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Effect of Filters on Autofocus Accuracy and Sharpness

TOC

Introduction

Telephoto Lenses Max Aperture

EF 50mm f1.4 Centre at f1.4 and f5.6

EF 50mm f1.4 Corner at f1.4 and f5.6

Ultra Wide Lens Wide Open Center and Corner

Introduction

It had been suggested that even multicoated filters could have a detrimental effect on either lens sharpness or autofocus accuracy. Being of a mind to always fit a protective filter I wanted to look into this. For the effect of filters on general contrast, flare and ghosting see this experiment.

The effect on colour balance is measured here and  vignetting with ultra wide angle lenses is looked at here.

The methodology here was to setup with a solid tripod and using mirror lockup shoot an orthogonal pair of centre ISO slanted line test targets. A group of 10 frames were taken with and without a Hoya Pro 1 UV filter. Before each shot the focus was thrown off either to minimum focus distance or to infinity and the autofocus activated (via the * button CF 4-1). The test target range was just over 4 metres.

The target had a window behind it with potential for out of image flair. Light levels were reasonably low with exposures of 1/13 @ f2.8 and 1/6 @ f4 for 100 ISO, the centre focus point only was used with the camera in AI Servo mode. The hood was used and the methodology was the same with and without filter.

There was no special focus target other than the two slanted lines or a USAF resolution target section in the case of the shorter focal lengths, the region of the centre AF point is shown below.

The individual test results are plotted in various formats for information on this page.

 

Telephoto Lenses Max Aperture

The test was run for an EF 200mm f2.8L II and a EF 300mm f4L IS with the results of the average of the horizontal and vertical MTF 50 % at maximum aperture presented as a histogram with a note of the mean and standard deviation for with and without filter.

NB The 200mm has a Hoya SHMC Pro 1 UV(O) fitted and the 300mm has a Hoya MC Pro 1 Digtial UV(O) fitted for these tests.

Now the odd thing here is the results, although very close are better with the filter. This I conclude to be due to the limited size of the sample.

So the conclusion is that good quality multicoated filters do not have any detrimental effect on image sharpness or autfocus accuracy.

Under extreme conditions of a light source in the image additional flair or ghosting is possible and it may be advisable to remove the filter if shooting under these conditions.

I also chose a pair of results with similar centre sharpness and checked the corner sharpness, there was absolutely no change by introducing the filter.

In the case of the 200mm with the 1.4X teleconvertor there was one low statistical outlier with the filter on but this is considered insignificant as on average the sharpness was better with the filter. 

An interesting point with the teleconvertor fitted is the increase of variation of sharpness over the case with no teleconvertor regardless of if a filter was fitted. The variation in sharpness with the teleconvertor was about 10X more in lp/mm terms despite there being a modest drop in average sharpness of ~48 lp/mm to 42 lp/mm. 

The exposure values were similar but 1.3 stops lower when the with the teleconvertor, typically lens only f2.8 0.1sec ISO 100 and with teleconvertor f4 0.5sec 100ISO. This may have contributed to an increase in sharpness variation in the teleconvertor test but most of the difference is thought to be due to the reduction in phase difference baseline for the AF sensors due to the 1 stop loss of the TC.

The difference in variation of sharpness between the filter and no filter cases is small enough to have no statistical significance.

The 100% crops and individual frame SRF plots for the 200mm + EF 1.4X are here.

 

EF 50mm f1.4 Centre at f1.4 and f5.6

Same test procedure as before, Hoya SHMC Pro 1 UV(O) fitted.

For information the 100% crops and SRF plots of each of the 20 test frames are here.

For information the 100% crops and SRF plots of each of the 20 test frames are here.

So again there is no significant difference in average sharpness or spread of sharpness. 

 

EF 50mm f1.4 Corner at f1.4 and f5.6

Analysis of same frames as above repeated for the lower right corner which seemed the best corner wide open.

For information the 100% crops and SRF plots of each of the 20 test frames are here.

For information the 100% crops and SRF plots of each of the 20 test frames are here.

Again there is no significant measurable effect from using the filter.

 

Ultra Wide Lens Wide Open Center and Corner

Here the test is repeated with the EF-S 10-22mm f3.5-4.5 USM @ 10mm and sharpness statistics presented for both the centre and corner to establish if there was an issue with ultra wide angle lenses due to the acute angle of light rays for objects in the corner. Hoya SHMC Pro 1 UV(O) fitted.

Although there is a considerable spread in sharpness across autofocus instances the spread was similar in both the centre and corner for filter and no filter cases. So once again it is concluded these is minimal degradation if a protective filter is fitted.

 

Last Updated 29/03/2008

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