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

[Terry O'Nolley]

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!

[Adey Baker]

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!

[Adey Baker]

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!

[kennygee]

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.

[Adey Baker]

Originally Posted by kennygee

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

[kennygee]

Adey

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

[Adey Baker]

Originally Posted by kennygee

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

[kennygee]

Adey

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

[LSB]

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?

[robski]

Originally Posted by LSB

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.