Saturday, November 25, 2006


focus testing with Pentax *ist DL and 35mm f1.9 lens

What this photo shows is using markers (crude) to show what is being kept in focus.

I shot this with a Pentax *ist DL with Vivitar 35mm f1.9 lens at f1.9 at ISO equivalent 1600 hand held.Basically, the boy's nose, and the tip of the toy coffee machine, and the marker labeled "FOCUS" are 36" from the approx. sensor plane.

Marker 1 is 8" in front of focus, marker 2 is 4" in front of focus, marker 3 is 4" behind point of focus, and marker 4 is 8" behind the point of focus.

From these results it shows that my eyes, or camera lens setup may be slightly forward focusing.
Using the tables from the online DOF calculator at if I were perfectly focused at 36", everything from about 35.1" to 36.9" should be in focus.

However, it is likely I was actually focusing with +/- 2 inches as there is no sensor plane mark on the camera, and I was hand holding using a floppy tape measure for a distance guide.Also dofmaster doesn't have a setting for f1.9, so I used their f1.8 setting.

Another variable is the circle of confusion is estimated at 0.02mm on this calculator, where in reality it may be slightly different for my particular lens.It is important for lens tests, especially at wide open aperture settings, and close focus distances, to ensure that the subject is in focus when judging images at different apertures.

Often casual lens tests indicate a particular lens is "soft" or not sharp wide open, when in fact, it is possible that the lens is not focused accurately wide open, and close-up. This is especially the case with rangefinder cameras which are most challenged at fast wide apertures, and at close distances.

I have about four lenses that are known to be "soft" wide open, which is sometimes preferred for portraits, but "sharp when stopped down, " yet I have many examples of them being extremely sharp wide open and not stopped down, and have seen many examples strewn across the web from these lenses wide open and "soft-looking" when they are actually not quite in focus. Posted by Picasa

Hi Ted,

You need to stabilize the camera on a tripod so that you cab carefully focus to make sure that you are focusing on the right plane. It also allows the use of a cable release {with a film based camera). The set-up gives me a good idea of what to do for my testing. Still, it will difficult to have a 3.5 year old child stay still for 72 exposures.

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