Do You Really Need a Zoom Lens?

Do You Really Need a Zoom Lens

Ken Osborn (c) 2013

If you have a camera that can take a 25 MP (mega-pixel) image, you may have asked yourself "why do I need a zoom lens?"  If, like me, you aren't printing posters and mainly show your photos on your website, why not just use a good prime 50 mm lens?  Maybe a 70 mm if you are doing portraiture.   With a zoom you can compose the way you want in the field, but if you are going to resize the image prior to posting, what is the difference between doing that and cropping from a larger image?  After all, you have all those extra pixels does it really matter how you waste them: by reducing resolution or by cropping them away?

I decided the only way to answer that question was to try it.  

I used an 18-55 mm zoom lens on a Pentax K5IIS to take two photos.  The first at a zoom of 18 mm and the second at 55 mm.  

Image 1: 18 mm zoom

Image 2: 55 mm zoom

The 18 mm image was cropped to the focusing chart for an image size of 423 pixels wide.  The 55 mm image was cropped to the focusing chart and then resized to 423 pixels wide.  So now both images have the same target filling the view and are both at the same final resolution.  The results are immediately below with the 55 mm zoom photo on the left and the 18 mm photo on the right.  (Click on the image for a larger view.)

Comparing Resizing (left) vs Cropping (right)

  

I have concluded I will keep my telephoto!