Article Technical & Testing

Comparison: Pentax FA 31mm f1.8 Limited vs. Pentax DA 18-55mm f3.5-5.6 @ 31mm

Today I’ll be trying to compare one of the cheapest lenses available for Pentax cameras to one of the most expensive. My spin on the article will be that I used the 18-55mm lens at around 31mm throughout the review to give some equality to the lenses. So let’s find out if the 31mm lens earns its reputation as one of the best lenses available in the Pentax mount.

Pentax FA 31mm f1.8 Limited vs. Pentax DA 18-55mm f3.5-5.6:

The 18-55mm I’m using for this test is the original version, not the AL II or AL WR. This lens came with a used camera (*ist D) I bought recently and seems like a pretty good copy for an original AL lens. As you can see, these lenses are pretty similar in size. The kit lens might be a bit easier to manage due to the small front lens element, but otherwise very similar in handling as both focus rings rotate when auto-focus is being used.

Nature photo analysis:
I went to a local park and took a few photos with each lens. My main goal was to keep the lenses around the same focal length and aperture throughout the test.

General landscape test at f5.6:

I didn’t quite get the focal length exactly the same. The 18-55mm lens was slightly wider angle (say 29mm or so) than the 31mm for that photo, but it is close enough to compare them. Both photos had the auto setting in Adobe camera RAW as well as auto-contrast applied to them, so exposure variation would not be a factor. The white balance and temperature were not affected, so we can compare color rendering between the two lenses.

– The FA 31mm rendered a much warmer looking image than the 18-55mm. Neither is bad and really depends on what you prefer.
– The 18-55mm has visible vignetting in the corners, especially visible in the top-left corner. The lens hood was not being used, so that was not the cause.
– While the focal length was slightly different, from this comparison the FA 31mm renders more detail in the 200×200 crop of the red house. The trees have more texture and the green street sign is slightly more legible.

Sign test at f7.1 aperture:

This time I had the focal length very close, but my framing was a bit off. I used a smaller aperture to give the 18-55mm lens more of a chance sharpness wise.

– The bokeh (background blur) of the FA 31mm lens is much more pleasing to the eye.
– Sharpness is pretty close this time with both lenses. You can see that bug on the sign looks pretty similar between the two lenses. The FA 31mm still seems to have a slight edge in sharpness.

Lens chart test:
These images were taken on a tripod with a 2 second timer to avoid camera shake. The photo is of a basic lens test chart printed with a laser printer at 600 dpi. I ran a photoshop action on the photos to extract out a 640 pixel square in the center of each image. The action also extracts out the corners of each image and includes them as an overlay on the final result.


At f4, the FA 31mm lens is noticeably sharper throughout the image. At f5.6 and f8 the two lenses seem very similar in the center of the image, but the FA 31mm is sharper in the corners with less chromatic aberrations. In our final test at f22, the lenses seem pretty equal (however I did find the 18-55mm lens better at handling small apertures like f22 in real-world images than the FA 31mm).

Comparison conclusion (For and against points):
They both have their respective good and bad points that are outside of the review context, but are important when considering one of the lenses. I’m focusing on the lenses as a whole now to keep those points in consideration.

For the 18-55mm lens:
– It’s cheap and is offered as a kit lens with new cameras or easily available dirt cheap used.
– When used in an optimal focal length and aperture, the lens performs nicely. Especially when looking at the lens chart photos where it was hard to discern a different at some apertures. Just keep in mind distance to the subject will have an affect on that. The ~30mm range is said to be the best focal length for this lens and it shows.
– It’s a zoom lens, so you have a pretty large and standard range of focal lengths to use.
– Good at very small apertures and generally pretty good for outdoor photography.
– Background blur isn’t bad and can be maximized with magnification due to the ability to change focal length to something like 55mm.
– A good lens to use in situations where you don’t want to risk an expensive lens.

For the FA 31mm lens:
– Good at large apertures in sharpness.
– It’s a wide aperture lens, so you have depth of field control and low light benefits. It’s really no contest when considering this feature.
– Build construction is very high.
– Good in pretty much any situation, either night or day.
– Image quality is good from corner to corner.
– While I didn’t make much note in the article, this lens has particularly appealing contrast and color rendering properties.
– Nicer bokeh even at the same aperture and focal length compared to the 18-55mm.
– It’s a full-frame lens.
– It has an aperture ring.
– in APS-C it is a standard focal length making it an optimal general purpose lens.

Against the 18-55mm lens:
– Noticeable vignetting even at around 31mm.
– Isn’t as sharp in most situations compared to the FA 31mm lens, especially in real-world photos.
– The maximum aperture available changes depending on your zoom setting. At 31mm, f4 is the maximum available and is noticeable less sharp in all situations than the FA 31mm.
– Prone to flare at wide angle (I don’t have a lens hood, so I’m not sure if that would help). Some purple fringing in wide angle and extreme light.
– Low quality build compared to the FA 31mm lens.
– It’s an APS-C sized sensor lens only.

Against the FA 31mm lens:
– It’s expensive to the point where you could buy around 10 (new) or 20 (used) kit lenses for the price of a single FA 31mm Limited.
– It can suffer from purple fringing in real-world high contrast situations. My least liked property of this lens.
– f22 in real world situations seemed underwhelming compared to the 18-55mm.
– It’s a prime lens, so you are limited compared to a lens with a range of 18-55mm.
– The lens hood is built-in and designed for full-frame cameras.

Overall, The 18-55mm lens compares pretty well with the FA 31mm when the zoom lens is used at optimal settings (~35mm, around f8) and situations (ample light). It’s up to you to decide if you need and can afford the abilities that the FA 31mm offers you. This comparison goes to show that understanding your equipment is truly your best tool for creating high-quality images. Even the kit lens can be used to produce quality images compared to more expensive gear.