Some questions about lens

[Staples]

I ordered a Digital Rebel yesterday from Dell and was able to get a great deal. I will probably get the body overnight if they ship it on Monday because I live 80 miles from Dell. I have yet to order a lens.

I am looking for one all purpose lens rather than many that cost $400 each.

The lens that I am considering are.

Tamron 28-75mm F2.8 -- $360
Canon IS (2nd gen?) 28-135mm F3.5-5.6 -- $400
Canon IS (3rd gen) 17-85mm F4-5.6 -- $600

Things I am looking for are.

Decent wide and telephoto.
IS (more on this later)
Speed (I figure I will have to get the 50mm 1.8 as a compromise)

What I hear about the Tamron lens makes me want to buy it. It is supposedly very clear at F4 and a lot cheaper than other lens of its quality. I have shaky hands so I am very interested in Canon’s image stabilization.

Question 1
IS sounds great but can someone explain it to me? They say 3rd generation will allow 3f stops lower than what you normally would have been able. What does that mean? Does it only help at high apertures or will it also help at the minimum aperture?

The scenario I am looking at (sounds weak but bear with me) is taking an indoor picture at 17mm (widest) at F4 (fastest). The camera estimates ¼ second exposure. The picture would be blurry at that speed in my hands but will IS help? They make it sound like it only works at higher apertures. The hell with that, I just want to get minimum apertures first, I will screw around with it if that works. I have no intention of taking pictures indoors at F8 (supposedly where IS is advertised to work).

I think I need a better understanding of IS before I decide to plunk down $600.

Question 2
It seems to me that when a camera is stopped down, the pictures are usually clearer and have less barrel distortion and chromatic effects. When a lens has an aperture of 1.8 only or a range of 3.5-5.6, how is that camera able to lift the aperture to 2.5 and 9.0 respectively? I thought the lens would be limited to 1.8 and 5.6. Maybe those numbers just specify the minimums. Is that the way it is?

 

[Tim_axe]

First, understanding a little bit about the apeture numbers could help a bit...

Some common f-stop number "stops" are:
f/1.0
f/1.4
f/2.0
f/2.8
f/4.0
f/5.6
f/8.0
f/13.0
f/16.0

What you'll notice for the most part is they are all powers of the square-root of two... 1.4 * 1.4 = 2.0. 1.4 * 2 = 2.8. 2.8 * 1.4 = 4.0, etc. The smaller the "f-number" is, the "faster" the lens, and the more light it can get to the sensor. IE, an f/2.8 lens lets in twice as much light as an f/4.0 lens. And f/4.0 lets through twice as much light as f/5.6, and f/2.8 is 4 times the light of f/5.6.

When you have twice as much light, you can double the shutter speed; essentially letting in light for half the time.

Common shutter speeds are:
1/2000
1/1000
1/500
1/250
1/125
1/60
1/30
1/15

You can see that the shutter speed is half/double the one right above/below it...

If you're indoors and can get 1/15 at f/4.0, and your lens lets you, you could open the lens up to f/2.8 and then adjust the shutter speed to let light in for half the time, and get 1/30 at f/2.8...



Now to address your questions:

1. IS is a system to stabalize the image. Basically when IS is on, it cancels out some shaking/vibrations from your hand to steady the image as it is recorded by your camera. The claim of "3 stops" or whatever comes from the following rule:

"Hand-Holding Rule"
Shutter speed must be 1/focal-length to hand hold it. IE, with a 50mm lens you need a shutter speed of at least 1/50 (1/60) to hold the camera still enough to not blur the image. When you consider the "digital cropping factor" of 1.6 on the digital rebel, the rule changes a little bit to be 1 / (focal-length * 1.6), needing a shutter speed of 1/80 to prevent some blurring from shaking...

By 3 stops, it means you could theoretically hand-hold a 50mm lens to 1/15th of a second (1 / 50 / 3), or 1/25th of a second (1 / 50 * 1.6 / 3) on a Digital Rebel.


2. The numbers provided by a lens are the fastest apeture possible. IE, the 50mm f/1.8 can open up to f/1.8, but it can also stop down to f/2.0, f/2.8, f/4.0, f/5.6, etc. Since that lens is designed to work pretty good at f/1.8, it generally gets better as you stop it down a stop or two. (IE, f/2.8, f/4.0, etc)

The f/3.5 - f/5.6 lenses, means that when zoomed to the widest it gets f/3.5, and when zoomed in closer gets f/5.6. If they're not sharp wide-open, you need to stop down a bit, and before you know it you're at f/8.0, and it might not be enough stopping down at times to really improve the image.

Generally stopping down will improve a lens. Just because it starts at f/5.6 doesn't mean it will be sharp at f/5.6.

 

[Staples]

Great explanation Tim. It also answered some other questions I had. The 1.4 (square root of 2) rule is new to me and it is something very handy to know. Glad you explained it.

The hand held rule you explained seems to assume that a person will move their hand x in y amount of time and based on this ratio, they calculated the 1/(focal length) equation. This is also very handy to know. I have thought of this myself but never really thought of this equation. I will use it from now on.

By your explanation of IS, IS would seem to compensate for more movements of the hand that you will endure for longer exposure times. Sounds like IS fits my bill and I see no reason that it wouldn't help with indoor shots.

24mm at F4 with IS (3rd gen)

Seems like it would have the potential to be clearer than.

28mm at F2.8

Although the difference isn't as big as I would have thought. I question if it is even big enough for me to still consider the EF-S 17-85m lens. The one thing I love about it is that it gets a good wide angle. I can always buy the kit lens (but I will have to kick myself for not getting it cheap with the camera.)