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Old Feb 1, 2007, 10:11 PM   #1
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This is my frst post on this forum. I've been lurking for a few weeks now and find the people here very friendly and helpful. I recently purchased an XTi . This is my first SLR camera, so I have a lot to learn! I've been a P&S user for many years. I've been spending all of my time lately reading online forums and reviews, etc. etc. and haven't spent a whole lot of time shooting. Part of that goes to the fact that I live in MN and the weather has been below 0 lately... not very fun to be outside shooting

Anyways, I'm trying to really understand the 1.6 crop factor. I understand that the sensor on a camera such as the XTi is not 35mm and that it is smaller.

What exactly does this mean as far as image quality, etc. ? I know when using lenses you are going to have to add .6 to the length to get the true 35mm lengh, but what does this really mean and what happens inside of the camera? If you use the same lens on a 35mm sensor and on a smaller one, where does the magnification happen?

Thanks!

Gary
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Old Feb 1, 2007, 11:12 PM   #2
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The size of the exposure on 35mm film is 24mm x 36mm. Lenses for 35mm SLRs are designed to create that size image on the film. The size of the sensor in the XTi is 14.8mm x 22.2mm, so the sensor only gets the center part of the image that the lens creates. As a result, the field of view is narrower in the XTi than in a 35mm SLR. This means that a lens on the XTi produces an image that is magnified to about 1.6 times what itwould be on a 35mm SLR (14.8 x 1.6 = 23.68 , 22.2 x 1.6 = 35.52) (Well, not exactly 24 x 36, but you get the idea.)

So a 50mm lens on a 35mm SLR would have a field of view of 46 degrees, but on the XTi some of the image that the 50mm lens produces is wasted because the sensor is smaller. On the XTi, the field of view would be about 31 degrees, or what you might get from an 80mm lens on a 35mm SLR. (Note that 50mm x 1.6 = 80mm.)

That's the "Crop Factor".

It doesn't affect image quality at all. The crop factor is just a crutch for people familiar with35mm SLRs, so they can understand how the smaller sensor affects the field of view. If you've never owned a 35mm SLR then it doesn't mean anything to you.
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Old Feb 2, 2007, 2:12 PM   #3
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TAcv is correct. I does not affect image quality at all. You actually don't really gain any reach, just a narrower field of view, so the subject will fill more of the view finder. nice when using long lenses, but for wide angle, you will loose some of that field of view. This is why a lot of lens makers have came out with 10 and 12mm lenses. All and all though everything will just seem normal to you as you are taking pictures, what you see in your view finder is pretty much what you get.
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Old Feb 2, 2007, 10:06 PM   #4
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For example: A 300mm lens on a 35mm camera is a decent telephoto but would be a normal lens on an 8x10 view camera. A 300mm lens on a point and shoot camera with a small sensor would be a telescope!
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Old Feb 5, 2007, 11:21 AM   #5
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Thanks everyone for the replies. This does help clear some things up! Also the diagram was very helpful, thanks TCav !

I have another question that's related to this issue.

I have read several times that 85mm or 100mm are the best focal lengths for portrait photography. If I want a lens for use on the XTi for portrait, am I going to have to use a 53mm lens so it has the same field of view as a 85mm full frame? I assume not, since from what I understand it's the 85mm focal length itself which makes that a good portrait lens, and not the field of view. Am I wrong?

Also, the lenses which are made for the small sensor slr's. If using at 85mm, is it going to give me the same field of view as an 85mm regular lens would on a full frame slr?

Gary
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Old Feb 5, 2007, 12:20 PM   #6
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hopkinsg wrote:

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I have read several times that 85mm or 100mm are the best focal lengths for portrait photography. If I want a lens for use on the XTi for portrait, am I going to have to use a 53mm lens so it has the same field of view as a 85mm full frame? I assume not, since from what I understand it's the 85mm focal length itself which makes that a good portrait lens, and not the field of view. Am I wrong?
When they were talking about 85mm and 100mm lenses for portraits,they were talking about35mm photography, and it is the field of view that matters. For portraits, you don't want a normal lens, because for portraits the subject usually fills the frame, and a normal lens will accentuate the nose. (Try it.) Also, telephoto lenses have a larger depth of field. On a shorter lens, the nose might be in focus but the ears are out of focus.

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Also, the lenses which are made for the small sensor slr's. If using at 85mm, is it going to give me the same field of view as an 85mm regular lens would on a full frame slr?
No. The focal length of a lensis the focal length of a lens. What determines the field of view is the size of the captured image, whether it be on film or on a CMOS sensor.

Cameras that use 2-1/4 film (twin-lens reflex cameras like the Yashica Mat-124 G, and large format SLRs like Hasselblad and Bronica) use 80 mm lenses as "normal" lenses, and 120 and 180mm lenses as portrait lenses. In fact, the Hasselblad 30mm lens is a "Fisheye" lens, but it would be a "Wide-Angle" on a 35mm SLR, and a "Normal" lens on your XTi.
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Old Feb 5, 2007, 8:05 PM   #7
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Caboose wrote:
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TAcv is correct. I does not affect image quality at all. You actually don't really gain any reach, just a narrower field of view,
This is really all any telephoto lens does, and why the effect is the same as using one with a longer focal length. It is the ratio between lens focal length and film or sensor size which determines the FOV.

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Old Feb 6, 2007, 4:08 AM   #8
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Some lenses even improve on a 1.6x crop camera.

Some lenses have small problems on the edge of the lens, but the 1.6 crop camera only uses the center part of lenses made for full frame camera's, and doesn't use the problem parts.
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Old Feb 6, 2007, 6:01 AM   #9
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Marc H wrote:
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Some lenses even improve on a 1.6x crop camera.

Some lenses have small problems on the edge of the lens, but the 1.6 crop camera only uses the center part of lenses made for full frame camera's, and doesn't use the problem parts.
Or, more correctly, the 1.6 crop factor crops out the flaws in lenses that have problems near the edges of a 35mm exposure. So a lens that isn't very good on a 35mm SLR might be ok on your XTi.

But this is Canon we're talking about. There aren't a lot of "bad" lenses for a Canon.
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Old Feb 7, 2007, 12:46 AM   #10
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Three issues here; perspective, depth of field, and field of view.

"Field of view" has been covered pretty well. You multiply the crop factor by the focal length to get the equivalent on a 35mm camera. The focal length doesn't change, a 50mm lens of course still has a 50mm focal length, but on a 1.6 crop factor camera it will have the same field of view as a 80mm lens on a "full frame" camera.

Perspective is what will cause noses to appear too large if you get too close on a portrait. Technically, it is not directly affected at all by focal length or crop factor . Perspective is determined entirely by distance from subject. For portaits you generally want to get more than 5 feet from the subject, preferably 7-10 feet. Longer distances can be even better, but tend to be impractical indoors. A roughly 85mm equivalent is thus generally recommended for indoor portraiture because it allows you to fill the frame with your subject from somewhere around 7-10 feet.

"Depth of field" should be mentioned because this is the other thing that changes with the crop factor. To get the same depth of field (the same region in focus), apply the crop factor also to the f-stop. Since the light gathering ability doesn't change, you would also need to adjust either the lighting, or the ISO. Thus, suppose you have a shot, on a "full frame" 35mm camera, at 80mm, f8, ISO 200. Without changing the light, or subject distance, you would get the same exact photo on a 1.6 crop factor camera by shooting it at 50mm, f5.0, ISO 80.

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