# Aperture Help! Difference in Lenses?

Question by Tom G: Aperture Help! Difference in Lenses?
I am an amatuer photographer, and am still getting to grips with everything

I know that aperture affects the amount of light entering the camera, but what I want to know is the limits

For instance, my Dad recently purchased a 70-200mm F/4 zoom lens for his Canon.
This means that the minimum F stop is 4 right?
But that mean that it is the only one he can use? What if he wants to use higher than that? Can he?

There was a 70-200mm F/2.8 zoom lens as well (but it was almost 4 times the price)
I again assume that this lens can also shoot at f/4? Or if you want that do you have to go and buy 2 separate lenses? Equating to almost £2000??

Lastly, on some zoom lenses, the f/ value changes, why is this?

Thanks
Tom G
Thanks for the great answers, that is what I thought, though I was scared I had got it wrong!

Yes, you can adjust the aperture on almost every lens that exists. The f/value changes on some zoom lenses because the focal length changes.

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### 3 Responses to Aperture Help! Difference in Lenses?

1. Jens says:

Yes, that’s just the minimum aperture. A f/2.8 lens can close down to f/4 and all the way to f/22 or even less than that.

On many zooms the maximum aperture changes because it is a function of the focal length.

f/4 means that the aperture opening diameter is equal to the focal length divided by four. So on 50mm that’d be 50mm/4=12.5mm, but on 20mm it’d be only 20mm/4=5mm.

Since the size of the lenses is one of the most important factors in determining the price of a lens, many zooms have that variable aperture. It opens wide enough to allow for e.g. f/3.5 at 18mm, but the lens diameter is insufficient to allow for such a wide aperture at 55mm, Hence they use a variable aperture, because the same maximum aperture opening as measured in mm is a different value when measured in f-stops when different focal lengths are involved.

2. fhotoace says:

“Lastly, on some zoom lenses, the f/ value changes, why is this?”

Pure economics. It costs more to make a lens with a single aperture throughout its zoom range

Here is a link to all the Canon lenses. I think as you look at their specifications, you will see how the wider the maximum aperture is, the more the lens costs.

http://www.usa.canon.com/cusa/consumer/products/cameras/ef_lens_lineup

3. deep blue2 says:

“For instance, my Dad recently purchased a 70-200mm F/4 zoom lens for his Canon.
This means that the minimum F stop is 4 right?”
Yes that’s right – the widest aperture this lens is capable of (ie most light it can let in) is f4.

“But that mean that it is the only one he can use? ” No – the lens can be stopped down (reduce light) to probably around f22 or so.

“There was a 70-200mm F/2.8 zoom lens as well (but it was almost 4 times the price)” – these are so-called ‘fast’ lenses – the lens is capable of its maximum aperture (f2.8) throughout the zoom range – but as you’ve seen, these are expensive glass!

“I again assume that this lens can also shoot at f/4?” Yes – it can be stopped down to a small aperture (usually about f22) so no, you don’t need to buy another lens. It will perform better at f4 than an f4 lens (like your Dad’s), because lenses generally can be a bit ‘soft’ wide open. At f4, the f2.8 lens is not wide open.

“on some zoom lenses, the f/ value changes, why is this?” It’s cheaper to make a variable aperture zoom lens. THe aperture changes according to the focal length. For example, the kit lens 18-55mm is f3.5 to f5.6 – this doesn’t mean it can just close down between these two apertures!! What it means is – at 18mm, the lens is capable of apertures between f3.5 & f22, at 55mm it can only manage f5.6 to f22.