3 replies to this topic

[Benjamin DeHaven]

Hello,

So I am trying to get a handle on how my camera compares to a scope. Keeping it as simple as possible (ignore crop factor) I am trying to understand magnification.

I have always heard a 50mm lens is equivalent to a human field of view and therefore considered 1x. This could be pondered about with film (pull out your loupe everyone) but is more obvious with digital: how does zoom (as in 1:1 v. 1:2 or 50% view) effect this overall magnification. When I have a 500mm lens on my camera it should be about 10x and it does actually sorta match my 10x binoculars. But that is through the viewfinder. When I bring it up at home or even on the small LCD review screen and zoom to 100% I am way beyond my binoculars 10x.

I feel like I know enough to be confused but not enough to understand whats going on. Any thoughts?

Thanks!

Benjamin DeHaven

[David Case]

Hello,

So I am trying to get a handle on how my camera compares to a scope. Keeping it as simple as possible (ignore crop factor) I am trying to understand magnification.

I have always heard a 50mm lens is equivalent to a human field of view and therefore considered 1x. This could be pondered about with film (pull out your loupe everyone) but is more obvious with digital: how does zoom (as in 1:1 v. 1:2 or 50% view) effect this overall magnification. When I have a 500mm lens on my camera it should be about 10x and it does actually sorta match my 10x binoculars. But that is through the viewfinder. When I bring it up at home or even on the small LCD review screen and zoom to 100% I am way beyond my binoculars 10x.

I feel like I know enough to be confused but not enough to understand whats going on. Any thoughts?

Thanks!

Benjamin DeHaven

I agree that a 50mm lens gives a view that is close to what the eye sees, at least they do on my crop sensor 50D which has a 1.6 crop factor. That same lens though will look wider on a full frame camera. I do not know what the 'x' means for scopes and binoculars but with cameras it is used to give the relative magnification of a zoom lens and is simply the ratio of the longest focal length (with the narrowest field of view) to the shortest focal length. So for example my Canon 100-400mm lens is a 4x lens since 400mm/100mm = 4, but a 24mm-96mm lens would also be a 4x lens, if Canon made such a lens. My 28mm-135mm lens is a 4.8x, my Canon 70mm-200mm is a 2.9x lens and so on. In any case, this is not intended to be an absolute measure of magnification. If you wanted to use the 50mm as a starting point then a 400mm lens on my 50D would be an '8x' lens but in using it this way we are departing from the normal camera meaning of 'x'.

There is one absolute standard in camera lenses and that is a measure of the macro capabilities of a lens. A true macro lens has at least a 1.0 magnification. This means that when photographing an object of say 5cm in length that the image projected onto the focal plane of the lens (where the sensor or film is) is also 5cm. So a magnification 2.0 macro lens would magnify that image twice making the projected image of the object 10cm in size and so on.

If one were to compare across different types of optical equipment such as cameras and scopes I would think that the most meaningful comparison would be the angle of view, since the narrower that is the more magnified the image appears. Camera manufactures publish the angle of view of their lenses (for my lenses it is in the manual) but I don't know about scope and binocular manufacturers. I hope that helps.

[Benjamin DeHaven]

Ok, I finally found part of what I was looking for after you pointed out "zoom" on a lens refers to the ratio between short focal length and long focal length. I wasn't getting that zoom and magnification were not interchangeable. For anyone else interested in this I will link to a forum discussion where they hash this out. The real info starts around post #8.

http://photography-o...&highlight=

But basically my D90 (1.5x crop factor) with my Tamron 200-500mm at 500mm will result in a "magnification" of (500*1.5)/50=15x. On my 400mm refractor scope this is approximately a 26mm eyepiece (400/26)=15.38x.

So what I am looking for now is based on above information what zoom (in the sense when you review an image in the camera's monitor it fits it to the screen but it has zoom buttons that you can drill down to 100% aka 1:1 (not in a macro sense mind you)). Assuming equal magnification as detailed above, if I looked at object A in the Scope then took a picture of object A in the camera's LCD review screen I would have to zoom in on the image to 100% and at that point object A should appear the same size on both instruments (field of view would be vastly different of course).

Does any of this seem vastly wrong? And sidebar, we need to invent some new words because in this conversation zoom, magnification, and power all mean too many different things.

Thanks for humoring me...

Benjamin

[David Case]

Ok, I finally found part of what I was looking for after you pointed out "zoom" on a lens refers to the ratio between short focal length and long focal length. I wasn't getting that zoom and magnification were not interchangeable. For anyone else interested in this I will link to a forum discussion where they hash this out. The real info starts around post #8.

http://photography-o...&highlight=

But basically my D90 (1.5x crop factor) with my Tamron 200-500mm at 500mm will result in a "magnification" of (500*1.5)/50=15x. On my 400mm refractor scope this is approximately a 26mm eyepiece (400/26)=15.38x.

So what I am looking for now is based on above information what zoom (in the sense when you review an image in the camera's monitor it fits it to the screen but it has zoom buttons that you can drill down to 100% aka 1:1 (not in a macro sense mind you)). Assuming equal magnification as detailed above, if I looked at object A in the Scope then took a picture of object A in the camera's LCD review screen I would have to zoom in on the image to 100% and at that point object A should appear the same size on both instruments (field of view would be vastly different of course).

Does any of this seem vastly wrong? And sidebar, we need to invent some new words because in this conversation zoom, magnification, and power all mean too many different things.

Thanks for humoring me...

Benjamin

Benjamin,

OK, I think that I see what you are after now. It seems to me that you are working from three assumptions:
(1) what we mean by '1x' is what the eye sees,
(2) scopes use assumption (1), and
(3) looking through a camera viewfinder is comparable to looking at its LCD screen

I am not sure about assumption (2) for scopes but for now let's run with it. First we find a lens for our camera that approximates assumption (1). As I mentioned, for my Canon 50D (1.6 crop factor) that is pretty near a 50mm lens and so it should be pretty much the same for your D90 with its 1.5 crop factor. Specifically what I mean by this is that when I look through my viewfinder with a 50mm lens on my 50D and compare that to what I see with my naked eye, things look pretty much the same size. So at 500mm your camera will give you 10x, not 15x. The crop factor would come into play only if you were comparing how the lens would look on your D90 as opposed to when the same lens was on a full frame camera. In other words, if you had a lens that gave you a 1x view on a full frame camera and you wanted to know what magnification you would see on your D90, then you would multiply by the crop factor, and the answer would be 1.5x. But since we are starting from 1x on the crop sensor camera, including the crop factor throws the calculation off.

Alright, so we have about a 10x magnification with your D90 at 500mm. Now to calculate the additional magnification achieved by zooming in all the way on the LCD screen. This is going to be the ratio of how wide your camera's images are in pixels to the pixel width of your LCD screen. I would guess this would be in the neighborhood of 4x but I don't know the specs of the D90 so that is just a guess. But surely you can get these numbers from your camera manual and work out the precise value. But assuming it's about 4x you then have a magnification of 40x, not quite what can be achieved with a 60x scope.

There is one more complicating factor, assumption (3). In doing the last step we were implicitly assuming that the x value of looking through the view finder is the same as the x value of looking at the LCD screen, and that may not be true. If your camera has a live view mode you could test this by setting it up on a tripod at 50mm and compare the two views to see which seems larger or if the two seem about the same. I can't offhand figure out an objective way to measure this. Let's say you decided that the viewfinder view seemed about a 1/3 larger than the LCD. Then you would have to divide your 40x by 4/3 which would bring you down to 30x and if the proportions were reversed you would get 53.33x. Again, this step seems very subjective to me. Looking through a view finder is a lot different than looking at an LCD screen.