Camera lens – fingerprints

The camera lens is an important component of the camera imaging chain. Enthusiasts spend large sums of money to get the best lenses available. Thus it does make sense to look after our lenses (and protect our investment), and ensure it can achieve its maximum performance. I decided to look at the effect of a fingerprint on the front lens element of a lens.

I chose to use a Soligor 35mm F3.5 lens which I had bought in the past to test its UV-imaging capabilities; the fact I am using it as the test subject, already tells you that it isn’t particularly good for UV-imaging.

Clean lens front element
Clean lens front element
fingerprint on front element
fingerprint on front element

Method:

I used my Sony A7R and its 36 megapixels for the test. I tested for close-up (macro), mid-distance and far distance, to see if it affected the sharpness of image etc. For close-up, I used my iPhone 5s, mid-distance was a bush and far distance was trees in the distance. All images are 100% centre crops. For each set, I shot with clean lens and then placed a fingerprint on the lens prior to shooting the second shot. Hence there is slight difference of composition between shots. Between sets, I did clean the lens initially with a blower, followed by a microfibre cloth and then the Lens pen.

Lens blower
Lens blower
Microfibre cloth
Microfibre cloth
Lens pen using carbon to clean lens
Lens pen using carbon to clean lens

These are the resulting images:

Closeup:

No fingerprint on lens
No fingerprint on lens
Fingerprint on lens
Fingerprint on lens

Mid-distance:

No fingerprint on lens
No fingerprint on lens
Fingerprint on lens
Fingerprint on lens

Far distance:

No fingerprint on lens
No fingerprint on lens
Fingerprint on lens
Fingerprint on lens

Conclusion:

There is reduction in contrast in the images with the fingerprint on the front lens element, like viewing through a dirty window. This can reduce the relative sharpness of the image, as the sharpness has to do with increased contrast.

But what is clear to me from this test, that the lens is the limiting factor, as it is clearly not able to get the most from the high resolution of the Sony A7R sensor (few lenses can); otherwise I would expect more difference between the two. But I am not risking my better lenses to test this theory though.

It is advised not to clean the lens elements unless absolutely necessary, as there is the possibility of scratching the lens element in the process of cleaning it. However, fingerprints by nature are acidic and oily and can damage the antireflective coating of the lens. Hence this should be cleaned off where possible. In this situation, it is simply the matter of weighing the risk vs benefit.

This is one of the arguments for using a lens filter to protect the front lens element – it will be less stressful to clean the lens filter (UV or protector), as some filters have coatings which enable dirt and fingerprints to be cleaned off much more easily e.g. Hoya HD filters. Also, it will be cheaper to replace a scratched lens filter than the front element. I have found the Hoya HD filters (and B+W) to function well and not impact on image quality (unlike some other makes), but these are also generally more costly to purchase than others.

Boon