Reply To: 50mm for A7 recommendation please

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#9228 Reply
viktor pavlovic
viktor pavlovic

Hi Juan,
thank you for moving here, I should be able now to respond faster 🙂

I should probably make a post about my suggestion how to test legacy lenses, but so far, I can only give you basic ideas…

First and most important is to think about what are you willing to test. It is not good/possible to test all aspects of the IQ at once.

1. Absolute sharpness (sharpness in the mid zone)
This test might tell you lot about the condition of your lens. Most older lenses that had been opened and cleaned from fungus or some sort of haze, would have some level of “blooming” or “glow” wide open. If you find that the amount of this “glow” is high, your lens is most probably subpar (especially both Planars) and you should consider return.
To test the lens, find high contrast area at a distance of about 5m, that will cover mid frame (around 20% of the frame in the center). Use tripod, focus magnification, remote control or self timer (2s should be enough), use electronic first curtain set to on, If you are using lens or body with image stabilization, switch it off. Shot in a good light (sun from the side, ideally overcast). Shot in A mode, set ISO to 100, WB to preset value (daylight, cloudy etc.) open your lens to its max aperture and repeat focusing and shooting at least 10 times. Shot in RAW (for better evaluation of the detail) and check your files in your RAW converter at 100%. Check only center of the frame. If you find at least one shot (out of 10) which is sharp enough and doesn’t show much glow there, chances of having good lens are very good.
2. Sharpness across the frame and de-centering
First, you need to understand “field curvature” and your lens native performance in that regard. Planar design is supposed to have low field curvature, or if you like – even sharpness across the frame.
To test your lens if its rendering suffer beyond native field curvature, you will need flat target at a distance of approx. 3-5m (for 50mm lens). This is tricky part, because 100% precise alignment (parallel to the target) is crucial for good results. People usually use brick walls, murals and graffiti walls. Most important is to try to perfectly align camera with the subject as mentioned above. You can use all sort of guides and tools to accomplish the mission, but take your time.
You are now looking for the even sharpness across the frame, so the aperture that you will select should be similar to the one that you will use in a real life situation. Wide open aperture will show rather field curvature issues than de-centering, to narrow aperture can hide any softness, thus I would suggest to go with f/2.8 or f/4.

Use tripod etc. (all settings as in the previous test) and take few shots. You don’t need to take 10 shots like in the previous test, because you are not looking for absolute sharpness now, but rather for flatness of the focus field. Therefore 3 shots should suffice more than enough.

Trying not to move anything much, take SD card from the camera and upload files in the computer. Look how sharp is center vs edges and extreme corners. If you find that one side (left i.e.) is softer than other side, it could be that either your lens is de-centered or that you didn’t align your camera perfectly against the target. First try to eliminate second option by making few more shots with a camera panned slightly to the opposite direction (right in our case) and in the same direction (left). Rotate the camera by approx. 1°per shot. Check your files again. If in all cases the same side is still softer, de-centering is most probably the problem. If in some shots the sharpness improves on one side (left in our example and decrease on other side, you should try to align camera with target again. PITA? You bet. But if you really want to know what is the problem with your lens, you’ll need to go through the torture. Forget about DOF theory, it wasn’t made for pixel peeping at the recent resolutions…

If you suspect your lens being de-centered, to be sure do the following. Go back to your set-up and turn the camera in the portrait orientation by rotating it (on the tripod head) at 90° CW. Take few shots at the same settings as before.
Check the images in the computer. If you see that the softer part moved in the same direction as you rotated the camera (in our example, the top part of the image will be now softer than bottom), you have de-centering problem, but…

it could be the lens as well as the adapter. If you have other lenses in the same mount, check if they shows similar pattern as the tested lens. If yes, you might need better adapter. If not… it is most probably the lens, but the problem might occur in the lens to adapter connection. Slightly deformed mount or similar problem e.g. To be 100% sure if the problem is in the lens optics, you will need to test it on the optical bench, which for most mortals is impossible.

Other tests:
You might want to shot against the sun or any other light source to check if the coatings are OK. (I had several lenses, on which dumb owners tried to clean some scratches or dirt and they almost completely removed coatings!) To test the lens, you don’t even need to make images, just pan and tilt around your light source and follow resulting loss of contrast. Both Zeiss Planar lenses should have very effective coatings, so you shouldn’t see large loss of contrast. (Lens flare are something else, they depend also on the sensor reflections, so don’t be too picky.

Your lens might (and will) have many native aberrations, such as Longitudinal and Lateral CA, Spherical aberration etc. but if the above tests didn’t show some extreme anomalies, you can be sure that you have good sample. Best way to test lens for most aberrations (to see the character of the particular design) is to shot high contrast targets, such as ISO 12333 test chart e.g. but I don’t think that you need it though…

I wish you good luck with your Planars, and let me know which one you’ll decide to keep.


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