MTF data

Lookings at the K20D manual I see that there is an MTF priority programme:

MTF priority program is a Program Automatic Exposure
that prioritizes the best aperture settings for the attached
lens and is effective when combined with a DA, D FA, FA,
or FA J lens.

In most cases this will be f8-f11, however I was wondering whether there was any database/documentation of the best aperture for each lens. Any thoughts?

Thanks in advance,

Jonathan

I don't think you'll find a complete database anywhere.

The reviews at www.photozone.de/pentax reveal the info you want for the 35 or so lenses for Pentax they have tested - not only for central resolution, but edge too, and at various focal lengths with the zooms.

In a lot of cases, the peak resolution is at apertures larger than F8.

Take any such data with a pinch of salt to allow for testing errors and sample variation.

The K10D also has an MTF priority program.

See also www.photodo.com

The MTF information is actually stored inside the lens
And it's not always f/8. Some of the lenses I have are f/5.6... and some other values I can't remember...

Matt

Thank you for your responses. On reflection I should perhaps rephrase my question - if there is an MTF programme then presumably the K20D will have been programmed with the MTF data for the relevant lenses.

So what I was wondering was whether there was a database with the MTF details that vthe K20D is programmed with?

each Pentax lens from the F series onwards contains a ROM chip which has (among other things) the MTF data stored on it. The camera acceses this data via the digital data contact on the lens mount.

this way, the camera doesn't have to be updated every time a new lens comes out, it simply asks the lens.

Which is what I said before


But the additional info is helpful

If you actually want to find out the MTF manually then it's a long winded process. Have a look at http://www.imatest.com/

Matt

Is there any way to determine what the MTF value would be?
For example, if I have MTF priority mode set and I shoot in program mode, shouldn't aperture and shutter speed both be adjusted "properly". Thus if the light is such that the best quality aperture can be used with a reasonable shutter speed/ISO, that should give you your answer, correct?

Thats how I understand it, but I never studied this much. So I would be interested in any more info on how this works, if I'm wrong.
I'm not sure I ever saw this work well in practice, so I could certainly be off on my assumptions.

[edit] Looks like Matt types faster that I can. I'll check out the link. Thanks![/edit]

Generally... but don't forget that the aperture chosen is based on a Ev graph (remember those printed in the old Pentax SLR manuals??) The MTF program mode has a kind of "shelf" where the MTF aperture is chosen, but only up to a point. If you fiddle around you can discern what the MTF of the lens is (ie you'll have to point the camera at a bright enough light source to suss it out).

Goodness only knows how auto ISO folds into the mix

Best thing is to download a Z1 PDF manual and take a look at the chart.
(e.g. page 127 of the Z1-p manual)
Matt

he, he, oh yeah I've looked at the charts in the past, sort of stared at them and let my eyes glaze over is probably more like it.
I never had any idea how to use that in the real world.

I really should take a look at them again. Its been a few years and they would probably mean something to me now.

In a generalised way, most prime lenses have an optimum aperture of maybe 2 or 3 stops down from open aperture. So your average f2 lens might perform best at f5.6. Older designs may be better at f8 or even f11.

Hgh quality zooms will be best around f8 or f11, but poor quality ones will continue to improve and may be at f16 before they are performaing at optimum.

This is all theory though, and it will depend on the decentering of the elements. A poorly constructed lens will not reach the potential of its design. Equally, a superbly centred low cost design could out-perform a poor example of a top design.

I don't think we need an MTF setting to know we need to stop down to f8 or f11 for best results...

This page shows why an MTF setting would be useful - resolution of the 16-45 is dropping by f/8 according to their test results, and I'm sure many other lenses behave in a similar way, or at least contrary to your assertion John.

Dan

I trust that the people that designed the equipment, knew what they were doing.

So I leave my camera set to "MTF" as default. I can rest assured that I'll get the sharpest image possible. Of course I only take the camera's advice as a starting point, a suggestion. From there on I take over, and change settings as needed, to make the shot!

Actually the information shown is only a very small part of the overall picture. Measuring at 50lpmm is measuring very fine detail and we need to know the 10lpmm and 30lpmm readings as well to pick up a more holistic view.

The graphs shown are actually suggesting behaviour more typical of a theoretically perefect lens, where the optimum resolution is obtained at or near open aperture. Stopping down reduces resolution because of diffraction effects.

It is much more usual for a lens to peak at middle apertures, older lenses at smaller apertures than more recent designs, so it makes me wonder how accurate the testing was.

Unless of course the 16-45mm behaves as a perfect lens. Perhaps it does.

I understand the Pentax 16-45mm F4 is really an F2.8-4 lens, but with a cam mechanism to stop it down to provide the constant F4 max ap.

It is therefore not extraordinary that Photozone find its peak resolution is F5.6 at 16mm, and F8 at 45mm. (Center & border averaged).