Maximum DOF with Pentax 45-85/4.5 at 45mm and f/32

[rayn]

I am considering getting a Pentax 645 and 45-85/4.5 lens for landscapes

and am wondering how much depth-of-field I can get with this lens at

45mm and f/32. The calculator says at an object distance of 4 feet,

everything from 1.89 feet to infinity will be in focus. The calculator

can be located at:

 

http://ue3.ee.nus.edu.sg/DOF.html

 

But that seems a little too good to be true. I am wondering what the

practical DOF is at 45mm f/32 based on user's experience. I know that

opinions about this can vary, so give me your opinion for prints up to

11x14. Unless I am mistaken, this is the only 645 lens in the 45mm

range that goes to f/32.

[andrew_schank]

Ray, my experience with the 645 format is that there is a tremendous loss in image quality that occurs beyond f16 with most lenses I have tested. Resolution and contrast were often only about 1/2 of what they were at f8 or f11, due to optical problems caused by interference patterns as light is diffracted passing through such a small lens opening. So there is a great trade off at one point between image quality and depth of field. As far as the depth of field scales go, the rule of thumb for a very small circle of confusion is to scale focus using the f stop 2 less than the one you are shooting with.

[scott_jorgenson1]

Ray, I own the Pentax 645 and the 45-85/4.5 zoom, and have tested it at all apertures greater than f/8 for DOF and general sharpness (diffraction, etc) by tripod-shooting it with high-resolution film (Velvia) at various targets, and comparing the transparencies under a 10X loupe.

 

Unlike the other poster, I have found at worse only negligible image degradation at almost every aperture. Specifically, I've never noticed any degradation proceeding from f/8 through (and including) f/19 (the 1/2-stop between f/16 and f/22) when testing against detailed flat targets and specifically looking for such things as diffraction effects. I do notice negligible diffraction effects appear at f/22 and f/26 (the 1/2-stop between f/22 and f/32) - by negligible I mean that I don't believe anyone would notice them without repeated side-by-side comparison. At f/32 the image softness is just a bit more pronounced than that, but again I believe that without repeated side-by-side comparisons anyone would be hard-pressed to tell. I myself don't hesitate to use the small apertures, even f/32, when I need the DOF.

 

Regarding your DOF question, I find that allowing for a circle-of-confusion (CoC) of 0.03 mm still gives me acceptable sharpness in 8 - 10X digital (drum scan + PhotoShop + LightJet) enlargements. This CoC is more conservative than the ones used for every other MF lens or DOF scale I've seen printed - typical values are 0.04 or 0.05 mm. So using this CoC, the hyperfocal distance at 45mm and f/32 is about 7 feet - ie, DOF from 3.5 feet to infinity. I've tested this in the field and the results are satisfactory in the aforementioned 8 - 10X digital enlargements. Conversely, I've tested shooting with a larger CoC - eg 0.04mm - and pushing its DOF to the limits, and not been satisfied with the results (noticeable near/far softness).

 

To summarize, it is true that the DOF calculators do not take diffraction effects into account, and so suggest that f/32 is as sharp as f/16 or f/64 even. Of course this isn't the case - eventually diffraction wins out. But as I've found in the field, up to f/32 at least the diffraction effects in my opinion are either unnoticeable or, at worst, minimal and acceptable. And within that range of apertures and diffraction effects, I've found the DOF calculators to give pretty usable results with a CoC of 0.03 mm.

 

Scott

[andrew_schank]

This particular lens must be unusual if it still holds on to details at f32, because all of the primes I've tested fall apart at their smallest openings. This holds true for my 35mm equipment as well. Your basically using the lens to turn the Pentax into nearly a pinhole camera at f32, so I'd be interested in your results after you try it out. I've gotten my sharpest high depth shots with my 35mm Pentax lens at about f16.