Newbie question - how do you convert the mm of a lens into a magnification factor?
 

For example, I have a Nikkor 300mm lens for my D-50. How does that translate into a magnification factor?

Like, my binoculars ar 16x. That, I understand. The images I see are 16 times larger than in reality.

But with a 300mm lens, how much magnification is there?

Do I assume 35mm is "real" and then divide 300mm by 35mm to come up with a 8.5x mag factor?

Thanks!

Good question. It depends on the size of the film/sensor. With 35mm, say, a 300mm would definitely be a telephoto but with large format cameras it's just a 'standard' lens.

The usual formula, as far as I'm aware, is to take the diagonal measurement of the film-frame and this will give you the focal length of the standard lens to get a 'natural' view.

With 35mm the frame is about 43.5mm so that would be the standard lens - I think the main reason that 50mm became the normal lens was that when Leica produced the first 35mm camera they had a 50mm design from another format (cine?) available, so didn't bother re-designing one just a few millimetres away. Other makers then just followed suit.

Any mathemeticians will be able to work out what the magnification is by dividing 300 by 43.5. And working out the diagonal of any other formats, such as the D50's is just a matter of finding the square route of the sum of the squares of the two quoted dimensions of the format size - easy isn't it!!

OK, it's a long time since I was at school but at least I can remember the formula even if I can't work it out!

I might add that when you look through the camera viewfinder you have to take the viewfinder magnification into consideration when comparing directly with binoculars and, of course when you print your photo there's also the matter of whether you crop the frame, how large you print it and at what distance you view it to consider, as well!

Terry

Depends on the camera make. Assuming a Nikon this has a crop factor of 1.5 which has to be taken into account.

The calculation is Focal Length of lens X Crop factor. Then divide this by 50mm which is the same as a 1:1 magnification.

So for a 300mm used on a Nikon DSLR -

300X 1.5 = 450mm

Divide this by 50mm

This gives a magnification of 9 which would equate to a binocular rated at 9X.

On a Canon camera with a crop factor of 1.6, the 300mm lens would give (300 X 1.6)/50 = 9.6.

Or, if you use my 43.5mm figure, you can have (near enough) 10x on a Nikon and 11x on a Canon!

Again, if you're using the camera as a sort of ad hoc binocular you need to consider the viewfinder magnification (or reduction, rather!). As an example, the spec sheet for my 20D says: viewfinder mag, 0.9x at infinity with a 50mm lens, but this doesn't take into account the 1.6x crop-factor - the 50mm is more akin to an 80mm in the 35mm full-frame format

Adey

No doubt your reasoning is more accurate but I am suggesting an easier rule-of-thumb method. You get a ball park figure.

Not sure whether 'near enough 10x' or '9.6x' is the ball park figure! but as you say an easy rule-of-thumb guide is more useful.

I've got a 400mm lens and I reckon it gives me something reasonably close to what I see through my 10x bins. Nominally, with the 1.6x crop factor it gives a higher magnification, but you always concentrate your gaze onto the subject at the centre of the bins, ignoring the outer area, anyway, so from a practical point it's about right

Adey

Its an interesting point you raise about the impact of the viewfinder in magnification issues. I must admit I'd never considered it.

The other side of the coin. I use a 10x optical digicam with a 1.7x tele convertor giving me 17x optical. What's this in mm?

We will need to know the model of camera because the lens and sensor details are required to calculate the equiv 35mm figures.

Most DSLRs with a 'cropfactor' seem to have smaller viewfinders than full frame models but I've got a Canon EOS50E film camera plus Olympus OM1 which are both 35mm full-frame cameras but the OM1's viewfinder image is noticeably bigger (and brighter).

Incidentally, if you've got something like the OM1 which has a range of interchangeable focus screens you can fit a 'clear' type and see just how bright your lenses really are - it comes as quite a shock to see how much light the normal screens soak up in order to give you a focussed image across the frame (you have to use the microprism in the middle of the clear screen to accurately determine correct focus).

I used to like the microprism/split screen focussing methods. As my eyes get worse - I'm very short sighted - these methods would be much better.

If it wasn't for autofocus, I'd be lost.

Ok Cheers
Its a Fuji S5500 and an Olympus Tcon 17 conertor..Does this help?

The actual lens on the S5500 is 5.7 - 57mm.

With the crop factor taken into account the 35mm Focal Length Equivalent is 37 - 370mm.

Now if you add the Olympus Tcon 17 TC to it (x1.7 ) it converts this to 62.9 - 629mm

Cheers Rob
I thought it was only around the 500mm mark..Im surprised.
Thanks

It's actually a crop factor and not a multiply ;)

Thank you all for this information! I was totally confused because printed on my lens is a red "1:4" and I had no idea what that meant since the lens is fixed focal length.

No, I use my binoculars for viewing. WHen I look through my D-50's viewfinder, it looks very tiny and I am lucky when I can see the bird at all. Unless I am very close to the subject, I can't even tell if the bird is in focus or not - I just guess and take a range of photos moving the focus a tiny bit each time. Autofocus never works because there is always brush and branches in the way.

I do miss the view finder on a full frame camera, as you say the view on the smaller formats is tiny. Many a time I have missed something in the background I did not want in the photo. For example a for sale sign appearing above somebodys head. You get it on the PC monitor and it is plain as day. Dam and blast and a few other bad words are expleated.

You might try looking up the specs for your lens and camera to find that information. For example the following link will give that info for Canon lenses.

http://www.the-digital-picture.com/C...ion-Value.aspx