# Total focal length and magnification

• I
• Aymangh994
In summary, you want to make an image of a cantilever with dimensions of (450x50) microns, using a self-constructed telephoto lens with a magnification of 4 to 8. The distance between the two lenses and the distance between the CCD-Sensor and the last lens need not be specified.

#### Aymangh994

I would like to realize an image of the cantilever. The dimensions are (450x50) microns. I would like to use a self-constructed telephoto lens. This consists of two lenses. The image magnification can be 4 to 8. The magnification of the image is nothing more than the ratio between the two focal lengths of the two lenses. E.g. for f1= 320mm f2=40mm the total f= 35.5mm and the imaging magnification is 8 . Since typical focal lenghts of telephoto are over 50mm, I would like to ask if the total focal length matters? I.e. for f1= 800mm and f2=100mm the total focal length is f=88.89 mm but the image magnification remains the same.

davenn
You need to specify more about the set-up. What is "the cantilever"? We need an annotated diagram if you want sensible answers.
Edit: PS I think you are assuming the 'telescope formula' applies but your object distance is not infinite and are you looking at an image or projecting it onto a sensor?

davenn and hutchphd
I would like to ask if the total focal length matters using two different combinations of lenses but the magnification still the same.
Combination 1 : f1= 320mm f2=40mm the total f= 35.5mm and M= 8
Combination 2 : f1= 800mm and f2=100mm the total focal length is f=88.89 mm and M= 8
and how to calculate the working distance ?

Sketch
following questions :
1. should I use the lensmaker's equation to calculate the working distance ? which focal length should I insert in the equation? the total f or f1 ?
2. The distance between the two lenses and the distance between the CCD-Sensor and the last lens ?

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You seem to be using a telescope rather than a microscope. In such a case, the objective lens should be as large as possible to collect maximum light, but may have to be stopped down to reduce distortion. The required magnification is very small - can this be obtained with a single lens?

Aymangh994