Lens speed question

[contoursvt]

Ok I'm trying to understand. So if I have lets say two lenses. One is an F2.8 and one is an F4 and lets assume both are quality lenses, will the only advantage of the F2.8 be when I've got it wide open?

So if I take both lenses and shoot at say... F5, will they produce similar results if all other factors are very equal? I assume that the lens that is F2.8 will only allow more light when its at 2.8 so if I set both for F4 or F5, then both will have to have the same shutter speed to produce similar exposure?

I was playing around with a canon 50mm prime F1.8 and I also have my 18-55 kit lens. Of course at F1.8, the 50mm allows tons of light in but it will only focus on a small area and blur the rest. If i want to get a larger area in focus, I'd be bumping up to say F8. Now if I took my kit lens and set it to F8 at 50mm, would it not be very similar? You can see I'm confused and maybe a little green

 

[Lovesong]

One thing you should remember is that the maximum aperture is what you're going to be seeing through the viewfinder. An f/2.8 lens will be twice as bright as an f/4 lens. This will have effects in what you're seeing when you're framing a shot (cropped sensor cameras are tough when it comes to the viewfinder (sorry- I come from film, and jumped on a 5D) and how your camera meters (especially in low light conditions). So while you're right in that at the same aperture, two lenses with various max aperture will give you the same shutter speed, there is a bit more to it.

 

[contoursvt]

Thanks lovesong, I think what I really need to do is play with the camera and some lenses. I picked up my 20D a while back but I've only got a few lenses and most are cheap so I think I need to buy one better lens and play with it to compare. I might go with the canon 17-40 F4. I think it will be a good walk around lens

One thing you should remember is that the maximum aperture is what you're going to be seeing through the viewfinder. An f/2.8 lens will be twice as bright as an f/4 lens. This will have effects in what you're seeing when you're framing a shot (cropped sensor cameras are tough when it comes to the viewfinder (sorry- I come from film, and jumped on a 5D) and how your camera meters (especially in low light conditions). So while you're right in that at the same aperture, two lenses with various max aperture will give you the same shutter speed, there is a bit more to it.

 

[ChrisA]

Ok I'm trying to understand. So if I have lets say two lenses. One is an F2.8 and one is an F4 and lets assume both are quality lenses, will the only advantage of the F2.8 be when I've got it wide open?

So if I take both lenses and shoot at say... F5, will they produce similar results if all other factors are very equal? I assume that the lens that is F2.8 will only allow more light when its at 2.8 so if I set both for F4 or F5, then both will have to have the same shutter speed to produce similar exposure?

I was playing around with a canon 50mm prime F1.8 and I also have my 18-55 kit lens. Of course at F1.8, the 50mm allows tons of light in but it will only focus on a small area and blur the rest. If i want to get a larger area in focus, I'd be bumping up to say F8. Now if I took my kit lens and set it to F8 at 50mm, would it not be very similar? You can see I'm confused and maybe a little green

Yes, you've got it right. One of the reasons you might want the faster lens is so that you can apply selective focus to blur out a background.

Also because the focusing is done wide open the focus sensor works better for two reasons: It has more light and it's easier to judge focus with the narrower depth of field.
and then there is the ability to shoot in lower light or with a faster shutter speed.

Are the shots at f/4 the same? They could be, maybe. But generally the prime lens will produce a better quality even if both are at f/4 and 50mm. The are other ways they could differ. mostly it's color, contrast flatness of field, geometric distortion and sharpness and most importently from an artistic standpoint how the out of focus background is renderd. Many low cost zooms will make haloo and even double images while the better lens will make a much softer looking background without these kinds of artifacts.

When shooting people snapshots indoors I like to use the 50mm nearly wide open to get rid of background clutter and I'll aim the flash backwards to soften up the shadows. So one more point.. The wider lens makes the flash more effective and gives it enough "reach" that I can use the bounce method.

 

[mchendricks]

Another thing to keep in mind is that usually a lens is sharper when it is stopped down from its max f-stop. In the case of a f2.8 lens, it will generally be sharper at f5.6 or f8 than at f2.8. So the faster lens will probably be sharper than the slower lens at the same f stop assuming it is near the max ratings of each lens. At f16 for example, there will be little difference compared to the difference at f5.6 for each lens.