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Lens: Pentax 100 mm f/4 bellows

Vintage: about 1970.

Lens Mount: M42

Needed Adapters: M42 to F-mount, easily found on eBay.

Preferred Mounting: Normal, not reversed

Filter Thread: 49 mm.

Street Price: About $150 in good used condition. Prices vary a lot online, shop around.

Controls: This lens only has two controls - aperture preset and aperture setting. The outer preset ring allows you to set the final position of the aperture, while the aperture setting ring allows you to adjust the aperture smoothly between wide-open (for focusing) and the chosen preset (for taking the picture).

Aperture: 8 blades, varies in shape from a near-circle to an 8-sided star, to an octagon as it is closed.

Basic Function: Requires a bellows to set focus and magnification

Extension Magnification Working Distance
9 cm 0.47 294 mm
13 cm 0.87 196 mm
16 cm 1.17 165 mm
19 cm 1.48 148 mm
Est. Focal Length: 99.7 mm

Appearance:

pentax 100 bellows M42 top view pentax 100 bellows M42 side view

aperture graphResolution vs. Aperture:

The lens has f/9.4 as its sharpest and highest resolving aperture while many of its competitors are sharpest at about f/8. As a general rule, you will get better resolution long as it is the sharpest).

For me this means that I would use f/9.4 when shooting images to get the sharpest and most highly detailed images.

corner sharpness graphCorner Sharpness vs. Aperture:

There is moderate fuzziness on the periphery at larger apertures, but the corner sharpness improves as the aperture is closed down. At the sharpest aperture of f/9.4, the corners are very nearly as sharp as the center - just what you want with a macro lens.

Testing done on a Nikon D200, a DX format digital camera (APS-C). On a full-frame camera the performance would tend be a little worse.

Note: I would say that any number under 10% will only be faintly visible on an image.

resolution graphSharpness and Resolution vs. Magnification:

This lens' resolving power in the center of the field is slightly lower than its competitors of similar focal length. Compared to modern macro lenses of similar focal length, the resolution is significantly decreased. The MTF50 of this lens at 1:1 magnification is about 800 lp/ph while a good modern macro lens will be anywhere from 1000-1200 lp/ph at the same magnification.

These numbers only apply to a DX format Nikon D200. With a full-frame detector or a higher megapixel camera, the numbers would tend be slightly higher.

As an example: If you are shooting an image at a magnification of about 0.5x or 1:2 (will just about fit a silver dollar full screen on a D200), you can expect to get about 1050 "sharp" pixels across the height of the image and no more than about 2200 pixels of real detail in the image across the hieght of the picture. These numbers will improve somewhat with the use of image sharpening in the photo editor (or in-camera when shooting jpg).

performance:sharpness graphPerformance: Sharpness vs. Magnification:

I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.

The Pentax 100mm shows sharpness performance that varies from good at the low-end of magnification to poor at the high-end.

performance:resolution graphPerformance: Resolution vs. Magnification:

I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.

The Pentax 100mm shows resolution performance that varies from good at the low-end of magnification to poor at the high-end.

resolving power graphResolving Power vs. Magnification:

This graph represents the smallest details that are able to be resolved by this lens at various magnificaitons. If the number doesn't get smaller as the magnification rises, there is little benefit to going up in magnificaiton with this lens. This situation is also called empty magnification.

Good resolution, although somewhat limited by the f/9.4 aperture.

Chromatic Aberration:

This lens show minimal color fringing in the center (about 0.10 pixel, mainly axial) and mild in the periphery (0.80 - 1.0 pixel, mainly lateral). Anything below 1.0 pixel is good.

Image Contrast: IIage contrast is good, but not spectacular for this magnification range.

Flare: This lens shows no significant flare during testing.

Distortion: This lens shows no significant distortion during testing.

Image Samples:

About 1:3 magnification, f/9.4, cropped and resized:

sample image

crop imagePixel level crop from the image above:

This image shows just a trace of unsharpness compared to its more modern macro lenses when viewed at full size, but not much. When viewed at a smaller size such as the above picture, the images are virtually indistinguishable.

About 1:1 magnification, f/9.4, resized:

sample image

crop imagePixel level crop from the image above:

Again there isn't much difference between the pixel level detail of this lens and its newer competitors such as the Nikon 105VR.

Conclusion:

This is a bellows lens that performs well, but less than typical modern 100 mm or 105 mm macro lenses - likely a symptom of the age and simplicity of the design. As seen in the above images, it is fully capable of taking high-quality images across its usable magnification range. As with most bellows lenses, this lens shows very good corner sharpness and little chromatic aberration.

This lens is useful to get more working distance and for use at lower magnification. Shorter focal length lenses will almost invariably have better resolution capabilities. That being said, modern macro lenses will almost always outperform bellows lenses in this focal length and magnification range, although that difference is small when looking at actual images.