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Old Dec 25, 2005, 11:46 PM   #1
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I was reading on the FredMiranda photography website a post about the 1.6 crop factor. The writer said that even though the crop factor makes a 300mm lens act like a 450mm(appx) lens, he said the image would be a lower resolution image becausethe imagefell on fewer sensor pixels.

Is that true...:? Seems backwards to me.

I thought the image froma normal EFlens was LARGER than the sensor, and therefore, the edge of theprojected image falls off the sensor, which is what made the captured image larger...

Or do I have it wrong? If so, the magnification benefit of the 1.6 crop isn't that cool.

SJ
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Old Dec 26, 2005, 9:13 AM   #2
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They are wrong, but what you're talking about in the post is unrelated to what you read on FredMiranda.

1 - The crop factor has basically nothing to do with the resolution of the camera. They clearly don't know how a sensor works to say that. A digital sensor is made up of photosites, which is the thing that really senses the light on a CCD or CMOS sensor. The Canon 1D (old camera) was a 2.7MP 1.3x crop factor camera. The 20D is a 8MP 1.6x crop factor camera. The 20D uses a smaller sensor but resolved WAY more detail than the 1D did with its larger sensor. The ability to resolve detail (what I would say "image resolution" really refers to) has to do with the number of photosites per centimeter (or some arbitrary fixed size.) Clearly the 20D has more photosites per the same unit of sensor surface than the 1D did, so the 20D will resolve way more detail.

The problem with what I said above is digital noise. Packing the photosites so close together can produce electrical/signal interference between them, which causes noise in the image. And noise obscures detail, producing a "lower resolution image". But the 20D is a very low noise camera. Canon continues to find ways of producing higher resolution sensors while reducing the noise. Isn't technology great?

So there is no way to make the blanket statement that just by having a smaller sensor you will have an image with less detail (lower resolution) than a sensor that is larger.

Now, everything else being equal, if you had two cameras with the same MP but one was a 1.3x crop and one was a 1.6x crop, the larger sensor might perform better. It should have less noise and perform better in lower light (the photosites could be larger.) But there would be fewer photosites per unit area, and therefor it would not resolve the same amount of detail. The 1.6x crop would resolve more detail (more sensors per unit area = more detail captured.)

2 - You are correct that an EF lens throws an image circle that is larger than a 1.6x crop sensor. Heck, it's even larger than the size of film/full frame sensors (but not by much.) So on lenses that have a lot of edge distortion that distorted part of the image will not be recorded because it won't land on the sensor. That is a good thing. I don't know how you get to the statement "…which is what made the captured image larger". Could you explain what you mean there? I truly don't know why you said that, so I can't comment on it.

The 1.6x crop factor is generally a good thing. The only downside that I know of is that wide angle lenses aren't that wide. So if you really want a wide-angle lens you have to buy a expensive super wide. For example; the Canon 28-135 is a fairy wide lens…. Until you put it on the 20D and it's a 44.8-216mm lens. 44.8 is almost what the human eye sees. If you really want a wide lens you need something like the 17-40, which is a 27.2-64mm lens. And the price difference between those lenses is about $200USD. (Ok, sigma makes some good wide angle lenses that are cheaper.)

I've read that you could make some wide angle lenses designed for the smaller sensor lenses that would actually have some real benefits (less distortion and other things) but I know fully understand it. So that is another benefit, if true.

Eric

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Old Dec 27, 2005, 11:54 PM   #3
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Thank you Eric, that was quite informative. I didn't think that guy was right. As for my mystery line(I don't know how you get to the statement "…which is what made the captured image larger". Could you explain what you mean there?) lets see if I can explain.

Everyone says that with the 1.6 crop, a 300mm lens acts like a 450mm (appx). Let's say you look in the camera at your subject sceneand zoom in, at, say, 3zebras drinking from a pool. Asyou zoom in,the lens brings a smaller and smaller area into view, and the world around what the lens looks at drops out. As youzoom in on the center zebra, the other two disappear outside the frame. Therefore, if you have, for example, the 28-300mm L lens I want, and zoom in, it will projectthe one zebra at FF into the camera. But because of the crop factor, the edge of the image captured by the lensfalls outside the sensor and is not recorded. For example, you just get a picture of the middle of the zebra. This is what allows an even smaller areato beprojected on the sensor, but at the full resolution of the sensor. Isn't this what makes the magnification of the image equal to a 450mm?

Now, I live out in the boonies and can't just take a quick jaunt to the local camera store, so I wonder also...when looking through the viewfinder of a 20D with the 1.6 crop,would it show me the full image captured by the FFlens or the cropped image thatwill beprojected on the sensor? If it just shows the FF image, that would require some mental work to make sure I had my image zoomed in and centered correctly for the sensor.

I agree about the wides. I was looking at the 16-35 or the 17-40as a good possibility to make up for the crop factor. I'll check the high quality Sigmas forthis as well. I wonder what the crop factor is going to be on the 20D's replacement...Are they sticking with the same sensor size?

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Old Dec 28, 2005, 10:38 AM   #4
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Is this a post comparing the 5D to the 20D?

Suppose you use the same lens on each camera. The lens projects a roughly circular image. The 5D crops a "35 mm" image out of the circle; that image is 23.9mm by 35.8mm and contains 12.8 megapixels. The 20D crops the circle to a 15mm by 22.5mm image containing 8.2 megapixels. You can take the 5D image and further crop it to the same dimensions as the image taken by the 20D. Because the 5D has a lower pixel density, you will end up with less than the 8.2 megapixels of the 20D. If you do the math, the cropped 5D image will be 2730x1820 pixels, or about 5 megapixels. So the "telephoto loss" going from the 20D to the 5D is just over 3 megapixels of resolution. But, of course, the 5D shines on wide angle.

Ken
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Old Dec 28, 2005, 10:02 PM   #5
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kenww,
Exactly right. Good to have a direct example to show the point.

speleojeff,
Your description of how the crop works to make the image look "larger" is exactly correct. That sentense isn't how I would have said it, but now that I understand what you're getting at you are exactly correct.

Using a smaller sensor is, all things being equal (which they aren't), the same as just cropping the center out of the image. As kenww pointed out in his post, you loose a lot when you crop. So you actually need a LOT more data/photosites in the full frame image so that when you crop it you get the same as the 20D's "cropped" image.

I own the 17-40 f4 L. I love it, but I also don't use it a lot. Kinda wasted on me, but I wouldn't give it up. When I need a wide angle (I almost always photograph wild animals, so I use my 600mm f4 95% of the time) it is great.

Sigma makes some good wide angles. I don't know which are good and which aren't, so be careful. But they shouldn't be ignored (and it looks like you aren't doing that.)

Eric
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