please explain in laymen terms the difference

in a lense with a 4-5.6 (like my 17-85 is) versus the 70-200is 4 Lseries)

and by this i mean i was looking at 70-300is from canon but will stretch for the 70-200 is 4 L series.

i am using it on for now my rebel xt which probably will be replaced by the 30d replacement in the future.

i want to know what benefit a fixed apeture (4) has over a varied apeture 4-5.6) as seen in many of canon consumer grade lens offer? of course i understand the differences in L series versus consumer series.

to me the pix look awesome in the 70-200 and feeling the lens in my hands you can feel/see the build quality difference so for me thats a no brainer.

i am just interested in why the fixed focal lengths and what benefit they offer in actual use.

thanx

edit: i read the faq which explains the change in exposure while zooming in manual mode...does this also carry over for Av/Tv mode too?

I'm sure someone here can explain this better than I can, but I'll give it a shot as to how I understand it.
The variable aperture lenses let in less light at the long end, meaning that you have to slow your shutter speed or boost iso (light sensitivity) to achieve the same results as a shot at the larger aperture (lower number). This may result in blurry or noisy photos. The advantage of the fixed aperture is that this is no longer an issue. The lens lets in the same amount of light to the sensor at 70 and 200mm, letting you keep your shutter speeds up and your iso down. The IS on the 70-300 somewhat compensates for this and if you search the forums you may find that many people believe the images to be comparable between these two lenses. The trade off for the fixed aperture is a bigger and heavier lens, the trade off for the IS is increased cost. I won't even go into the "L" vs. consumer lens fight, or this thread will get to 10 pages before it gets locked.

Don't get confused between fixed aperture and fixed focal length, which is something else altogether. Fixed focal length lenses are just that- 50mm, 85mm, 135mm, etc.- no zoom. Image quality is generally better in the prime lenses than zooms for reasons I'll let someone else expand on.

To answer your last question, the answer is yes, it affects Av/Tv mode as well. Simply put, you can shoot in Av mode at f5.6 at 70mm, but you can't shoot f4 at 200mm, and shutter speeds will adjust the same way.
That's how I understand it anyway. I'm sure someone will correct me if I'm wrong.

txdude35 wrote in post #2285706
I'm sure someone here can explain this better than I can, but I'll give it a shot as to how I understand it.
The variable aperture lenses let in less light at the long end, meaning that you have to slow your shutter speed or boost iso (light sensitivity) to achieve the same results as a shot at the larger aperture (lower number). This may result in blurry or noisy photos. The advantage of the fixed aperture is that this is no longer an issue. The lens lets in the same amount of light to the sensor at 70 and 200mm, letting you keep your shutter speeds up and your iso down. The IS on the 70-300 somewhat compensates for this and if you search the forums you may find that many people believe the images to be comparable between these two lenses. The trade off for the fixed aperture is a bigger and heavier lens, the trade off for the IS is increased cost. I won't even go into the "L" vs. consumer lens fight, or this thread will get to 10 pages before it gets locked.

Don't get confused between fixed aperture and fixed focal length, which is something else altogether. Fixed focal length lenses are just that- 50mm, 85mm, 135mm, etc.- no zoom. Image quality is generally better in the prime lenses than zooms for reasons I'll let someone else expand on.

To answer your last question, the answer is yes, it affects Av/Tv mode as well. Simply put, you can shoot in Av mode at f5.6 at 70mm, but you can't shoot f4 at 200mm, and shutter speeds will adjust the same way.
That's how I understand it anyway. I'm sure someone will correct me if I'm wrong.

don't understand your last paragraph but got all above it.

i am only interested in what f4 offers me in the 70-200 is versus what 4-5.6 offers in 70-300. i realize the extra 100mm in focal length which i don't care about at this time.

fixed versus variable is all i am wanting to know about.

thank you

What he is saying is that at 200mm f5.6 is the most you can get on that lens. But at less zoom such as 70mm you can use f5.6 or stop down more.

Ill try to take a crack at this.

The 70-200 f4 is an f4 lens throughout, meaning, whatever focal length you use the lens at, it is an f4 lens. So, f4 at 70mm, and f4 at 200mm.

The 70-300 is only f4 at the shorter end of the lens, 70mm-(i dont know what exactly). If you use the lens at 300mm, it becomes an f5.6 lens, you will not be able to change AV to f4 at that focal length.

Same with your 17-85mm lens. It is only f4 at the wider end, if you zoom to 85mm, then it becomes a 5.6 lens.

A lens with a fixed f/4 aperture lets the same amount of light into the camera at all focal lengths. The variable aperture of 4-5.6 reduces the amount of light by one stop, or half, as you move from 70 to 300.

chevysales wrote in post #2285638
in a lense with a 4-5.6 (like my 17-85 is) versus the 70-200is 4 Lseries)

and by this i mean i was looking at 70-300is from canon but will stretch for the 70-200 is 4 L series.

i am using it on for now my rebel xt which probably will be replaced by the 30d replacement in the future.

i want to know what benefit a fixed apeture (4) has over a varied apeture 4-5.6) as seen in many of canon consumer grade lens offer? of course i understand the differences in L series versus consumer series.

to me the pix look awesome in the 70-200 and feeling the lens in my hands you can feel/see the build quality difference so for me thats a no brainer.

i am just interested in why the fixed focal lengths and what benefit they offer in actual use.

thanx

edit: i read the faq which explains the change in exposure while zooming in manual mode...does this also carry over for Av/Tv mode too?

One benefit is that if you have established an exposure at 70mm (let's say 1/500 at f/4.5) because you need to stop action, perhaps; and then you zoom to 200mm, your exposure is still good. However, if your lens is a variable aperture lens with a maximum aperture at the long end of f/5.6, then your shutter speed would need to be about 1/300 in order to maintain the same exposure. If you were shooting in AV the shutter speed would be reduced by the camera, maybe causing motion blur. If you were shooting in TV, the image would be under-exposed due to the smaller than necessary aperture.

Actually, the term "fixed" aperture is incorrect and quite misleading. The proper term is "constant" aperture.

Most zoom lenses have a variable aperture as the focal length is changed. The maximum aperture in a lens such as Canon's EF 28-300mm f/3.5-5.6L IS USM, for example, changes by more than a full stop when adjusting the focal length from minimum to maximum. If you have chosen f/8, though (as an example), today's automated cameras will adjust the lens' iris to maintain f/8 as you adjust the focal length. However, if you are shooting wide open at 28mm and zoom out to 300mm, the shutter speed will have to slow down to less than half the speed it was when at 28mm.

With a constant-aperture lens, you can shoot wide open and adjust the focal length without any change in exposure settings at all.

With today's automated cameras - and the lack of an aperture control ring on the lenses - the advantages of a constant-aperture lens are not as significant as in the old days when you had to crank on the aperture ring to set your exposure. The typical zoom lens in those days was marked with aperture settings for the shortest focal length and the photographer had to remember that when using the longer focal lengths he/she had to compensate for the variable aperture AT ALL SETTINGS.

A significant advantage of the constant-aperture zoom lens, even with today's cameras, is the fact that the viewfinder will stay at the same brightness level even when zooming from one end to the other.

chevysales wrote in post #2285638
i am just interested in why the fixed focal lengths and what benefit they offer in actual use.

I assume you meant fixed aperture not focal lengths. Basically, the advantage is that it's a fixed aperture, no matter what focal length you choose, your exposure will not change.

thanks to all who responded... i get it now

looks like i am definetly opting for the new 70-200 is L over the 70-300 is also fairly new. and i at the same time i also understand the differences i couldn't understand properly... ie constant vs. variable.