Is F2.8 the same on SLR and P&S?

[GaryS]

I have a Canon S2 now and after a year, I am at the point where I think its time to move to a SLR. It looks like I actually enjoy this hobby and will stick with it!

I know that ISO settings and shutter speeds mean the same thing on different cameras (and I do know that different camera have different quality levels at various ISO settings). Are F stops the same?

Lets say that with my S2 in my living room.... ISO100 and F2.8 gives me a shutter of 1/10th. If I was using an XTi at ISO100 and F2.8, would I get the same shutter speed?

Like most newcomers to SLRs, I'm trying to figure out what lens to start with, and knowing this will help me decide.... Thanks all!

[cdifoto]

It's a ratio so yes they're the same mathematically and in how much light is passed through. What they do in and/or to your images (ex. depth of field) is not the same.

[Dawoofo]

And bumping the ISO up to 400 on a Canon DSLR will give you 1/80 shutter speed and you'll get excellent quality without a whole lot of noise...unlike with the S2. :cool:

[GaryS]

Thanks Don, I suspected it would be the same. And Dawoofo, that is one of the things I am looking forward to! I find I can only use ISO200/400 in the most dire of emergencies on the S2.

Knowing that they will be similar leads me to spending more money on F2.8 lenses. I'm always shooting indoors, looking for more light to get higher shutter speeds.

[cdifoto]

Thanks Don, I suspected it would be the same. And Dawoofo, that is one of the things I am looking forward to! I find I can only use ISO200/400 in the most dire of emergencies on the S2.

Knowing that they will be similar leads me to spending more money on F2.8 lenses. I'm always shooting indoors, looking for more light to get higher shutter speeds.

An alternative (sorta) to going full tilt (and possibly broke!) right away with the f/2.8 zooms is a couple primes with light is at a premium. 3 good non-L primes can be had for less than a single f/2.8 L zoom. You lose convenience but not much else.

A couple OEM primes I can recommend are (all prices approximate off the top of my head):

1. Canon 28mm f/1.8 - $400
2. Canon 50mm f/1.4 - $325
3. Canon 85mm f/1.8 - $325
4. Canon 100mm f/2.0 - $375

I have no direct experience with 1 and 4 but I've seen great stuff out of 'em. I've owned both 2 and 4. Sold the 50mm merely due to non-use. No problems with it whatsoever. Still have and really like the 85mm.

[coldrain]

The maximum f-stop figure is about the size of the pupil (aperture) in relation to the lenght of the lens. The longer the (focal lenght of) lens, the less wide the field of view is and the more diffucult, heavy and expensive it will be to let e lot of light in.

If you have a 50mm lens which has a maximum aperture of 50mm, you have an 50/50 or 1/1 f-stop: f1.0.
If you have a 35mm lens with a 17mm wide max. aperture, you have a 17/35 or 1/2 f-stop: f2.0.
If you have a 200mm f4 lens, you have a 200/4= 50mm size of the max. aperture.
For a 200mm f2.8 lens the max. aperture already has to be 200/2.8 = 71.4mm

The S2 IS has a very short lens... it is not 36-432 mm, but 6-72mm. With a maximum aperture range of f2.7-3.5. This means that the maximum aperture size will be 1/2.7 ratio, aperture/6mm, aperture = 6 / 2.7 = 2.2mm.
And the max.aperture at 72mm: 72 / 3.5 = 20.6mm.

The amount of light falling on the SAME size area on a sensor or film with an f2.8 35mm lens on a full frame DSLR would be about the same as with your S2 IS at 6mm at f2.7. What is actually different is the size of the aperture itself... giving the S2 IS a MUCH bigger depth of field.

So, while the aperture f-stop figure means about the same for the exposure time needed at a same ISO rating, it will act totally differently in regards to depth of field, isolating your subject, bokeh.

[GaryS]

Hmm. I don't think primes are on the list for me yet. The quality always looks great, but I'm not ready to give up the flexibility yet.

Wow, thanks for all that great info Coldy! Thats great. I knew that F2.8 would have a big effect on DOF, but I was really hoping that it would be brighter too!

I want top quality images when I am trying to take good pictures, but I still want flexiblity when I'm chasing the kids around... Is it crazy to start with:

Tamron 18-200 F3.5-6.3 (for chasing the kids, would give me similar flexibility I have with the S2 now)

Sigma 18-50 F2.8EX (for my images, when I am trying to get the best photo possible)

I realize that the Sigma duplicates the entire range of the 18-200, but its sharper and brighter.... Is that nuts?

[cdifoto]

Hmm. I don't think primes are on the list for me yet. The quality always looks great, but I'm not ready to give up the flexibility yet.

Wow, thanks for all that great info Coldy! Thats great. I knew that F2.8 would have a big effect on DOF, but I was really hoping that it would be brighter too!

I want top quality images when I am trying to take good pictures, but I still want flexiblity when I'm chasing the kids around... Is it crazy to start with:

Tamron 18-200 F3.5-6.3 (for chasing the kids, would give me similar flexibility I have with the S2 now)

Sigma 18-50 F2.8EX (for my images, when I am trying to get the best photo possible)

I realize that the Sigma duplicates the entire range of the 18-200, but its sharper and brighter.... Is that nuts?

Well you said you want f/2.8 and that first one doesn't have it. It's useless in lower light and not all that great IQ but it's an ultra convenient lens at a fairly low price.

The Sigma has the f/2.8 and some say it's great and sharp and such but with focus issues on Canon bodies. Others say focus is fine. Depends how hard you push it I suppose.

In broad daylight either lens would be ok. The Sigma, while having f/2.8 and allowing more light in, might still have focus issues in that same lower light so it could be moot.

Neither lens will be great in the house unless your home is really well lit (mine isn't!) and/or you don't mind shooting at ISO1600/ISO3200. Maybe my home is a cave compared to most though...I dunno.

Personally I'd skip the Tamron completely since the idea in paying bucks for a dSLR is not just for speed but also image quality. The Sigma is better than the Tamron by far, but still not in the "top end" - but of course they're both priced accordingly. Spend a little more than the Tamron and you can have the Canon 70-300IS...an excellent lens by seemingly all accounts. You wouldn't have 50-70mm covered but a macro prime later can do that, or just not worry about it.

[coldrain]

Yeah, what cdi said! (EF 70-300 IS USM, you will not regret it)

[GaryS]

Alright, the more I think about it, the more I agree. 18-200 is off the list. I hadn't even looked into the 70-300, but I will now.

As far as photography goes, I've found everyone's house is "lit like a cave"! I've given up on natural light and gotten a slave flash for the S2 that I can bounce. I will probably add a fast prime to the mix at some point, mostly for indoors.

Thanks again!