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can someone explain the purpose of constant f/4?

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backpackerPT said:
Thanks guys - this is very helpful.

my RX100 mk iii has a great lens that is nice and wide at the wide angle, but I see that it changes f-stop when i change focal length. A constant aperture lens would not do that...

is there any reason they call it constant f/4? is that because that's as wide as these lenses go? that seems awfully small if that's the case...or is it just a naming convention?
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Christof21 said:
Gerry Winterbourne said:
Christof21 said:
Personally, for my shooting, I don't really care whether the max aperture is constant or not. It does not prevent me from getting the full potential of the lens.
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If you think about it that statement is true of any lens, because the potential is what the lens is capable of delivering.
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My answer is about adjusting the exposure settings, the auto iso for instance makes it really easy to adjust it to its best.
Member said:
The difference between fixed and variable aperture zooms is that the fixed aperture ones have a greater potential (when the widest aperture is the same for both).
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Sorry, I don't understand what you mean. You mean constant max aperture ? Why greater potential ? Just curious to know, I am not aware of it.
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Yes; the thread is about constant max aperture lenses. What I mean is that if a lens has a max aperture of, say, f/2.8 at its widest zoom and f/2.8 at its longest zoom (constant aperture) while another lens has the same max aperture at its widest zoom but f/4 at its longest zoom (variable), the constant one will have a stop more potential at the longer end.

The actual limit doesn't matter - every shot and every user might have a different limiting light-gathering requirement; but whatever the limit is an extra stop gives greater potential.
 
backpackerPT said:
I see this for several zoom lenses - does this mean I have no other apertures other than f/4? or does it mean f/4 is the widest it goes at either end of the zoom?

Why is this a good thing?

Thanks, from a learning noob!
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Robyn Banks said:
Janoch said:
Christof21 said:
backpackerPT said:
I see this for several zoom lenses - does this mean I have no other apertures other than f/4? or does it mean f/4 is the widest it goes at either end of the zoom?

Why is this a good thing?
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It is not really an advantage anymore. I don't think people buy a lens because it has a constant aperture...
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Some of us actully do! :-P

Got tired of ev compensating, so ended up in manual mode. Which forced me into constant aperture lenses... was going nuts having to change exposure just because of a little zooming...

So a big advantage here! :-D
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Constant aperture
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'Constant f-number' really, of course as the focal length changes, the aperture has to to keep the f-number the same.
Robyn Banks said:
is a huge advantage, most of us buy them for that very reason.
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Well, depending on what the constant f-number is. I would say that an f/2.8-4 has it over a constant f/4, all else being equal.
Robyn Banks said:
Between my 12-40 and 40-150 I can shoot at F2.8 right from 12-150mm (24-300 eq)

I expect I'll also be getting a 7-14 f2.8 (14-28) before too long.
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Now that Sigma's pushed past the f/2.8 barrier, I wonder how long before someone works out that while an f/1.8 constant might be too large and expensive to be practicable, there's mileage in an f/1.8-2.8.
 
bobn2 said:
Now that Sigma's pushed past the f/2.8 barrier, I wonder how long before someone works out that while an f/1.8 constant might be too large and expensive to be practicable, there's mileage in an f/1.8-2.8.
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For sure there is mileage in going the wrong direction

This is as practical as mega-megapixel sensors, yet 'pro'sumers lap this numbers stuff up. Go figure.

I don't know if it's funny or just plain destructive, watching all these idiots chasing their tails to own >20 megaportions and faster than the weight of lead lenses just so they can take a snapshot to print on crappy quality canvas or just to be viewed on a monitor with a fraction of the resolution.

Any rational explanation?
 
bobn2 said:
Jacques Cornell said:
backpackerPT said:
I see this for several zoom lenses - does this mean I have no other apertures other than f/4? or does it mean f/4 is the widest it goes at either end of the zoom?

Why is this a good thing?

Thanks, from a learning noob!

--
https://backpackerpt.smugmug.com
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These constant f-number zooms really test out the terminology
bobn2 said:
Others have already provided good explanations that I won't duplicate here. I'll just note that generally a constant-aperture zoom has a bigger aperture at the long end of it's focal length range than a variable-aperture zoom of the same range.
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I'm guessing you meant 'bigger f-number'
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No, I meant what I said, a bigger aperture. A bigger aperture has a smaller f-number. An f4 constant zoom has a bigger max aperture (f4) at the long end than a variable f3.5-5.6 zoom (f5.6).
bobn2 said:
though what you said is generally the case.
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I did say "generally".
bobn2 said:
So, if you took something like a 24-85/3.5-4.5., its aperture at 24 is 6.9mm and the aperture at 85 is 18.9mm
bobn2 said:
It's common to see constant f4 and variable f3.5-5.6 zooms of the same range. "Fast" zooms tend to be constant-aperture, and it's often pros who want the big apertures,
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See now where the confusion's leading you. You used 'big apertures' in the opposite sense to the previous statement.
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No, I didn't. I think you need to read it again. In both cases, a "big" or "bigger" aperture is one that's physically larg(er) / bright(er) / "fast(er)".
bobn2 said:
bobn2 said:
so constant-aperture is often associated with "pro" quality, and constant-aperture zooms tend to be bigger and more expensive.

Aside from their generally being faster and of high quality, I can't think of any other advantages to constant-aperture zooms. If you've got an f3.5-5.6 zoom and you don't want the aperture to change as you zoom, all you have to do is manually set the aperture to f5.6 or smaller.
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Moder cameras, with the f-number set from the body, keep the f-number the same as you zoom. The only advantage is that you lose availability of the wider aperture at the short end.
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Losing the wider aperture is an "advantage"?

--
The way to make a friend is to act like one.
http://jacquescornell.photography
 
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Andy Cann said:
bobn2 said:
Now that Sigma's pushed past the f/2.8 barrier, I wonder how long before someone works out that while an f/1.8 constant might be too large and expensive to be practicable, there's mileage in an f/1.8-2.8.
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For sure there is mileage in going the wrong direction
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It depends what you think is 'the wrong direction'.
Andy Cann said:
This is as practical as mega-megapixel sensors, yet 'pro'sumers lap this numbers stuff up. Go figure.
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mega-megapixels sensors give better pictures, just like mega-mega-sharp lenses. Whethe people need pictures that better is another question.
Andy Cann said:
I don't know if it's funny or just plain destructive, watching all these idiots chasing their tails to own >20 megaportions and faster than the weight of lead lenses just so they can take a snapshot to print on crappy quality canvas or just to be viewed on a monitor with a fraction of the resolution.

Any rational explanation?
Click to expand...
Yup. If camera specs were trimmed to the capabilities of most photographers, the camera companies wouldn't sell any cameras.
 
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