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6 Days Ago - 1 Like   #1
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Does zoom lens change physical aperture during zoom ?

Hi there,

I just find that by looking into the lens my DA 17-70 has 7 blade aperture, which is perfectly round wide open only from 60mm and up, is this normal or just my lens ?

6 Days Ago   #2
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Lenses have different aperture blade configurations. Some will be round at certain focal lengths others all the way through the range, others might be pentagonal or some other shape. I don't have DA 17-70 to check but if the lens is operating properly I would say that is normal. Hopefully someone with the lens can verify.
6 Days Ago - 3 Likes   #3
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It's normal for a zoom lens. The f-stop is a ratio of focal length to area of the opening. If the focal length changes the physical opening of the aperture has to change correspondingly.
6 Days Ago   #4
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QuoteOriginally posted by lotech Quote
Hi there,

I just find that by looking into the lens my DA 17-70 has 7 blade aperture, which is perfectly round wide open only from 60mm and up, is this normal or just my lens ?
Your lens is a constant aperture zoom lens. When you change focal length, your lens must adjust to maintain the same effective f/stop. Because it has seven straight blades, that's why you're seeing a heptagon instead of a circle when wide open and increasing the focal length.


Last edited by Alex645; 6 Days Ago at 02:29 PM. Reason: correction; my bad
6 Days Ago - 1 Like   #5
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At 17mm the entrance pupil needs to be. 17/4=4.25mm. At 70mm it needs to be 70/4=17.5mm. If it stayed at 4.25mm then at 70mm you would have, 70/4.25=f/16.47. Also if it stayed at 17.5mm at 17mm you would have a very fast lens. 17/17.5=f/0.97.
The entrance pupil and physical aperture are not the same, but related. The optics do change the appearance of the physical aperture so this is simplified.
6 Days Ago - 1 Like   #6
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QuoteOriginally posted by Alex645 Quote
Your lens is a constant aperture zoom lens. When you change focal length, your lens must adjust to maintain the same effective f/stop. Because it has seven straight blades, that's why you're seeing a heptagon instead of a circle when wide open and increasing the focal length.
Does that mean in order to maintain constant aperture through the range, the lens has to be designed to stop down a little at the wide end ? I've no other zoom of similar range so can't compare, and what about zoom with non fixed aperture ? I just thought that all lenses should be designed to make the best use of the max aperture even not preferable in terms of image quality, but aid focusing.
6 Days Ago   #7
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QuoteOriginally posted by lotech Quote
Does that mean in order to maintain constant aperture through the range, the lens has to be designed to stop down a little at the wide end ? I've no other zoom of similar range so can't compare, and what about zoom with non fixed aperture ? I just thought that all lenses should be designed to make the best use of the max aperture even not preferable in terms of image quality, but aid focusing.
Yes, @lotech you can see for yourself how your aperture stops down when at the wide end.

On a non-constant aperture zoom, like a common 18-55mm zoom f/3.5-5.6, the aperture itself is fixed and thus when you do the math, at one focal length it's f/3.5, at others it's f/4 or f4.5 or f/5.6, etc.

The confusing part about this is that constant aperture zooms have wide apertures diameters that are not constant while on non-constant aperture zooms, the diameter of the aperture is actually constant or fixed.

Although light transmission and large apertures are an important criteria, it is only one aspect the engineer must consider when designing the optics.
5 Days Ago   #8
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QuoteOriginally posted by lotech Quote
I just thought that all lenses should be designed to make the best use of the max aperture even not preferable in terms of image quality, but aid focusing.
When the focal length is shorter than the registration distance you need a reverse telphoto design to bring the lenses in front of the camera. This takes some complex designs. Canon has a 28mm f/2.8 that has 5 group of 5 lens construction. To make their 28mm f/1.8 requires 9 group 10 lens construction.
More info here, but not much more.
lens - Why do wide angle prime lenses have relatively small apertures? - Photography Stack Exchange

5 Days Ago   #9
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Thank guys for the detailed explanation, that is more than I expected on lens design great to learn more !
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