Effective focal length is a myth!

Started Jan 18, 2009 | Discussions thread
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Effective focal length is a myth!
Jan 18, 2009

Crop cameras and effective focal length

My first DSLR was a Canon 1.6-crop, and I really went in for the multiplier factor. My 17-55 was actually a 27-88, and my 70-200 was actually a 112-320, and my 150-500 was actually a mighty 240-800! WONderful!

Then I got a 5D2 and learned the truth, and the truth is what I’ve read on this forum at least 100 times – that 50 mm is 50 mm is 50 mm! If I’m taking a portrait of my daughter from 8 feet away with the 30D, I’m pretending it’s an 80 mm, and therefore giving me the perspective I want for a portrait. NOT! From 8 feet with a 30D I’m getting EXACTLY the same head size and perspective I get with the 5D2, but with the 30D I just get less of the surrounding area in the image! So 50 mm is 50 mm, no matter what sensor or film you’re projecting it onto. I suggest everyone get a 5D2 and make your own experiments.

I’m saying this because, although I do have a basic grasp of simple physics and optics, it took using a full-frame camera to really drive that understanding into me. There Ain’t No Such Thing As A Free Lunch (TANSTAAFL), and pretending you get a focal-length multiplier by getting a smaller sensor is a total mind-game, with no basis in reality. What really happens is that you lose a lot of the picture, but do NOT gain any higher telephoto!

Rant mode off.

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BJCP National

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