Full Frame Lens on APS C?


I'm very confused as to how focal lengths and depth of field changes on an APS C sensor camera, so I've been conducting some experiments with my kit lens (18-55mm) and a 28mm full-frame lens.

When I set my kit lens to 28mm and take a photo and then take another one with the 28mm full-frame lens and compare the 2 pictures, the focal length doesn't seem to change at all. 

So what does it mean that the depth of field changes when I use a full-frame lens on a crop sensor camera? And does the focal length really change?

Thank you for making it easier to understand

4 Answers

  • keerok
    Lv 7
    2 months ago
    Favourite answer

    Confused no more!

    Focal length does not change. A 28mm lens on a full-frame camera is STILL a 28mm lens on an APS-C camera. No matter what you see, focal length NEVER changes.

    Angle of view changes. A 28mm lens on an APS-C camera has a similar angle of view of an ~18mm lens on a full-frame camera. The other way around, a 28mm lens on full-frame "sees" like a 42mm lens on APS-C. To put it another way, using the same focal length, you see 1.5X less on an APS-C camera compared to full-frame. The lens goes narrower on APS-C.

    DOF changes too. Since full-frame is bigger, DOF is shallower given the same lens at the same aperture setting. There is math to all of these but since I only had one math subject in college, I will inhibit myself from going further.

  • 2 months ago

    I think lenses show actual values not equivalents - so 28 is 28. To see a difference you have to compare a picture taken at 28mm on the crop sensor and one taken at 28mm on the full frame sensor -  the crop sensor will show a narrower angle of view, thus resulting in a seemingly a longer lens.

  • 2 months ago

    The focal length doesn't change. 18-55 APS-C lens is equivalent to 28.8-88mm on a 35mm sensor. So you're just taking a (sensor magnified) centre cropped view of the bigger image. Perspective compression with telephoto is a bit of a myth because what really changes between wide angle lens image and telephoto lens image is the depth of field.  To keep an image subject  the same size as it is in a telephoto shot, you'll have to go a lot closer with a wide angle lens and put up with considerable distortion.

    See the interesting article in the link, and the bit about sensor resolution lower down as you scroll. https://photographylife.com/what-is-crop-factor#:~...

    NB: I have used old Minolta 35mm lenses on a Sony APS-C camera for many years. Nikon allows the two formats to swap too.

  • Anonymous
    2 months ago

    Full Frame Lens on APS C?

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