Shooting With a Scratched Lens Rear Element

Jun 9, 2018

6 min read

If like me, you have been wondering how a scratch on the back element of a lens would affect an image, you came to the right place!

This article was not on my to do list but I figured that we could all benefit from my misfortune when I bought from a store this Contax G1 with a 28mm Carl Zeiss Biogon T* 2.8 with a nasty scratch on its back element.

Quickly after coming back home, life hit me hard in the face with a small scratch on the rear glass. Probably because of my bad habit of trusting people too quickly, added with the excitement of finally owning a G1, I didn’t pay enough attention when inspecting it and ended up with a damaged lens that was worth close to nothing.

After a few F*** words against myself and a series of deep breaths, the idea came to share this misadventure with you and the two valuable lessons it taught me:

Table of Contents

LESSON 1: Never Trust 100% Sellers, Even Pros

The first one is obvious: Pay attention when buying used gear, even from a professional seller.

Don’t assume because it’s coming from a pro that it won’t have any faults. I honestly don’t know if the seller knew the lens was damaged but we can share the blame of both of us as I should have been more careful with my inspection.

Long story short, after informing him about the lens condition, he kindly accepted to take it back a few days later. This could have been a costly mistake so be careful guys and don’t make the same mistake than I did !

Even if you think you are making a great deal and have the cameras or lens of your dream in hands, don’t rush it out and take your time to inspect it as much as your knowledge allows before laying down the cash.

Even a tiny mark like this can ruin a lens!

LESSON 2: Front VS Rear Element Scratch

In case you were not convinced yes, the second one should finish to persuade you that a thorough inspection is a must when buying second hand lenses. With the lens in hands, I figured that it would be interesting to do a rear element scratch test and see how it would affect our images.

Whilst a small damage on a front element will barely affect an image, the rear element has to be absolutely flawless, especially in the middle. When the light enters through the lens, it goes through a series of elements before hitting the film or sensor. If there’s a scratch or dent on the front element, chances are that you won’t notice it because it will be evened while passing through all the layers of glass and coatings.

On the other end, the back element is the last stop before your image is created. All the light rays will pass through this curveted glass to expose your film. That’s why it’s absolutely crucial to keep it free of marks and scratches.

Lens Rear Element Scratch Test

So I was left with only one thing to do: Shooting a roll of Agfa Vista 200 at different apertures to see its effect.

The test was pretty straight forward. Using the Aperture Priority mode on the G1, I exposed the film from f2.8 to f22 in one stop increments. This gave us 7 pictures showing the evolution of the mark.

As I suspected, you can’t see a thing at f2.8 and you have to be aware of the mark to notice it at f4, and even with that in mind it’s not clearly visible.

From f5.6 to f8, we can tell something is going on as a black tumour is starting to appear in the center and spreads out.

Once you reach f11, f16 and f22, it becomes obvious that we have a serious problem! As you can see the black spot is clearly visible and we can also see other marks appearing right above in the sky. I suspect that these other marks may be there because of other tiny damages around the main scratch OR because of flare inside the lens.

Then I have repeated the same sequence with 2 other scenes under different light conditions and orientations but the results were more or less similar. For the sequence below we can notice a minor improvement as the image is slightly brighter where the impact is reflected.

Here it kinds of blend in thanks to the busy pattern so we can have something usable up to F8 but beyond that point the black mark is really noticeable.

How To Fix a Scratch On a Lens

Fixing a damaged back element is not easy task and that will depend a lot of bad and deep is the scratch. You will find different solutions online to reduce the impact going from smoothing it with toothpaste, have it polished professionally (which I don’t recommend as it may affect the lens properties), use of those Scratched Lens Repair Kit or masking the scratch with a black paint to block the light passing through. These solutions may work with minor scratches but in my case, the impact was too deep to consider any of them.

If you have an expensive piece of glass in your hands, it may be worth trying to get the rear element replaced but that will cost you a lot of money and most of the time you’d be better buying a new one instead. Like with my lens, there was absolutely no point in trying to get it replaced.

Another thing you should look out if you are considering buying a damaged lens is, where is located the scuff. In my situation it was placed in the worst possible place: right in the middle. You may come across some good deals on eBay with lenses that suffers from scratches.

When located at the edge, chances are that you will barely notice it, except maybe at the small apertures. But if it’s too close to the center, like here, you should stay away from it.

If your lens suffers from this incurable illness, that doesn’t mean you should throw it away or sell it for a 1/10 of it’s price. Just learn its limits and stick with the wider apertures.