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03-16-2007, 05:49 AM   #1
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What the difference between a 50mm and a 50mm zoom

I'm new and I can't figure out the difference taking a picture with a prime 50mm f2.8 and using a 18-50 zoom f2.8. Or doing the same thing with a 100 or a 50 200 zoom. Is the glass that much better? Does any body have photos taken that compare the differences?


03-16-2007, 06:30 AM   #2
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Hey hmcfly,

A prime usually has less elements and has a simpler design than a zoom lens.

More elements in a lens may introduce more distortion because the elements are more than likely made of different materials and allow light to pass through them differently.

A quick and dirty analogy I use is to look through a beer mug. Now look at the same spot and put another beer mug in front of that.

A simpler design (less elements) introduces less variables to distort the light coming through the lens to the sensor (or your eye), and should produce higher image quality.

I'll use Sigma as another example. Take a look at the construction diagrams in the links below or take note of how many elements are in each lens.

Sigma 70-300 zooms have 14 elements...

Sigma 70-300/4-5.6 Macro
Sigma 70-300/4-5.6 APO Macro

You should get higher image quality with the second lens because of the introduction of a different type of element designed to minimize problems that may be detected with the first model. It's more expensive too.

While a fixed focal length prime such as the 70mm Macro has 10...

Sigma 70/2.8 Macro

and the 300mm has 11
Sigma 300/2.8

You'll notice less elements in both of these primes compared to the zooms. Unfortunately these cost an arm and a leg because of the much higher image quality and especially the bigger aperture.

Hope that helps.
03-16-2007, 06:39 AM   #3
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I don't have any direct shot to shot comparisons, but I can say that my FA 50 1.4 and my K 50 1.2 are both sharper than my 18-55, at least for apertures up to about f8 or maybe even f11. The other main difference (aside from the low light capabilities of the 'fast 50's' is that the bokeh (out of focus areas of the photograph) is much more pleasing to my eye in the primes than in th kit lens. However, that is not to say the kit lens is a bad lens. It has good close up ability, and is "fairly sharp". The main reason I use the primes is their low light capability and the bokeh helps let the subject stand out better.
Here are two examples of what I mean:
This first picture was taken with the 18-55 at 50mm, see how the background is somewhat busy? The lily in the foreground stands out, but the out of focus areas aren't smooth, they catch the eye a bit.

This next shot was taken with the FA 50 1.4 See how the background is smooth? The subject stands out better, there is nothing to catch the eye in the background. The flower is also definitely sharper.

Unfortunately it's not a completely valid comparison, the first shot was taken at f9.5 and the second at f5.6 which will change the character of the background a bit. But the principle is ths same. The "fast 50" primes have much smoother bokeh than the kit lens.

NaCl(hope this helps)H2O
03-16-2007, 06:54 AM   #4
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If you want to do a bit of browsing you can check out a couple of different galleries of mine. The first is 23 photos of the Pentax SMC 50mm 1.2 Found here:
SmugMug - saltwater : K 50 1.2 lens test

The next is 22 photos all taken with the kit lens at various different focal lengths and at different times during the summer.
SmugMug - saltwater : My Sister's Garden

NaCl(not direct comparisions but I personally think there is a difference in the IQ of both sets)H2O

03-16-2007, 07:11 AM   #5
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You have got some good replies and opinions. You can add this:

A lens has a lot of properties. One of them is the focal length (FL). When a zoom is set to 50mm it has the same FL as a 50mm prime lens. There is no use in showing a picture from a zoom and then the same picture from a prime taken at the same place and so on: they will look the same. So, with regards to FL there is no difference.

Then if you check the speed of the lens it is often, but not always another thing. Most prime lenses are built to be "fast", meaning they will have a large maximum aperture opening. It is common for 50mm lenses to be fast and they often have a max aperture opening of f/1.4 or f/1.8.

In the not showed picture above I assume the lenses are stopped down to the same aperture value. When you open up your prime lens you can use a shorter shutter time and also get a more shallow depth of field (DOF).
A prime usually flares less and also (usually) weights less.
From reading your question I understand you allready know all this.

Is the prime better? Withall the above in mind my answer is yes. Then for picture quality with regards to sharpness (a combination of resolution and contrast) the prime is usually better. Most lenses perform better when stopped down a couple of stops (f/8 and be there...) and then you'll probably not see much difference between the prime and the zoom. Many primes have better bokeh than zooms. This is not always true. Here is an example of four lenses taking the same picture (warning: 2.7 MB):

What is bokeh? Here:

Flare can be important as it lowers the picture quality in general, minute details can get lost and the contrast lessens. Zooms in general are not as good as prime lenses when it comes to handle flare. Here is a compilation of a "torch test" showing what flare can be like with the venerable SMCP A50/1.4 (290 kB):
Compare that to these two kit lens examples (small picture):

I hope that helps somewhat. (I don't show any kit lens resolution tests as I don't want to not to embarras it.) For the rest of the facts (conveniance and possibilities with zooming function, weight, price, needs) you surely answer them better yourself.

03-18-2007, 12:59 AM   #6
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Maybe it didn't help?
03-18-2007, 01:38 AM   #7
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Jonas, even though the OP hasn't replied yet, I'm sure your post has helped many random viewers Sure was a good read for me!
03-18-2007, 04:44 PM   #8
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Thank you Jon. I guess you got some good material from the posts by Salty and Alvin as well and it is good to hear you had a good read.

Now I can speak for myself only but when trying to help by providing some text and links I just dislikes it when I get treatened as a search engine or something.

A reply in this style; "Why not do some basic reading, surely there is a library somewhere around" probably doesn't deserve a Thank you! When community friends, albeit virtual of course, trying to help get the same response it doesn't make it interesting to help next time. Or try to help.

my take/opinion/feelings,


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