Blur with HC100mm + 52mm Extension Tube

[chaosphere]

Hi !
Recently I bought a 52mm ext tube to do some macro. I was surprised of the bad quality in the corners, at full aperture but even at f8 I have this sort of "excentric" blur. Has anyone noticed that ? Ok, I don't need f2.2 for macro but f8 is not as good as I thought it would be. The focus was made at minimum distance (0.90 on the lens, 42cm in reality). Someone told me to try at infinity focus, but it loose a bit of its interest if I can't focus at mini.

Here are some exemples :

f2.2


f8

[Brian Hirschfeld]

The results don't seem terribly surprising with that long of an extension tube, although I can't explain it technically it seems perfectly reasonable that soft corners would be exacerbated in this situation....possibly?

[chaosphere]

thanks Brian !

[Ed Foster, Jr.]

Matias,
I've not seen this with an HC100 and either the 13mm or 26mm extensions. However, if the color checker is not perfectly parallel to the plane of the sensor, this could occur. With a 52mm ring and the 100 mm at its minimum focus distance and wide open, any misalignment is bound to show.

Ed

[theguywitha645d]

This is not that surprising. Many lenses are not optimized for a flat field and the extra magnification coupled with an object distance the optics are not designed for can result in this. You could try using a close up lens/filter. This will present the lens with a column of light at an optical distance it is designed for. I know these things get dissed, but they can work really well. You might not even need the extension tube and you f-number will not be impacted by the extra magnification. These filters are given in diopters and are effectively a fraction of a meter. A 1 diopter filter will allow you to focus at 1m when your lens is at infinity; a 2 diopter to 1/2m (50cm); a 3 diopter to 1/3m (33cm). Where the close focus is is a little harder to judge. You could use your tube as well.

The other solution is to get the 120mm macro lens.

[chaosphere]

thanks guys ! It's true that I did not notice the same issue with my other lenses (80mm and 50-110). Yeah a 120 would be great (the V.2) but it's not my priority (HTS first...) then the 120 maybe. 

[esox]

C'est classe une charte à son nom tout de même...

[chaosphere]

C'est classe une charte à son nom tout de même...

[hasselbladfan]

I got a similar issue with a 52, but got it fixed by ensuring the surface was perfectly in a 90 degree angle. A slight deviation and it re-appeared.

[chaosphere]

Hi Hasselbladfan,

I thought you were speaking about the planarity of the sensor and the extension tube and lens. But you're speaking about having the subject and the sensor perfectly parallel isn't it ? This, it was ok when I shot this image... 

[Louis Novak]

The extension tube creates more distance from the rear lens element to the camera's sensor effectively changing the minimum focus distance and magnification of the lens. However this also drastically changes depth of field, making it shallow even at smaller f stops.

[BobDavid]

The 120 Macro M1 is excellent. The MII is necessary when using an MS-200 megapixel back.

[jerome_m]

When I first read about this topic, I wondered whether the problem only mattered with the HC100 or whether the HC80 had the same field curvature when used with extension tube. I was able to try it with the 13mm and 26mm tubes stacked today and this is the result at f/4. Since I don't have a chart at my name, I used a page from the "Victor" magazine. Obviously the HC80mm and the HC100mm give the same poor corner sharpness when used with extension tubes…

[EricWHiss]

I've seen similar with the 110/2 when used with lots of extension. It appeared to me that the plane of focus is more spherical with that lens - maybe your 100 is similar?  If you shoot into some plants or something with depth instead of a chart you might find some sharp things on the edges just not at the plane of the center if that makes sense.  Some lenses don't take extension as well as others.

[jerome_m]

Actually, all this lenses (including your 110 f/2.0) are modified double-Gauss designs. The unmodified double-Gauss gives very good results with extensions, because it is a symmetrical design and therefore works very well when the distance between the subject and the lens is similar to the distance between the sensor and the lens.

When the lens is modified to become asymmetric, it may give poor corner sharpness with extensions. Between the HC80 and the HC100, the 80 is more symmetric than the 100. It was thus interesting to test whether it would give better results.

The 110 f/2.0 modifies the symmetric design by adding a further element on the front side. It is thus not surprising that it gives poor corner sharpness on extension rings.