HD Pentax-D FA 28-105mm F3.5-5.6 ED DC WR

General Image Quality

On full-frame cameras, the D FA 28-105mm F3.5-5.6 offers a zoom range that covers a host of commonly used wide-angle, normal, and telephoto focal lengths. At 28mm the field of view is 65° horizontally and 75° diagonally, while at 105mm, the field of view is 19° horizontally and 23.5° diagonally.  Being able to focus on subjects as close as 50cm (about 1.5 feet), the lens doubles for close-up shooting and it can deliver a maximum magnification of 0.22x, or 1:4.5.  With its compact size and light weight, it truly is a jack of all trades.

Using the 28-105mm as a close-up lens: photo by Setophaga magnolia

The small physical size does come at the expense of aperture speed.  At 28mm lens the widest selectable aperture is F3.5, with F22 being the smallest.  The aperture range narrows as you zoom and progresses as follows:

Since the full-frame format allows for greater high-ISO flexibility, and the Pentax K-1's built-in Shake Reduction compensates for up to 5 stops of camera shake along 5 axes, the slower aperture is not necessarily a handicap for low-light shooting.  Hand-holding at shutter speeds such as 1/8s or 1/15s is not unrealistic at the wide end of the zoom range.

You can of course also use this lens with APS-C cameras.  While the smaller sensor ensures that only the "sweet spot" of the image circle gets used, it also results in a considerably narrower field of view.  28mm on APS-C results in an angle of view of just 46° horizontally and 53° diagonally, which is closer to "normal" than wide-angle.  Thus, we wouldn't recommend it over something like the DA 18-135mm unless you rarely find yourself going wide.  The benefits of better image quality simply wouldn't be worth the loss of versatility at both ends of the zoom range.

Wide Enough?

28mm on full-frame delivers roughly the same angle of view as an 18mm lens on APS-C— the latter has become quite a common focal length in the digital era, being found in crop kit lenses from most camera manufacturers.  But is this field of view wide enough?

The D FA 24-70mm F2.8 is the 28-105mm's main competitor.  The 4mm difference at the wide end gives you an angle of view that's 9 degrees wider.  We demonstrate this in the comparison below:

Frequent landscape shooters who don't mind the added bulk or higher pricetag of the fixed-aperture 24-70mm may thus be interested in it as a wider alternative, or to at least consider a wider supplement to the 28-105mm.

Colors and Exposure

We noted slight variations between wide-open exposure at 50mm and 70mm due to the variable aperture design.  In general use, however, we've found the lens to meter accurately on the Pentax K-1 both in live view and through the viewfinder.  You likely won't encounter exposure inaccuracies unless you're shooting wide-open in Av mode.

The lens also does a fine job of reproducing colors accurately, and white balance proved to be consistent during our outdoor shooting.

Accurate colors and correct exposure- click to enlarge

Consistency

Some may see the 28-105mm as a cheap or budget series lens due to its low price tag compared the other D FA zooms.  We are happy to report that the 28-105mm does not fall into this category; it performs quite well from an optical standpoint.  To this end, we've found the D FA 28-105mm to be noticeably sharper on full-frame than the DA 18-135mm was on APS-C, though the latter does cover a longer slightly zoom range of course.

The D FA lens delivers consistent image quality throughout the zoom range in addition to bringing the latest lens technologies to the table (HD coating, silent autofocus, weather sealing, etc.)

Sample full-size photo from RAW, 68mm F8- click for full version

This photo was developed from RAW and not cropped, with only a small degree of sharpening applied.  The details are nice and crisp from corner to corner.

Of course, no lens is free from optical imperfections, and it's inevitable that a small wide-to-tele zoom such as this one can't perform as well as a bag of fast primes would.  In the following sections, we will analyze the optical performance of the 28-105mm in detail.