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PostPosted: Wed Jul 20, 2011 12:05 pm 
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Joined: Fri Jul 08, 2011 12:02 am
Posts: 58
I have a superzoom at the moment, but I am considering an upgrade to a DSLR. One of the things which is confusing me is lens specs.

My superzoom, zooms x15 but lens specs for DSLR's are in mm's, can anyone explain what the mm's mean?


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PostPosted: Wed Jul 20, 2011 1:00 pm 
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Joined: Tue Oct 13, 2009 1:16 am
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The mm gives the focal length of the lens. On a full frame sensor*, 50mm focal length matches the natural focal length of your eyes. Anything wider than 50mm (that is, shorter than 50mm) will be wide angle. Anything longer than 50mm will be telephoto. If a lens gives two focal lengths, that means it can zoom all the way from one to the other. You can calculate the amount of the zoom by dividing the longest focal length by the shortest. For example, an 18-55mm lens is approximately a 3x zoom.

It's actually the non-DSLR specs that should be confusing. Five different compact cameras that all claim to have 4x optical zoom might actually have five different focal length ranges.

* Many DSLRs have a smaller sensor than a full frame sensor. These are called cropped sensors, and they have an effect on the effective focal length. Such a DSLR will give a crop factor. For example, my Canon 7D has a crop factor of 1.6. So a 50mm lens on my 7D will act like an 80mm lens instead.

_________________
Body: Canon Rebel XS, Canon EOS 7D
Lenses: Sigma 17-70mm f2.8-4.0 OS HSM DC Macro, Canon EF-S 18-55mm f3.5-5.6 IS II, Canon EF 50mm f1.8 II, Canon EF-S 10-22mm f3.5-4.5 USM, Canon EF 85mm f1.8 II USM


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PostPosted: Wed Jul 20, 2011 1:47 pm 
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Joined: Fri Jul 08, 2011 12:02 am
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And that's why I love this forum!

Like having an encyclopaedia of photography, but with extras.

Thanks origamist :)


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PostPosted: Wed Jul 20, 2011 2:06 pm 
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Joined: Thu Oct 08, 2009 11:24 am
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Watch out for Nikonfreak chiming in about a 50mm being a 50mm, whether it's on a crop or full frame!

Basically the crop sensor will only use the centre of the image circle, giving an equivalent field of view of 1.6 (Canon) or 1.5 (Nikon) times the focal length on a 35mm or full frame sensor. This is why you sometimes hear of lenses vignetting or having soft edges on full frame but it not being an issue on crop sensor bodies.

So a 50mm prime lens on an old 35mm film camera will be 50mm, 50mm on FF, the equivalent of 75mm on Nikon or 80mm on Canon.

When you see Micro 4/3 lenses quoting focal length in mm, the crop size is 2 x. therefore a 14-140 zoom will be 28-280mm on full frame, or a 10x zoom when talking about compact cameras.

Advantages of crop sensors - you can get more bang for your buck. A Nikon 70-300mm VR lens wil set you back hundreds of dollars, giving the equivalent of 450mm on Full Frame. To get that range on Full Frame you'll need thousands of dollars. The image quality will be better, but the weight and portability can be compromised. That's why you'll see many wildlife photographers sticking to crop sensor bodies as even if they have the really hardcore fast long prime lenses, they have the 1.5 or 1.6 crop to give them more range. That's also why the likes of Nikon have the D300/D300s or equivalent, which has high quality focussing, weather sealing etc in a crop sensor package.


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PostPosted: Wed Jul 20, 2011 2:11 pm 
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Richard, to get a feel for focal lengths try downloading a freebie EXIF viewer. This easily installs on your PC and then you can right click on an image to view the details of the shot.

You'll see focal length, aperture, shutter speed, everything (depending on where the picture is hosted)

Google Opanda iEXIF 2 - it's a good learning tool


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