Tech. Question, ...Digital depth of field vs. film?????????

[bensonphoto]

Hello,
I recently adapted an 85mm 1.5 lens, and a 50mm 1.4 lens to my Sigma SD-9. I noticed that my focus was off and realized that my depth of field was perhaps one half or one third of what my viewfinder image displayed. I am viewing the image with the lens stopped down and believe what I see should translate onto the image. I placed a row of about 10 nails into a board. They were about 2-3 centimeters apart, and I focused on the center nail. My viewfinder showed 1 or 2 nails in front of my focal point to be sharp, and about 3 or 4 behind the focal point. This coincides with my knowledge of depth of field (1/3 in front and 2/3 behind focal point) ...But, my image showed FAR less depth of field, with a total of 2, and sometimes 3 nails appearing focused. My shooting distance was about two meters.

...Can anybody explain this to me? I can also be reached at: bnsnphoto@aol.com Sincerely, Neil Benson

[castledude]

Are you pushing the Depth of Field (DOF) preview button? (on the front of the camera).

[ukexpat]

Hello,
I recently adapted an 85mm 1.5 lens, and a 50mm 1.4 lens to my Sigma SD-9. I noticed that my focus was off and realized that my depth of field was perhaps one half or one third of what my viewfinder image displayed. I am viewing the image with the lens stopped down and believe what I see should translate onto the image. I placed a row of about 10 nails into a board. They were about 2-3 centimeters apart, and I focused on the center nail. My viewfinder showed 1 or 2 nails in front of my focal point to be sharp, and about 3 or 4 behind the focal point. This coincides with my knowledge of depth of field (1/3 in front and 2/3 behind focal point) ...But, my image showed FAR less depth of field, with a total of 2, and sometimes 3 nails appearing focused. My shooting distance was about two meters.

...Can anybody explain this to me? I can also be reached at: bnsnphoto@aol.com Sincerely, Neil Benson

Did your adaptation take into account any differences between the camera that the lens was originally designed for and the SD-9? For example, if the rear element to film/sensor plane distance is different on the two cameras, the lens will not focus properly...

[bensonphoto]

The lens adapter is such that my lens is always stopped down, so the depth of field preview button is unnecessary. The adapter is commercially made and I can only assume the appropriate measurements have been taken into account. It is produced by C. Point Incorporated, and it allows me to mount M-42 screw mount lenses on my Sigma SD-9. I like to blur the background in outdoor portraits and glamour photos. I have an 85mm 1.5 and a 50mm 1.4 that bridge the gap in the Sigma lens lineup. At all apertures, my actual depth of field is less than what I see through my viewfinder. I don't understand why, but a friend who is an engineer suggested that perhaps it has to do with the prism and/or focusing screen. I also have an adapter for the Canon EOS, and I am about to borrow a digital rebel from a friend to see if it is a universal phenomenon or simply an engineering fault native to sigma. I do love the camera though. The color and resolution are not at all exaggerated in the manufacturer's claims. Thank you for your input!

Neil

[ukexpat]

Neill

Do you have a website URL for C Point?

Thanks.