[rdenney]

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Series Contents

Considering how much I was amazed by the 120 Macro, I decided to look at another lens with a reputation for extreme performance. The A* lenses were Pentax's premium line in the manual focus era. I managed to snag this one from ebay for $379 from a Japanese seller, which makes it the most expensive of my manual-focus 645 lenses.

The first thing I noticed while handling it with my NII (I bought this lens before the 645z arrived) was that it was heavy. The weight was ungainly, and the often-heard complaint that it lacked a tripod ring certainly rang true. I posted elsewhere that the Canon 70-200/2.8L tripod ring would fit with just a bit of shimming, which I did with some stick-on leatherette. It certainly seemed firm enough. But it just didn't work--motion blur in every shot taken with a slow shutter speed. Not that much motirion blur, but enough. So, here is a picture of what NOT to do:



Another poster suggested a ring used for mounting telescopes, or perhaps finder scopes. I was a bit dubious--my own telescope hardware has never proved all that strong (and I've never been willing to spend enough to rectify that). Nevertheless, I ordered them because they were cheap: $33 (not including the lens plate, of course). The part is available on Amazon and is called "Orion 7363 76mm ID Telescope Tube Rings," and they are designed for the Orion Observer 70. Only one of the rings has the 1/4"-20 thread for a lens plate, which they added for piggybacking a camera for astrophotography. When they arrived, I was pleasently surprised. They are heavily constructed and fit the lens barrel like a (felt-lined) glove. It's easy to adjust and turn, and there is no wiggle whatsoever.



There is only one hole for the lens plate, but eventually I'll drill a hole for an anti-twist screw.

But I had not installed that when I conducted the test, and I made the photos by attaching the camera to the tripod rather than the lens.

Okay, back to lens performance. Here's the full test image made using the 300 A*. As with the other tests, the day was dull, but there wasn't much wind. As usual, the focus point is the lamp, and I used the focus confirmation beep (I have, since then, installed a microprism screen for an NII into the 645z as an experiment--so far so good). These images gets the basic default correction from DXO Photolab, but nothing else.



Unlike the 120, this lens is not diffraction limited at all apertures. This one gets sharper--let's look at the series.

1:1 center crop at f/4--remember these would be part of a 7-foot-wide print if displayed at 100 pixels/inch, which is most computer monitors.


Nope, it's not perfect wide open, but it's very good. Here it is at f/5.6:


That's a lot better, and it would look very sharp in a smaller print or from a little distance.

Here's f/8:


Okay, this is about as good as anyone might want.

Here's f/11:


But this is still a bit better--a tiny bit. So far, f/11 is the sharpest aperture on this lens.

Let''s look at f/16:


Nope, f/11 is still the best. But I will always stop down if that's what I need for depth of field--being slightly below the optimum of the lens is trumped by being out of focus because it's too far from the focus plane.

Finally, here's f/22:


And now we see the effects of diffraction, small though they are.

I will definitely use this lens, but I'm not sure it's going to Alaska. It's better than the 400, except that it's shorter than the 400, and in my teleconverter test, I confirmed that the 400 is better than the converted 300. The 200 is also excellent by f/8, and with a converter is only a stop slower than this lens. And the 200, with converter, is lighter than the 300A* by a long shot.

This is a great lens, but I'm still more astounded by the 120.

Rick "two more lenses to go" Denney

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Last edited by rdenney; 06-30-2018 at 12:18 AM.

[Sandy Hancock]

Do you have the A or FA version of the 120 macro Rick?
I'm mightily impressed with my A, but the FA rates even higher in the database reviews despite the two looking to be the same optical design.

[rdenney]

Do you have the A or FA version of the 120 macro Rick?
I'm mightily impressed with my A, but the FA rates even higher in the database reviews despite the two looking to be the same optical design.

It’s the A, manual focus. Ratings are subject to systemic biases of various kinds, and sample variation, too. I suspect objective testing of the type I’m not doing here, with enough samples, would show that the A and FA perform similarly.

Rick “no trouble focusing manually” Denney

[AstroDave]

Unlike the 120, this lens is not diffraction limited at all apertures. This one gets sharper--let's look at the series.

Don't understand. For a given camera, ALL lenses (regardless of focal length/front lens diameter) will show the same diffraction effects as a function of f-stop.

If your 120 was looking blurry, don't blame it on diffraction (until f22 or so for medium frame).

[kaseki]

Don't understand. For a given camera, ALL lenses (regardless of focal length/front lens diameter) will show the same diffraction effects as a function of f-stop.

If your 120 was looking blurry, don't blame it on diffraction (until f22 or so for medium frame).

As a point of pedantry, all lenses with the same aperture shapes (circularly symmetrical in theory but in actuality whatever messy 2D function the iris shape provides) will show the same diffraction effects at the same specified wavelength. The first null (transverse distance at the focal plane) of a circular aperture diffraction pattern is at 2.4 x wavelength x f/#. Comparing two lens assemblies with different iris leaf patterns may yield different results for the same f/#.


In addition, although this should be minimal when changing among the smallest apertures, there might be chromatic aberrations balanced over the lens aperture that change in value with f/#. These might be mistaken as a diffraction effect as the iris is closed. Similarly, the point of best focus (compromise) could change with aperture.

[desertscape]

there might be chromatic aberrations balanced over the lens aperture that change in value with f/#. These might be mistaken as a diffraction effect as the iris is closed.

Are you talking spherochromatism here, because it you are, the color correction is improved when stopping down and should not be mistaken for diffraction.

[rdenney]

Don't understand. For a given camera, ALL lenses (regardless of focal length/front lens diameter) will show the same diffraction effects as a function of f-stop.

If your 120 was looking blurry, don't blame it on diffraction (until f22 or so for medium frame).

The 120 was at its sharpest (in the focal plane) at f/4. It became progressively less sharp when stopped down, but that isn’t the same thing at all as “looking blurry”. Take a look at that test—I showed all apertures. When I say “diffraction-limited”, I mean that the improvement in residual apertures as the lens is stopped down is less than the effects of diffraction, which increase with smaller apertures.

It’s hard to compare different focal lengths with my samples, because the photos were all made from the same position, and the comparison details are therefore magnified different amounts with changes in focal length, especially when viewed at 100%.

Rick “diffraction exists even wide open—it just may not be noticeable” Denney

[AstroDave]

The 120 was at its sharpest (in the focal plane) at f/4. It became progressively less sharp when stopped down, but that isn’t the same thing at all as “looking blurry”. Take a look at that test—I showed all apertures. When I say “diffraction-limited”, I mean that the improvement in residual apertures as the lens is stopped down is less than the effects of diffraction, which increase with smaller apertures.

I did go back and look now. For sure, as you stop down, things get less sharp (~ my "more blurry"), but I think (without doing some calculations) that this is happening faster than diffraction would be kicking in. This may be due to other effects such as commented upon by kaseki.

[rdenney]

I did go back and look now. For sure, as you stop down, things get less sharp (~ my "more blurry"), but I think (without doing some calculations) that this is happening faster than diffraction would be kicking in. This may be due to other effects such as commented upon by kaseki.

I think you're right. I'm moving this discussion over to the 120mm thread (Grand Lens Test 9).

Rick "this needs to be said there" Denney

[kaseki]

Are you talking spherochromatism here, because it you are, the color correction is improved when stopping down and should not be mistaken for diffraction.

I was hoping that that would be recognized as my point.

[johnha]

Thanks for the review Rick - I've acquired a refurbished A*300 but have yet to really put it through its paces. As you say, it would benefit from a foot (what where Pentax thinking!).

In the original review you mentioned this mounting: "Orion 7363 76mm ID Telescope Tube Rings," and they are designed for the Orion Observer 70.

The closest I could find on Amazon was the Orion 7369 76mm ID... rings for the same 'scope (Which showed a review with them around a Pentax 300/4) - I'll report back whether they fit or not when I receive them.

[SteveinSLC]

I opted to get a Neewer Long Lens support for my 300A. Seems to work pretty well, but a good quality telescope collar might be a better option yet.

[texandrews]

Thanks for the review Rick - I've acquired a refurbished A*300 but have yet to really put it through its paces. As you say, it would benefit from a foot (what where Pentax thinking!).

In the original review you mentioned this mounting: "Orion 7363 76mm ID Telescope Tube Rings," and they are designed for the Orion Observer 70.

The closest I could find on Amazon was the Orion 7369 76mm ID... rings for the same 'scope (Which showed a review with them around a Pentax 300/4) - I'll report back whether they fit or not when I receive them.

I don't see that review....

[johnha]

I don't see that review....

The first review (from Italy 21st June 2019 - 4 star) on this page: Orion 7369 76mm ID Telescope Tube Rings: Amazon.co.uk: Camera & Photo

Not sure which 300mm f/4 it might be though?

[rdenney]

I don't see that review....

Post 1 in this thread is where I identified the part, from my Amazon invoice.



And, as shown in Post 1, this is for the 645 300 f/4 A*.

And I see I had a typo in my first post, which you guys had already discovered.

Rick “there were other threads about this, of course—not my original discovery by any means” Denney