[herzzreh]

I remember reading somewhere that the focusing scale on lenses designed for 35mm sensor/film are not exactly accurate when used on 1.5x crop. Does anyone have any reference to this?

[dgaies]

I remember reading somewhere that the focusing scale on lenses designed for 35mm sensor/film are not exactly accurate when used on 1.5x crop. Does anyone have any reference to this?

As far as I understand the focusing scale should be accurate regardless of the sensor size.

If the lens (designed for a full-frame sensor or film) has a DOF scale, that might be inaccurate as I believe a crop sensor has more depth-of-field at a given aperature than a larger sensor.

[RioRico]

As far as I understand the focusing scale should be accurate regardless of the sensor size.

Quite correct.

If the lens (designed for a full-frame sensor or film) has a DOF scale, that might be inaccurate as I believe a crop sensor has more depth-of-field at a given aperature than a larger sensor.

Indeed inaccurate, but in the other direction. DOF *should* be about the same for larger and smaller sensors, but in practice it ain't, especially if a small-frame image is enlarged to the same size as a large-frame image.

I have a rule-of-thumb: allow 1+ stops over the distance range indicated by the DOF scale. So let's say I've put a full-frame 24mm lens on my APS-C (half-frame) K20D, set to f/8. I'll set my distance range to just inside the f/5.6 marks. So if I want infinity DOF, I'll set the infinity mark just below one f/5.6 mark, which sets my hyperfocal to the high edge of 5m, and the near end of my range shows as around 2m, just inside the other f/5.6 mark. Try it and see.

[alohadave]

Indeed inaccurate, but in the other direction. DOF *should* be about the same for larger and smaller sensors, but in practice it ain't, especially if a small-frame image is enlarged to the same size as a large-frame image.

I have a rule-of-thumb: allow 1+ stops over the distance range indicated by the DOF scale. So let's say I've put a full-frame 24mm lens on my APS-C (half-frame) K20D, set to f/8. I'll set my distance range to just inside the f/5.6 marks. So if I want infinity DOF, I'll set the infinity mark just below one f/5.6 mark, which sets my hyperfocal to the high edge of 5m, and the near end of my range shows as around 2m, just inside the other f/5.6 mark. Try it and see.

DoF is the same because it's a physical characteristic of the lens. The apparent DoF increases because you are cropping the image.

[rhodopsin]

When a full-frame lens is used on a crop-sensor camera, the angle of view is reduced, the apparent dof decreases, and the dof scale on the lens is inflated, i.e. the dof obtained will be less than the lens dof scale indicates. How much less depends on the crop-factor. On crop-sensor, setting the lens aperture to the aperture indicated on the scale multiplied by the crop factor will give the scale-indicated dof. Example, if crop factor is 1.5x, and you want the dof distances indicated on the lens for f/8, set lens aperture to f/8 times 1.5x, to f/12. How about that, a use for the crop factor other than comparing lens views on differing format cameras!