4.1 - Magnification Flashcards
Why consider image size?
- Leads to important results about blur = understand refractive error
- Gives an insight into the magnification produced by different refractive corrections (spectacles, contact lenses, refractive surgery) – don’t all produce same level of magnification
- Has direct clinical relevance to aniseikonia
What is transverse magnification
- Ratio of image height to object height ( h )
- Gives a relative size of how big image ( h’ ) is compared to object
What is equation for transverse magnification - for finite object
M = L/L' M = h'/h
What can the equation for transverse magnification be applied to
Any single surface or thin lens ( one surface where refraction takes place )
Why is transverse magnification called transverse
As its measuring image and object heights across the optical axis i.e. vertical heights
What values do we need to find out mag and image height
L and L’
How to work out retinal image height
- Object location l: L = n/l SO l = n/L
- Refraction at eye ( image vergence ): L’ = L + Fe
- Magnification: M = L/L’
- Retinal image height: h’ = M X h
What is the power of the eye
+60D
Is eye low or high power
High
What is magnification for real objects forming real images on or near the retina
- Negative therefore image on back of eye from real object is inverted
- Less than 1 cause power is high and positive = retinal image is much smaller than the object looking at
What does image on retina have to be for it to produce a response
Real
What happens in multiple surface/thin lenses
Image formed by one surface acts as object for the next ….
-If we can work out what the 1st lens does in terms of image it produces we take that info/image and use it as the object for the next surface and keep going through the surfaces all the way through the system
What happens when sp lens in front of reduced eye
Have 2 surfaces to take into account
How do you work out magnification for finite object for multiple surfaces i.e sp lens before eye
- Object location l1: L = n/l SO l1 = n/L
- Refraction at lens ( image vergence ): L’1 = L1 + Fsp
- Step along i.e. vergence at eye: L2 = L1’/(1 − dL1’)
- Refraction at the eye: L2’ = L2 + Fe
- Magnification: M = L1/L1’ X L2/L2’
What is finite object transverse magnification equation for multiple surfaces
M = L1/L1’ X L2/L2’