Lenses terms

[mutukumaar]

hi i m very new to DSLR and i need help understanding the terms used in the lenses e.g 75-300mm, 4.5 - 5.6. pls advise. thanks

[DonSchap]

Originally Posted by mutukumaar

hi i m very new to DSLR and i need help understanding the terms used in the lenses e.g 75-300mm, 4.5 - 5.6. pls advise. thanks

Well, 75-300mm describes the lens' focal length or "reach" A standard or "normal" lens is roughly 50mm and it can see roughly what the human eye sees. When you jump to 75mm, you begin to get into the "telephoto" classification. The image begins to look closer, but in reality ... you are seeing progressively less of the overall image as you scope in on it. The lens allows you to apparently "grow" the image as you "zoom in", which is the term for transitioning from one focal length to another.

At 75mm ... the subject seems further away. As you rotate the zoom ring of the lens towards 300mm, the subject appears to rapidly be drawing closer, allowing you closer inspection of it.

f/4.5-5.6 is the change in "base aperture" that lenses goes through as you zoom from 75-300mm or whatever range they are. Because the lens is lengthening, the amount of light down the lens tube is decreasing.

A good way to understand this change is to get a toilet paper roll tube and a paper towel roll tube. They are the same diameter inside. Place your eye at the end of the shorter tube and look down it. Then do the same by looking down the longer roll tube. The light appears diminished, even though it is the same amount of light at the other end. So it happens with the lens you speak of. As you lengthen it to 300mm ... the light decreases accordingly. At 75mm, you can open the aperture to f/4 (its "brightest setting"), at 300mm ... you can only get f/5.6 out of the lens, as its "brightest setting" as the lens tube has lengthened and "naturally" gotten less light.

I hope this helped.

A more detailed and diagramed explanation can be found HERE (<-click this)