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I just recently bought "Canon EF 50mm f/1.8 II" for a Canon rebel T3. This was about $100. I just came across "Canon EF 55-200mm f/4.5-5.6 II USM" which is only about $40 more right now.

My wife wanted the 50mm because a friend of hers recommended it for portrait shots and yes the pictures are better than the standard lens the camera came with.

What I am trying to figure out is whether the 55-200mm would give me the same great pictures with zoom. That's the part missing in the 50mm.

The part I am completely confused about is "f/1.8" vs "f/4.5-5.6"

The reason this lens was bought was to take portrait pictures.

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I think we have room for newbie questions — it's not like the site is overloaded. When one doesn't know exactly what one is doing already, it's sometimes hard to formulate a search. This question is really about aperture and What is a “fast” lens?, but if one doesn't know those terms it's hard to even ask about it. –  mattdm Dec 12 '11 at 19:12
    
Quick note: both the 50mm f/1.8 and the 55-250mm are considered one of the best lens for the money. I would happily get both. Actually, I do have them both, and they perform very well for their relatively low price. Anything significantly better will easily cost 5 times as much, discovering that, I learned that I don't need to spend that much for better pixels. –  Gapton Dec 13 '11 at 2:07
    
Thanks for the responses but as @mattdm pointed out, I didn't even know what the right question to ask was. However, I will try doing extra research next time. –  Pranav Shah Dec 13 '11 at 15:46
    
Just want to clarify that it is you that takes the pictures not the lenses, although they help make them even better. But without good composition, lighting and Mis-en-scene you will not have a good photo what ever lens you use. (I won a Bronze Award with a photograph taken with an iPhone) –  Graeme Hutchison Dec 14 '11 at 15:53

3 Answers 3

up vote 7 down vote accepted

The numbers 1.8, 4.5 and 5.6 refer to the aperture of the lenses. The smaller the number, the bigger it's light gathering power, and the smaller the minimum depth of field you can achieve with it. The 50mm is then a lot faster (i.e. gathers more light) than the zoom, by almost an order of magnitude.

The pros of the 50mm are then:

  • better ability to take pictures hand-held in low light
  • better ability to get a shallow depth of field.

One of the reasons the pictures taken with the 50 look so good is the fact that with this lens you can throw the background out of focus. The background-defocussing ability of a lens is proportional to the diameter of it's entrance pupil, thus:

  • for the 50mm/1.8, the entrance pupil is 28mm across
  • for the 55-200 @ 55mm, it's 55mm/4.5 = 12 mm
  • for the 55-200 @ 200mm, it's 200mm/5.6 = 36mm

This means that the 55-200 will also allow you to defocus the background. You will, however, need to go to the long end of the lens, which in turn means a long working distance: you have to stand far from your model. With the 50mm you would stay closer.

Although I don't know this particular zoom, it's also common for this kind of zooms not to provide a very good sharpness when used at their long end and full aperture. In summary I would expect the 50mm to provide nicer images and the 55-200 to be more comfortable to use.

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Are you sure the background-defocussing ability is dependent only on the diameter of the entrance pupil? How does this relate to focus distance, as the closer you focus, better defocused-background you get? –  fahad.hasan Dec 14 '11 at 10:29
    
Edgar is right , to get a portrait to 'pop' the background needs to be out of focus whihc is much harder to accomplish with a longer lens. In addition there is the problem of minimum focus distance with the longer lenses, with a cheap 200mm zoom at max zoom you will need to be at least 2-3m away from the subject meaning that your real constraints become the space you're taking the portraits in. –  user7226 Dec 14 '11 at 10:50
    
@ShutterBug: if the background is far enough (significantly farther than the subject), then the level of background blur depends only on two factors: (1) how tight you frame your subject and (2) the diameter of the entrance pupil. The first factor is your photographic choice, and the second is the “raw” background-defocusing power of the lens. –  Edgar Bonet Dec 14 '11 at 20:24
    
@Robin Brown: if the background is far enough (let's say you are outdoors), it is easier to get it defocused with a long lens because, for a given F-stop, the diameter of the entrance pupil is proportional to the focal length. If you are indoors and the background is close to the subject, then you don't care about the entrance pupil but only about the F-number (because the depth of fiend is dependent on it) and, as you say, the available space will indeed be an issue. –  Edgar Bonet Dec 14 '11 at 20:32

The "f/1.8" vs "f/4.5-5.6" part is key as it denotes how wide open the aperture will go on each lens. The 50mm lens, with a maximum aperture of f/1.8, will allow you to shoot in much lower light conditions or with much faster shutter speeds than the zoom lens, which is designated "f/4.5-5.6" to let you know that the maximum (widest) aperture is f/4.5 at the 55mm end and f/5.6 at the 200mm end.

To compare the 55mm end of the zoom lens with the 50mm lens, if you took a photograph at 1/125s at f/1.8 with the 50mm lens you'd need to use a shutter speed of 1/15s or slower with the zoom lens in identical lighting conditions (or change the camera's ISO setting by three stops to compensate), as it will not open wider than f/4.5. This makes the lens much less versatile as far as lighting is concerned. Additionally, using the f/1.8 aperture on the 50mm lens will result in a much shallower depth of field, something that is often highly desirable in portrait photography.

The 200mm zoom range would not be much use for formal portraits either, as your subject would usually be close enough to allow you to shoot at 55mm.

My advice would be to buy the 50mm lens if its primary use is for portraiture. The zoom lens sacrifices a lot for that zoom range and should only be used if you need the full range of the zoom and you have a restricted budget.

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Agreed, if you're on a budget and want to do portraits only then the 50mm is probably the best choice. –  user7226 Dec 14 '11 at 16:03

I've used both types of lenses for portraiture so this is my personal opinion:

With the 50mm lens you will get nice sharp shots but you will have to physically move back and forth to get the right framing, manual focusing will also be tricky.

With the zoom lens you will get easy framing but the shots will not 'pop' because the depth of field is too high as the aperture is limited

For easy portraits I use a Sigma 28-105mm F2.8 lens (from memory so might be slightly different specifics), this has the benefit of zoom for framing and short depth of field to give you sharp snappy pictures. Unfortunately it's at least twice the price of the other two lenses. Expensive but worth it.

Another point to remember is that the longer the lens the less distorted the resulting image will be. A short lens used close to the face can give a big nosed and bug eyed effect while a long lens will negate this and make the subject appear more natural. Taking a portrait is not just about the lens but alos about where you take it from.

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