[GibbyTheMole]

Hey all.

I've noticed when examining a few photos at full resolution, they don't seem to be totally in focus when according to both the AF confirmation & the split screen, they should be. In an effort to find out what's going on with some of my zooms, I decided to conduct a little test.

I took a piece of grid paper & taped a couple playing cards with fine detail to it & taped it flat to a wall. then I aimed a couple of photo lights at the target, put my K-x on a tripod to avoid camera shake & auto focused on the target. The shutter was tripped by remote to prevent shake. The following lenses were used:

Pentax 18-55 kit lens
Cosina 28-210 AF zoom
Tamron 28-300 AF zoom (model 185D)

I took separately auto focused shots at varying focal lengths through the range of each lens, and at a variety of distances from the target. All shots were with a wide open aperture. Here's what I found:

Pentax 18-55 kit lens - Very soft (out of focus, actually) at 18mm f3.5 at a distance of about 6 feet. Sharpness improved at close distances. It was hard to evaluate sharpness from farther away, as the detail at 18mm was too small to really make out. Excellent sharpness at all distances from about 28mm on up. I was kind of surprised by this. I expected this lens to be sharp all across the range.

Cosina 28-210 AF zoom - Somewhat soft (a bit out of focus) at 28mm, f4.2 at 6 feet. Otherwise, sharp from about 35mm on up.

Tamron 28-300 AF zoom - Somewhat soft (a bit out of focus) at 28mm, f 3.5 at 6 feet.. Slightly soft at 300mm, f6.3, but acceptable. Sharpness was good to excellent from about 40-250mm. The results were a little surprising. I expected this lens to be better at 28 than 300mm. The reverse was true.

After seeing the results, I repeated the tests with the K100D body to eliminate the possibility that it was the camera's AF to blame. The results were essentially the same.

I didn't test my manual focus lenses. I only tested the auto focus lenses, because I wanted to know why some of my AF shots were out of focus... I figured the camera's AF should give me correctly focused shots under well-lit conditions. Since the results were the same with two different model bodies, I guess the issue is just weak lens sharpness at certain focal lengths... But at least the camera body isn't malfunctioning.

Just thought I'd share in case someone else experiences blurry shots when viewed at full resolution and wonders as I did what the heck is going on. I guess zooms just have some weak spots, some more than others.

Now what I really want is a 12-600mm constant f 1.4 zoom that performs superbly at every FL, and costs $100. Hear that, Pentax? So gimme.

Cheers,
Bobbo :-)

[lytrytyr]

Might be interesting to repeat the test using contrast AF with live view.
That's what I use with the Tamron 17-50/2.8 A16P on my K-x
when there's plenty of time and focus is critical.

[GibbyTheMole]

Might be interesting to repeat the test using contrast AF with live view.

I'm not sure I'm familiar with that routine. Might be interesting to try, though. Of course in a real-world application, I doubt I'd ever use live view for focusing.

[lytrytyr]

I'm not sure I'm familiar with that routine. Might be interesting to try, though. Of course in a real-world application, I doubt I'd ever use live view for focusing.

It's easy to do on the K-x,
and it really nails the focus
(at least with a sharp lens
like the Tamron 17-50/2.8 A16P).

Make sure you've set up
contrast AF for live view
and center point focusing beforehand.

Out in the field,
you just press the LV button,
aim the image center where you want to focus,
half-press the shutter button,
and give the camera a couple of seconds
for the white rectangle to turn green.
Then press the LV button again to get out of live view,
switch back to MF so you don't change the good focus,
and shoot.

For moving targets,
the technique helps you get
an accurate prefocus on a certain spot,
and of course it's ideal for static subjects.
Takes about 5 seconds.

[GibbyTheMole]

Thanks for the tip. I might give that a try. I haven't really used the live view feature on my K-x yet, and I've had it for several months. Maybe it'll be useful for something.

Cheers,
Bobbo :-)

[lytrytyr]

I haven't really used the live view feature on my K-x yet, and I've had it for several months.

I've had a K-x for more than two years now
(so long, that even its replacement is obsolete),
but still keep coming across new ways to use it.

Maybe one day I'll try the toy camera feature,
and save having to shell out for a Q.

[GibbyTheMole]

To me, the word "obsolete" means nothing. If it does the job, that's all that matters. The K-x is a terrific camera. I probably will keep it 'til it shells out on me, then I'll buy another one, or a K-r... Unless there's an even better cost-effective Pentax body available by then.

I still use my K100D as much as the K-x, and get great shots with it. And my wife has two *ist Ds, which are older yet. Still great cameras, though!

Cheers,
Bobbo :-)

[Frogfish]

Maybe a couple of points worth noting :

1. If you are focusing on a vertical flat subject then you can't tell whether the subject is actually in focus (and soft) or is front or back focusing. Usually a ruler or test chart positioned at around 45 degrees works best so you can see where the focus really falls.
2. Some zooms have focus creep at different FLs.
3. The 18-55 has been noted as being a little soft at WA and much sharper within the mid-range until softening up slightly again at 50-55mm (i'd imagine your other zooms will be the same : soft at WA and sharpening up in the mid-range before tailing off again at the end of their ranges). The same goes for the aperture, if you are shooting wide open then the vast majority of lenses will not be as sharp wide open than at (and this depends on the lens of course) anywhere from f2.8 thru f8.

This is almost identical for all zooms, unless you are paying for pro-level glass you are always going to find this. Even the US$1,000+ Pentax 60-250 is much sharper at 220-230mm than at 250mm (unless stopped down to f5.6 / f8) and you are better off cropping a shot at 220mm to the FOV of 250mm than shooting at 250mm itself. Zooms with large multipliers especially (x4 and upwards) will suffer from this which is why the best zooms are x2 or x3 max.

[GibbyTheMole]

Hi Frogfish.

I've used a 45 degree focusing chart and as a rule, the focusing is correct. Your findings with regards to WA focal lengths on zooms being less sharp wide open just mirrors my test. (or vice versa.) Good to know that's the norm.