Why does a cheap circular polarizer seem sharp at wide angles but soft at longer focal lengths?
Asked 1/13/2016
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I tested an inexpensive circular polarizer on a full-frame camera and found that image quality looks fine at 24mm, but sharpness drops as I zoom in. Around 50mm it becomes noticeable, and by 100–200mm the images look much softer. What could make a CPL appear to perform worse at longer focal lengths? Is this mainly due to autofocus/light loss, poor filter quality, or some physical limitation of polarizers? Can a better-quality CPL avoid this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Hard to know from your post, but note that a CP filter will loose you between 1 and 2 stops of light, depending on it's setting. This significantly reduces the amount of light reaching both the sensor and the all important autofocus system.
If you have a variable aperture lens, this means that longer focal lengths have a smaller maximum aperture and already let in less light than wider settings. This compounds the loss of light from using the CP.
If there is not enough light for the autofocus system to work well you will find significant loss of focus accuracy.
In general you should remove any filter, especially a CP, when shooting unless you specifically require the effect the filter provides for a given shot.
Originally by user46861. Source · Licensed CC BY-SA 4.0
user46861
10y ago
0
Generated from our catalog & community — verify before relying on it.
This is usually not a limitation of circular polarizers themselves, but of a cheap filter and the shooting conditions it creates.
A CPL typically reduces light by about 1–2 stops. That can hurt autofocus accuracy, especially at longer focal lengths where many zooms already have a smaller maximum aperture. If focus is slightly off, softness becomes much more obvious at 100–200mm than at 24mm.
Low-quality CPLs can also add optical softness. At longer focal lengths, any imperfections in the filter glass/material are more visible because the image is effectively magnified, so blur and loss of detail stand out more.
Also, using a CPL may force you to shoot at a wider aperture or slower shutter speed, moving the lens away from its sharpest aperture or increasing blur from motion.
So the likely causes are:
- light loss reducing AF performance
- poorer optical quality in the cheap CPL
- aperture/shutter changes caused by the filter
A better-quality CPL can greatly reduce the optical degradation, but you should still expect the normal light loss. Use a CPL only when you need its effect, and check focus, aperture, and shutter speed carefully at longer focal lengths.
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