Roger Cicala tests Lomography's Petzval lens

Pontus · Expire: 2016-08-25

That is more than sharp enough for me. And I never experienced corner softness with my copy either.

iangreenhalgh1 · Expire: 2014-01-07

Corner softness is very much a non-issue with many shots.

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

scsambrook wrote:

Arkku - I think Gerald's point was probably that he wanted to make sure your picture really [b]was from the corner . . .

I did not doubt the words of Arkku. I just wanted to know the exact reproduction scale he had used. In principle it would be possible to deduce the scale through the crop, but it seems the forum software automatically resizes large images to 1024 pixels wide, what complicates to figure out what scale was really used.

Arkku wrote:

My personal conclusion: no worries about corner sharpness for me.

I agree. The corner sharpness is much better, for example, than the samples from the test by Kurt Munger. Why such a difference in performance? I don't know, but an obvious difference in the test procedures was that in the test conducted by Arkku, the focus distance was about 1m, while Munger tested for infinity.

The Sony STF lens is an Ernostar type, a highly asymmetrical optical structure of quite variable performance with focus distance. It is possible that the Sony STF was optimized for very close distances (1 to 2 meters) at a loss of performance for longer distances.

Unfortunately I do not have a Sony STF lens to clarify those doubts, but I did some tests with a few lenses I own in order to get a better idea about the level of performance of the Sony STF. I tested using the same reproduction scale used by Arkku. The RAW images were converted with ACR with basic settings. There was some variation in exposure and white balance because I did not make a special effort to equalize the results. The camera used was a full-frame with 24MP (Sony A99) so the 100% crops are comparable.

Sony 135mm T4.5 STF versus Pentacon 135mm F2.8
The first comparison was with a pedestrian Pentacon 135mm F2.8, which is a good 135mm lens for general use. The Pentacon minimum focus distance is 1.7m without accessories, so I had to use an extension ring to focus to approximately 1m.

Sony 135mm T4.5 STF at T4.5:

Pentacon 135mm F2.8 at F2.8:

Pentacon 135mm F2.8 at F4:

Comments:

1. No doubt the Sony STF was sharper than the Pentacon in this test. The details are there, but the images produced by the Pentacon are hindered by oblique spherical aberration and coma. The images produced by the Sony STF are cleaner, even though sharpness is not outstanding.

2. The Pentacon 135mm worked as an improvised macro lens, which is not the strong point of an Ernostar-type lens optimized for infinity. Macro lenses typically use a highly symmetrical optical formula, whose performance is much less sensitive to the focus distance.

3. The Pentacon 135mm was probably optimized for infinity, but as the design is also of Ernostar type, the Pentacon worked far below its capabilities in the test.

Sony 135mm T4.5 STF versus CZJ Biometar 120m F2.8
The CZJ 120mm F2.8 Biometar has a focal length that is not too different from the Sony STF 135mm T4.5 so the comparison is valid provided the same reproduction scale is used. As in the case of Pentacon 135mm, an extension tube had to be used for the required reproduction scale.

The CZJ Biometar is a lens which was originally designed for the medium-format Pentacon Six camera. This causes the CZJ to have a possible disadvantage when adapted to the format 135. When a lens is designed for a larger format, the optical designer attempts to correct the aberrations optimally over a larger area. Of course, if a lens which is optimized for a larger area, it tends to have lower performance than if the same optical formula were used to cover less area. Well that's the theory. Let's see how the practice is.

Sony 135mm T4.5 STF at T4.5:

CZJ Biometar 120mm F2.8 at F2.8:

CZJ Biometar 120mm F2.8 at F4:

CZJ Biometar 120mm F2.8 at F5.6:

CZJ Biometar 120mm F2.8 at F8:

Comments:

1. To my eyes, the CZJ Biometar is sharper than the Sony STF even when CZJ Biometar is at wide open. This result is somewhat surprising because the CZJ Biometar is medium-format lens, and its performance for infinity is not better than the Pentacon 135mm F2.8 also for infinity.

2. The CZJ Biometar is a double Gauss lens, which is a symmetrical configuration. That explains why the good performance maintains even for short focus distances.

3. The pictures for F5.6 and F8 were also included to show how performance varies with aperture. The peak performance occurs for F8, when the sharpness is uniform for the entire frame. It was not shown but from F8 the performance begins to drop due to diffraction.
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If raindrops were perfect lenses, the rainbow did not exist.

Ultrapix · Joined: 06 Jan 2012 Posts: 551 Location: Italy

I hate to say that, but the first "bill test" is totally wrong if done in this way...

many lenses known for their corner weakness are only affected from field curvature... their corners are sharp, but at different distances!

a proper test needs a flat surface, perfectly parallel to the sensor, focused on center; then you'll know the outer areas performance...

Just my 2 cents

Arkku · Joined: 28 Feb 2007 Posts: 1416 Location: Helsinki, Finland

Ultrapix · Joined: 06 Jan 2012 Posts: 551 Location: Italy

calvin83 · Joined: 12 Apr 2009 Posts: 7554 Location: Hong Kong

Field curvature are vary by focus distance. The resolution of many lens does not drops suddenly but the field curvature makes the MTF drops suddenly. The test used by Arkku is a practical test showing the lens is capable deliver at the corner.

I won't surprise see the Biometar is better at the corner because it is a double Gauss with less field curve at the middle of the image circle. In my experience, many good double Gauss medium format lens performs good on a 35mm camera too.

Gerald · Joined: 25 Mar 2014 Posts: 1196 Location: Brazil

In the previous comparative test with the Sony STF 135mm T4.5 it was necessary to add an extension ring for Pentacon 135mm F2.8 because the focusing distance was approximately 1m. The performance of the Pentacon was not very good, but as I have mentioned before, in these conditions the Pentacon was working in unfavorable conditions since its design was probably optimized for infinity, not for short distances.

The performance of lenses with very asymmetrical design as Sonnar, Ernostar, retrofocus, etc., usually degrades much when the focusing distance is very short. Would it mean a lens like the Pentacon 135mm cannot be used for close-range photography or macro- photography?

The answer is: A lens optimized for infinity can be efficiently used for short-distance photography and macro photography if used together a close-up lens of high quality. To prove that, I ran some tests using a 1.6 diopters Sigma achromatic close-up lens. I also took photos with an extension ring at the same reproduction scale to show the differences in performance. The differences in performance are larger in the corners, so the crops from the center were not shown. The focus was adjusted to the upper left corner.

Full image:

Pentacon 135mm F2.8 at F2.8 plus close-up lens � 100% crop:

Pentacon 135mm F2.8 at F4 plus close-up lens � 100% crop:

Pentacon 135mm F2.8 at F5.6 plus close-up lens � 100% crop:

Pentacon 135mm F2.8 at F8 plus close-up lens � 100% crop:

Pentacon 135mm F2.8 at F2.8 plus extension ring � 100% crop:

Pentacon 135mm F2.8 at F4 plus extension ring � 100% crop:

Pentacon 135mm F2.8 at F5.6 plus extension ring � 100% crop:

Pentacon 135mm F2.8 at F8 plus extension ring � 100% crop: