Lens / Aperture Question

[Tim Spencer]

Hello all:

I have a question on largest apertures for lenses. What is it exactly that determines what the largest opening (smallest f-number) can be on a particular lens? I know fast lenses (i.e. f1.4) are expensive, but I don’t know why.

For example, I have a Canon 75 – 300 f4–5.6. What is it that makes it so that the largest opening of this lens is only f4? What precludes a larger opening?

Hope this makes sense!

Thx,

Tim

[RichardTaylor]

Basically you need big pieces of high quality glass to let more light in.
That ups the cost and if you have a lot of money in the glass you may as well make the whole lens better mechanically (and possibly weathered sealed) as well as making them a lot faster to focus and possibly adding image stabilisation as well.

[Thunder_o_b]

What Richard said.

[cleamon]

Originally Posted by Tim Spencer

Hello all:

I have a question on largest apertures for lenses. What is it exactly that determines what the largest opening (smallest f-number) can be on a particular lens? I know fast lenses (i.e. f1.4) are expensive, but I don’t know why.

For example, I have a Canon 75 – 300 f4–5.6. What is it that makes it so that the largest opening of this lens is only f4? What precludes a larger opening?

Hope this makes sense!

Thx,

Tim

An f-stop is the ratio of focal length to lens opening (Aperture). In simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. For example, a 50mm lens with a 25mm opening is a 2.0 f-stop. If the opening is 12.5mm, then the f-stop is 4.0, etc. Going the other way, if the opening is 35mm on a 50mm lens, then the f-stop will be (approximately) 1.4. Obviously, to get a LOWER f-stop (larger aperture), you need to get a GREATER ratio, this more often than not means BIGGER glass -- more $$.
In the case of your 70-300, the ratio of the MAX aperture varies with the focal length to which you are zoomed. Zoom lenses with FIXED MAX aperture are generally more expensive.
Hope this helps.

[ravncat]

The only real factors limiting aperture size is diameter of the lens and how big you can make the front element. There are a few others if we care about projecting an image, as aperture exceeds f1 it becomes harder to correct distortions and keep a sharp image. Other practical factors include register distance (lens mount to sensor) mount diameter and weight . One could build a 500mm f1 but cost for the glass and weight would limit something like that to the observatories

[Tim Spencer]

Thank you all for answering - def. clears it up!