### Author Topic: Focus Distance  (Read 458 times)

#### garry23

• Hero Member
• Posts: 1280
##### Focus Distance
« on: September 25, 2017, 09:51:26 AM »
@anyone

When I get a chance I will attempt to measure this myself, but until then, does anyone know th reference plane for the focus distance reporting?

That is the sensor or the lens principal plane?

I know it won't make that much difference in calculating depth of field etc, but I'm being picky ;-)

Cheers

Garry

#### Walter Schulz

• Hero Member
• Posts: 5713
##### Re: Focus Distance
« Reply #1 on: September 25, 2017, 10:16:25 AM »
Sensor plane!

#### garry23

• Hero Member
• Posts: 1280
##### Re: Focus Distance
« Reply #2 on: September 25, 2017, 10:59:56 AM »
:-)

Thanks

#### rpt

• Freshman
• Posts: 83
• 5DIII, 24-105, 40 2.8, 100-400, 600EX
##### Re: Focus Distance
« Reply #3 on: September 26, 2017, 02:38:16 AM »
There is usually a marking on the camera for that. I see them on my AE1, 5D3 and 7D2. It is a circle with a horizontal line through the middle.

#### garry23

• Hero Member
• Posts: 1280
##### Re: Focus Distance
« Reply #4 on: September 26, 2017, 08:58:16 AM »
@rpt

I needed to confirm that ML Lua reporting was referenced to the sensor ;-)

I needed to do this as the DoF equations I use are referenced to the lens principal plane, i.e. about a focal length difference, which for macro becomes important.

Cheers

Garry

#### rpt

• Freshman
• Posts: 83
• 5DIII, 24-105, 40 2.8, 100-400, 600EX
##### Re: Focus Distance
« Reply #5 on: September 27, 2017, 04:40:47 AM »
Oh dear! So you will need a table of lens and the distance of the principal plane from the sensor. But then you knew that already

#### garry23

• Hero Member
• Posts: 1280
##### Re: Focus Distance
« Reply #6 on: September 27, 2017, 07:25:20 AM »
@rpt

Yes, an 'impossible' task, so I'm simply going to use the focal length as an approximation.

After all, the distance between the sensor and the principal plane of the lens, for non-macro work, is insignificant compared to the principal plane of the lens to the focus distance; especially for wide angle work.

Also, the depth of field equations are already an approximation, i.e. assuming a simple, thin lens and ignoring the pupil magnification.

Cheers

Garry