Camera Specs – Resolution & Zoom

Posted: January 18, 2012 in Photos
Tags: , , , ,

Sometimes the simplest of questions can easily stump you.

On the trip to Tanzania, Rags asked me some simple questions about my camera specs – its resolution and maximum zoom – that I had no clue of.

The main reason was the different way in which specs are quoted for SLR and point-and-shoot cameras. She had a point and shoot which had been sold to her with specs of 15x optical zoom and 8.1 megapixel resolution. I, on the other hand, had never thought about my SLR in those terms. All I knew was that my camera sensor had good enough resolution for all my reasonable needs, and that the zoom depended on lens I used.

I gave her that answer (mixed in some gibberish to make it sound intelligent and accurate) and left it at that. Still, somewhere in my mind the two questions had been ringing around. Bored of work this evening and looking to take a non-TV break, I went Googling  for the answers, and this is what I found.

First, the easy one. My camera, Nikon D90, has a resolution of 12.3 megapixel.

Now the more complicated bit – the zoom. Most point-and-shoot cameras mention their zoom as a multiple, say 10x. Most consumers take this to mean magnification, i.e. a 10x zoom camera will make an object look 10 times bigger than with naked eye on full zoom. The reality is a bit different.

A 10x zoom, in reality, only means that the maximum focal length of a P&S camera is 10 times the minimum focal length, and thus an object will look 10 times bigger at maximum zoom than at minimum zoom. Useful, but not quite the same. The P&S marketers publish this number because it is a fixed number for each P&S camera depending on the range of lens fixed to it.

SLRs, on the other hand, have changeable lenses, so there is no single number for any particular camera. However, each SLR lens has a X-times zoom factor based on its minimum and maximum focal length.

E.g., for my 3 lenses, it’d be:

  • 50mm prime: 1x
  • 18-105mm: 5.83x
  • 70-300mm: 4.28x

Another way to look at it for SLRs is the potential zoom across lenses. So, with my 3 lenses, I have an available range of 18-300mm giving a zoom of 16.67x compared to 15x on Rags’ P&S  (35mm eqvt: 31-465mm, crop eqvt: 19-290mm).

Of course, changing lenses on an SLR is nowhere as easy as flicking the zoom button in a P&S, so the numbers are not strictly comparable.

Finally, if that 10x zoom is not really ‘zoom‘, what is? It’s called magnification, and it depends not only the maximum focal length of the lens being used, but also on the sensor size. Hence, even within SLRs using the same lens it can differ depending on whether the SLR uses 35mm film, a full frame sensor or a regular crop sensor.

On a 35mm film or a full frame SLR, a 50mm lens is believed to give approx the same image as human eye. So, a 50mm prime lens, like the one I have would have a magnification of 1x. But that’s on a full frame SLR. Most of the SLRs amateurs use, including my D90, have a crop sensor. That is because the image on them is a cropped version of what a similar lens and setup would give on a full frame SLR. The crop ratio is fixed – 1:1.6. So, the same 50mm lens that is 1x on a 35mm film or full frame SLR, has a magnification of 1.6x on my D90. Now that I’ve explained the basics, here are the maximum magnification percentages for my various lenses and Rags’ P&S:

  • 50mm prime: ~1.6x
  • 18-105mm : ~3.3x
  • 70-300mm: ~9.6x
  • Rags’ P&S (19-290mm): ~9.3x

There. I learned a lot today researching these small bits of info and hope the few who read this do too.

If you want better, deeper, explanations, these are the best two resources I found: Photography-On-The-Net Forum (specially Jim_T’s post on 5th Apr 2004) and the Luminous Landscape Tutorial.

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