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12-08-2014, 01:16 PM   #1
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constant vs variable aperture

So what I think of as a variable aperture would be the kit lens as it goes from f3.5-f5.6... But in reality wouldn't the 17-70 f4 be considered variable because f4 is different at 17 than it is at 70 right? I understand that the f number is constant even though the aperture is changing. Am I overthinking something. I am fine when I think of a prime lens like 50mm 1.4 but the zoom is confusing me.

12-08-2014, 01:22 PM   #2
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QuoteOriginally posted by soycory Quote
But in reality wouldn't the 17-70 f4 be considered variable because f4 is different at 17 than it is at 70 right? I understand that the f number is constant even though the aperture is changing. Am I overthinking something
Nope, just misunderstanding. The f-number is the aperture. Constant aperture means that at the same shutter speed and ISO, f4 (for example) will give you the same exposure (brightness). Focal lengths is irrelevant. So this is why it is called constant aperture, because you can use the same minimum aperture for all focal lengths. Variable aperture means you can choose f3.5 at one end, but only down to f5.6 at the other end, for example.
Aside from that, high end zoom lenses are usually constant aperture, like f4 or f2.8. But cheaper, consumer zooms are variable (guess they are cheaper to make, but they are also much more limited, because one end of the zoom is darker, slower).
12-08-2014, 01:26 PM   #3
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Constant aperture is defined as a lens having the same aperture at all focal lengths. So your 17-70 f4 lens would be a constant aperture lens because if you set it to f4 at 17mm and zoom to 70mm the aperture would still be f4. Your kit lens example is a variable aperture because if you set the kit lens to f3.5 at 18mm and zoom to 55mm the f3.5 will change to f5.6.

Constant aperture zoom lenses are harder to produce and cost more...and, in general are better lenses.
12-08-2014, 01:26 PM   #4
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QuoteOriginally posted by Na Horuk Quote
Nope, just misunderstanding. The f-number is the aperture. Constant aperture means that at the same shutter speed and ISO, f4 (for example) will give you the same exposure (brightness). Focal lengths is irrelevant. So this is why it is called constant aperture, because you can use the same minimum aperture for all focal lengths. Variable aperture means you can choose f3.5 at one end, but only down to f5.6 at the other end, for example.
Aside from that, high end zoom lenses are usually constant aperture, like f4 or f2.8. But cheaper, consumer zooms are variable (guess they are cheaper to make, but they are also much more limited, because one end of the zoom is darker, slower).
so it's a constant aperture (rating) even though the aperture opening is changing...

12-08-2014, 01:33 PM   #5
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QuoteOriginally posted by soycory Quote
so it's a constant aperture (rating) even though the aperture opening is changing...
I don't know what you mean with that. The aperture is not changing. What is changing is the focal length. Aperture: F-number - Wikipedia, the free encyclopedia
Aperture = focal length / diameter

The only thing is that a constant aperture zoom lens might not use the same diameter of the optics throughout the zoom range. But the actual aperture (ratio of focal length and diameter) stays constant. The aperture is a rating, it is a ratio, and it is used because it gives a certain EV (exposure value), which can be compared to shutter speed and ISO (you can raise aperture one stop and shutter speed one stop in the other direction to get the same total exposure/brightness)
12-08-2014, 01:37 PM   #6
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soycory may be thinking of the t-stop (actual light transmission). That can change from one end to the other of a constant-aperture zoom lens.

My Tamron 17-50, even though it is a constant f2.8, will change ISOs as I zoom in or out (ISO increases as I move towards 50mm). I assume it's because the lengthening of the barrel causes less light through, so the camera has to compensate.

Maybe someone more knowledgeable in this regard can chime in.
12-08-2014, 01:45 PM   #7
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The diaphragm of the Da*16-50 is not fully open at 16 mm. It is a 16-F2 to 50f2.8 non constant aperture zoom, and the diaphragm is used to change it to constant f2.8 zoom.
12-08-2014, 01:49 PM   #8
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QuoteOriginally posted by soycory Quote
so it's a constant aperture (rating) even though the aperture opening is changing...
The f-stop is the thing that's constant. "Aperture" can refer to the hole the physical light goes through, but it's also used to refer to the f-stop in the photography world. Note the entrance pupil (how the 'hole' looks from the front of the lens) does change size as you zoom.

12-08-2014, 01:54 PM   #9
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QuoteOriginally posted by soycory Quote
so it's a constant aperture (rating) even though the aperture opening is changing...
For discussions of exposure, DoF, etc., the absolute aperture width per se does not matter; only its relation to the focal length does. That's why aperture is always expressed as a ratio: F / whatever.
12-08-2014, 01:55 PM   #10
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QuoteOriginally posted by EarlVonTapia Quote
soycory may be thinking of the t-stop (actual light transmission). That can change from one end to the other of a constant-aperture zoom lens.

My Tamron 17-50, even though it is a constant f2.8, will change ISOs as I zoom in or out (ISO increases as I move towards 50mm). I assume it's because the lengthening of the barrel causes less light through, so the camera has to compensate.

Maybe someone more knowledgeable in this regard can chime in.
I'm saying at 17mm f4 is 4.25 mm aperture opening while at 70mm f4 is 17.5 mm. If the aperture was actually constant (not just the value) and could stay 17.5 through all focal lengths, at 17mm the lens would almost be f1.0.

As the value remains constant at f4.0 the aperture value is staying the same while the aperture is changing
12-08-2014, 01:57 PM   #11
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QuoteOriginally posted by goubejp Quote
Sometimes they are chitting...
The diaphragm of the Da*16-50 is not fully open at 16 mm. It is a 16-F2 to 50f2.8 non constant aperture zoom, and the diaphragm is used to change it to constant f2.8 zoom.
Very interesting - I've always found that constant aperture zooms seem to have better IQ than variable aperture ones. Many forum members respond to me that theoretically it should make no difference. But the DA*16-50 is weakest at 16mm already. I certainly wouldn't want to shoot it at f/2 - even if I could. So perhaps this "artificial limiting" of the aperture ends up being a benefit anyway, and there are legitimate reasons why fixed aperture zooms almost universally end up being the better quality ones!
12-08-2014, 02:08 PM   #12
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I thought that the absolute aperture diameter could change with different focal lengths depending on the lens design. However, the focal length ratio will always be f/4.0.
12-08-2014, 02:39 PM   #13
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QuoteOriginally posted by 6BQ5 Quote
I thought that the absolute aperture diameter could change with different focal lengths depending on the lens design. However, the focal length ratio will always be f/4.0.
And that is correct! I believe the term "constant aperture" as used in this thread is misleading. What is constant is 'the focal ratio' or 'f-number' but NOT the absolute diameter of the aperture as measured in milimeters or inches.

Actually, we are talking about the 'entrance pupil' and with a constant focal-ratio zoom lens one can easily convince oneself that the aperture = entrance pupil does change with focal length: Just look down the lens (wide open) through the front element of the lens as you zoom in and out. It follows the fomula (entrance pupil diameter) = (focal length)/(f-number)
12-08-2014, 02:48 PM   #14
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I've always wondered why Constant Aperture (constant f-stop) lens are built to have a constant f-stop. If you think about it, a 17-50 mm constant aperture f/2.8 lens in theory can open up to a diameter of 18 mm based on f/2.8 and 50 mm. If the lens can open up to the absolute 18 mm, then when zoomed at 17 mm, you would essentially have an f/1 lens.

That sounds appealing, but image quality would likely suffer and the depth of field would be quite shallow even at a focal length like 17 mm. You'd probably end up stopping down all the time anyway.

I'm guessing that is what has led to just leaving things as constant aperture for the high level lenses anyway. The lenses probably wouldn't be too good anyway and would lose their value. That's probably part of what make variable aperture lenses cheap too, or the fact that they don't have to build in the constant f-stop.
12-08-2014, 03:17 PM   #15
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QuoteOriginally posted by emalvick Quote
If you think about it, a 17-50 mm constant aperture f/2.8 lens in theory can open up to a diameter of 18 mm based on f/2.8 and 50 mm. If the lens can open up to the absolute 18 mm, then when zoomed at 17 mm, you would essentially have an f/1 lens.
Unfortunately, wide angle lens designs are much more complicated than that. In particular when we talk about zooms: As you reduce the focal length you need a lot of elements and lens groups to "project" the image over a constant flange/registration distance of some 45mm. Just look at fast, ultrawide primes: They are enormous, bulky and heavy beasts. Now, try to build such design into a reasonably compact 17-50mm zoom with constant focal ratio...........

The entrance pupil is very rarely the same as the diameter of the lens front element of a prime and it never is neither for any wide angle prime nor for any zoom.
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