Originally posted by Jewelltrail [I]ChipB Lens factor??
....
3) a 50mm lens, from medium format system, acts like a 75 mm lens too (due to 1.5 crop factor of aps-c sensor)
Is the third statment above true? Thanks.
No. you have been following the explanations here. But what there is a subtle but important difference between what was/is been said and whats true.
The 50mm lenses NEVER act like a 75mm lens. focal length is focal length is focal length. As someone else pointed out above. Look at the nice picture with crop lines above. This is the best way to understand whats going on.
The focal length of the lens (and aperture and subject distance etc..) establish what the image looks like. i.e whats in focus, how big things look. The format is now just a question of how much of the image to crop out. If we had and infinite format camera, and an infinitely large 50mm lens, then you could capture the universe. but instead we have to crop something out. The different crop lines above illustrate how much you cut out by using a smaller format camera.
The only reason to ever think about crop factor is if you are transitioning from a different camera system, and are trying to learn the new system
Once you have learned the new system, forget about crop factor. It has nothing to do with the lens the camera or photography. Its a language tool. If you know english and are learning italian, you carry around a book to translate. Once you have learned Italian, you can forget about the book.
The Crop factor is only used to translate form your old system that you know to the new one that you are learning
If you know that you used a 24mm lens on your old MX film camera to capture a sunset scene years ago on vacation. But you are now going on vacation with your new K20 instead... what lens do you need to take along to take in the same scene? If you put the 24mm lens on the K20, you will get the same depth of field etc... but the edges of the photo will be cropped out. So instead, you can take along a 16mm lens and get the same scene captured. The 16mm is more wide angle than the 24mm. But since the k20 sensor is smaller, the effect is it crops the center out, you are left with the same image elements as with the 24mm on the MX.