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Old Dec 24, 2003, 9:38 PM   #1
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Default Perspective musings

We all know that the 1.6 crop factor means that the camera cuts out a smaller rectangle out of the area of light falling on the back of the camera than a full frame sensor does. But since this smaller area is all you see, it is as if you zoomed into this smaller area using the full frame sensor.

So far so good, right?

We also like to compare to full-frame sensor cameras, since that is what we've used in the past. Thus a 50mm lens on a 10D becomes a "80mm" lens. Because a true 80mm lens on a full-frame camera would produce the same picture (field of view).

But does this 50mm lens really become a true 80mm lens on a 10D or digital rebel? I don't think so. The perspective (is the picture compressed or enlarged) is still that of the 50mm lens. After all, we just cut out a bit smaller rectangle out of the circle of light that fell on the back of our camera.

Is that right?

Barthold
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Old Dec 25, 2003, 12:06 AM   #2
-jb
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Default Re: Perspective musings

Quote:
Originally Posted by barthold
But does this 50mm lens really become a true 80mm lens on a 10D or digital rebel? I don't think so. The perspective (is the picture compressed or enlarged) is still that of the 50mm lens. After all, we just cut out a bit smaller rectangle out of the circle of light that fell on the back of our camera.

Is that right?
I don't think so... As I remember perspective is also affected by distance.

If you shoot the same picture, at the same distance, with both a 50mm and a 100mm lense, and then crop the 50mm shot to the same image size as the 100mm shot, their perspective would be the same.

So... If you use the 50mm lens on a 1.6 ratio DSLR, you would frame the image as though it was an 80mm lense, so you would probably be further away, so the perspective would be different.

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Old Dec 25, 2003, 9:41 AM   #3
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I agree with -jb.
The apparent perspective is governed by the relative size of near and far objects in the image. This would depend on the camera to subject distance and not the focal length of the lens. AS -jb pointed out, to capture a given object of the same size with a 10D you would be farther away with a 50mm lens than having the same lens on a full frame camera; therefore a different perspective would result in the image.
This can be demonstrated with a zoom lens by shooting something like a line of poles or fence pickets using two different settings of zoom without moving the camera and then cropping the wider angle zoom image to match the narrow zoom image. They should match in perspective.
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Old Dec 25, 2003, 9:06 PM   #4
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But I believe a difference that is true is that DOF will be different.

The 50mm on a 10D (1.6x crop) will not have the same DOF as a 80mm on a 1Ds (full-frame.) Because optically it isn't a an 80mm lens. The field-of-view will be the same (I believe. I defer to the others.)

Eric
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Old Dec 26, 2003, 1:03 AM   #5
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You guys assume too much :-) It wasn't a constraint to frame the subject the same. Yes, if you move further away (or closer) to the subject the perspective (and DOF) will change.

I believe jb has it right in his example, as long as you stay at the same distance.

To say it differently:

If you put a 80mm lens on a full frame camera and a 50mm lens on a 1.6 factor camera, and snap a subject at the same distance, will the perspective be different?

Barthold
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Old Dec 26, 2003, 7:38 AM   #6
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No as long as the camera to subject distance is the same.

Ted
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Old Dec 26, 2003, 8:54 AM   #7
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wow, this is too messed up for me to take in this morning...
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Old Dec 26, 2003, 9:33 AM   #8
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Quote:
Originally Posted by barthold
If you put a 80mm lens on a full frame camera and a 50mm lens on a 1.6 factor camera, and snap a subject at the same distance, will the perspective be different?
I'm sorry, but I don't understand what you mean by "perspective" in this context. To me that means, what I will see. What you see in the pictures will cover the same area (field of view) but the DOF will be different (for the same settings) because (I believe, please correct me) different focal length lenses with the same settings (apetures) will have different DOFs.

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Old Dec 26, 2003, 10:45 AM   #9
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Quote:
Originally Posted by barthold
If you put a 80mm lens on a full frame camera and a 50mm lens on a 1.6 factor camera, and snap a subject at the same distance, will the perspective be different?
You almost got it right the 1st time... Think about it this way: snap a picture @ 80mm on a full-frame camera, and then put a 50mm also on the same full-frame camera, and snap the subject again at the same distance, but crop its image for the field of view of an 80mm (same as what a 10D does) -> the perspective is different (ie still a 50mm) and so is the DOF (ie also still a 50mm's DOF) that Eric has pointed out, even though the field of view is the same as an 80mm after the crop!

This is very noticeable especially with super wide angles where the perspective is more dramatic! Think of it as you're putting a smaller APS sized film into a 35mm film chamber camera, it's more of a crop factor than a focal lenght multiplier...

...BTW this works in your favor too in the 1/true focal-lenght (ie before the 1.6x), shutter speed rule of thumb! :lol:
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Old Jan 1, 2004, 10:12 PM   #10
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Thanks NHL, that all makes sense!

Barthold
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