Roger Cicala tests Lomography's Petzval lens

[scsambrook]

Okay - let me get this clear . . . the edges of the frame are less sharp than the centre ? Is that correct? And going by the lack of depth of field, the aperture must have been pretty wide open? So . . . is this fall-off in sharpness at wide apertures unusual? It doesn't look too bad to me.

[ZoneV]

Probably there is a big sample variation - on these images the borders looks better.
https://www.flickr.com/photos/clwongwilliam/8118689939/sizes/o/
https://www.flickr.com/photos/belmoniaco/13966650819/sizes/k/
https://www.flickr.com/photos/shunsuke_onji/8545250094/sizes/o/
https://www.flickr.com/photos/clwongwilliam/8536361549/sizes/o/
https://www.flickr.com/photos/104852879@N08/10176836466/sizes/o/
https://www.flickr.com/photos/bk_avocado/4934894820/sizes/o/

Regarding the MTF curves from manufacturers - one does not know wheter the published MTF curves are typical, or the best hand optimized result. Or only the theoretical MTF the design software gives - without all production tolerances. One of the several reasons why I recommend not to compare MTF curves from different ressources.

Softer edges is exactly what I see in the MTF curves for the Sony STF 135

[Gerald]

About homemaking an apodization filter.

Ideally, the apodization filter should be placed at the position of the diaphragm of the lens. However, with some restrictions the apodization filter can be placed just in front of the lens. The possible vignetting should not be very significant for a 135mm F2.8 lens. One big advantage of an external filter is that it is not necessary to open the lens to install the filter. Another is that it is extremely easy to experiment with different filters.

As you know, the apodization filter is basically a filter with a density gradient, which is zero at the center and maximum at the edges. Instead of using a filter with a density gradient, an alternative is to use a disk with multiple small holes, as used with the Imagon lens. This guy built a very rustic "filter", but it gives a good idea of how it can be implemented:
https://www.flickr.com/photos/richard314159/4705300207/

[ZoneV]

[Gerald]

[dan_]

[Gerald]

[Gerald]

[dan_]

Of course it was always wide opened.
Just made a quick test right now to show the effect :


What is interesting:
- the position of the center circle in the whole spot of little circle lights is de-centered as the effect appears further of the center of the image
- the effect only affects the out of focus area
- the effect is clear if the glow that creates it is more close to be in-focus
- the effect doesn't appear at all the glows in the image

[dan_]

[Gerald]

[Gerald]

[visualopsins]

[ZoneV]

[ZoneV]

[Gerald]

Out of curiosity, I did a quick-and-dirty quick experiments with a homemade apodization filter. First I drew a pattern with Photoshop and print it with a LaserJet printer on a transparency. Then I cut out the filter from the transparency sheet, mounted on a UV 55mm filter and put the "filter" on a 135mm lens.

The result? Terrible!

The problem was not the filter itself but the transparency that diffuse light too much. I think that a B&W negative film negatives, photolithographic film, or a slide could be a little better, but not much. The biggest problem to improvise an apodization filter seems to find a proper substrate to print it.

Below the pattern I used. If printed with a 600 dpi printer it should generate a circle with diameter of 52mm.

[ZoneV]

[dan_]

[ZoneV]

Additional glass inside the lens is a problem. This is a reason why I used film which is thin relative to most glass.

[Gerald]

[dan_]

Thanks for the info, Gerald.
I think a front-mounted apodisation filter is feasible this way.
Probably the Photoshop image should be transformed in duo-tones and printed in B&W for the best results and the glass of the filter should be pre heated in order to resist.

[Gerald]

Sony 135mm T4.5 STF mistery solved!

What I mean by "mystery" is: Why does such an expensive lens like the Sony 135mm T4.5 STF have a performance below average in the corners?

When Kurt Munger reviewed the Sony STF, he made the following remark: "The soft corners wide open with a full frame camera are a little surprising, but look fine at F/8."

My analysis of photos taken with that lens wide open confirmed Munger's observation. If you disagree with Munger, and are really convinced that the Sony STF is a super sharp lens, you do not need to keep reading. In any case, please make sure that I do not want to belittle the Sony 135mm T4.5 lens, which I think is an interesting lens. However, it is clear to me that its design implied a trade-off between sharpness in the corners and pleasing bokeh.

The key to understand the "mystery" of the Sony STF is the following statement from the Sony Alpha lens book:

"The 135mm F2.8 [T4.5] STF has an exceptionally large diameter (72mm filter size) for a fixed focal length lens, ensuring high aperture efficiency with virtually no vignetting. Even with the aperture fully opened, defocusing is superb right up to the periphery of the image."
Excerpt from: http://www.dyxum.com/columns/articles/lenses/sal-135f28/sony-af-135-stf-sal-135f28_review.asp

Consider that a 135mm F2.8 lens requires an aperture of only 135 / 2.8 = 48.2mm, so 72mm appears to be overkill. So why does the Sony 135 STF use a 72mm filter, which is much larger than the typical 52-55mm filter used in most 135mm F2.8 lens?

The reason for Sony STF uses oversized front optical elements is to minimize vignetting.
If you take a look at the graph below you will notice that the illuminance on the Sony STF (red curve) is nearly constant across the field. In comparison, the Zeiss Sonnar 135mm F2.8 for Contax/Yashica suffers from strong light fall-off more wide open. For the Sonnar, the illuminance in the corners drops to almost 40% compared with the center.






Why uniform illuminance (no fall-off) is so important in a STF lens
The best way to understand why uniform illuminance is important to a STF lens is observing how a non-STF 135mm lens renders the image of an defocused point source. Take, for example, a Pentacon 135mm F2.8 lens when wide open. The photograph below shows that the light blue point source in the center is rendered as a uniform circle, while the red point source in the corner as an oval shape due to vignetting by the front and rear lenses of moderate diameter (incidentally, the ratio between the areas of the two circles gives the relative illuminance, which is around 50% for the Pentacon lens).

Pentacon 135mm F2.8 at F2.8 (wide open):



The pictures below show the blurs when the lens diaphragm is closed to F4, F5.6 and F8. The out-of-focus images acquire a hexagonal shape as the Pentacon has a 6-blade diaphragm. The vignetting disappears completely by F8 or smaller apertures.

Pentacon 135mm F2.8 at F4:


Pentacon 135mm F2.8 at F5.6:


Pentacon 135mm F2.8 at F8:



A key point is to understand what happens when if STF suffers from vignetting

The figure below shows a simulation of the image of a defocused point source, as produced by a STF lens with a gradient filter. The blur at the left is for the case where no vignetting occurs (center of the frame always, or corner when there is not light fall-off). Note that the blur shape is circular and edges are fuzzy, what should produce excellent bokeh.

The blur at the right represents the defocused image of a point source in the corner for a lens that suffers from significant vignetting. Note that vignetting causes the blur to gain a well-defined boundary, which will certainly have a negative effect on the quality of the bokeh.




A conclusion we can draw is that an STF lens should not suffer from any significant vignetting; otherwise, the quality of the bokeh at edges and corners will be greatly impaired.

At this point, I believe it is quite clear why the designers of the Sony 135mm STF lens had to use front and rear optical elements with much larger than usual diameters. If they had not done so, vignetting would occur and the bokeh at the corners would be nothing better than an average135mm lens.

Unfortunately, the increase in the diameter of the optical elements has a price to be paied, which is the increase in the aberration at the edges and corners. Indeed, the lens designers often introduce vignetting to reduce, among other reasons, the off-axis aberrations. The vignetting is beneficial because it cuts the most oblique rays that produce the most horrible aberrations. By allowing the passage of these rays to reduce vignetting, the designer necessarily increases the level of aberrations at the edges and corners.

The Sony STF lens could produce a high-quality image at the edges and corners if the designers had used a larger number of optical elements to better correct off-axis aberrations. Unfortunately, the Sony STF lens already has 8 optical elements (two elements are to implement of gradient filter, so 6 elements are left to correction of aberrations), so adding additional optical elements would greatly increase the weight, size and cost of the lens. The designers of the Sony STF probably thought that the sacrifice of quality was acceptable for most users that think that resolution in the edges and corners is less important than a pleasing bokeh.

Just out of curiosity, I show below the pictures produced by a CZJ Biometar 120mm F2.8, whose focal length is only a little smaller than the Sony STF. Note that the shape of the blur at the corner is almost circular at F2.8, showing that the vignetting is small even for the lens wide open. For F4 on, vignetting is simply nonexistent. Hence, the Biometar 120mm would be a good candidate for implementing an STF lens.

CZJ Biometar 120mm F2.8 at F2.8 (wide open):


CZJ Biometar 120mm F2.8 at F4:

[calvin83]

It is a well known fact here Minolta use oversize lens elements to reduce optical vignetting on the STF.

The Biometar is a medium format lens. The vignetting will be much less visible when it is used on 35mm camera.

[dan_]

[Gerald]