Focusing Screen rant/question

[photophorous]

Just when I think I understand the basic optical principles that govern photography, I come across something like this.

The short story is that I noticed the factory focusing screen in my 450D shows the view through my lens with greater clarity, or maybe greater DOF, than what the sensor actually captures, at large apertures. IOW: I can set the aperture to it's largest setting of f/2.8, focus on something nearby, and see that the back ground is out of focus. But when I actually take the picture, the out of focus background is much blurrier than it appeared through the viewfinder. Waaa??? How can that be?

I've been reading about aftermarket focusing screens, because I want to improve manual focusing capability. There's a company in Taiwan that cuts down replacement 35mm SLR screens for use in DSLRs and they carry a variety of screens from Canon, Nikon, Minolta.... In researching these screens, I found several comments from users who say that when using the Nikon F3 screen (for example) it is much easier to see what is in and out of focus, even outside of the microprism collar or split image section of the screen, than it was with their stock screen. This is what caused me to check mine, and sure enough, my stock screen lies to me. If you haven't already noticed this, check it out and you'll see what I mean.

I don't get it. How can the focusing screen effect DOF like that?...or is it something else that makes it look sharper? And why would we want that? If you could more easily see how out of focus things really are, then you could also more easily see what was in focus. I'm starting to think the DSLR makers are making manual focusing difficult on purpose.

Paul

[Speed]

Hey Paul,

I don't mind telling you - I haven't got a clue!
Have you posted this on the Canon forum? One of the Canon guru's may have an answer. The only other thing I can think of is call Canon tech support.

I do agree that manual focus with a DSLR does indeed seem to be more difficult than it is with a manual focus film camera. Again, I don't have a clue as to why, I only know that it is harder.

[Wild Wassa]

"How can the focussing screen affect DOF?" - Photophorous.

It doesn't actually. Optical illusions are also problem. It isn't just about depth of field it is also about depth of focus from the optical properties of the screen, and the size of the image that is misinterpreted by the brain.

The focussing screen with its light gathering properties is not as highty corrected as the camera lens, which doesn't help. A focussing screen only has to give acceptable visual coverage over the whole screen. The brain fills in the gaps.

Depth of field and depth of focus are much reduced in an attempt to make focussing more critical and more obvious, here in lies the issue, compounded by the smale scale.

To make an image brighter on a focussing screen, the focussing screen is lightly etched, minimizing light absorption. Screens have limited diffusion properties, which makes the light brighter in the centre. Which creates another issue and illusion called the Constancy Effect.

The Constancy Effect also known as Subjective Constancy and Perceptual Constancy and is a visual illusion, which happens when concentrating on something, The overall size of an object (in particular) is falsely magnified by the brain and seen differently in the brain to how the camera actually records it.

How often have you photographed something that was well proportioned in the view finder only to see that same image as a much smaller size if not an insignificant size in the final exposure/capture? ... (or what ever term is used nowadays for digital recording).

The brain misinterpreting what is actually seen in the viewfinder at the time of tripping the shutter is an ongoing issue with photography ... and always has been.

The higher the quality of the screen, the less likely that problems will arrise. The more time you take to assess what you are seeing, the less likely misinterpretation will occur.


Warren.

[brmill26]

As Warren said, the focusing screen has no effect on your DoF. Nor is this a Canon, Nikon, or any other brand issue. All DSLRs that I know of use a clear focusing screen. The reason is three fold. 1) Autofocus systems are extremely good and they are used at least 90% of the time by the vast majority of users. 2) APC-C sensors have very small, relatively dark viewfinders. A clear focusing screen allows for the maximum light transmission through the viewfinder, which is important when framing in low light. 3) Cost. I imagine a plain screen is cheaper to produce that a precision-etched piece of glass.

Without much scientific backing, I think the difference between a matte screen and clear screen has to do with the way the light hits it. A matte (or ground glass) screen gives the light somewhat of a surface to hit and diffuse across - thus why they are darker, b/c not as much light flows through. However, because it gives the light a surface to hit, you get a clearer picture of what is and is not in focus. I would assume this is because the screen effectively creates a plane that intersects the light beam at a fixed point, thus giving you a very clear idea as to what is and is not out of focus. Whereas, a clear does not create a plane because 99.9% of the light simply passes through it. Thus, instead of seeing a fixed point where the focus is or is not, you simply see a general field of focus that is not nearly so well defined. But when you take the photo, the sensor (or film) is at a fixed point in that area of focus. So DoF is much more defined in the picture than it looked to you in the finder, because without a matte screen, you were unable to tell exactly where the plane would be.

Hopefully that makes a little sense...

[photophorous]

I understand that the focusing screen can't actually change the depth of field, and I understand that depth of field and depth of focus are the same optical phenomena occurring on the opposite sides of a lens. However, I'm convinced that the optical properties of the focusing screen can effect the way depth of field appears in the viewfinder. I'll have to go home and look again tonight, but I'm pretty sure I could see details in the out of focus backgrounds in the viewfinder that were completely obscured by blur once I actually captured the image on the sensor.

Brad, I don't really understand how more light passing through the focusing screen could give that effect. Only the light that is diffused at the focus point, by the focus screen, is seen by us as an image. If the focus screen were just a clear piece of glass we would see no image at all, just a bright blur, because our eyes are too far from the point of focus. The only explanation that I can imagine is that the focus screen somehow gathers light from throughout a larger area (or depth) within the "depth of focus" region than what the sensor would gather. Or IOW, the focus screen is somewhat three dimensional and the sensor photosites are truly (or at least closer to) a flat surface. I'm not sure I buy that explanation, and I'm not sure how such a focus screen design would actually be implemented, but that's all I can come up with at the moment.

What this all comes down to, as far as my concerns go, is that the image in the viewfinder is not a true representation of what you are capturing. If I'm right, even the DOF preview wouldn't be accurate, except at the exact point where the actual depth of field was equal to the perceived depth of field caused by the screen design. Anything less would appear to have greater depth of field than it really does and anything more would all appear to have equally deep depth of field. Or, I could be completely wrong.

If you can't tell, things are kinda slow where I work.

Paul

[Wild Wassa]

"I'm convinced that the optical properties of the focusing screen can effect the way depth of field appears in the viewfinder." - Photophorous.

Not just DOF but also the eye's ability to define any detail is based on a limit, called the 'Circle of Confusion' or 'Circle of Indistinctiveness'.

This isn't only our ability to see the smallest point in the real world which is governed by a limiting distance, that limits what the eye can resolve, it is also dependant on scale, the smaller we reduce an image the greater is the illusion that we have gained an increase in overall sharpness ... this gives the illusion of an increase in the DOF also when the image of a subject is decreased in size, depth of field increases (and depth of focus is reduced). These two factors giving false interpretations.

"I understand that depth of field and depth of focus are the same optical phenomena occurring on the opposite sides of a lens." - Photophorous.

Depth of Focus applies to the movement of an image plane, Depth of Field concerns zones within the subject. The two depths are continually confused as being proportionally, mirrors of each other.
As a subject approaches a lens the Depth of Field decreases, where as the Depth of Focus for the image increases.

Warren.

[photophorous]

....the smaller we reduce an image the greater is the illusion that we have gained an increase in overall sharpness ...

Are you basically saying it looks sharper in the viewfinder because it's smaller?

Paul

[Wild Wassa]

Photophorous, yes ... exactly.

Warren.

[photophorous]

Photophorous, yes ... exactly.

Warren.

Warren,

I know what you're referring to, and I've noticed that when viewing images on my computer screen at different sizes, but that's not what is happening with my focus screen question. I just got home and grabbed my camera to take a few test shots, just to make sure that I'm not going crazy. The differences I'm seeing are not because the viewfinder image is smaller.

I can pear through my viewfinder and clearly make out details that you simply can't see once I take a photo, because the background of the photo is blurrier than it appeared in the viewfinder. This is not an illusion. Try it for yourself. The focus screen is doing something that makes DOF appear greater than it really is.

Paul

[Wild Wassa]

Photophorous, I think that you will find that if you reduce your photographic images that you have taken, to the same size as your focussing screen, the only difference that you will see will be the differences in the light gathering qualities between the lens and the focussing screen. What you are seeing is normal. When we realize the difference, between the screen image and the realized photographic image, it is a most surprising revelation.

It is a shame that Wikipedia is a tad light in describing the nature of real and virtual images. If you google the characteristics that I wrote above, you might find Wiki's explanations less clumsy than mine.

What you have described is something that when I studied photography, the lecturers put much emphasis on and gave several warnings about images seen in focussing screens. What we think that we see isn't what we get.

You are totally right in what you are describing and seeing.

There is nothing that is a greater waste of time when camera handling, than stopping down the aperture of the lens to check to see, if there is enough depth of field before a shot is taken ... and trying to assess the DOF in the viewfinder.

Warren.

[photophorous]

I'm sorry Warren, but I have to disagree. I'm certain that what I'm seeing is not just an illusion. There is more to this. I can read text through the viewfinder that is so blurred in the photo you can barely tell it's text, and viewing it at a smaller size does not make it any more legible. I can see distinct bokeh circles on highlights in the photo that are quite clear in the viewfinder.

Paul

[photophorous]

Warren, I just used "the google" to find this article. A 40D user illustrates what I'm talking about, in making a case that live view is more accurate...which of course, it is. The 40D viewfinder is essentially the same as my 450D and what he illustrates is exactly what I'm seeing. I never noticed it with my f/2.8 zoom until an f/1.4 prime made it obvious and I went looking at the zoom. Anyway, read this article. He doesn't really explain what's causing it, but this is what I'm talking about.

http://www.dphotoexpert.com/2007/09/...lr-viewfinder/

Paul

[Wild Wassa]

Duplicate posting.

[Wild Wassa]

"I'm certain that what I'm seeing is not just an illusion." - Photophorous.

Photophotous, I'm sorry that I gave you the impression that I viewed it as only an illusion. The brain filling in visual gaps consistant with our visual experiences is only one part of what comes into play when we view an image. I thought that I had given equal weight to the optical and other factors influencing what we see as well.

"A 40D user illustrates what I'm talking about, in making a case that live view is more accurate...which of course, it is."- Photophorous.

Because 'live view' alleviates the Constancy Effect that looking through a view finder and viewing a focussing screen keeps problematic.

"The 40D viewfinder is essentially the same as my 450D and what he illustrates is exactly what I'm seeing."- Photophorous.

Yes I now understand, the pseudo oil bath effect, like "oiling a viewing screen" or better still, having an oil bath replacing air between the image and the subject, to change the refractive index of the air/lens interface. The Poms, the Kiwis, we Aussies, Canuks and you Yanks, we're all separated by a common language.

The author has only said what I have described, but differently ... although I haven't described the oil bath effect that he did, until now. Which will extend the detail on a viewing screen "by bending light," was how he put it?

We are (both of us) on the same plain. I wrote about optical features that determine how we interpret what we see.

Being a 40D user, which I am, I'd hate to be lagging on the goss. I don't see a difference when I view through my 40D screen, to what should be totally different through my F-1s fresnel screen, because my brain has adapted over the last 3 decades to my visual experiences ... I see what I expect (want) to see, I do not see what the viewfinder shows me.

Photophorous, I'm going to read sections of my old texbooks on optics to see why we view and interpret things differently ... but firstly I'm going back to read the article that you sited.

Warren.

[drg]

This is why they make viewfinder magnifier optics. Angled or simple diopter attachments, by magnifying the image it is easier to achieve critical focus manually.

All of the technical reasons and the small size of perceived image included, this has also been a reason for differing format sizes of film (and now with digi-backs) cameras from Medium to very Large format (i.e. 8x10) cameras. It is much easier in some ways to focus 'under the bag' than through a tiny eyepiece.

You will still find, and I have done it for larger format frequently, ( for me mostly 4x5) photograpers actually measuring the distance from the film plane to the midpoint of desired focus with tape. A larger lens is easier to 'set' the correct distance. It also helps to know how the camera/lens combo performs.

One other factor that is always overlooked is the magnification factor and the type of prism or mirror in use in a Thru The Lens focusing system. The bigger and brighter, the better! Some low to moderate range cameras (expense primarily) have not had the most effective viewfinders for anything other than composing/framing the shot.

There is also the 'slight' difference with some lenses in their design for where the proper plane of focus is (difference between EF and EF-S as an example) and some cameras have the viewfinder system adjusted to compensate for anything from a third party lens to the top of the line pro-optics. Others just do it right.

This was always a complaint about the 35mm format was being able to 'see' well enough to focus.Really it also was just as much that the adjustments on the lens are so small that it is hard to 'tweak' it the right amount.

The DOF issue has some components to it beside CoC and the other perceptual issues. The optical path is as hinted above is not the same for the lens focusing on a flat surface, film or sensor as it is to be bent around and sent in to your eye. The viewfinders I've seen critical tests performed on with a very few exceptions actually increase the depth of field to control the 'eyepoint' or how far away you can hold the camera and see an image in focus through the finder. That means you have an additional mechanical optical problem in the path that is different from the final image.

The viewfinders are an underated point of evaluation in most cameras. There is a huge difference between cameras at different price points, and interchangeable screens are available for certain models. Precision matte screens, at least, are becoming fairly prevalent in mid to upper range DSLR's even if not interchageable.

A magnifier is still the best way at times for manual focusing or at least using the AF points to help indicate when focus has occured and one can rock back and forth to get the range. Or use a tripod for every shot.

[freygr]

Just when I think I understand the basic optical principles that govern photography, I come across something like this.

The short story is that I noticed the factory focusing screen in my 450D shows the view through my lens with greater clarity, or maybe greater DOF, than what the sensor actually captures, at large apertures. IOW: I can set the aperture to it's largest setting of f/2.8, focus on something nearby, and see that the back ground is out of focus. But when I actually take the picture, the out of focus background is much blurrier than it appeared through the viewfinder. Waaa??? How can that be?

I've been reading about aftermarket focusing screens, because I want to improve manual focusing capability. There's a company in Taiwan that cuts down replacement 35mm SLR screens for use in DSLRs and they carry a variety of screens from Canon, Nikon, Minolta.... In researching these screens, I found several comments from users who say that when using the Nikon F3 screen (for example) it is much easier to see what is in and out of focus, even outside of the microprism collar or split image section of the screen, than it was with their stock screen. This is what caused me to check mine, and sure enough, my stock screen lies to me. If you haven't already noticed this, check it out and you'll see what I mean.

I don't get it. How can the focusing screen effect DOF like that?...or is it something else that makes it look sharper? And why would we want that? If you could more easily see how out of focus things really are, then you could also more easily see what was in focus. I'm starting to think the DSLR makers are making manual focusing difficult on purpose.

Paul

YOUR Brain does that trick. There is not a think wrong with any of the equipment.