Tuning a lens

Hi,

For interest, is it possible to have a lens tuned or adjusted for optimal sharpness, I'm thinking in particular about my Pentax 80-320mm f4.5 zoom. I know there are better zooms available but I've got this one.

I've only used prime telephoto lenses and am a disappointed with the zoom's resolution and would like to get the best out of it. I don't expect prime sharpness.

Greg

I don't think you can have the lens tuned, although if you can I expect it would cost almost as much as the lens!

I presume you have already tuned your camera's AF for this lens?

Hi Tim,

Been doing a bit of reading up on the subject and came across all the articles about collimation which made me start to wonder about tuning the lens. link

I have been playing with the auto focus adjustment on the K200D using Debug Mode, in fact I've spent the morning in the workshop making a permanent DOF scale (45deg target) which will mount on a tripod and make it easier to use with lenses of different focal lengths.

Thanks for the feedback

Greg

You can get pre-made charts for lenses of various focal lengths from here.

Thks Peter, I've tried those charts but they are very flimsy and unstable.

Greg

You could print it all out, and then carefully (as flat as you can, without wrinkles) paste it on a thin sheet of cardboard, and then it should probably work out ...

I had printed out focus-charts too, and then kept them as flat as I could on a table, and used a tripod to get a 45 degrees line of sight from the camera to the chart ...

Focus testing can be tedious, so do keep in mind stuff like subject-distance, aperture and what DOF you get for the combination ...

rkt, thanks for the tip, I spent the day making something similar.







I haven't tried it yet.

Greg

My 80-320's sharpness seems ok at the short-end but struggles beyond 280 or so, but I thought this was due to my lack of experience (not using monopods/tripods etc) with 'long' lenses. If there is a fix I would be interested to hear more!
I could ramp up the asa to 1600 or the shutter speed to 1000th but I can't do this when taking pics of propeller driven aircraft. Someone link recently posted some cracking shots on FLICKR using the 300 digital prime but the cost is out of reach to many.
My 80-320 stuff ( http://www.flickr.com/photos/28204187@N08/sets/72157622639681232/ )

I suspect you'll find that it simply isn't that sharp at the end.

However... eliminate shake as a cause first, as I think you might be surprised how much it affects the end result.

Bret

The FA80-320 I had was sharp, even at the long end
There are a few things to bear in mind:

1. Contrast can be low - particularly if the day is hazy. This is easily seen in the histogram, and quickly corrected in PS.

2. There is a definite possibility of camera shake. If you're using SR, then ensure that the "hand" icon in the viewfinder is lit before finally pressing the shutter

3. If you are having problems with near items at the long end, then you may need to adjust the focus in the camera body (K20D, K7). For that test you can use a focus aid chart. Hint: rather than putting the chart at 45 degrees sloping backwards, just pin the chart to the wall, and use a tripod a the same height, but 45 degrees into the wall (i.e. viewing from the side of the chart). This is the way the Pentax service manual shows

4. If you are having problems with far away things, then the infinity setting on the FA80-320 may be slightly out (it was on mine!). This link shows the disassembly - you need to get to this step where the little metal clamps are seen. You loose all three, and adjust for infinity by twisting the extended barrel slightly (you'll need a nice clear day, and an object on the horizon). When happy, just re-tighten the screws and reassemble.
(Usual disclaimers apply )

I found the 80-320 to be an admirable performer, and that the newer DA55-300 is only slightly sharper than the one I had.

Hope that helps!
Matt

Thanks Matt,

Those picture are a most helpful, I'll do some proper sharpness tests at 320mm this coming weekend.

Greg

Greg - I'll post a shot from your part of the world taken in the Kruger, near Sabi Sands, which was done with my K10 and the FA 80-320 @ 320mm, F11, 1/500, ISO 400 and hand held. Good at 50%.

Took 3 in total - next shot not as good - target fairly small for the AF, even at narrowest setting.

HTH


Steve

Please do.

Greg

Done - should be able to see it now in Gallery - title is SA Giant Heron & sole search tag is FA80-320mm


Steve

Thanks Steve,

They are beautiful birds and not common.

To help me get the correct perspective, how far away was the bird and did you use RAW format, also did you do any PP to the picture.

Please don't misunderstand me and I am probably being pernickety but the photo is showing exactly the same characteristics as my lens, which prompted me to post my original question.

The lack of contrast is easily fixed but I find it to be very soft and lacking in detail, see the grass, I would have liked it to be much more clearly defined. maybe I'm expecting too much from the lens.

Are you satisfied with it's performance?

Greg