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If I want to have a good, and most of all objective, idea of what a view finder looks like compared to another one, which numbers of a DSLR's characteristics must I pay attention to?

For example, when looking at the characteristics of a D300 and those of a 6D, I feel that they are very similar. In fact the D300 seems better than the 6D.

respectively 6D/D300: coverage 97%/100%, Magnification 0,71x/0,94x.

But when looking inside the viewfinder, there is no doubt that there is a huge difference and the 6D IS better. I wear glasses, so that may be a factor.

Before spending lots of money into a DSLR, I'd like to be sure "on the paper" that what I feel when looking into a new camera is real and as important (or not) as I can see it, and not only felt because of the excitation caused by the novelty.

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I've never encountered a camera store that won't allow me to handle a demo copy of a camera that I'm interested in buying... That's the best way to find out about the viewfinder in reality. – John Cavan Jun 27 '13 at 15:02
I disagree that the Nikon D300 and the Canon 6D are similar at all. One is a 6 year old 12.5MP crop body, the other is a brand new 20.7MP Full Frame body that has four stops higher maximum ISO, and a much higher DxO Mark score, especially in terms of low light/high ISO performance.… The viewfinders are far from the most significant differences between these two cameras! – Michael Clark Jun 27 '13 at 20:36
@MichaelClark:That's not correct, the technical points you are talking about have absolutely no impact on the viewfinder. Worse, on the paper, the D300 regarding the asked question seems better than the 6D, respectively 6D/D300: coverage 97%/100%, Magnification 0,71x/0,94x. Both are supported with a pentaprism. So where do you see a so huge difference for the 6d ? – Oliver Jun 28 '13 at 8:02
@JohnCavan:yes, but when you have a new dslr in the hands, you automatically feel that it is better than yours, even if this is not correct. When differences are subtile, an "on the paper" basis comparison can be helpfull. – Oliver Jun 28 '13 at 8:22
Because the coverage of a lens on a FF camera covers a much wider angle of view than the same lens on a crop sensor camera, the magnification of the FF camera must be less in order to show the wider angle of view provided by that lens on a FF camera than the magnification in a viewfinder of a crop sensor camera that only shows the reduced angle of view that same lens provides to the cropped sensor. Since magnified light is also spread out further, the crop sensor viewfinder (D300) will be noticeably dimmer than the FF viewfinder (6D). – Michael Clark Jun 28 '13 at 12:01

There are two (maybe three) major things to look for in a viewfinder. The first and most important is coverage. Viewfinders provide an approximation of what the sensor sees. While they are looking through the lens, they don't necessarily match exactly with the sensor size. This is why there is normally a % coverage associated with the viewfinder. 100% coverage is the best, but often on cheaper cameras, it is more in the mid-90s which means you don't see some of what the sensor will see.

The second is pentaprism vs pentamirror. This question has a lot more useful information about the specifics of the difference, but the short version is that a pentamirror is cheaper and lighter, but passes through less light and is less clear. A pentaprism is higher quality and brighter, but more expensive and heavier. Pentaprisms are generally better for there superior visibility.

The third, though only important for people who wear glasses, is the amount of diopter adjustment. Diopter adjustment allows for some correction for vision issues to allow more direct use of the viewfinder.

As far as your original question about the "bigness", that really isn't measurable. How big it seems to you is going to depend on how you look through it to some extent. The biggest recommendation I can think of is to stop somewhere like a Best Buy or some camera store and take a look at the demo models to compare for yourself. The way you hold the camera and how comfortable the viewfinder is for you to use is going to make a far larger impact on ease of use than "bigness".

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Bigness is the magnification factor, which is usually relative to the sensor size itself. On a FF camera, a 0.76x mag means the viewfinder is about 76% the size of the sensor. In a lot of APS-C cameras, magnification can be higher, however it is relative to a smaller sensor area, so in an APS-C, the viewfinders are usually quite small. – jrista Jun 27 '13 at 14:27
@jrista - it would also be a factor of how close your eye gets to the viewfinder itself, which is going to be dependent on how you hold the camera and how well it fits your face. – AJ Henderson Jun 27 '13 at 14:32
The magnification issue is covered here. In brief, it's not relative to the sensor, but to the naked-eye view of the world with a given lens (usually 50mm; it'll be stated in the spec). – user2719 Jun 27 '13 at 20:02
Right, the sensor size comes in because the lens isn't chosen to keep angle of view is constant across formats when measuring. That makes the crop-sensor viewfinders sound more impressive on paper, which is the crux of the question here. – mattdm Jun 28 '13 at 12:47

As has been well covered in the question What does "viewfinder magnification" mean?, the magnification rating number of any viewfinder is based on a specific focal length lens. If you double the focal length of the lens from 50mm to 100mm, you also double the amount of magnification the viewfinder provides compared to looking at the world with just your eye.

Of the two cameras you are comparing, one is a Full Frame body with a 24mm X 36mm sensor, and the other is a crop sensor body with a 15.8mm X 23.6mm APS-C sensor. Since magnification numbers are based on a specific lens at a specific aperture focused to infinity, if both manufacturers do not use identical lenses (hardly likely since they do not share the same mount), the magnification numbers provided by the manufacturers can not be directly compared since differences in the characteristics of the lenses used will affected the calculated magnification number.

If you look at the Nikon D300 review at DPReview, you will see that with a 50mm f/1.4 lens the D300 was measured at approximately 0.94x. The Canon 6D review at DPReview lists the 6D viewfinder magnification as 0.71x but does not include the lens information used to obtain that number. 50mm is generally the standard used.

What you must realize when comparing the viewfinder magnification of these two cameras is that the same focal length lens on each will NOT provide the same angle of view since their sensor sizes are markedly different! In the case of the APS-C sized sensor in the Nikon D300, you must use a lens of approximately 33mm to provide the same field of view (FoV) in a picture as the use of a 50mm lens on the full frame sensor of the Canon 6D. Since the arithmetic here is linear, to calculate the magnification factor of the D300 viewfinder with a 33mm lens you would divide 33mm by 50mm. 33/50 X 0.94x = 0.6204x. What this reveals is that when both cameras have lenses mounted that provide the same FoV, the 0.71x image in the 6D viewfinder is larger than the 0.62x image in the D300 viewfinder. Even when taking the viewfinder coverage onto account (6D=97%, D300=100%) the image in the 6D viewfinder will still be about 11% larger when viewed from the same distance behind the viewfinder.

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The size of a viewfinder is you see it through the standard eye-piece is magnification divided by the crop-factor. So a 0.9X on a 1.5X crop camera has the same size as a 0.6X on a full-frame.

This gives a very good approximation of the comfort of using a viewfinder. However, a better measure is how big are details shown through the viewfinder. This correlates to your ability to judge focus and fine details. For example, for a group portrait, if you see details better you will be able to notice more easily when people are blinking or not looking the right way.

In order to determine the size of details shown through the viewfinder, coverage needs to be included in the calculation. The simple reason is that if you see less in a fixed sized window, then what you see must be bigger and vice-versa.

This is what I explained in my original answer:

The size of a viewfinder is determined by its magnification. It is measured relative to sensor-size and coverage. The former has a significant impact while the latter a small one, since coverage rarely varies by more than 5%. so you have to normalize it in order to compare it in absolute size.

The absolute size is labelled as Effective Size on the DSLR Viewfinder Size article. This article is fed right from the database and computed live, so it always contains the latest data supplied to me by manufacturers. You can sort by clicking on any column heading. A difference of 0.1 is significant and 0.05 is still easily noticeable. I would not expect to notice anything less than 0.02.

Since you mentioned wearing glasses, there is another number which is rarely supplied by makers and that is the eye-relief point. That is the distance at which your eye can see the entire viewfinder.

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Hey Itai. You didn't happen to write that article or be affiliated with that site without mentioning it..... – mattdm Jun 28 '13 at 2:21
Ha ha. The fact that the data is supplied to me and appears in the article is purely coincidental ;) Using the site logo for my avatar is probably not. – Itai Jun 28 '13 at 2:49
It's hard to trust those numbers as they seem to include the coverage, and as it's said "Magnification is a relative measure which compares the size of what we see through the viewfinder to what we would be seeing without looking through a viewfnder". So whatever the coverage is 100% or 10%, it's not because you only see a small percentage of something that the said something size is different. Same thing for the crop factor : take a lilltle window, and zoom whatever is displayed into that window, the size of the window will remain the same. And what I want to find is that Window's size. – Oliver Jun 28 '13 at 8:39
@Oliver - Ah, OK. Yes, the Effective Size is the size of what you see through the window which is what magnification should measure if there were not normalized for a 50mm lens mounted on the camera. As you noted, this is not the size of the window. I guess that is because people are interested in see details. Otherwise a big window could still make it hard to discern your subject if it has a wide field of view. You can see this well in Bright-Line viewfinders on a rangefinder which can show in excess of 140% coverage and no less improve your ability to see your subject. – Itai Jun 28 '13 at 12:50
(Cont'd). If you want to only know the size of the window as seen by the viewfinder lens, you can probably divide magnification by crop-factor this cancels out the normalization to using a 50mm which is used by manufacturers when computing those numbers. In any case, this will give you close numbers to those in effective size since coverage only accounts of minor variations in most cases. – Itai Jun 28 '13 at 12:56

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