Pentax-FA 645 120mm f/4.0 MACRO on the 645D

A lens that is quickly becoming a favorite for image quality on the Pentax 645D is the 120mm Macro. I’ve been shooting with the autofocus version as I felt I’d rather have the option of using autofocus than having the strictly manual version. Given the relative bargains on the used market for Pentax 645 lenses, there wasn’t significant financial penalty to opt for autofocus.

I know the brick wall test takes a lot of heat online. And it should. Really? Is this what photographer’s spend their time doing? (FWIW, the tests below were shot tripod mounted, self-timer with mirror lockup, ISO 200, lens corrections disabled in LR 3.3 and sharpening set to Amount at 50%, 0.8 px radius, Detail at 50, Masking at 7.)

My approach has been to test the lens at a variety of apertures, then use it for personal as well as professional work. The product photography I have used it for leaves no doubt in my mind about using the 120mm Macro for critical jobs. It is sharp! As with any lens, you need to know where it shines. Wide open at f/4 isn’t where it shines. (100% crop from center of image)

There is vignetting and an overall softness. At f/4, you are definitely not getting 40MP worth of resolution out of the 645D. Stopping down to f/8, the lens really starts to shine. (100% crop from same center portion of the image)

Much better results. Vignetting is virtually gone at f/5.6, clearly gone at f/8. Sharpness is strong and remains so at f/11. The lower right corner is shown below.

It is not quite as sharp as the center. This improves slightly by stopping down to f/11.

Once the lens is stopped down to f/16, loss of sharpness due to diffraction starts to become visible. I don’t think it is unacceptable at f/16, however it’s there. Some users might object while others will not. At f/22, diffraction is clearly an issue. I’ve included a center crop from f/32 to illustrate the issue. Unusable in my opinion. If you need considerable DOF, then hopefully focus blending is an alternative.

Maybe some black gaffer’s tape over f/32 is appropriate? This isn’t going to cut it.

To summarize the Pentax-FA 645 120mm f/4 MACRO on the 645D:

f/4 – vignetting, soft image

f/5.6 – minimal vignetting, center almost matches sharpness at f/8

f/8 – very good sharpness in center, corners still slightly soft

f/11 – excellent sharpness in center, corners as good as they get

f/16 – slight loss of sharpness compared to f/8 or f/11

f/22 – strong loss of detail due to diffraction

f/32 – all but useless

In the coming days, I will post results from the 35mm f/3.5, 75mm f/2.8, and 200mm f/4 – all in the autofocus versions. A 45mm f/2.8 is on its way. I returned the 55mm “digitally optimized” lens that was purchased with the 645D. For much of the professional type of work I do, corner sharpness is critical. That lens just doesn’t have it.

And don’t worry, there will be more that just brick wall photos!