Doug Kerr
Well-known member
As new kinds of cameras emerge, with new kinds of viewfinders, we are often treated to various numbers that are supposed to tell us about the "geometric" properties of the viewfinder image. I thought I would review what these tell us.
The two parameters we usually see quoted are:
Viewfinder image magnification - This (usually expressed directly as a number) tells us the ratio of (a) the linear equivalent of the angular width of a scene object as seen through the viewfinder to (b) the linear equivalent of the angular width of that object as seen directly from the camera position, with a certain lens focal length in place.
It is often a little bit short of 100%. 100% is essentially "perfect", but if it is not practical to assure exactly 100% in all cases, then a lesser value "fails safe" in that everything seen in the viewfinder will be captured in the taken image.
An important parameter almost never stated is:
Size of viewfinder image - This refers to the angular dimensions of the viewfinder image subtended by the view from our eye (often expressed in degrees, perhaps a single number pertaining to the diagonal size of the viewfinder image).
This is in fact the property that we first notice when we look into a viewfinder - "Wow, that is a nice big image".
And we rarely, if ever, see it stated.
Can we determine this from other parameters we are given?
Yes, if we have all the pieces.
What we need to know is the viewfinder magnification, the focal length upon which that is predicated, and the sensor dimensions.
We rarely are told the focal length upon which the magnification is predicated. Does that make knowledge of the magnification useless? Essentially.
Perhaps we can infer that from customary practice. What is customary practice in that regard?
Well, in full-frame 35mm film cameras (43.3 mm frame), the custom was to state the viewfinder magnification based on a 50 mm focal length. The rationale is that this is the "normal" focal length for such a camera. Why? Because that is what is said.
What about cameras with, say, an "APS-C" frame size? Well there, the custom is to state the viewfinder magnification based on a 50 mm focal length.
Woof! That would not have the same implications. Wouldn't it be better to base it on:
- A focal length equal to the format diagonal, or
- For consistency with the full-frame 35-mm practice, a focal length with 50 mm full-frame 35-mm equivalent.
Yes. But neither of those are what is done. For "35-mm family" digital cameras (APS-C and such), the basis is 50 mm, regardless of the actual format size.
But what about cameras with, say, a 7.7 mm sensor? If we see it said that one has an EVF with a magnification of 0.8, what does that mean? There is no telling.
On my Panasonic Lumix DMC-FZ200, by observation, the viewfinder magnification is 1.0 for a focal length whose full-frame 35-mm equivalent is 126 mm.
If we consider 50 mm ff-35 equivalent focal length to be the "normal" focal length for this camera, then at that focal length, the viewfinder image magnification would be about 0.4.
What is the stated viewfinder magnification? I'm not sure it is stated by the manufacturer.
What is the size of the viewfinder image? I haven't calculated it yet.
So when we see in the DPR quick look at the newly-announcxed Nikon Df:
The Df's optical viewfinder is very large - the same size as the D800. The magnification is 0.70x and, naturally, coverage is 100%. We've also shown the viewfinder of the film-era F3 here for reference. As you can see, the F3 offered an impressively large finder, but this did cause issues for wearers of glasses.
and see that illustrated by this figure:
Linked from Digital Photography Review
What does this tell us?
Well, it in fact tells us that, if for each of those cameras the viewfinder magnification was stated predicated on a focal length that is a consistent ratio to the frame size (and that might well be in the case of these cameras), then those rectangles represent the relative size of the viewfinder image as seen to our eye.
And if not, they don't. Then what do the sizes of those rectangles tell us? The relative sizes of the numbers 0.69, 0.70, 0.74, and 0.8.
Best regards,
Doug
The two parameters we usually see quoted are:
Viewfinder image magnification - This (usually expressed directly as a number) tells us the ratio of (a) the linear equivalent of the angular width of a scene object as seen through the viewfinder to (b) the linear equivalent of the angular width of that object as seen directly from the camera position, with a certain lens focal length in place.
We note that rarely today is that focal length expressed (more on that shortly).
Viewfinder coverage - This tells us what fraction of the scene that will be captured in the image is seen in the viewfinder. It is normally expressed as a percentage, the implication being that the fraction is the same for height and width.It is often a little bit short of 100%. 100% is essentially "perfect", but if it is not practical to assure exactly 100% in all cases, then a lesser value "fails safe" in that everything seen in the viewfinder will be captured in the taken image.
An important parameter almost never stated is:
Size of viewfinder image - This refers to the angular dimensions of the viewfinder image subtended by the view from our eye (often expressed in degrees, perhaps a single number pertaining to the diagonal size of the viewfinder image).
This is in fact the property that we first notice when we look into a viewfinder - "Wow, that is a nice big image".
And we rarely, if ever, see it stated.
Can we determine this from other parameters we are given?
Yes, if we have all the pieces.
From here on I am going to ignore coverage, as it is usually close to 100% and as such has little effect on this issue.
What we need to know is the viewfinder magnification, the focal length upon which that is predicated, and the sensor dimensions.
We rarely are told the focal length upon which the magnification is predicated. Does that make knowledge of the magnification useless? Essentially.
Perhaps we can infer that from customary practice. What is customary practice in that regard?
Well, in full-frame 35mm film cameras (43.3 mm frame), the custom was to state the viewfinder magnification based on a 50 mm focal length. The rationale is that this is the "normal" focal length for such a camera. Why? Because that is what is said.
What about cameras with, say, an "APS-C" frame size? Well there, the custom is to state the viewfinder magnification based on a 50 mm focal length.
Woof! That would not have the same implications. Wouldn't it be better to base it on:
- A focal length equal to the format diagonal, or
- For consistency with the full-frame 35-mm practice, a focal length with 50 mm full-frame 35-mm equivalent.
Yes. But neither of those are what is done. For "35-mm family" digital cameras (APS-C and such), the basis is 50 mm, regardless of the actual format size.
But what about cameras with, say, a 7.7 mm sensor? If we see it said that one has an EVF with a magnification of 0.8, what does that mean? There is no telling.
On my Panasonic Lumix DMC-FZ200, by observation, the viewfinder magnification is 1.0 for a focal length whose full-frame 35-mm equivalent is 126 mm.
If we consider 50 mm ff-35 equivalent focal length to be the "normal" focal length for this camera, then at that focal length, the viewfinder image magnification would be about 0.4.
What is the stated viewfinder magnification? I'm not sure it is stated by the manufacturer.
What is the size of the viewfinder image? I haven't calculated it yet.
So when we see in the DPR quick look at the newly-announcxed Nikon Df:
The Df's optical viewfinder is very large - the same size as the D800. The magnification is 0.70x and, naturally, coverage is 100%. We've also shown the viewfinder of the film-era F3 here for reference. As you can see, the F3 offered an impressively large finder, but this did cause issues for wearers of glasses.
and see that illustrated by this figure:
Linked from Digital Photography Review
What does this tell us?
Well, it in fact tells us that, if for each of those cameras the viewfinder magnification was stated predicated on a focal length that is a consistent ratio to the frame size (and that might well be in the case of these cameras), then those rectangles represent the relative size of the viewfinder image as seen to our eye.
And if not, they don't. Then what do the sizes of those rectangles tell us? The relative sizes of the numbers 0.69, 0.70, 0.74, and 0.8.
Best regards,
Doug