma 30 1.4 (this is just an example) for interior church shots where flash is forbidden, wouldn't I simply be getting shots with a very narrow depth of field with this lens?

If I wanted to capture details with f8 (again, just using this for example) aren't I going to get the same photo from a Sigma 30 1.4 at f8 as I would any other lens (kit lens) at f8 too?

Or will the 1.4 lens be faster no matter what f-stop it is at as opposed to a lens that is 3.5-5.6?

F/8 is f/8 is f/8, no matter what the maximum aperture of the lens.

For architectural interiors, where you need lots of depth of field, a good Kirk or Really Right Stuff L-bracket on your camera will provide flat surfaces against which you can brace the camera.

For example, in the picture below, I used f/16 to get the depth of field needed, and even at ISO800 I needed six seconds of exposure. I rested the camera against a rail, using the side of the L-bracket to give me a flat surface to rest on the rail.

this may help you: http://www.dofmaster.c​om/dofjs.html

You guys are awesome. I was wondering why the < 2.8 lens were so much in demand for interior shots since the f-stop that is (normally) used for these kind are always much higher than 1.4, 2.0, etc..

ClickClick wrote in post #8122017
You guys are awesome. I was wondering why the < 2.8 lens were so much in demand for interior shots since the f-stop that is (normally) used for these kind are always much higher than 1.4, 2.0, etc..

You mean "> f/2.8." The number is a fraction, so the smaller the denominator, the larger the amount of light it's passing. F/2.8 is "faster, larger, wider" than f/4. F/4 is "slower, smaller, narrower" than f/2.8.

AB8ND wrote in post #8120644
No, f8 is f8 no matter the speed of the lens. A faster lens will, or should, focus faster than a kit lens, the kit or slower lens may not even gain focus on low light. Remember a lens is wide open until the shutter is pressed. Try manually focusing with a f/1.4 lens and a slow f3.4 or f4 lens, you will see the difference.
What a fast f1.4 or f1.2 will give you in low light is a faster shutter speed, maybe but not necessarily allowing you to hand hold the camera. It will make the depth of field narrow therefore making focus precision critical

jack

Yes, not to mention that Canon consumer cameras switch off autofocusing completely below f5.6 and switch on high-precision focusing at f/2.8.

Even though the amount of time the shutter is open may be very short, you actually do use the lens at maximum aperture most of the time.