This topic was covered before in a variety of forums, however my impression is that many people don't understand the phenomenon when annoyed 7D users attempt describe it.
So let me be clear about it: this is not about dust or any sort of dirt collected in the viewfinder. It is not about diopters, not about general blurryness or softness, not about empty or missing batteries, not about parts of the viewfinder visible range (but the entire field of view), and not about the picture quality that comes from the image sensor. The effect I am describing here happens exclusively in the viewfinder (eyepiece) with no impact to the general image quality.
However, it is quite infuriating because Canon seems to have reduced the visual resolution of the viewfinder artificially and deliberately. It is hard to describe how, and even harder to take pictures of it. So forgive the bad image quality of those that follow, I had to do these images with my smartphone because the 7D is the only other camera I own. Well, I think the effect becomes apparent enough.
So let's start at the beginning. I have recently bought a Maksutov 1000mm mirror telescope lens for astro photography. I am entry-level, not expert, about astro photography, but the Mak 1000 is a good lens to start with. It is 100% manual, no AF or IS available.
Lately I discovered that a wind turbine is being built a few kilometers away and was curious about the construction work, so I thought I'd give the Mak 1000 a chance to prove its qualities in daylight. This is where Canon's 7D viewfinder design is becoming really troublesome.
Trying to focus a remote object manually requires a razor sharp viewfinder. The EOS 7D however adds an analog-style grain to what is seen in the viewfinder which makes it virtually impossible to adjust the focus precisely. It's rather down to guessing, because even if the focus is bang-on, the entire viewfinder impression is still completely imprecise. And with a 1000mm lens, it's fractions of millimeters on the focus ring that decide whether the focus is on or off the object.
The effect becomes stronger as the aperture becomes smaller. As the Mak 1000 does not have an adjustable aperture either, I took the photos below with a different analog lens that has a manually-adjustable aperture, and set that way down to 11. There is nothing specific in focus (distance set to infinity) to avoid distraction. Then I put my phone behind the viewfinder and attempted to capture the impression.
Note the darker areas to the left and bottom. The original image from the smartphone was pretty miserable because only a fraction of the field of view was covered (maybe a 16th or so in the very center, with everything else around it being pitch black), consequently only a fraction of the available image sensor resolution of the smartphone was used at all. JPEG adds its own artifacts but you might get the idea anyway.
The next photo includes the bottom display section in the viewfinder to prove that the camera was powered on and the battery far from empty:
The fine grain that can be seen across the entire view (except the display line below) is definitely not a JPEG compression artifact or something else my smartphone added. It is actually there and looks just like old analog film grain. For the eye, it resides on the same focal plane as the focus indicators and green illuminated info display at the bottom, so it is best perceived by focusing the eye on one of these items instead of the subject you are about to capture. In long-range photography the grain blurs the entire viewfinder image to a degree that makes manual focusing virtually impossible. Let me emphasize one more time that if I create an actual shot, the RAW image does not reflect any of this grain, it is purely a viewfinder issue.
There are some forums where members claim this grain effect is actually something useful and may aid manual focusing. Well, I cannot confirm that at least for long-range tele photography. And I wouldn't know how this supports me in manual focusing even in short range. Personally, I feel this is an annoyance and nothing else.
For comparison and reference, let's have a look at the viewfinder when the battery was pulled:
This is clearly different. I needed to apply some heavy image processing to get it as close as possible to the real thing. Just ignore the colors. Without power the display becomes a dark grey 'overlay' with some bright speckles in it, and pretty blurry, too. The reason is that the translucency of the LCD display in the optical path drops considerably when power is removed. It looks totally different from the grain shown above, so that grain is not battery related, I am 100% sure of that.
The new kind of in-line LCD display introduced with the 7D is my main suspect. It's just a thought, but the display setup might cause the grain effect directly, just like it causes the grey curtain when power is lost. I wish Canon could be a little clearer on this matter, particularly about its being deliberate or accidental.
If the grain is a desired effect, I regret that Canon did not make it optional. This 'feature' renders the 7D pointless (in my humble opinion) for long-range photography unless one of the professional lenses with AF is used, and the AF performance needs to be really stellar to get the best out of remote objects, too. Not a concern here, unless you are a millionaire.
I fear the moon photos I intended to make with the Mak 1000 will all be considerably out of focus because it is virtually impossible to get it right. The LiveView mode may serve as a workaround - at 10x it gives some details that the naked eye cannot see anyway, no matter if the viewfinder is crystal clear or grainy, but it's hard to work with again because I need to be behind the camera at all times to see what the sensor gets, and there is some lag between adjusting the focus and the consequence of that appearing on the screen. It's painstaking and time-consuming. With remote objects, adjusting the focus will always move the camera off the subject as well so it comes down to a lot of trial and error. And the moon never stands still either so while struggling to adjust the focus, it may have moved out of the image center. A mobile HDMI display with its own power supply might be a good yet costly way out. LiveView also drains the battery rapidly so I'm not convinced of that workaround at all. I would still prefer a clear and non-grainy viewfinder over anything else.
I cannot say what other models might have the same effect. The 7D seems to be the first model to introduce this kind of LCD integrated into the viewfinder optical path, and they may have continued to do the same with all camera models built ever since. There's no knowing whether the grain effect is also found in other (newer) models. As there is not much talk about this matter at all, I think it is best to test the EOS cameras thoroughly for this effect before buying.
I am all amateur so this topic might be plain BS. I might misunderstand the intention behind the grainy pattern. So if anyone knows better, I'd be more than happy to learn. Anyway, thank you for any contributions :)
That's it for now. Thanks for reading and have a nice day.