Originally Posted by Daniel Browning
<div><span style="font-size: small;"]I found this insteresting...
The difference in focal length at the short end (17mm vs. 24mm) is only 7mm but the difference in FOV is about 18 degrees.
The difference in focal length at the long end (55mm vs. 70mm) is 15mm but the difference in FOV is under 6 degrees.</div>
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It's not the numerical difference but the ratio of the focal lengths
that is important, though that doesn't directly translate into FOV in the same ratio. It
does translate into magnification, which can be related to field of
view.
Here's an example: 10mm to 20mm is a factor of 2, so, if an object at infinity has an image 1mm long on the sensor at 10mm focal length, the image will be 2mm long on the sensor at 20mm focal length. (The magnification doubles--but that's strictly true only for objects at infinity, i.e., pretty far away.)
Now start at 100mm. If the image of an object at infinity is 1mm long on the sensor at 100mm, at 110mm (same numerical difference as between 10mm and 20mm), the image will be 1.1mm long. To get the image to be 2mm long, you'd have to go to 200mm.
To put this another way, an image that fills the frame in one dimension at 20mm will be only half as wide/long at 10mm. The same ratio would hold for 200mm and 100mm or any other pair of focal lengths that are in the ratio of 2:1.
Field of view can be related to magnification with trigonometry. You can look this up on Google, if you like.
The same sort of relationship can be found between f-stops. f/1.4 and f/2 are only 0.6 apart, but that's the same ratio as between f/11 and f/16, which are 5 units apart. Both are "one stop" apart. An exposure that requires 1/30 sec at f/1.4 would require 1/15 sec at f/2. An exposure at f/11 that requires 1/30 sec would require 1/15 sec at f/16. In the case of f-stops, the operative comparison is the square of the ratio of the stop numbers. (They go up in steps of the square root of 2. The "standard" f-stops are: 1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8, 11, 16, 22, 32. Each of those is one stop from its neighbors. (One stop = a difference of 1 EV.)
The square root comes in because f-stop is defined as the ratio of the focal length to the effective aperture diameter. Thus, an f-stop of 2 would mean that the effective aperture is half the lens focal length. An f-stop of 4 would mean an effective aperture 1/4 the focal length. Thus, the ratio of the effective aperture diameters for f/4 vs f/2 would be 2. However, it's the AREA of the aperture that affects exposure. The amount of light that reaches the sensor or film is proportional to the time the shutter is open and the area of the aperture, which is proportional to the SQUARE of the diameter. Each successive "stop" represents a ratio of DIAMETERS by the square root of two and thus a ratio of AREAS by 2.
For example, the effective area of an lens's aperture at f/2 would be *4* times the area of the same lens at f/4--two stops = 4x the light. For the same exposure, you could make the shutter speed 1/4 as long (e.g., from 1/15 sec to 1/60 sec).
Then, there are Guide Numbers for strobes ...
George Slusher
Lt Col, USAF (Ret)
Eugene, OR
Funny thing is that i was about to ask the same question - i have the 10-22 as well and a 70-200 f4 L on an XTI and was thinking that the 24-70 would be it. The only issue is that it is really big and top heavy on the XTI for a walk around lens but the downside of the EF-S 17-55mm f/2.8 is the 17-22 overlap with the wide angle and the 15mm you lose on the top end. The "you can crop 55 to get 70, you cannot
"uncrop" 24 to get 17" is a good point but if i'm gonna go wide i'll prob just change to the 10-22 and shoot with that.
I'm still leaning towards the 24-70 and getting the next version of the 50d but i'll probably change my mind about 5 more times before i do something about it.
keep us updated on what you think about the EF-S 17-55mm f/2.8 IS as i would be interested in your take and if you think the 55 isn't long enough on the top end.
My thought is that the EF-S 10-22mm f/3.5-4.5 is more of a specialty lens. When I put that on my camera it's usually when I am trying to achieve a specific look. Like Bryan said, "The similarly-built Canon EF-S 10-22mm f/3.5-4.5 USM Lens turned in similar optical results to the Canon EF-S 17-55mm f/2.8 IS USM Lens in the short range of focal lengths that overlap. I see these lenses more as complementary than competing." I'm not concered about the overlap in focal length either. The full-frameL series counterparts tothese two lenses(EF 16-35mm f/2.8L & EF 24-70mm f/2.8L) have overlap between them as well.
I believe my EF-S 17-55mm f/2.8 IS is scheduled to arrive tomorrow so once I get to play with it for a little bit I'll be sure to report back!
Here's a picture taken with my new EF-S 17-55mm f/2.8 IS. It's not very represenative of what this lens can do but I am at the office and it's the only file that I have with me. I was more concerned with catching this smile than taking a technically sound picture
I don't have acess to the EXIF data but the best that I can remember is :
f/2.8, 800or 1600ISO, 1/50
[img]/cfs-file.ashx/__key/CommunityServer.Components.UserFiles/00.00.00.26.13/2009_2D00_05_2D00_13_5F00_0011.JPG[/img]
I have the 17-55mm f2.8 and it is an awsome lens. I use it on my 40D and when I recently added a 1D to my line up I found the need to buy another lens to use with my 1D. If you really plan on upgrading to a 1D body get the 24-70. While the IS is nice it is not indispensable and most of my pictures require a high shutter speed anyhow.
I had the same decision to make. A local pro advised me to get the 24-70. Then again I already have a 17-40 f/4L to get down to 17mm. I ended up with the 24-70 2.8L and I'm not sorry. Its heavy but comes with L series pedigree and drips quality. On the downside its not IS so you are back to 1/focal length shutter speeds unless you are on a *pod. Dont use it outside without the hood.
These two lenses have been compared left and right a lot. I was in the same predicament a few months back.
I decided to go for the 17-55 2.8 ISU because I need the wide end. I've been to events and social gatherings and I sure can say that 24mm isn't going to cut it when you want group shots not unless you're outside and/or have a lot of space to move back on. Most of the time, the events I've been to are in limited space. The IS is a plus too. A good low light lens, 2.8 + IS combo.
But I voted for the 24-70 because you've mentioned that your going full frame, though still not sure when. So I'm voting based in the long run of the lens you'll be using. If you're going to keep your APS-C camera then I would suggest you go get that 17-55 now then get a copy of the 24-70 after you get your full frame camera. Also, though the 17-55 has the same glass as that of L lenses, having a lens with a designated L and a red ring is something. I hope to get one of those Ls myself
Either way, you won't go wrong. It all comes down to your preference on shooting. Would you need the wide end or an extra reach on your shots?
Go pick your poison [:P]
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