Is this the right way to check focusing for a G2?

[jbtran]

I taped some patterned paper on a wall, and set the "Distance Reference Index" mark (circle with horizontal line through it, on top of camera) exactly 1.00 meter from said wall. When I press the shutter button half way down to engage autofocus, the LCD readout on top of the camera reads "1.02 m", which is 2 cm off. Does this mean that my camera has inaccurate focusing and needs service? I have a 45/2 Planar and a 90/2.8 Sonnar. Is this inaccuracy going to be a problem, especially for the 90 mm when wide open and focused at the minimum distance? Please help. Thanks.

 

[Socke]

DoF at 1 meter with a 90 at f2.8 is 0.99 to 1.01 meter given a CoC 0.025.

So it may be visibly out at print sizes exceeding 20x30cm.


If you need critical focus as this with a long lens, a SLR with a bright screen is better suited to the task than the Contax G2.

Edit,

Just looked at that a bit closer and why I never had a problem with critical focus and the 90.

At 1 meter your object may not be wider than 0.35 meters and not higher 0,22 meters to fill the frame, that's a bit bigger than A4 an I tend to shoot bigger prey 🙂

 

[jbtran]

Thanks for the reply, but what is CoC?

 

[akptc]

Thanks for the reply, but what is CoC?

Circle of Confusion

 

[Socke]

Circle of Confusion - a group of photographers talking about DoF 🙂

The smallest point a human eye can discern at usual reading distance is roughly 1/16th of a millimeter. Most lens makers use 1/6th of a milimeter to calculate their lenses DoF markings.

For DoF calculations an enlarging factor of 5x is used, roughly a 5x7" print with 35mm film, and so you get a CoC of 0.0333 from 0.1667/5. Zeiss uses a smaller point than most lensmakers since they expect either bigger enlargements or users with better eyes 🙂

 

[Socke]

Bang on head!

I haven't answered your question!

Yes, this is a good way to check focusing, or better it is the important part of it.

But don't rely on the cameras display, shoot a sharp film and enlarge with an enlarger you can trust. Or shoot slides and project on a very flat screen.

 

[jbtran]

Socke,

OK, I will do as you recommend in your last post. Thanks for replies, everyone.

 

[weoul]

When I press the shutter button half way down to engage autofocus, the LCD readout on top of the camera reads "1.02 m", which is 2 cm off. Does this mean that my camera has inaccurate focusing and needs service?

you are wrong completely 🙂 .
LCD has a little common with exact distance.All you need is to shoot 45 degree tilted target and determine the focusing point.For more detailed explanation look here
http://www.rangefinderforum.com/forums/showthread.php?t=7891

 

[jbtran]

Thanks weoul. I will check it out.

 

[pvdhaar]

you are wrong completely 🙂 .
LCD has a little common with exact distance.All you need is to shoot 45 degree tilted target and determine the focusing point.For more detailed explanation look here
http://www.rangefinderforum.com/forums/showthread.php?t=7891

For a manually focused camera, shooting a tilted target is useful, but with an AF camera there's a caveat.

The AF-sensor has a certain area. You can't really tell where in the AF sensor the contrast was highest, and where the camera actually focused. You may draw very wrong conclusions, while the camera actually worked perfect (or the other way round).

I'd say that for checking AF, a perpendicular test is best. If the camera tells you it's 1.02 meters, and you measure it out to 1.00 meters, then you need to take two pictures. One with AF at the indicated distance (1.02), and one where you set the distance yourself at 1meter. Compare sharpness and then decide whether AF accuracy is good enough.

 

[jbtran]

pvdhaar,

What you suggest makes good common sense, don't know why I didn't think of it. I will try this out first chance I get, but I will also try to do the 45 degree target test for learning purposes. Between the 2 methods here, I figure I'll learn a few things about my camera and its AF. Thanks.