[Naturephoto1P]

Guys

Take a look at the following focusing test and let me know what you think. I just got a Pentax 300mm 2.8 FA* and I noticed that most of my pictures were soft. This is not at all what I expected as my 600mm FA* is tack sharp, same camera body.

I started testing this lens and found severe back focus issue, I adjusted it to +10mm and it still looks off to me. I just want to make sure I am not too picky as the lens came from a reputable source but lets say I am concerned - this is not cheap glass.

Pentax 300MM F2.8 FA - Naturephoto1P's Album: Lens Tests - PentaxForums.com

This test was done with inside back light as it is raining today. The calibration for the K20D body is 0 (other lenses work just fine) and I adjusted the lens to +10; tripod, delayed release, F2.8 - this is the maximum AF adjustment and this is the results. To me it appears to be about 10 to 14mm back focus.

What do you guys think?

[rvannatta]

Guys

Take a look at the following focusing test and let me know what you think. I just got a Pentax 300mm 2.8 FA* and I noticed that most of my pictures were soft. This is not at all what I expected as my 600mm FA* is tack sharp, same camera body.

I started testing this lens and found severe back focus issue, I adjusted it to +10mm and it still looks off to me. I just want to make sure I am not too picky as the lens came from a reputable source but lets say I am concerned - this is not cheap glass.

Pentax 300MM F2.8 FA - Naturephoto1P's Album: Lens Tests - PentaxForums.com

This test was done with inside back light as it is raining today. The calibration for the K20D body is 0 (other lenses work just fine) and I adjusted the lens to +10; tripod, delayed release, F2.8 - this is the maximum AF adjustment and this is the results. To me it appears to be about 10 to 14mm back focus.

What do you guys think?

Is there a filter in the filter holder...???? I've heard reports elsewhere that the AF assumes the presence of a filter.....

[Naturephoto1P]

PF filter in front - filter in filter slot - all elements of optical formula present.

[borno]

I test my lenses in the back yard in daylight with a flat target some distance away, tripod, mirror lock up self timer afc and pick the best one on the computer. That being said, nothing was sharp on the fa*300mm f2.8 at a distance even manually focused (I think I set mine to +10 too) I hope you didn't get the one I traded in a few years ago. The reason I got some canon gear was Pentax (did their own service then) wouldn't own up to it and fix it. I hope you get it sorted, maybe it will be better in daylight.

[Naturephoto1P]

Do you have the serial number of the lens you got rid of, I need to run some better tests but from everything I see it is too off.

[borno]

3911073 I sent it in twice and pentax said it was in spec. and the second time the box had almost no padding in it, almost as if they were hoping for damage in shipping. That did it for me. I really loved my pentax bodies too.

[Asahiflex]

What's a "PF filter"? Please remove this filter if it's not part of the original optical system.

Also, try a second body (despite your camera body working perfectly with the 600mm).

[Marc Sabatella]

It appears you invalidated the test by marking up the focus target such that you cannot guarantee the camera actually focused on the horizontal line. Repeat with a clean copy of the chart - and be sure to be at a 45 degree angle (at least; doesn't have to be exact, but it has to be steep).

[Naturephoto1P]

Ordered a lens align and I have a couple of bodies I can run some tests with it later this week when it arrives that are more scientific.

Like I said I just got this lens and this is an initial test based on what I have available which is a sheet that I had used in the past. Not optimal but two lenses same body gave very different results and if you do the tree test (what I call it) - shoot at the center of a tree where you can see the rest of the diameter of the tree and see what you got - it is focusing about 12 mm behind center - half inch whatever you want to call it behind the point of focus depth wise, this is at +10 on the internal camera lens specific adjustment. Not scientific but a reasonable mans approach to test something quick and dirty with minimal effort. The second lens I used has worked great for many years for me as has other lenses on this and other bodies with no major issues.

I am looking for similar experiences and what was done to resolve the issue. I would prefer not to ship this lens out and have it calibrated (if that can be done) and was not really needed and hope that there is a better and simpler fix. The seller is wiling to cover repairs but that is not the issue as I hate to waste his time and money as well as my time. However; if I need to send it out where is the best place to send a FA* lens for calibration that is reasonable and are these lenses able to be calibrated successfully?

I need to check it out but I know on my 600mm F4 FA* the front filter is part of the optical formula, I am not sure on this lens if that is the case but it looks like it came with the lens and is not something added on later.

I have had a couple of 300MM f2.8's in the past and you are right in that the front filter can and does throw out the focus and can distort the image slightly. I know the rear filter on this lens is part of the optical formula and I checked that it is in place and what appears to be the original filter.

So more testing and hope I can get this puppy to live up to its reputation.

Will post better tests and some thoughts I got from other members that I received PM's from that I know have a copy of this lens and see how things work out. Any thoughts are appreciated.

[borno]

What's a "PF filter"? Please remove this filter if it's not part of the original optical system.

Also, try a second body (despite your camera body working perfectly with the 600mm).

There is a protective filter on the front with a rubber rim and it's part of the lens.

[Clinton]

Naturephoto1P and I have been PMing about this all day, however after helping another forum member through a similar set of issues, and dealing with a set myself, maybe it's time to put out the how this works with this lens once and for all.

It seems to be a trend with this lens to require a +8, +9, or +10 AF adjust; but if you are reading this post, and thinking about tuning or troubleshooting your FA* 300/2.8. This should help.

Something very important to be aware of is how narrow the DoF is on this lens at MFD. It's .17 inches or .43 cm. What this means is that even a slight change in distance due to shake, or even the angle of what you are focusing on, can have a dramatic impact on what you are seeing in an AF test.

After extensive tests, my copy requires a +9 af adjustment, but when I first received it, I was convinced it would need to go to CRIS for an AF adjustment because +10 didn't seem to have a strong enough impact.

When I first got it, this was the untuned result:

This image was with me pointing the lens at the center focus target on the lensalign. Clearly there is an issue here.


Following this, was a lot of testing, lost nights of sleep, and a bit of stress.

This test was done at AF+10:


Clearly, this is quite a bit better, but when you look especially closely, the in focus point on the ruler appears to be at about 4, not at zero where it should be.

Also what appears to be lateral CA between green and purple appears to really transition at 5 or 6 on the ruler, not at zero where I thought it should be.

This is when you have to boil it down to some science, also until we get to the end, assume that +10 is actually one too far.

Clearly this lens has some lateral CA color changes, but it also has some purple fringe that masquerades as lateral CA. I was attempting to use the color to help me ascertain the correct focus point, but that wont work for this lens, so, strip the color:


That clearly helps us visually better see what is in focus, but it still looks like the 4 is the best focus. Also on the plate, it still looks like the actual plate is less in focus than even the texture of what is below the lensalign plate.

With the lensalign system, you use an opening in the back plate to make sure you're perfectly centered before you shoot the trial shots, then once it's perfectly centered, you slide the back plate down and the camera is supposed to see the bright contrast lines of the front plate. In this case, somehow the lens is actually able to focus on the dark back plate which is a slight distance behind the front plate that you see in the image.

Once I focused on the bottom middle target in the front plate, this noticeably improved, but that simply moved what I perceived as the correct focus point down to a solid 3 on the ruler. Not good enough. (Sorry I didn't keep a sample of this.)

This was when I made my discovery that even in live view, the AF seemed to be off. AF in live view shouldn't ever need to be adjusted since it's contrast AF. Also since I started playing with focusing on the lower targets, I came to understand that even the little bit of angle between the middle range of targets on the lensalign and the bottom row had a *dramatic* impact on the AF.

In fact, even using the P on pro, right next to the ruler as a focus point wasn't good enough. The angle of the line away from the P skewed the results.

My lensalign also comes with an extended ruler that slides together in two parts. I used that to insert a target at the zero mark, almost dead center in the ruler. I also found that lighting played quite a role in what AF chose to target, so I tried to get my spotlights to highlight the target and show the ruler, but not reflect directly into the camera in any way.

This is the result of spot metering, B&W shooting, AF+10, center AF point, remote trigger, 2 second delay, and as much sliding things back and forth as I could stand to try to get the ruler to appear nearly straight up and down:

100% crop


Note the fibers from the top of this piece of paper where I tore it.

Full image:



From the 100% crop, you can tell the focusing is actually slightly in front of the target, which is why I now say +10 was one too far.

I probably spent 30 hours coming up with this process, trying in all sorts of lighting, focus points, and with a pro tool like the lensalign. This is by far the most technical AF adjustment I've done, and it really makes me wonder how people ever shot with AF lenses of this type with film. Hopefully this will save some heartache and time to the rest of you who purchase a copy of this lens.

[Ash]

Clinton, that's a very comprehensive and useful explanation of the focusing characteristic of your lens. Thanks for sharing.

[borno]

Mine was about +10 as well, but not sharp even stopped down. I sent it to pentax with a body (twice) and other than a focus adjustment it was no help. I don't think I've ever heard of Pentax (or cris) adjusting a lens for sharpness and should be easy for an optical company repair department. All they seem capable of is adjusting front/back focus or replacing focus motors. (or swapping a lens) Here's what pentax wrote on my repair order. ( I sent it in with a disc and long explanation on the fact it just wasn't sharp at distances over about 30 feet)

Quote from Pentax:
"The image sensor in your D-slr has different chromatic characteristics than film does. For this reason, lenses designed for film cameras will not perform as well as lenses designed specifically for Digital SLR cameras. Your camera body has been programmed to correct for overall back focusing."

The same company that used backwards compatibility as a big selling point. After telling me they would fix it under warranty and then blowing me off, I was pretty mad and traded it to a dealer for a bit of a loss and got a couple canon lenses and a refurbed 40d I can still mount my old pentax lenses on my canons and I gave my son my k100dsuper so I still hang around as this is the best forum

[Clinton]

@Borno, I believe the op and even the forum this is in is requesting help with the af.

I see you had trouble with yours back in the day. Hopefully the technology is better now. if you still had the lens, I would suggest the same things to you. As you can see from the sample, mine is diamond sharp when tuned correctly.

In any event, I would rather give the op hope that it can be tuned, and the tools to do so. Cris also told me they can perform this type of repair.

[borno]

What I was trying to say is, if it needs more than a BF/FF adjustment, good luck.:ugh: