# Life-size objects in the viewfinder

Discussion in 'Digital Photography' started by sfeher@gmail.com, Jul 16, 2006.

1. ### Guest

Hi All,

As I am trying to educate myself in the physics of the camera, I have a
question regarding the focal length for which you get objects to
*appear* life-size in the viewfinder? Is this important in any way?

Looking through the viewfinder the objects appear to be life-size at
aprox. 70mm ?! This happens for both DSLR (D70 - 18-70 lens) and 35mm
SLR (Elan7 28-90
lens - which has a bigger viewfinder).

I thought the focal length for real life-size would be 43mm for 35mm
camera and somewhat proportional(1.5x) for the DSLR, hence my confusion
when I actually got 70mm for each one of them.

I hope my question is somewhat clear. It could be that I am looking at
the problem from a weird perspective as I am learning these things. If
so please let me know.

Regards,
Sebastian

, Jul 16, 2006

2. ### k-manGuest

You might be confusing focal length and magnification. Magnification
is where you get "lifesize" images. This is particularly important
with macros, where often you would want a magnification of 1:1. This
is not a very detailed explanation, I know. But there are a lot of
sites out there that have all the details.

Kevin

wrote:
> Hi All,
>
> As I am trying to educate myself in the physics of the camera, I have a
> question regarding the focal length for which you get objects to
> *appear* life-size in the viewfinder? Is this important in any way?
>
> Looking through the viewfinder the objects appear to be life-size at
> aprox. 70mm ?! This happens for both DSLR (D70 - 18-70 lens) and 35mm
> SLR (Elan7 28-90
> lens - which has a bigger viewfinder).
>
> I thought the focal length for real life-size would be 43mm for 35mm
> camera and somewhat proportional(1.5x) for the DSLR, hence my confusion
> when I actually got 70mm for each one of them.
>
> I hope my question is somewhat clear. It could be that I am looking at
> the problem from a weird perspective as I am learning these things. If
> so please let me know.
>
> Regards,
> Sebastian

k-man, Jul 17, 2006

3. ### Guest

wrote:
> Hi All,
>
> As I am trying to educate myself in the physics of the camera, I have a
> question regarding the focal length for which you get objects to
> *appear* life-size in the viewfinder? Is this important in any way?
>
> Looking through the viewfinder the objects appear to be life-size at
> aprox. 70mm ?! This happens for both DSLR (D70 - 18-70 lens) and 35mm
> SLR (Elan7 28-90
> lens - which has a bigger viewfinder).
>
> I thought the focal length for real life-size would be 43mm for 35mm
> camera and somewhat proportional(1.5x) for the DSLR, hence my confusion
> when I actually got 70mm for each one of them.
>
> I hope my question is somewhat clear. It could be that I am looking at
> the problem from a weird perspective as I am learning these things. If
> so please let me know.
>

This will vary with the camera. Most DSLR viewfinders are essentially
a telescope. The prism re-inverts the image, so the type of telescope
is the same as a 'spotting scope'.

A telescope does have magnification, though not the same mathematically
as the 'magnification' of a simple lens. The telescope 'magnification'
is in angle space. The magnification of the scope depends on the ratio
of the fl of the eyepiece lens to the objective lens. Thus if two
cameras have different eyepiece lens fl, the magnifications will be
different even with the same fl objective lens.
> Regards,
> Sebastian

, Jul 17, 2006
4. ### Ed Ruf (REPLY to E-MAIL IN SIG!)Guest

On 16 Jul 2006 11:13:31 -0700, in rec.photo.digital wrote:

>I thought the focal length for real life-size would be 43mm for 35mm
>camera and somewhat proportional(1.5x) for the DSLR, hence my confusion
>when I actually got 70mm for each one of them.

You are forgetting to also take into account the magnification of the
viewfinder. For the D70, copied from the fine manual:

Magnification Approximately 0.75Ã— (50-mm lens at infi nity; â€“1.0 mâ€“1)
--
Ed Ruf ()
http://edwardgruf.com/Digital_Photography/General/index.html

Ed Ruf (REPLY to E-MAIL IN SIG!), Jul 17, 2006

### Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.