Focus Screen/Fresnel Lens/Prism ???

Canon 7D

I am seeing all manner of conflicting information and am somewhat confused.

Are these three items just different names for the same thing; or, are they separate items ?
Focus Screen, Fresnel Lens, and Prism.

I hear all sorts of scare stories why one should never touch the focus screen; and then, I see well-respected professional photographers say that these things should be cleaned and they wrap a cloth around a finger and reach in there and go to wiping.

Ever since we have owned it, looking through the viewfinder on one of our 7D, it is as if looking through matte plastic; and, the closer in I zoom, the worse it gets.

With a 18-250mm lens at 18mm, the graininess is there, but not nearly so noticeable.
With the 50-500mm lens, it is quite noticeable at 50mm and downright awful at 500mm.

I am familiar and quite adept at cleaning sensors and such.
I don't want to screw something up; but, I am going to have to do something.

Perhaps this page can give you some better understanding of where the parts are.

The prism is a part by itself, and normally not accessible.
The focusing screen may be made by grinding glass, or by using a Fresnel lens, or sometimes by combining the two.
I have one camera where a gentle touch permanently damaged the focusing screen, so somtimes they are sensitive.

apersson850 wrote in post #19392374
Perhaps this page can give you some better understanding of where the parts are.

Thanks; maybe I missed it or it just never sunk in before; but, I have a head-high stack of photography books and spent countless hours on the internet looking at photography stuff and I have never had it made as clear to me as that simple picture did.

From what I gather from that diagram, even with the absolute newest cleanest DSLR, it is a miracle one can see anything looking through the viewfinder.

I have found new replacement focus screens on EBay for $25; and, form what I have seen, they are very easily replaced.

I am not made of money and right now $25 looks as big as a wagon wheel to me; but, in the interest of fixing my problem, if a good mirror cleaning doesn't cure the problem, I am tempted to buy a new screen to have on hand in the event I screw mine up in trying to clean it.

In an almost dark room, holding my super-bright LED spotlight sort of at an angle and looking at the mirror sort of askance, it appears to be absolutely filthy.
Vigorous application of my big Rocket Blower didn't seem to help.

When I bought this camera second hand, the vendor had over thirty 7D to choose from.
I chose the one with the lowest shutter count and later investigation proved it to be accurate.
When I received the camera, it looked Mint to me = clean as a whistle with nary a blemish.
This pristine appearance didn't carry over to the internals; the sensor looked like someone had been using it as a spittoon.
From what I am seeing on the mirror, it is as to be expected considering how filthy the sensor was.

BuckSkin wrote in post #19392421
From what I gather from that diagram, even with the absolute newest cleanest DSLR, it is a miracle one can see anything looking through the viewfinder.

Why would it be that? You look via one single mirror and then a prism, which has total reflection. That's as good as it gets, optically. It was the same with SLR cameras for film.

I am seeing new "Matte" focusing screens and also "Split Image" focusing screens.

Is the original Matte ?

What does the Split Image do and is it desirable ?

Thanks.

On film cameras, a split-image screen was standard as a manual focus aide. A little circle in the middle where, when you lined up 2 sides of a split image, you'd be in focus at that point.

Two questions. Is a split-image screen the same thing as it was on film cameras? They make a split-image screen for a 7D?

drsilver wrote in post #19392631
On film cameras, a split-image screen was standard as a manual focus aide. A little circle in the middle where, when you lined up 2 sides of a split image, you'd be in focus at that point.

Thank you for explaining that.
I would expect that the split-image would interfere with auto-focus abilities, true ?

drsilver wrote in post #19392631
They make a split-image screen for a 7D?

Yes; there are many more options for split-image 7D screens than for "Matte", which I am assuming is normal/original.
Amazon only lists a single option for the 7D, a split-image; EBay lists several of both options.

I have noticed that mirrors in my cameras look nasty. It's easier just to not look at them.

Here's the inside of my 100-400 lens. DOF is very thin so it's only the crap on one lens element.
If I could get all the depth of field to see the all the crap in the lens, there would be several times this much in the photo.
I just took this photo to show how bad the internals of a lens that produces fantastic results can look.

Are you sure there's really a problem with your 7D?

Photo from Choderboy's gallery.
Image hosted by forum (1003622)

Choderboy wrote in post #19392808
Are you sure there's really a problem with your 7D?

I tried the lens, 50-500mm Sigma, from the 7D on three other cameras, a T3/1100D, another 7D, and a 7DMkII, and the view was crystal clear.

I tried the 18-250mm Sigma, that usually resides on the 7DMkII and is always crystal clear, on the 7D and the view was just as filthy and gritty as with it's usual 50-500mm.

I believe this affects my focus as well; as, on any of the other cameras, those lens will achieve focus instantly, no matter how far out of focus the lens may initially be.

On the 7D in question, if the lens is way out of focus, I must help it get in the ballpark manually before the camera will find focus; and, it still hunts around a lot even when close to being in focus.

This is not just an annoying little speck or two; it looks like a handful of grime has been tossed in there.

OK, so it's not overall grain, it's more like the photo of my lens internals.
That sounds like it needs a clean.


Not shown in the link Anders posted are the sub mirror (or secondary mirror) and the PDAF sensor.
They don't use the focus screen or prism.

https://photographylif​e.com …detection-autofocus-works

So if AF is affected by dirt, it could be main mirror, sub mirror, or the PDAF sensor itself (or a combination of all three), but not the focus screen.

Me, I'd look for a shop to check it and I'd be more concerned about the AF than the view through the viewfinder.

3 is Sub mirror
7 is PDAF sensor

So several lenses are "crystal clear" on multiple bodies but one body (7D) seems lacking. Did you purchase the 7D new? Has the sensor ever been cleaned? (See https://id.canon/en/su​pport/8202665700).

Worth reviewing the content at https://www.dpreview.c​om/forums/thread/35708​68 as well.

John from PA wrote in post #19392844
So several lenses are "crystal clear" on multiple bodies but one body (7D) seems lacking. Did you purchase the 7D new? Has the sensor ever been cleaned? (See https://id.canon/en/su​pport/8202665700).

Worth reviewing the content at https://www.dpreview.c​om/forums/thread/35708​68 as well.

"When I bought this camera second hand,..."

Dirty sensor won't affect AF or the view through viewfinder. (As I'm sure you'd know)

If you are going to try cleaning yourself, you can probably make things easier by checking something.

The photo of my 100-400 lens internals, I shone a bright light on a white wall and pointed the lens at it.

You can probably see the dirty focus screen (assuming focus screen is dirty) by pointing camera at white wall with bright light illuminating it. (looking through viewfinder)
This saves continually mounting then dismounting lens.
This is for achieving a clean view through viewfinder, not cleaning light path to the PDAF sensor.
(except the front surface of main mirror, the only surface shared by viewfinder and PDAF sensor).

Another thing to be aware of. Dirt and dust can find it's way between the focus screen and the prism and or the condensor lens.

See here:
https://photography-on-the.net/forum/showthre​ad.php?t=839102

Note the opening posts suggesting your camera may not have a condensor lens, but the thread could be relevant.

Finally, be aware.
Your 7D focus screen, I think, was the first of the focus screens that need to be powered on.

ie, with a 40D, there is no change in view when you power up the camera.
With a 7D, there is.

Canon state that the 7D focus screen is not interchangeable. It is though.
That info may give you a head start.

You need to read at least to the bottom of the first page...
https://photography-on-the.net …hread.php?t=817​959&page=1

Choderboy wrote in post #19392846
"When I bought this camera second hand,..."

Dirty sensor won't affect AF or the view through viewfinder. (As I'm sure you'd know)

Thanks for pointing out that the OP had bought the camera used. Personally, looking at the balance of what he said, if returnable, I would make an attempt to return the camera and make a selection based on both external condition and internal condition. Quoting the OP...