How to Figure Out the Lens Multiplication Factor

Hi,

Another noob question.

I have a 7D with a 17-55mm lens. How do you figure out what the multiplication factor is?

For example, I have a telescope with a 1500mm focal length. If I put a 5mm eye piece in it I get a multiplcation factor of 300x (1500/5).

How do you do this for a camera and lens?

I'm asking because I want to know how much closer (bigger a 100mm Macro lens is gonna get me as the 55mm end of my current lens is woefully under powered for real close up macro work. I would like to see the tiniest bug eyes!!

General_T wrote in post #13153002
How do you do this for a camera and lens?

You don't.

Some folks seem to think that what they see through the viewfinder (relative size of the object in the viewfinder as opposed to what they see with their naked eyes) would be the basis for a multiplier based on the focal length attached to their camera. That's wrong, though, because different cameras have different magnification optics in their viewfinder systems.

The most common method that would be used to come up with a magnification factor is to compare the focal length being used to the "normal" focal length (approximately equal to the diagonal of the film frame or sensor).

The bottom line is that I've never once talked to an experienced photographer who ever really considers an "X" power number for any lens.

If you want to compare two focal lengths, you'll find that the size of the image of a subject changes pretty much linearly with focal length. For example, if an object takes up 1 inch horizontally in a 4x6 print made with a 50 mm lens, then it would be roughly 2 inches wide if you shot the same scene from the same location with a 100 mm lens.

On a full frame sensor, it goes roughly like this :
50 mm lens = x1
100 mm lens = x2
500mm lens = x10
Etc ...

TC_Fenua wrote in post #13153202
On a full frame sensor, it goes roughly like this :
50 mm lens = x1
100 mm lens = x2
500mm lens = x10
Etc ...

Thanks for the replies;

So, if i get a 100mm Macro lens I can only expect to be able to double the size of an object shot at 55mm with my 17-55?

Yes, approximatively that, with a ratio of 1:1. Here is a good article on that, hope that helps

General_T wrote in post #13153292
Thanks for the replies;

So, if i get a 100mm Macro lens I can only expect to be able to double the size of an object shot at 55mm with my 17-55?

Lenses typically project an object onto the sensor about 1/6 to 1/4 the size as it is in real life. The 100mm macros are life size meaning that an object 1cm long will be 1cm long as projected on the sensor. That's 4-6x as large. The macro lens lets you focus much closer than a non-macro lens.

Edit: Just looked it up and your 17-55 IS has a max magnification of 0.17x so a 1:1 macro gets you an image 5.8x larger.

General_T wrote in post #13153292
Thanks for the replies;

So, if i get a 100mm Macro lens I can only expect to be able to double the size of an object shot at 55mm with my 17-55?

Yes, if the distance stays the same then an object photographed with a 100mm lens will take up twice as much of the frame as it would when viewed with a 50mm lens. Basic math.

At macro distances and when discussing magnification with macro lenses, things are a little different. You can have a 50mm macro, a 100mm macro and a 200mm macro that will all provide 1:1 magnification when they are set to their minimum focus distance. Since they are 1:1 the subject will be the same size regardless of which lens is used. The difference between them is that the longer focal lengths will give that amount of magnification longer distances so you can stay 12" away from that bug instead of 6", for example.

mike_d wrote in post #13153312
Edit: Just looked it up and your 17-55 IS has a max magnification of 0.17x so a 1:1 macro gets you an image 5.8x larger.

The above is correct when talking about using the lenses at their minimum focus distance, which is much closer when using the macro lens. If using them for portraits at 10 feet, the person will simply appear twice as large with the 100mm as with a 50mm.

you can use the calculations at the bottom of this page to show you what going from 55mm to 100mm will do for you:
http://www.juzaphoto.c​om …al_length_compa​risons.htm

but since you mentioned macro the 100mm macro will allow you to get a lot closer...it doesn't really have to do with the focal length more so the fact that it's a 1:1 macro lens....

Hi,

Thank you all.

I understand, appreciate the information.

Cheers