Technical info about crop factors and why....

Discussion in 'Beyond the Basics' started by battletone, Oct 14, 2009.

  1. battletone

    battletone TPF Noob!

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    So as far as I am aware, lens design for DSLRs are just updated designs from the film cameras, with a few changes for lenses like the EF-S which won't work with a FF.

    But what I am not understanding is why we are dealing with say an 18mm lens that gives us a crop/zoom whatever you want to call it that is 1.5x or 1.6x. How come a lens like Canons EF-S, which only works with their crop bodies, isn't designed so that an 18mm lens gives a view like that of an 18mm on a FF? Is it just so people don't get confused?..isn't the mm distance on components inside the lens itself?

    I mean, you see people who used fisheye lenses on 35mm and on FF cameras, and they sell the same lens for use on a crop, and people use them. But if you are loosing so much of the image that makes it what it is, and crop frames are not a complete minority of the market by any stretch....you would think they would make lenses for them.

    I guess I am asking because I read that since we are squeezing all these pixels on a sensor that isn't getting bigger, we are getting the extra information out of the same area of the lens....which in general means you need higher quality glass to really realize this, but the professional glass is designed around FF cameras, so APS-C users are by and large using mediocre glass and then pulling more and more information out of only that cropped portion of it. Does it makes sense at what I am getting at?
     
    Last edited: Oct 14, 2009
  2. Garbz

    Garbz No longer a newbie, moving up!

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    Why they don't design 18mm lenses that give us the 18mm FOV on a FF? They do they call them 12mm lenses. What you fail to realise is that this notion of mm is not something invented by 35mm film. It's just a measure of a proportion which together with a known sensor size gives you a certain angle of view. Just like 28mm is a really wide angle on full frame cameras, and 90mm is a really wide angle on a 5x4 large format camera.

    Some companies do make EF-S Fisheyes for APS cameras. But unfortunately you are a minority. Fisheye's always have been and always will be. The effort that goes into making a lens sometimes makes it not worth while. Especially when rehasing a new standard zoom with new features is far far more profitable.

    The real issue is that aside from full frame fisheye, full frame lenses are 100% compatible with their APS counterparts. All that high quality 70-200mm glass works just as fine on your APS camera as it does on a 5DMkII. All you get from a lens that is dedicated designed for APS cameras is a small reduction in size and weight, hardly a metric any pro uses to select their lenses.

    So why waste an R&D budget to bring the same thing again only slightly smaller and weighing 300g less to an audience with clearly less money (or they'd own full frame cameras) and to an audience where half the people are unlikely to buy it since they will consider moving to full frame down the line anyway? E.g. Nikon has a DX 17-55mm f/2.8 for their pro APS-C users. I don't know anyone who has one because everyone bought the 17-35mm full frame version to give them an upgrade path for their camera later.
     
  3. GeneralBenson

    GeneralBenson TPF Noob!

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    Great answer, Garbz.
     
  4. smn_xps

    smn_xps TPF Noob!

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    Sorry Garbz I gotta disagree here. the sensor size has nothing to do with the focal length of the lens, nothing.

    the focal length of the lens is the distance between the optical center of the lens and the focal point of the image when the object focused on is at infinity.

    I think the rest of your post though tells the right story of how focal lengths and sensor sizes relate, so good job there. :thumbup:

    jerry
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    Yeah I should have worded that better. The focal length is not the proportion based on sensor size, distance, and angle of view, but it is part of the equation when working backwards. I.e. given the focal length, projection size and distance you can calculate angle of view.
     

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