Lens magnification factor?

[Frank Kolwicz]

How should I calculate the magnification factor of a camera lens, like the factor given for spotting scopes, for instance?

For example, I have a 600mm lens with a 1.4x teleconverter on a 70d which combination has an effective focal length of about 1325mm according to the EXIF data from a .CR2 file.

Thanks

[Bob Atkins]

You can't really. Magnification factor for a telescope relates the image size seen though the telescope with the image size seen by the naked eye.

Since you don't look through the camera (except to point it), there's no directly comparable "magnification factor". Magnification has to be relative to some standard which is defined to be a magnification factor of 1x

Sometimes people use a 50mm lens as a standard, so a 500mm lens would be 10x and a 1000mm lens would be 2x. That's about as defined as things get. It gives you the image magnification relative to that given by a 50mm lens. Not sure if anyone takes sensor size into account!

If you can better define "magnification factor" for a lens, I'd be happy to give a shot at figuring out a way to calculate it!

[Frank Kolwicz]

Thanks, Bob, I was hoping you had a better way of figuring it.

[Frank Kolwicz]

I've been thinking about this some more and it seems to me that the way to make some kind of a valid comparison is to do a comparison to a spotting scope of similar focal length. That brought me to the following website:

 http://www.analyticalsci.com/Astronomy/Telescopes/imagescCelestron%20Web/About_SpotScopes.htm

that has a discussion of this topic under "Magnification (Power)". The scope being used as an example has a focal length of 1000mm, more or less similar to my 840mm focal length lens with the 1.4x extender. So, if I put a scope adapter on my lens with an arbitrary focal length eyepiece I could visually compare what I see in it to an image file and get a fair idea of it's "magnification power".

In the field I'd photograph a convenient subject and download the file to a laptop and bring the image up on-screen, then simply compare the full-frame photo to what I could see in the lens with the adapter and eyepiece lens. The diagonal of the image file should come closest to the eyepiece image, so maybe I'd rotate the camera to make that horizontal and count the number of boards in a fence, for instance, to make the comparison or mount a tape measure or ruler and take readings from that. So many inches visible in the eyepiece with the calculated magnification as a scope versus whatever number I see in the image file, presto!, relative magnification.

That is, unless I've missed something crucial which I hope you will helpfully point out.

By the way, I'd also like a recommendation for a fairly inexpensive eyepiece that might work well with my 600/1.4x combo and provide about 40x power as I'm planning to get an adapter - what a spotting scope that would make (even if the adapter is crappy)!

Thanks,

Frank

[klindup]

Assuming that you could get the adaptor, I think you need an eyepiece with a 21mm focal length.  This is based on the assumption that the power of a telescope with eyepiece is given by the focal length of the telescope divided by the focal length of the eyepiece.

[Bob Atkins]

Frank - you haven't been following this site closely enough! Everything is in here somewhere...

http://www.bobatkins.com/photography/technical/Lens-scope-adapter.html

I'm not aware of any commercial adapters for Canon EOS lenses that allow an eyepiece to be used to turn the lens into a high quality spotting scope. They would need to use relay optics to extend the back focus distance of the lens so that an image inverter of some sort could be used in the optical path (as they do in binoculars). You could do it, it's not a technical problem, but it would not be cheap to do it right.

The angular view through a telescope depends not just on magnification but on the eyepiece design. The angular FOV of an eyepiece can range from around 50 degrees for a Plossl design to 110 degrees for an Ethos design. Since the real angular coverage is the eyepiece FOV divided by the magnification, you can see that estimating "lens magnification" by comparing what you see through the telescope to the image recorded on the sensor would depend on the type of eyepiece you use.

[klindup]

Fortunately for us amateur astronomers with good refracting telescopes it is easy to use them as a long focus lens.  I am not suggesting for one minute that one should buy a telescope to use as a lens but if you happen to have one it works.  i have a Televue NP101 which gives me a really superb 540mm lens.  OK It is expensive, heavy, bulky has no autofpcus and operates at a fixed aperture (f5.4) and requires a very heavy and bulky mount but as a lens it is just great.  Once I have set it up!  Good luck Frank, but as Bob says it is not an easy path you are heading down.

Ken

[Bob Atkins]

If you have the telescope, then that's a good way to go. I used a Televue Genesis (500mm f5 fluorite) for several years as my main telephoto lens but as Klindup comments, it's heavy, big, fixed aperture and manual focus - plus it's expensive. I think that type of telescope currently retails at around $4000.

I dug around and actually found an eyepiece adapter for Canon EF lenses. Must be fairly new since I wasn't aware of it before. Price isn't bad at $179. There are straight through and 45 degree angle versions. Here's are links:

Kenko Lens2Scope Adapter for Canon EF Mount Lenses - 45 degree angle

Kenko Lens2Scope Adapter for Canon EF Mount Lenses - straight through

I'll have to update my article to include these. They also have them for Nikon, Pentax and Sony mount lenses. They seem to have a fixed eyepiece of 10mm focal length, so a 600mm lens becomes a 60x spotting scope. They are a 5 element/3 group roof prism design which should be fine for terrestrial viewing. A straight through design with no correction optics would be better for astronomy (where nobody cares if the image is upside down or switched left to right!).

[Frank Kolwicz]

Bob said: ". . . for astronomy (where nobody cares if the image is upside down or switched left to right!)."

Yeah! Back in the early '90s I tried adapting a 50mm normal as an eyepiece for an FD600 as a spotting scope and gave myself a real headache trying to follow swimming ducks going one way through the scope and the other way by eye.

I was aware of the Kenko adapter for EF lenses, but didn't realize they came with a built-in eyepiece, so it'll have to go on my 100-400L instead and only on the 600 for 60x or 600/1.4x combo when I need 84x magnification and can stand the image wobble.