Lens aberration impacts TechArt LM-EA3’s auto focus accuracy, part 4 & End

 

Image quality of 55mm f1.2 lens

I have been using the Canon FD 55/1.2 and the Nikon 55/1.2 lens quite often with a TechArt adapter, and as explained I thought the image quality at wide open was not so good as I expected.

The problem was not the focus-shift. Real problem was the focus-shift is so small because these lenses are relatively well corrected that the problem not easily noticeable. It just moves focus a tiny bit in front side, but combined with ‘shallow depth of field’ at f1.2 aperture, become much poorer image quality than original lens performance. And from time to time, I noticed obvious focus shift on 55/1.2 too but I believed it was subject or camera moving after AF locked.

AF accuracy testing

I have already confirmed the ‘front focus’ symptom when I tested Sigma with book shelves but want to double check 55mm lenses with a little different setting. Here I made AF test chart as above. Nothing’s difficult. Just make bunch of lines and put an image at center to make AF focus, then print it on A4 paper.

 

 

Canon FD 55mm AF test result at f1.2 vs f1.8

(Left) shot at f1.2   (Roght) shot at f1.8    That….. that’s right. At the wide open, it come to focus in front side.

On this FD 55 lens, at f1.2 actual focus located around -1 and -2 of scales. And one stop down, at f1.8 it moves to zero point. Nikon 55 /1.2 lens also shows the same trend, front focus at full opening and accurate focus at f2.0 (Nikon 55/1.2 has click stop at 1.2, 1.4 and 2.0 therefore f1.8 not tested)

After this, I did not mix up FD 55mm lens aperture and tested with only f1.8 setting to see AF accuracy. This is an example photo of the real life & most of shots show pin-point accurate AF in this time.

 

Canon FD 55mm f1.2 @f1.8 (click photo for large size view)

 

‘Hello, man. If it is so, why don’t you use f1.8 lens from the beginning instead of stop down f1.2 lens to f1.8, Huh?’ you might ask. It is good idea indeed, especially because Canon FD 50mm f1.8 lens costs less than 1/5 of the FD 55mm f1.2 standard lens. Only thing is the FD 55/1.2 still performs excellent with manual adapter. The problem, ‘front focus’ happens only with LM-EA3 AF adapter.

There are classic lens users because it has excessive aberrations which in turn delivers a character, mood and unique feeling on the photo. The state of the art modern lenses are so good in my opinion that it tends to make all the same image, always sharp & clear, no matter which one is which. Character doesn’t come from perfection I believe   :- )

In conclusion, avoid to use LM-EA3 with old manual lenses at wide open when you expect some degree of lens aberration exists. Use manual adapter & screen magnifier in that case for the best result.

 

TechArt LM-EA3 adapter + Sigma XQ 135/1.8 Scalematic

How to improve

Tech Art LM-EA3 is only thing to make old manual lens into auto focus capable. It is amazing to use last century’s masterpieces in AF, like 50, 55, 57 & 58mm f1.2 lenses, 75mm f1.5 Biotar, 85mm f1.5 Helios, 85mm f1.4 Planar & 135mm f1.8s.

However if photographer have to stop down always these exotics lenses to avoid focus shift, it loses main attraction quickly. To overcome this problem, I propose a solution as below to TechArt company;

(1) Utilize both, phase detection and contrast AF in combination

When operating the adapter, use phase detection AF for the initial 90-95% positioning, then switch to contrast AF to do final confirmation in last 5-10% positioning.

It is because the contrast AF reads the image without looking at phase detection pixels & find peak value of contrast to get focal point. Therefore this specific problem which caused by phase detection method would not come at all. Drawback is using 2 different mode potentially slow down overall AF speed, so need to add a switch to make user selection either ‘phase detection AF only’ for speed or ‘phase + contrast AF mode’ for accuracy.

(2) Switch to turn off the adapter

Current TechArt adapter does not have a switch, so cannot power off. It always operates in automatic mode even if user want to switch to manual focus sometimes. This is a very uncomfortable situation …

By adding ON / OFF switch to the adapter, user can turn off power and use it like a normal manual adapter with focus peaking or screen magnifier. In addition to this benefit, it will solve one of inherent problem of current TechArt adapter which sucks battery juice even the camera is turned off. The bluetooth module of the adapter is always on, drains battery constantly when the camera power switch off & stored.

In summary, adding a 3 way switch to the adapter, (1) Power OFF (2) Phase detection AF mode (3) Phase + Contrast AF mode will solve the problem structurally.

 

 

 

TechArt adapters is a revolutionary tool that allows last century’s classic lenses working in AF mode. I have no doubt this adapter is great, but it can be Greater if focus-shift problem resolved or at least TechArt provides ‘switch’ to turn off the adapter.

End

글쓴이 Leo_KHIMME

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