Tamron 90mm Macro vs Pentax 100mm WR: Review

Image Quality Comparison

On this page we present a number of tests through which we compared the optical performance of the 90mm and 100mm.

Resolution

Resolution (the capacity to render details, also called sharpness) is of paramount importance for macro photography. The following pictures allow comparison of resolution for both lenses at various apertures. The target is a DSC Labs Pocket CamFocus Test Chart. The target was photographed with a K20D set on a tripod, with 3 seconds mirror lock-up. It must be noted that the f5.6 picture of the Tamron lens turned out slightly out of focus and has been rejected. Our test setup was not accurate enough to evaluate centering defects; corner differences should be blamed on the target alignment instead of lens manufacturing. The test represents a graphical evaluation of the modulation transfer function (MTF) of the lenses. The DCS labs target does not provide a scale so no quantitative measurements have been made.

The lenses are a close match at apertures larger than f16. The Pentax seems to have the edge but by a hair-thin margin. The Tamron does produce chromatic aberration in the corners at wide apertures, a disappointing observation.

By f16 diffraction appears to take its toll on the Pentax, and the Tamron takes the lead. At f22 the situation is reversed and the Pentax comes out sharper again. By f32 both lenses show a significant decrease in sharpness.

The essence of these observations is that both lenses are extremely sharp, and quite comparable regarding resolution. We encourage you to form your own opinion by viewing the test samples provided.

Bokeh

One of the most desirable attribute of a macro lens is a smooth, creamy background blur. The Tamron lens has nine straight aperture blades, while the Pentax has eight rounded blades. The optical designs of the lenses are also different. Do these differences account to anything noticeable in real life?

The differences, when discernible, are minimal. The following images illustrate a representative example of busy out of focus areas taken with both lenses. Very close inspection shows that the rounded blades do create shapes that are more completely circular, but the nine straight blades come so close it’s almost a draw. The transition from in-focus to out-of-focus is also very smooth in both cases.

As can be seen in the test shots provided at the end of this review, the two lenses perform quite similarly in most cases. In only one occurrence did the Tamron deliver a surprisingly dynamic bokeh. The following image illustrates the phenomenon, which occurred when a background with strong textures was photographed with the lens wide open. It was never observed in other situations, so it is a marginal issue.

Vigneting

Both lenses are full frame designs, and it shows. Vigneting is not noticeable, for either lens, in real life shooting situations. The slightest corner shadowing is visible in each case at f2.8, but becomes almost irrelevant at f4. The following chart illustrates the measured vigneting behaviours of both lenses.

Exposure

The main critic that can be made of the Pentax lens is that it has a habit of underexposing in harsh lighting, such as a beach. This must be blamed in part on the camera body, but the Tamron lens produced more accurate exposure evaluations in the same situation. An exposure correction +0.7 was generally enough to bring the Pentax back on line with the Tamron.

In macro situations, both lenses exposed reliably, with well distributed histograms. Flash photography also yielded spot-on exposure in both cases.

Aberrations

We were quite surprised to find evidence of chromatic aberration with the Tamron lens. Most of the time CA was not a problem at all, but the two following examples (crops in both cases) illustrate that CA can be present in specific cases. Note the purple bands on both pictures, and the green band on the lens mount detail, to the right (the second picture links to a high-resolution version). In similar scenes, the Pentax lens showed no signs of chromatic aberrations. As mentioned earlier, CA (in the form of purple fringing) is also visible on the resolution test shots of the Tamron lens, mostly in the corners and at large apertures.

As can be expected of macro lenses, distortion (be it pincushion or barrel) is essentially non-existent in each case. We have not been able to observe any sign of distortion in our test pictures.

Sample Images Comparison

The following images are real-life comparison examples showing each lens' performances in various situations.