Pentax M42 Takumar 28mm v Sigma 17-50mm on SD15

Tried the recently-acquired Asahi Co Pentax Takumar 28mm this past week outside but had some difficulty getting good focusing and of course AF is not available (lens is manual only).

So I brung it inside and shot my target instead. First I tried twiddling the focus ring with the exposure button half-down until the SD15 beeped - a kind of semi-AF, so to speak. That didn't work, oddly enough (registration distance?) - serious blur resulted.

Then I measured the distance to the target from the image-plane mark on the body, set the lens to that, and got a much sharper shot. So I focus bracketed around there and picked the sharpest shot. At that point, I discovered how much better the SD15 LCD (same as SD1) is, compared to the SD14.

Out of interest I took one shot with the 17-50mm set at 28mm:

Please view original size

The colors look a bit more saturated on the Takumar but the image is a little darker too, so who knows?

As to sharpness, they came out about the same with a slight edge going to the Takumar.

Plots, plots, gotta have plots:

Sigma on top, key values arrowed

As can be seen by the wider spacing of the Takumar RGB channels, it's CA is quite bad. The Sigma's is quite excellent.

In plot form, the differences in sharpness and contrast are quite evident:

The Takumar edge is steeper (steeper is gooder) going from 10-90% in 1.15px as opposed to 1.30px.

The Takumar MTF50 is closer (closer is gooder) to Nyquist (0.5 cy/px) at 0.410 cy/px than the Sigma at 0.344 cy/px. The LPH values for MTF50 also tell the tale.

MTF50 is the spatial frequency at which the contrast of the image is half that of the original scene.

This post is not saying that either lens is "better" with the possible exception of that CA - too few trials to be significant but obviously more care is needed in focusing the 40-yr old Takumar - i.e. resurrecting skills lost with the use of modern lenses and electronics.