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Is it the same if you take a picture setting 50mm with a 18-55mm lens and a 50mm f/1.8 prime lens? Assuming that ISO, aperture, and shutter speed are the same, will both the lens give same results?

I tried searching to see any actual comparisons, but didn't find any. If it may help, I am talking about a Canon body.

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Just to add, does anyone know of link or picture that shows a side-by-side comparison? –  deppfx Jul 29 '13 at 14:05
    
I edited the word "wide angle" out of your question, because actually, 50mm is not wide angle on an APS-C (or full-frame) DSLR. An 18-55mm lens is often characterized as a wide angle because the 18mm end is, but it actually zooms from that all the way through the range of "normal" (about 25-40mm on an APS-C Canon body) to "short telephoto" at the longer end. The same range on a larger-sensor camera would be considered ultra-wide to wide to normal. –  mattdm Jul 29 '13 at 14:31
    
You may also be interested in Which scenarios are better shot with a prime lens versus zoom lens or macro lens?. And, in general, many of the questions in the field of view tag will probably help with understanding the subject. –  mattdm Jul 29 '13 at 14:35

4 Answers 4

up vote 15 down vote accepted

Provided you keep focus distance, ISO, aperture & shutter speed the same, and you zoom your 18-55mm lens to exactly the same focal length as the 50mm prime (which wont be exactly 50mm) then the images will very extremely similar when viewed as a whole.

On closer inspection you will see differences in the level of distortion, sharpness, contrast and possibly colour balance, as well as a possible change in the focal plane in the corners of the image (due to different levels of field curvature). Bokeh and out of focus highlights (if present) will look slightly different due to differing number of aperture blades.

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Will the bokeh look different? –  Unapiedra Jul 29 '13 at 14:02
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@Unapiedra yep, most 50 f/1.8s I've seen have 5 aperture blades which leads to distinctive bokeh –  Matt Grum Jul 29 '13 at 14:09
    
Excellent answer. I can only add that prime lenses usually enable to shoot with a wider aperture, which is a big advantage :) –  Itay Gal Jul 29 '13 at 14:12
    
The new Pentax 50mm f/1.8 follows the recent trend and has seven rounded blades. (FWIW, Pentax's classic 50mm f/1.7 had six.) But I also wanted to add that aperture blades aren't the only thing that will affect bokeh; spherical aberration is likely to be significantly different. –  mattdm Jul 29 '13 at 14:24

Focal Length is Focal Length and focal length determines the field of view and thus perspective. The minimum focus distance and lens sharpness may differ. The prime lens is likely sharper than the zoom, but the field of view and perspective will be the same.

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Focal length combined with sensor/film size determines FoV. Perspective is determined by subject distance (which of course is indirectly determined by FoV combined with the photographers selection of position based on framing choice). –  Michael Clark Jul 29 '13 at 21:05
    
@MichaelClark - Yes, but in the context of the given question, we're talking about being on the same camera body, so sensor size is implied as the same. Similarly, while less direct, the same shooting position is implied as well, thus same distance to subject. Clearly the shot would be different if you used a different camera body or moved the subject or camera between them. That much is obvious. –  AJ Henderson Jul 29 '13 at 22:35

All that is likely to be the same is the field of view. Distortion, chromatic aberration, color, glare, depth of field (allowed by a wider maximum aperture), bokeh, and general sharpness can all vary. This is why some lenses cost thousands of dollars.

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Given the same aperture, how will depth of field vary? +1 to the rest, though. –  mattdm Jul 29 '13 at 14:35

Focal distance is the same for all lenses. If, for example, you get two hypothetical lenses; one fixed focal-length 50mm & set it to f/8 & a 18-55mm zoom lens & set it to exactly 50mm @ f/8 & assuming that both those lenses have the exact same color saturation, sharpness.. etc. image quality & use the exact same settings on the same camera & shoot the subject from the exact same angle/distance @ the same lighting conditions, you'll get the exact same image out of both lenses.

Focal length is focal length, it's a way to describe the angle the lens covers, so the angle covered by a 50mm lens should be the same angle that is covered by any other lens, if both are used on the same type of sensor ie. Full-frame, aps-c, aps-h, micro four-thirds... etc.

PS: f/8 was just an example, instead of using f/X, to show that you need to have the same lens settings on both lenses.

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