HFD question~

[Zephyrize]

Hi

I've been reading about hyperfocal distance and downloaded a
customizable chart that calculates the point of focus for me.

I'm using a 10-20 mm on a XSi, so there is a 1.6x crop factor.
On the chart, should I look at 10mm or 16mm when I'm using my lens on the wide end (10mm) ?
There's no option in the program where I can specify that I'm using a APS-C sensor, so it assumes that I'm on full frame right?

Anyway, just a little confusion about this
thx!

[Sorarse]

Haven't got a definitive answer, but I would have thought that the DoF for a given lens and focal length is independent of the size of the sensor that is used to capture the resulting image.

[tzalman]

Haven't got a definitive answer, but I would have thought that the DoF for a given lens and focal length is independent of the size of the sensor that is used to capture the resulting image.
Not so, because DoF is also a function of the magnification of the capture image in order to make a print. More magnification, less DoF. The convention is for reference charts to report the DoF in an 8x10, which for 35mm. full frame is an x8 enlargement. For a 1.6 crop it is x13.

[20droger]

The depth of field is a function of four variables:
The focal length of the lens;
The aperture;
The focus distance; and
The diameter of the circle of confusion.

The diameter of the circle of confusion, however, is itself a function of sensor size.

So, yes, the sensor size affects the depth of field.

For a detailed explanation of how all these variables interact, and the formulas to compute hyperfocal distance and the depth of field, look here: http://doug.kerr.home.att.net/pumpkin/Depth_of_Field.pdf

[argyle]

Hi

I've been reading about hyperfocal distance and downloaded a
customizable chart that calculates the point of focus for me.

I'm using a 10-20 mm on a XSi, so there is a 1.6x crop factor.
On the chart, should I look at 10mm or 16mm when I'm using my lens on the wide end (10mm) ?
There's no option in the program where I can specify that I'm using a APS-C sensor, so it assumes that I'm on full frame right?

Anyway, just a little confusion about this
thx!

Look for a better program. :D The equation for HFD takes into account the circle of confusion for a specific type of sensor.

HFD = (FL)(FL)/(f x C)

where:
FL is the focal length
f is the aperture number
C is the circle of confusion for your particular sensor

This will give you an HFD in millimeters...to convert that to feet, divide the result by 304.8

As long as you use the proper C value for a crop sensor, you don't need to worry about the crop factor as it pertains to your focal length (don't multiply the focal length by 1.6). If your lens is 10mm, that's what you use.

And yes, as someone else has already posted, most C values are based on an 8x10 output. For larger than 8x10, rather that calculate the HFD with a different C value, just stop the lens down another stop for the next larger print size (for example, if your HFD calls for an aperture of f/8, stop down to f/11). I just basically set my own chart up as an Excel spreadsheet quite a while ago, laminated it, and tossed it in my bag...very simple using the above equation.

[Zephyrize]

HFD = (FL)(FL)/(f x C)

[...]

As long as you use the proper C value for a crop sensor, you don't need to worry about the crop factor as it pertains to your focal length (don't multiply the focal length by 1.6). If your lens is 10mm, that's what you use.


wow I've had the formula right in front of me... but didn't connect the dots. -.-' /fail
thanks for reminding
problem solved