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Old Dec 8, 2003, 8:58 PM   #1
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Default 1.6x Focal length multiplier... more like a crop

I've just recently discovered the concept of a focal length multiplier. And while I first it seemed like a great "feature" for telephoto pictures, do to comments like this one, "The 75300mm zoom becomes a 120480mm lens!" it now seems as if there is no real benifit.

From my understanding the CCD or CMOS simply isn't large enough to capture all the image a traditional lens projects, so a 44mm lens with a 1.6x multiplier creates an image that is framed as if it was shot with a 70.4mm lens. However the distortion of the image is still the same as a 44mm lens. Unless I'm missing something this isn't any different than shooting a 44mm shot, then cropping part of the image and getting excited that I some how got a 70.4mm image out of a smaller lens.

Am I missing something. Or is it really fair to say "The 75300mm zoom becomes a 120480mm lens!"?

I just placed an order for the digital rebel with lens kit, and realize that even though the 15mm lens will frame the picture like a 28mm lens I'm stilll going to get a much greater distortion of the image compared to if it was a true 28mm picture.
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Old Dec 8, 2003, 9:38 PM   #2
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While this statement is true:
Quote:
From my understanding the CCD or CMOS simply isn't large enough to capture all the image a traditional lens projects, so a 44mm lens with a 1.6x multiplier creates an image that is framed as if it was shot with a 70.4mm lens. However the distortion of the image is still the same as a 44mm lens. Unless I'm missing something this isn't any different than shooting a 44mm shot, then cropping part of the image and getting excited that I some how got a 70.4mm image out of a smaller lens.
In general the good part of the lens is toward the center... so in fact you're cropping the "bad" outer edges out of the image (ie you're only using the "better" part of the lens)!

The best feature IMO is really cost of the lens. Imagine what a 300mm f'/2.8 would have cost in a full-frame/film camera now that you can use a comparable 200mm f/2.8 with a 1.6x crop factor... (lighter and much more compact) 8)

... What you're paying for is really at the wide, but then you still have the 1.6x cropping factor in your favor.
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Old Dec 8, 2003, 11:36 PM   #3
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Actually there is less focal distortion than what you get in normal 35mm focal lengths. Here's some links to sample photos shot at the full 18mm wide angle length of the 18-55mm kit lens.

http://www.steves-digicams.com/2003_...s/IMG_0404.jpg

http://www.steves-digicams.com/2003_...s/IMG_0532.jpg

http://www.steves-digicams.com/2003_...s/IMG_0534.jpg


-Steve
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Old Dec 9, 2003, 8:06 AM   #4
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If you took the same sensor and made it full frame (and made no other changes) then it would be the same as croping. But the sensors that are used in cameras like the 10D, D Rebel, D100 are fairly densly laid out with photosites ( the actual individual sensors on the sensor chip.) In fact, they are laid out denser than is done in the larger chiped cameras (this is for manufacturering/cost reasons.)

For example, the Canon 1Ds has a full frame sensor. And a nice one. But you can not take its output and crop it down to the same framing as the output from the 10D and get the same results. The picture will have less data (i.e. its less MP) than the 10D's picture.

So I find the crop factor very handy. When the day comes that I can get a reasonable price on a camera with better AF than the 10D and if it has the same or better density sensor layout... I would consider buying it. But it has to have the same sensor density or I will loose focal length reach on my lenses (i.e. it needs the same inherent crop factor or I can make it in my PC.)

That is something I will not give up (for the reasons that NHL says.) I mostly photograph distant things, so I need as much reach as I can get.

Eric
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Old Dec 9, 2003, 8:30 AM   #5
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Eric

IMO Steve got it right, while the tele shooters win overall with the cropping factor... the poster is really wondering about the perpective distortion of an ultrawide (like a fisheye effect) even though the smaller sensor is going to crop the image out. Some effects of the WA lens are still there but not as much on the EF-S 18-55mm which is specifically designed for this type of "digital" camera.
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Old Dec 9, 2003, 10:39 AM   #6
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First, Thanks for all the replies.

NHL, my little rant started with the intention of understanding if it really is fair to use the multiplier to up you focal length. The whole point of my comparison of a 18mm shot and the distortion it has was to bring home the point that it's not the same as a true 28mm shot. And I'm afraid I'm still a little fuzzy, though I am glad to know the distortion factor is going to be minimal.

NHL, you're saying that you'd have to buy an expensive 320mm lens on a full frame camera, to get the same results as a 200mm lens with a 1.6x multiplier. To that I would say, no you wouldn't. You'd have to buy a 200mm lens for a full frame camera and crop the result.

Now, I can take Eric's point that the image would be better on the digital rebel than another full frame 6.3 camera. But, I guess I'm looking at it in less of picture quality terms, and more in terms of final image results.
i.e. Digital or not If I take a 320mm full frame shot, take a 200mm multiplied shot, then line the 200mm on top the 300mm they wouldn't be the same due to the, for a lack of a better work, distortion. A 320mm shot is going to flatten objects more, correct?

If you agree with this example then how can article after article say that you're getting a larger telephoto lens out of the camera? Heck I could get a 1000mm shot if I cropped the image small enough

The only thing I can think of about my comparison that isn't fair is that it doesn't take into consideration the digital element, which holds the only real advantage, being that I really couldn't get a 1000mm shot by cropping as it won't retain the same number of pixels as a camera with a 5x! Focal factor.

-Nathaniel
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Old Dec 9, 2003, 11:10 AM   #7
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original full frame image and 35% crop and 145% crop

http://www.pbase.com/image/24016966

http://www.pbase.com/image/23291403

http://www.pbase.com/image/24017452


the 1Ds has much more resolving power in the first place so cropping will give a better image in equivelent focal lengths when cropped at that equivelent. if i were to shoot an image at 300mm equivelent focal value with both cameras and then crop after that the 10D quality would fall off earlier. the main difference is that it would cost me more in lens to shoot that image

note the "Tiempo" on the shoe tongue flap.

shot at 280mm with a 70-200L IS w/1.4x extender.
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Old Dec 9, 2003, 11:59 AM   #8
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Quote:
NHL, you're saying that you'd have to buy an expensive 320mm lens on a full frame camera, to get the same results as a 200mm lens with a 1.6x multiplier. To that I would say, no you wouldn't. You'd have to buy a 200mm lens for a full frame camera and crop the result.
It's not the same like Eric and numerous others have point out:
http://www.stevesforums.com/phpBB2/v...highlight=crop

ie you are throwing out the resolution which are not the same pixels/area! (unless you have much more pixels like a DCS-14n for example). The other way is like Sjms to get a more expensive lens...
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Old Dec 9, 2003, 12:37 PM   #9
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sjms
Thanks for putting up the images, much apperciated, though neither of my two browsers can seem to load them I'll try it again at my home.

Ok, Ok I do think I'm on the same page NHL and the rest, to get as sharp of an image from a full frame I would need either a 16mp (1.6 squared x 6.3) and a 200mm lens or a 6.3mp camera with a 320mm lens.


I've always understood the the resolution would be better as the cameras packed more pixels in the area, but I don't think I really realized how much of a difference that really makes until I did the math. Either of the other two options above require major $$.
I'll have to give that other link a read, though I'm sure by the end of it I'll be all screwed up again

-Nathaniel
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Old Dec 9, 2003, 1:26 PM   #10
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Yes, now you are getting it. The easiest way to think about it is with much smaller numbers.

(Note, this logic has a HUGE assumption. That one photosite maps to one pixel. This is very wrong, except for the Fovon sensor... and I don't know if they even call the light sensitive part of the sensor a "photosite" like they do in CCD and CMOS sensors.)

If I had a full frame sensor with 10 photosites in the X and Y, you'd have a 100 pixel image.

If you have a 1/2 size sensor (1.5x crop), but you still have 10 photosites in both X & Y then you still get a 100 pixel image.

But to simulate the FOV of the smaller sensor on the larger on, you'd have to crop the picture to the center 1/2 only. But that would only have 25 pixes (5 in the X and 5 in the Y.)

The full frame sensor would have to have 20 photosites in each direction or 400 pixels in the resulting image. That is a much larger, more expensive sensor.

You are right, though. The issue about image compression and other things are attributes of the lens (which is exactly the same no matter what size sensor the camera body uses.) So to say that "using a 200mm lens on the 10D is the same as using a 320mm lens on a full frame camera with similar chip properties" is not true.

I've heard people refer to it as a "1.6x crop", and believe that is a better term.

Eric
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