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focusing techcamera with distometre

jlm

Workshop Member
i briefly saw a few things today at lance's get together re focusing.

1. the distometer D5: i ordered one after seeing Woody's. I had no idea it had a video screen with crosshairs where you are aiming the laser, and it zooms! this will get your distance number, eliminating one variable. may need a tripod adapter as it is tough to hold it still at long distance measuring
2. the Arca fine focus ring and look-up table: looks like with #1, above, this will work very well
3. the alpa HR lens rings: they read out directly in distance and closely enough marked to work very well also with #1
4. focus mask on the IQ180; you snap a shot and peep it, it shows with a highlight exactly what is in focus. seems like a great feature, especially if using T/S
 

Jae_Moon

Member
It definitely should be the sensor plane.
Could you explain why the sensor plane instead of lens plane? I thought the equation was 1/A + 1/D = 1/F where A is lens to sensor distance, D is lens to PSF distance and F is the lens focal length.

Thanks,

Jae M
 

jlm

Workshop Member
I assume this is a convention accepted by camera/lens makers?

probably only really matters for closer distances as the difference is a lens focal length, more or less
 

Jack

Sr. Administrator
Staff member
Marked lenses are for the sensor plane. Fixed lenses with no focus mounts like for LF shooting, use the focus formula 1/f = 1/i + 1/o where f is the lens focal length, i is lens to image distance (usually measured from the optical center which is generally assumed to be the lens flange), and o is lens to object distance. One needs to consider that as distance o increases distance i decreases, so there are two variables; then secondarily that the formula does not work for retro, tele internal focus lenses. Fixed-body cameras use the sensor plane convention for helical markings since it is the one constant in the fixed body camera -- which can at the same time be using conventional, retro, tele or internal focus lens designs.
 

Jae_Moon

Member
Marked lenses are for the sensor plane. Fixed lenses with no focus mounts like for LF shooting, use the focus formula 1/f = 1/i + 1/o where f is the lens focal length, i is lens to image distance (usually measured from the optical center which is generally assumed to be the lens flange), and o is lens to object distance.
Jack:

How about RM3Di? How is its helical focus ring calibrated, ie., the subject distance from the lens plane or from the sensor plane?

thanks,

Jae M
 

Jack

Sr. Administrator
Staff member
Jack:

How about RM3Di? How is its helical focus ring calibrated, ie., the subject distance from the lens plane or from the sensor plane?

thanks,

Jae M
I honestly don't know because I've never measured that close, but I'd assume the sensor plane.
 

delled

New member
From analysis of Arca's calibrations for a few lenses, the Rm3di calibration seems to be based on distances measured from the front nodal point (i.e., the lens, not the sensor). However, the data on Arca's crib cards appear to be based on calculations for lenses with specific focal lengths -- not necessarily the nominal focal length for each lens. So you have to know the actual focal length of each lens.

Dave.
 

Jae_Moon

Member
Jack:

Thank you for info. I just ordered my RM3Di and am trying to learn about it as much as possible while waiting.

Jae
 
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