DOF and more....

1. Does autofocus work better when increasing ISO? 2. How are full-frame sensors related to depth of field and its control? 3. Does focussing accuracy vary when closing down aperture? 4. When saving a Raw file as TIFF, how many brightness levels/bits I get from the TIFF? Maxim D via E-mail 1. No. 2. Depth of Field depends on the magnification produced on the film/sensor. Greater the magnification, lesser the DOF and vice versa. If you have two cameras (irrespective of the sensor size) fitted with identical focal length lenses or non-identical focal length lenses, the DOF will be the same as long as you manage to keep the magnification (image size on the sensor) the same. 3. When you ‘close down’ the aperture (that is, make the aperture narrower), the depth of field increases (assuming focal length and distance remains constant). Hence, to ensure the maximum focussing accuracy, all modern cameras focus at the widest aperture, irrespective of the aperture that will eventually be used to take the shot. When you fully press the shutter release button, the aperture drops down to what you have set. After the exposure, the aperture blades open once again to the widest opening. The idea behind this is that if good focus (or nearly good focus) is achieved with the aperture that provides the least DOF (that is, the lens wide open), then stopping down the aperture will definitely take care of any shortfall in the focussing accuracy. So coming back to your query, if you were able to focus the lens at a narrow aperture, chances are, that due to extended DOF, your focussing accuracy could suffer. 4. ‘Bits per Channel’ (12-bit or 14-bit) is responsible for the number of colours, and not the Brightness. In ACR (Adobe Camera Raw), you can set the ‘ bits’ by clicking on the blue workflow options tab at the bottom of the screen. In the dialog box that opens, select 8-bits/channel or 16-bits per channel from the ‘Depth’ box. After the edits in ACR, you press the ‘Open’ button and the image will open in Photoshop. If you have selected 8-bits per channel, the image in Photoshop will be at 8-bits; if you select 16-bits per channel, the image in Photoshop will be at 16-bits. After working in Photoshop, you have the choice to save the image as JPEG or TIFF. If you save it as a JPEG, it will save as a 8-bit per channel file; if you save it as a TIFF file, it can be saved at 16-bits/channel.