Zim: Today I'm bringing back Dr. Ellen Keister, a physicist at Colorado University. Hey Ellen, welcome back to the podcast.
Dr. Ellen Keister: Thanks so much for having me again.
Zim: Okay, today's question is again regarding filters, but not about polarizing filters. Everybody is familiar with filters in photography. It's the little round thing we generally put on the front of our lenses. But as it turns out, there are two other places we can put a filter. One is in the lens itself. It's called a drop-in filter. Some of our longer lenses, especially when you get to the bazooka size, like, you know, the 400 or 800 millimeter lenses, have a little slot in the middle of the lens, or actually it's usually towards the end of the lens, where you can drop a filter into the lens itself. And then the third place you can put a filter is actually right in front of the sensor inside your camera. And that's called a clip-in filter.
My question to you is, are there any advantages for putting a filter somewhere other than the front of our lens? So this question is not purely to do with physics. I can talk a little bit about some of the physics that's involved, but a lot of it has to do with the cost of the filters and the simplicity of the system and how quickly you can change the filters, which I think you would know a lot more about than I do. So are there any advantages to putting your filter at the front, the middle, or behind the lens?
Dr. Ellen Keister: A lot of the pros and cons related to the placement of the filters have to do with the cost of the filters and the ease with which you can change the filters and how much you can use them with different cameras. So front filters are relatively easy to change and they're also sort of widely compatible with a large variety of lenses. The filters that go in the middle, the drop-in filters, are generally specific to one specific lens, and the ones in the body itself, the clip-in filters, would be specific to that camera body. So why would you want to put a filter somewhere other than the front? Putting a filter on the front of the camera can lead to filters being much more expensive because, especially for telephoto lens, the diameter of the front element can get very large, and so a larger filter is going to be more expensive. And it can also be, in terms of the optical construction, more difficult to get a perfectly flat surface, or other characteristics of the filter itself can be harder to achieve across that large an area. You also have, with a front filter, two parallel surfaces to each other, the two sides of the filter. And so that's where you can get internal reflections and ghosting and things like that if you have the filter on the front of the lens.
Zim: For those who don't know, most large zoom lenses, especially beyond 200 millimeters, the filter size on the front can be 95 millimeters, which is fairly large. And I recently looked at a lens that had 112 millimeter filter size, which is huge. What I'm hearing from you, Ellen, is to make a filter that is that large, it takes a lot more quality control to make sure that that surface is even and sharp all the way across the filter. And one of the reasons why it's escalating the price, but also this can be problematic.
Dr. Ellen Keister: Yes, that's correct. So the large diameters present manufacturing challenges that don't exist for the smaller filters. Also, you simply have a larger area of fragile glass in a lot of cases. So they're probably a lot more susceptible to breaking just because they're more fragile. There's more glass.
Zim: Yikes. Now, one thing you said earlier that kind of caught my attention is you kind of mentioned the reflection between the filter and my lens itself. You think that by putting a filter at the front, there might be an issue there? Is that what you said?
Dr. Ellen Keister: Yes. So anytime you have what's called an optical element, so a filter or a single lens, you will get reflections from all of those surfaces. So most optical elements in cameras have what are called anti-reflection coatings, which are very special coatings to eliminate this effect, but they're not perfect, especially across the entire visible spectrum. So you will, or you may, still get reflections both inside the filter, so between the front surface and the back surface of the filter, and then between the back surface of the filter and the lens. You can get reflections in there as well, which could cause artifacts in your image that you did not want.
Zim: This is now another reason why I would advocate that anybody who's using something like a polarizing filter or an ND filter, you need to remove your UV protective filter off of your lens before you mount anything else. Try not to stack filters as much as you can. Ellen is definitely saying this can be a problem. It may be minor or it may be increased. And of course, if your filters are dirty at all, you're definitely increasing problems with your images. So that's the problem with putting a filter at the front. So what happens when we put that filter in the center of the lens or behind the lens?
Dr. Ellen Keister: So it's interesting you mentioned dirt or damage to the filters, because one of the things that happens when you move the filter closer to the sensor, the closer to the sensor the element with a scratch or whatever is, the more that will impact the image. You can think about it. One way to think about this is that if you block off part of the front of the lens, you don't get a sharp line on your image. But if you imagine putting a mask essentially in front of the sensor, you'll get a very sharp edge. It's kind of like a shadow. So the closer you are to the sensor or where the shadow is falling, the sharper the edges are. So the clip-in filters that are right next to the sensor, any damage or dirt or anything on that filter will have a larger impact on the image than it would if the filter was at the front of the lens.
Zim: So you actually kind of answered a question that I was going to ask at the end, which is a lot of people spend a lot of time trying to protect the front element of their lens while exposing the rear element of their lens. And I tell people, if you scratch the front element, you're probably going to be OK. But if you scratch the rear element, you're screwed. And you basically have confirmed that, yes?
Dr. Ellen Keister: Yes, that's correct. I mean, I wouldn't know how screwed you are, but it would be a much bigger impact.
Zim: So now, one of the problems then with a clip-in filter or a drop-in filter is it needs to stay clean. The benefit is they're really small and they can be a lot more inexpensive. Are there any other disadvantages or advantages to moving that filter closer to the sensor?
Dr. Ellen Keister: So we just talked about how damage to filters in those locations will have a larger impact on the quality of the image due to the fact that they're closer to the sensor. In addition to that type of damage having an impact on the image, you're also introducing the possibility of dust, particulates, hairs getting either into the body of the camera and potentially on the sensor, or into the interior of the lens construction if you're using a drop-in filter. Both of these things are, again, close to the sensor, so they'll have a large impact, and they're also very difficult to clean. And so those are significant disadvantages unless you're only changing filters in a very clean environment.
Zim: Okay, so that's a lot of disadvantages. Now the question is, what are the advantages to having the filter in the middle or at the end of the lens?
Dr. Ellen Keister: I think one significant advantage is that the filters are a lot smaller. As we've mentioned, they can get very expensive if they're very large, and drop-in filters are also fairly quick to change. You don't have to unscrew them. You don't have to take the whole lens off the body, but you can just drop them in and out. And so that ease and convenience is really nice. However, that means that those filters are specific to a lens. So an advantage of the clip-in filter is that it will work for all of your lenses because it's only specific to the camera body.
Zim: Okay, just so everyone knows, drop-in filters are not available for all lenses. Your lens had to have been constructed specifically for that. Very often, I find that manufacturers who offer lenses with a drop-in system, they will also sell you those filters. It's somewhat difficult to get those from a third party. And yes, I would have to say Ellen is absolutely correct. They are significantly smaller, and I would love to carry around some filters that just fit in the palm of my hand instead of these large, unwieldy things. But is there a visual advantage to putting that filter in the back? Does the light scatter differently at all?
Dr. Ellen Keister: So one advantage, optically speaking, to having the filters near the back is that the light that is hitting the filter is all coming from approximately the same direction, namely along the body of the lens. So that means that you're much less likely to get interference in terms of the reflections from the two sides of the filter as we discussed previously, which on the front of the lens, the light is coming from everywhere because it's just on the outside, but inside you don't have those impacts.
Zim: And in the middle, it would be the same thing?
Dr. Ellen Keister: Yes. So in the middle, the light is still primarily traveling along the axis of the lens. There's some small angle differences, but it's mostly traveling along the body of the lens.
Zim: So in other words, there are definitely advantages and disadvantages to having your filter at the front of the lens. And there are advantages and disadvantages to having your filters at the end of the lens as well. At the front of the lens, you might get a little bit of softness, whereas at the back of the lens, you might get scratches and things might get dirty.
Dr. Ellen Keister: Yeah, that is a good summary of the pros and cons of the different filter locations, not counting the cost factor, which we've also discussed.
Zim: And also the size factor too.
Dr. Ellen Keister: Yes.
Zim: Actually, I want to mention one more thing. I recently helped somebody buy a clip-in filter for his Nikon Z7 because he's shooting the 14 to 25 millimeter and it has a convex shaped lens at the front of that lens. So there's no way to actually attach a filter to the front of that lens. So the only place that we can put an ND filter for him is to clip it in in front of a sensor. So there's that issue as well.
Now this brings up one last question, if you can please hang on with me. My question is this. Mostly Nikon, yes, there are a few other manufacturers out there doing this, but I mostly see it in the Nikon. For the very wide angle lenses, they're making this very convex shaped front element to get to their 14 millimeters. My 14 millimeter Canon lens is flat at the front element. Is there a reason why they're manufacturing these lenses with these huge bulbs at the front? Is it easier for them to make these lenses? I mean, it sure makes life harder for us as photographers keeping it clean and adding filters to them, et cetera.
Dr. Ellen Keister: So the curvature of the lens is necessary to achieve that focal length. The shorter focal lengths will have a larger curvature. Whether it's facing the outside sticking out or whether it's facing inside and therefore the front surface is flat, there will always be that heavily curved surface somewhere in the lens. A single camera lens can contain many, many what are called optical elements or single lenses. So in scientific community, we typically only work with single lenses at a time, but in a camera lens, there are at least four or five, possibly more, that are arranged in a particular way. Some of them are glued together. Typically this is done for a number of reasons, to make the overall lens shorter in a lot of cases and also to address different types of aberrations. So there's two main aberrations that lens designers are trying to mitigate when they are arranging the optical elements. One is what's called chromatic aberration, which is due to the fact that different colors in the visible spectrum travel differently and bend differently through the optical elements. And that means that if a certain color is focused perfectly at the sensor, other colors will not be if you have an uncorrected lens system. So the chromatic aberration corrections are to make sure all of the colors are focused at the sensor in the same location so that you don't have fringes on the edges of certain objects in your image.
The other aberration that camera lenses are typically designed to mitigate is what's called spherical aberration. Spherical surfaces don't actually focus perfectly. For spherical aberration, it actually matters which direction the light passes through the lens or essentially on which side of the lens. So you will get less spherical aberration if the curved side of the lens is facing the world as opposed to facing the sensor. Now that's for a single lens, a single what's called planoconvex lens.
Camera lenses, as I mentioned, have many, many optical elements. So I don't think that it would be fair to say that a lens that has a flat surface on the front will have bad spherical aberration correction because it depends on the other optical elements in the system. But in general, it is probably easier to develop or to design a lens system if the curved side faces out because that's better for spherical aberration correction. So in other words, there may be some manufacturing issues that will make it more inexpensive to manufacture by having that curve facing the world versus facing in.
Zim: And it sounds like you're going to have fewer engineering hours necessary to design that lens, maybe.
Dr. Ellen Keister: Yes. So the design is probably significantly simpler with the curved side facing out.
Zim: Thank you so much for joining me today. It was really informative to have you on. Hopefully I can get you on another time with another question.
Dr. Ellen Keister: That sounds great. Thank you so much for having me.
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