The 24-105 f/4 is a better general usage "walk around" lens.
The 24-70 f/2.8 can collect twice as much light (exactly twice as much when used at f/2.8). This gives it an advantage in those situations... even though it doesn't have image stabilization and it doesn't have that extra 35mm of focal length taking it up to 105mm.
Image Stabilization is more of a big deal when you are using long focal lengths. As the focal length gets longer, the "angle of view" becomes narrower. This means a very tiny amount of movement of the lens will translate into what appears to be a very BIG movement when looking through the camera. Consequently it's more difficult to hold a camera steady enough when hand-holding a lens that has a very long focal length.
But it's actually easy to hand-hold a camera steady enough at short focal lengths. This is why shorter focal length lenses tend to not bother with image stabilization. It's generally not needed.
The "normal" focal length for any camera can be found by taking the diagonal measurement of the sensor in millimeters. In the case of your camera it's about 29mm (you could round off and call it 30mm). IF you are using a lens whose focal length (in mm) is the same as the diagonal measure of your chip (also in mm) then that angle of view is considered a 1x magnification and "normal". You are neither "wide angle" nor "telephoto" at that focal length. Anything shorter is considered wide angle. Anything longer is considered telephoto.
That means the 24mm end of these lenses is only going to provide a very moderate amount of wide-angle (not much). The standard working zooms that Canon produces specifically for the cameras with APS-C sensors (like the 7D) are using in the 18mm range on the wide end. Canon makes an 18-135mm zoom which is a good "walk around" lens. It's not as good as the 24-105 and not nearly as good as the 24-70 because that lens has a "variable focal ratio" and doesn't collect as much light. The 24-70 and 24-105 are both "L" series lenses -- Canon's top-end glass. But one of the rules for qualifying as an "L" series lens is that it must work with EVERY Canon EOS camera made... past or present (and presumably future). Since the "EF-S" lenses can't be used on Canon "full frame" sensor cameras (like the 6D, 5D III, 5Ds, 5DsR, or 1D X) they don't qualify as "L" series lenses.
If you are ONLY going to buy one lens, then the 24-105mm is probably the better choice as it's a bit more general purpose.
People who buy the 24-70 f/2.8... also usually buy the EF 70-200mm f/2.8 as well ... so their focal length range is covered all the way up to 200mm. I bought both the 24-70 and the 70-200mm lenses and skipped the 24-105. But if you were only going to buy one lens... I'd probably go with the 24-105.
I've bought the EOS 7D Mark II and I've been stuck as to which lens to buy, it's between the EF 24-105 f/4L IS USM and the EF 24-70 f/2.8L II USM. I like that the f/4L has a stabilizer, and the f/2.8L seems to be more durable but what purpose does each lens serve? Which would you buy, please provide your reasoning. (Yes, I am an amateur, this is my first camera and lens but I'll be utilizing my equipment when I go off to college and take some classes. For now, I'm using the lens for when I go to France and Spain this summer)
Both of those lenses are intended for a full-frame camera (24 x 36mm sensor). On a FF camera 24mm is a wide angle, and 105mm is a moderate telephoto. 70mm is a very mild telephoto, at the low end of what you might consider to be a portrait lens. 50mm is considered "normal", and the 24-70 lens is designed to have about the same reach on either end. The 24-105 is intended as a "walking around" lens on a FF camera, and those of us who own some variant of the 5D or 6D pretty much all have one. It has good reach and excellent image quality, and its size and weight are manageable. The 24-70 is a very high quality general-purpose lens that is good for any type of photography within its range. It's very large and very heavy, and it doesn't have IS, so it isn't an ideal walker. But its f/2.8 aperture is an advantage, so if I had one, I'd use it as an indoor event lens on my 5D3. (And if I were using two FF cameras at an event, I'd pair it with the 70-200mm f/2.8L IS II, with whose range it neatly dovetails.)
But your camera has the smaller APS-C (not FF) sensor. And on that camera, "normal" is around 30mm; 24mm isn't particularly wide; and 70mm is in the telephoto range. 105mm is pretty long. So while either of the lenses you're considering will work on your camera, you'd find your equipment somewhat biased towards the telephoto end. If that fits your style of photography, you might as well go for it. But if you want a lens that behaves on your camera as those two would on a FF, you'll need one centered around about 30mm. The Canon lens that best fits that criterion is one that Ernie Biggs recommended in the other thread: the 17-55mm f/2.8 IS, a stabilized lens with excellent optical characteristics. I bought one with my first 7D and liked it enough that I bought one for my wife. It's not quite an "L" lens, but it also costs about half as much as the 24-70. So I'd be inclined to second Ernie's recommendation.
If you don't follow our suggestion and buy one of the FF lenses instead, you'll be off to an excellent start if you really catch the bug and advance in a couple of years to a FF camera. So you can't really go far wrong.
One further point: If you were to get the 17-55 f/2.8, you shouldn't count on using the 7D2's built-in flash. The lens is so big that it casts a shadow in the image, so you'll need an external speedlight. That doesn't bother me much, because I prefer an external speedlight; but for walking around Europe, it might be an annoyance. That said, I suspect that the gigantic 24-70 would have the same issue, the 24-105 possibly not so much.
Limited to the two choices you have asked about and you can afford the lofty price tag, get the 24-70mm f2.8. This lens is the best lens of this type on the planet. The 24-105mm f4 is the best buy in a Canon "L" lens but it is by no means in the same class as the 24-70mm f2.8.
That said I am going to still recommend you seriously consider the Canon EF-S 17-55mm f/2.8 IS USM. This is a very good lens and has a very useful focal length on a 7D as a do all lens. Later you will want a real tele and a UWA. But it is a good start.
If you do decide on the 24-105mm f4 a faster lens, as suggested for low light capability, is not necessary with the 7D Mk II. At least as a reason for getting one. Of course I will never tell someone not to buy any lenses or gear.
Another thought, remember f4 to f2.8 is only one stop. It sounds like a big difference but in reality it is not.
And IS on a fairly wide focal length lens becomes less important. Of course if I had a choice between to identical lenses, one has IS and the other didn't I would buy the IS version. But a lens that doesn't have IS will have no effect on my buying it.
I have spent more of my life using lenses without IS than I have with lenses that have IS.
"Plus the primes are simply cool. "
Yes they are. I have the EF 50mm f1.2, remember? And it is all in what you want but to say the 7D Mk II will be challenged with low light is doubtful.
Two words; ISO to 16000 and dual DIGIC 6 processors. f4 should present no problem.
I should chime in on Scotty's comment.
The biggest difference in the quality of a photograph comes from (in this order)
1) The photographer's knowledge and skill
2) The LIGHTING quality (yes... lighting trumps lenses, but hardly anyone asks about lighting. But those photos that you see and adore... they have great lighting and the lighting contributes significantly to the reason why you like them.)
3) The LENSES you use.
4) The camera BODY (in 4th place here). I ran around for about 3 years with my T1i... but using a 24-70mm f/2.8L and a 70-200mm f/2.8L lens and got great results... even though the camera body itself wasn't anything spectacular.
With this in mind, I had to choose between a body with better ISO performance or a lens that can convey more light to the sensor, I'd take the LENS!
Scotty is right that when people quibble over light senstivity, ISO performance, dynamic range, etc. the amounts of these differences are usually not large (usually less than 1 full stop). But the light gathering power of a lens... that can be huge (often at least 2, and possibly 3 or even 4 full stops of difference.)
Full frame bodies are a bit different. When I moved from my T1i to my 5D II, the ISO performance and noise level difference was HUGE. My 5D II could shoot at ISO 6400 with probably less noise than my T1i had at ISO 1600 (at least two full stops difference in my typical results.) My 5D III is even better. I survived with a T1i that had noticeable noise at ISO 1600 and unbearable noise at ISO 3200 ... ONLY because I had f/2.8 lenses and most other people were shooting with glass where f/5.6 was the widest aperture they could get (so my glass is pulling in 4x as much light as their glass -- and that's how I could get away with a low performing body when shooting in very low light.)
But even at two full stops worth of difference... it's hard to argue against a lens that can pull in three stops difference. That means you can shoot at a lower ISO value (significantly less noise) or you can shoot at higher shutter speeds (a big help for situations where you might be trying to prevent motion blur.)
I should also caution -- since Adam may be able afford lenses that cost $2k+... Canon ALSO makes an EF 50mm f/1.2L lens... do NOT buy this particular version of the 50mm lens (even if you can afford it). Get the f/1.4 version instead.
The difference has nothing to do with money (I would still recommend the f/1.4 even if the price tags were identical.)
The 50mm f/1.2L is a fabulous lens IF you are doing the sort of slow, deliberately paced, "studio" type work. The lens is more difficult to work with and focus... but if you take the time to set up and focus your shot, it's results are suprior (it is the "better" lens in that regard -- no question.)
The "problem" is it's not designed for casual shooting. If you're walking about trying to do street photography (just as an example) you would be much better off with the 50mm f/1.4 version which is going to focus faster, more accurately, and be easier to work with in these sorts of situations. If you try to do that with the 50mm f/1.2L you'll probably be frustrated with the results.
Similarly, there's a 50mm f/1.8 version (aka the "nify fifty"). But the primary purpose of this lens is to create a low focal ratio prime at an affordable "within reach" price tag for entry-level buyers -- strong emphasis on affordability. The build quality is poor and if droppd, it probably will break into two pieces (it is well know for how easily it breaks apart). The optical quality is actually decent for the in-focus areas, but poor for deliberately out-of-focus areas (the bokeh has a jittery/nervous quality to it that many do not like -- blamed primarily on the fact that the lens has a simple 5-blade aperture that isn't well rounded so the out-of-focus points blur to pentagonal shapes that overlap and creates a bit of a strange effect we don't like.) It also has a slow and rather noisy auto-focus motor (again... Canon is trying to make this lens very affordable to entry-level users -- hats off to them for that, but if you can afford the $400 f/1.4 version... it really is the one you want. The f/1.8 version is the one you "settle for" when you don't have $400 to spend on a lens.)
I usually think your insightful and accuratly supplied information is great. But not this time. Your answer is too simplistic.
A cpuple points. Nobody walks around using their 50mm f1.4 lens at f1.4 all the time. I suspect it is on f4 or f8 or f5.6 a great deal of the time. This renders the f1.4 moot. I use my 50mm f1.2 but it isn't on f1.2 very often. The only up side is you can say it is there when absolutely needed.
You certainly didn't mean to cpmpare a T1i to a 1Dmk IV for instance? So it does matter what camera body you buy. I agree if bodies are close to each other than go with the better lens. But not in the early Rebel to a 1 series comparison for instance.
In the example, a 7D Mk II is not going to be hampered with a f4 lens compared to a f2.8 or a f2 or even a f1.4. Yes, three stops is beginning to be substantial but one stop or even two is not that great. A 7D Mk II should be able to handle 1600, 3200 and 6400 and even higher without much problem. Right, a Rebel T1i could not.
Cameras get better and better all the time. Sensors get more sensitive and will require even less light to make great photos. We both know and see that.
Now if you want to switch the subject from light to DOF or Bokeh, sharpness, etc, go for it. But f4 is not going to seriously handicap a 7D Mk II.
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