|Pentax-A* 300 F/4|
|Pentax-A* 300 F/2.8|
|Pentax-FA 80-320 f4.5/5.6|
|Pentax-A* 300 F/4|
|Pentax FA 80-320 @f/5.6|
|Pentax FA 80-320 @f/9|
Lens quality tests are always a matter of controversy. There are a lot of variables, and to get something meaningful you have to lock as many of them down as possible but then this provides an 'isolated case' example. My testing here is subject to this criticism but I did it anyway because I wanted a guide for my own work.
Within my lens collection I have 3 lenses which generally will support the 300 mm telephoto range and I was looking for indications of how much difference I could expect as the value of the lenses varies radically. My 'test' in this instance was a sheet of paper (8 1/2" x 11) which I stuck up outdoors on a cloudy day about 50 feet from a good camera rest. The paper was mostly white with various sized red lettering, not a real good test pattern but the first thing I cam across when looking for a test pattern.
The three lenses vary widely in value. The 80-320 zoom is not very well regarded (properly, it seems), and sells on the second hand market on EBAY (It, like all of the lenses shown here is discontinued) sometimes for under $100.00, even though it is the newest of the 3. The other two lenses were introduced at the same time in the early 1980's, though obviously aimed at a different market. Both are KA mount lenses (no auto focus), but otherwise fully functional on the digital camera bodies. Both are "A*" lenses which means that are suppose to be of superior quality, but alas the f2.8 monster sold new for thousands of dollars and still commands a value (at this writing) of $2000 and up. The F/4 lens on the other hand seems to sell for $500 to $1000 on the second hand market.
The first thing I learned as I tried to test these lenses is that I had an exposure disaster with a small white piece of paper against a dark green background. At 50 feet or so, the white paper was about the size of the center spot in the fresnel screen and when I used the default metering mode of averaging the whole photo for exposure the white piece of paper was over exposed by 2 or 3 f/stops. Even with aggressive photo shopping, it was barely possible to read the 'days' on the calandar. Interestingly the zoom lens was the worst of the 3 with the numbers for the days being burned beyond being readable.
I then switched the exposure mode to 'spot meter' and put the white paper directly on the 'spot'. I then got reasonable exposures, and although the photos shown here are the center clips, and photo shopped the best I could to make them legible, once the went to spot metering, a slight tweak in the contrast was all that I could do.
My conclusions were after spending most of a day fiddling with this, was simply that the two primes produced very similar results. I tried different F stops, and different ISO's, and I couldn't really see much difference although maybe just a little. By contrast, the degraded quality of the FA 80-320 is obvious---and clearly worse when the lens is "wide open" as opposed to being stopped down to f/9. With the prime lenses you could freely use any f stop you wanted and the results were not markedly different, but with the cheap zoom, raising the ISO so you can get the lens stopped down a stop or two from wide open pays visible dividends. This is why I show results for the FA 80-320 at 2 different F stop settings. Also for what its worth, I used the Zoom at a full 320mm and then resized the result all to the same size, so there is a little less 'optical zoom' in the Zoom lens renditions which should work to the Zooms benefit.
One last thing I did to attempt to differentiate the two prime lenses was to add a converter. Unfortunately I didn't (couldn't) test with the same converter. I used the Pentax Rear Converter-A 1.4x-L on the big f/2.8 300 lens. It's a dedicated converter that only works on a few long Pentax lenses and is not usable on the 'little 300'. Instead on the 'little 300, I used the "Pentax-F 1.7x AF adapter. For what it's worth, I convinced myself that the 'Big 300 with the 'dash L' converter had a better result than the 'little 300' with the AF adapter. I've thought all along that the value of the 'Big 300' prime lens was its superior ability to support teleconverters, and my research here seems to support that.
Sometimes one gets so possessed with the process that things get out of perspective. Just to do a reality check, I decided to take a 'control photo' with a little Pentax I-10 camera which is about the size of a cellphone. It's suppose to have a good optical zoom with a digital zoom beyond that. All I can say is that from the same 60 foot distance the photo really sucks. Even at 20 feet I couldn't read the days of the month. The exposure is haywire also but I couldn't find a way to override it so this is what you get.
This may not be fair to the I-10 because it probably isn't made to take pictures 60 feet away, but I couldn't read the calandar when I moved up to 20 feet either.
I did this first round of testing at about 60 feet. Next I reduced the distance to about 40 feet and dug out my Pentax-A f/4 prime and compared it to the FA 80-320. I got the same results. The prime was better, and the zoom was visibly improved when stopped down a couple of stops from wide open, but still inferior to the prime. On the third go around, I cut the distance to 25 feet and went with a Pentax-A 135 f/2.8 prime, a DA* 50-135 f/2.8 zoom (at 135), and the 80-320 zoom at 135. the FA 80-320 continued to suck big time while I felt that the prime and the DA* were similar in results. Essentially I can say the obvious. You get what you pay for, and those lenses that are over a $1000 do produce results. If you rely on a cheap zoom for all your long shots you will be disappointed.