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Pentax medium format vs 35mm lens test
Purpose I designed and executed a test in order to measure the sharpness difference between medium format and 35mm A series lenses in the Pentax range so that when posed with a critical application I could make a well founded decision on the most appropriate lens to mount on my 35mm Pentax camera bodies. And secondly just because I could :-) Method I picked an area of shaded wall during the morning of a clear sunny day and taped up four A4 sized LASER printed USAF targets at the centre, the top left, the top right and the bottom left of the proposed test area as per the instructions found at Ralf's Photography Page. The test set-up was not optimal since in the chosen test location the test camera had to be situated on a slight gradient plus I was not able to place a target bottom right of frame due to some foliage. However the purpose of the test could still be achieved as the comparison is relative and all the test frames were made sequentially and without interruption so the point of reference was virtually constant. I used a Pentax LX camera body with SE20 Focusing Screen, FE-1 Magnifinder and cable release mounted on a Manfrotto ART #055 tripod, Head #029 and Quick Release Hex Bracket #030 which then had a weight hanging from the centre column. The lenses were mounted using a combination of Adapter K For 645 Lens and Adapter 645 For 67 Lens. The LX mirror was locked up as soon as critical focus was achieved just prior to the exposure. The lenses were cleaned, had no filters fitted and their front elements were shielded from incident sun light.
To determine appropriate test target to film plane distances I pre-loaded the test lens focal length in my USAF lens testing worksheet which calculates a test distance for a user prescribed magnification. I also inserted the test f-stops and printed the worksheet so that I could use the sheet as a guide and check-list during the test. I used Illford 100 Delta Professional 135/36 film and then hand processed it in Illford Ilfosol-S liquid developer diluted (1+14) using the prescribed developing times and temperatures in a Jobo 1540 inversion processing tank. After the negatives were fixed, dried and cut they were inspected using a Labax 40x binocular microscope with back illumination via a Cabin CL-5000P 5" x 4" fluorescent Light Panel and external DC supply. Results table The following lens resolution table was generated using the USAF lens testing worksheet
Results distillation From the table above we can see that there is inconsistency between the three sets of corner measurements in each frame, this most probably indicates that the film plane was not sufficiently parallel to the target plane. As mentioned in the Method section of this document; the test environment was not optimal. I suspect that some of the results with the longer lenses were also corrupt by residual camera vibration even though the tripod was not fully extended, was supporting added weight and the camera mirror was locked. Visual inspection of the negative sheet readily revealed significant contrast loss and vignetting of the fast lenses when their apertures were fully open. I was quite surprised that the difference was quite so dramatic, I suppose I have not really noticed the effect before since in most instances I don't make multiple images cycling through the apertures. Possibly a reduction of absolute contrast is a positive attribute for me in the environments and with the films with which I tend to use these lenses wide open. The PKA50f1.2 fully open although low in contrast behaves similarly to the 67 45f4 fully open, but the PKA50f1.2 at f5.6 is far better than the 67 45f4 at f5.6 which shows little improvement from f4 to f5.6. The PKA85f1.4 at 5.6 exhibits high centre sharpness but the 645 75f2.8 at f5.6 displays reasonable sharpness and low distortion from edge to edge. The PKA135f1.8 exhibits similar performance to the 67 135f4 over a similar range of apertures with very consistent edge to edge performance at the middle apertures, however, visually the 67 135f4 contrast remains consistently lower than any of the other lenses on test. Conclusion The absolute differences in sharpness between the lenses above 50mm appears to be relatively insignificant since the results seem to be most affected by variables such as the test film grain, camera body vibration, support instability and focus errors that are not aberations of the lens under test. Since I don't believe these other variables are going to be realistically bettered in the majority of real world shooting I will definitely continue to mount medium format lenses on my 35mm format equipment (Pentax LX) and shoot with the knowledge that only testing in a stringent test environment could produce measurable improvement. The main reasons I can sight for mounting the medium format lenses on a 35mm body are where I may need to be weight conscious and are doubling up on focal lengths or where I may wish to make use of a particular characteristic of the medium format lens ie 67 135f4 macro linearity or working distance or perhaps I might use the 645 75f2.8 wide open instead of the PKA85f1.4 if I need linear sharpness across the frame. However given the test results obtained I will not use the 67 45f4 lens with my 35mm format equipment unless it is coupled to an adaptor that allows tilt and/or shift since its performance in normal use appears to be far superceded by most 35mm format lens equivalents. Pentax lens adapter gallery Clicking on a thumbnail will yield a magnified image, use the browser back button to return.
Disclaimers and other information The images in the gallery section of this article were captured under fluorescent lighting using a small video camera that was connected directly to a computer and video capture card. The text contained in this document represents my opinions only, and as such I will not be held liable for detrimental consequences that arise from the application of any information within or relating to this document. All references to trade marks and or trade names are acknowledged as belonging to their respective owners.
This complete document may be copied, re-posted, linked-to or used within other publications providing that permission from the author is sought and granted and that there exists a visible reference to Rob Studdert as author in any referring text. I do not represent Boris, Pentax or their affiliates. Rob Studdert Design & Copyright © - Digital Image Studio 1998 |
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