Filter for protection?

[fenger]

No comment on the front element question? Is it a stupid question?

[DaveB]

I have heard people referring to front element often. What exactly is it? For Canon lenses, there is a filter like thin piece of glass in front of the lens. If it is the front element, what is the purpose of it? To protect the lens inside. If so, then using a filter is just to protect the protector. Why can't Canon make it like a filter and can be replaced?

It depends on the lens design.

Literally the front element is the element of glass that's at the front of the lens. In most lenses this piece of glass is curved and is part of the optical design of the lens. I think in some of the super-telephoto designs the front element serves a purely-protective purpose, but even then replacing it is not a job for the home user (but still cheaper than replacing the next element in). Those are lenses where there is no front filter thread: it's simply too big.

Note that even on the smaller lenses, the front element can be replaced (not especially cheap, but cheaper than a whole new lens).

[fenger]

Then lens manufacturer should make the protective front element user replaceable. We then do not have to use filter to protect it. If it gets very dirty or scratched, we just replace it or choose not to use it.

[gstark]

Note that even on the smaller lenses, the front element can be replaced (not especially cheap, but cheaper than a whole new lens).

Probably not true on something like a 50mm f/1.8, where the labour costs invloved would probably make buying a replacement lens a viable option.

But we're going off-topic.

[fenger]

What I am thinking is that, Canon make the 50mm/f1.8 with no protective front element and just give us the protective glass as a filter. No labour cost for us to replace it. Unless of course you think that the protective glass is superior to any filter you can buy and cannot be replaced by standard filter.

[DaveB]

Then lens manufacturer should make the protective front element user replaceable. We then do not have to use filter to protect it. If it gets very dirty or scratched, we just replace it or choose not to use it.

They do. For example you have the right to purchase and use Canon Protect filters for your lenses. Many people use UV filters instead.


Sorry Gary, I was generalising and forgot to install the words "many" and "often" into that sentence.
The labour cost of working on a 50mm/1.8 is probably going to be close to the cost of a replacement lens! I must admit I was thinking of some of the shorter zooms...

[fenger]

For example you have the right to purchase and use Canon Protect filters for your lenses. Many people use UV filters instead.

Using Canon filters or UV filters means there are 2 layers of protecting glass. As far as I remember, all my Canon lenses have protecting glass front element. I use UV filters to protect my Canon lenses, that is, to protect the protecting glass front element. The original question in this topic asks if we should use UV filter to protect our lenses. People who do not want to use filters give the reason that extra layer of glass degrades the picture quality.

Other non Canon lenses I have do not have the protective glass. The front element is the actual concave part of the lens. I use UV filter to protect these lenses too. When I use a filter, there is only one layer of glass to degrade the picture quality. I can always choose not to use a filter.

I guess what I want to say is that why Canon must use protecting glass as the front element. As far as I know, it serves no optical function.

[DaveB]

Using Canon filters or UV filters means there are 2 layers of protecting glass. As far as I remember, all my Canon lenses have protecting glass front element.

Which lenses are those? Right now I'm looking at the:

• EF-S 18-55mm
• EF 17-40mm/4 L
• EF 28-135mm IS
• EF 28-80mm
• EF 50mm/1.8 Mk.I
• EF 75-300mm II
• EF 70-200mm/2.8 IS
• EF 100-400mm IS

In all of these the front element is part of the optical design and is not just protective.

If you go to the Canon Camera Museum, select an individual lens then drill down to the "Block diagram" at the bottom of the page (although it's not available for all lenses, it is for most) you can see which elements are which.

Other non Canon lenses I have do not have the protective glass. The front element is the actual concave part of the lens.

Don't you mean "convex"?

I guess what I want to say is that why Canon must use protecting glass as the front element.

I've only noticed it on lenses with very large front elements. Have a look at the block diagram for the EF 500mm/4 L IS USM.

[gstark]

As far as I remember, all my Canon lenses have protecting glass front element.

I would be very surprised if that was the design for most of their lenses.

Dave,

No worries; I thought that would be the case, but felt that a small amount of clarification may have been in order

[Greg B]

Fascinating. We had a very similar discussion in late '04, and it was just as polarising (no pun intended) as this one. It seems there will always be those who go with the filter, trading protection for a possible slight loss of quality, and those who trade the slight risk of damaging the front element for purity of light path.

Personally, I go for the filter, and the main thing I am trying to avoid is touching the front element. The oils on our skin, so we are told, can have a negative effect on the delicate coating on the lens element. Plus the myriad other misfortunes which can befall the front element. The problem is, you only need one slight mishap, and any perceived loss caused by a filter will seem like a bonus.

[fenger]

Don't you mean "convex"?

yes

Which lenses are those? Right now I'm looking at the:
...

From your list, I have the
EF 17-40mm/4 L
EF 28-135mm IS

In all of these the front element is part of the optical design and is not just protective.

Ooops. Have just checked the lenses. The front elements are all not flat glass. I don't know why I have this impression that they is a flat piece of glass in front. Maybe it's because I have filters put on them since day one and never take them off. Sorry for all the confusion guys.

[gstark]

The front elements are all not flat glass. I don't know why I have this impression that they is a flat piece of glass in front.

A couple of points here: just because the glass looks flat to us doesn't make it so.

And even if one surface of the glass is flat, that doesn't mean that it's other surface will also be flat.