fstops/aperture values
Posted 08/03/2009 - 21:28
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Perhaps it refers to the use of zooms, where the set aperture value could be say f8 but the actual f stop will vary as you zoom.
If this is the case, with a prime the values should always be the same.
If this is the case, with a prime the values should always be the same.
Best regards, John
Posted 08/03/2009 - 21:43
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The photos were taken on a zoom lens john so you may be right there ... It just confused me as I would have thought if the f-stop was set at F8 it would not differ.
Pentax K7 and K5 and several black tubey things with magnifying glass in that twist around and move in and out - to be honest I am not sure exactly what I have. Will list the proper names one day..
Posted 08/03/2009 - 23:21
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f-stop is also called focal ratio.
It's the ratio between the focal length and the diameter of the entrance pupil (aperture).
finally something that's easy to explain
It's the ratio between the focal length and the diameter of the entrance pupil (aperture).
finally something that's easy to explain
Stefan
K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Posted 09/03/2009 - 08:29
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I'm glad Pete asked as this has confused me also.
So, just to take this one step ( or should that be stop? ) further; why are the aperture settings marked as f3.5, f22 etc if they are not directly the f-stops? Or is this a throw back to the early days when all lenses would have been fixed focal length.
Also, again from looking at older lenses, I'd been led to believe that the f-number was the approxomate DoF range. So, at f22 I'd get 22metres in focus spread out as 1/3 in front of object and 2/3 behind. Is that correct?
So, just to take this one step ( or should that be stop? ) further; why are the aperture settings marked as f3.5, f22 etc if they are not directly the f-stops? Or is this a throw back to the early days when all lenses would have been fixed focal length.
Also, again from looking at older lenses, I'd been led to believe that the f-number was the approxomate DoF range. So, at f22 I'd get 22metres in focus spread out as 1/3 in front of object and 2/3 behind. Is that correct?
Getting there! Thanks to you guys
Pentax K3ii, Pentax K10d, Kit lens ( 18-55mm ), 50mm f1.7 lens, Tamron 70-300mm lens, Prinzflex 70-162 manual lens, Various old flashes.
Pentax K3ii, Pentax K10d, Kit lens ( 18-55mm ), 50mm f1.7 lens, Tamron 70-300mm lens, Prinzflex 70-162 manual lens, Various old flashes.
Posted 09/03/2009 - 09:45 - Helpful Comment
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Quote:
So, at f22 I'd get 22metres in focus spread out as 1/3 in front of object and 2/3 behind. Is that correct?
No.So, at f22 I'd get 22metres in focus spread out as 1/3 in front of object and 2/3 behind. Is that correct?
The f stop is a ratio, f1.4 is one stop wider than f2 and we know that the square root of 2 is 1.4 (OK, 1.414 or so before someone jumps on that!) so that shows the mathemeatical relationship.
If you have a simple lens of 25mm diameter and a focal length of 50mm this nice easy example is f2. (50 divided by 25)It's as simple as that. The beauty is that it holds true for any lens of any foacl length. If the same 25mm diameter lens had a focal length of 100mm it would be f4. (100 divided by 25). So to make a f2 100mm lens we would need a lens diameter of 50mm...much larger glass.
This is why long fast lenses are enormous and wide angle fast lenses can be much smaller. This latter point falls apart because SLR wide angles have to be retrofocus in design to clear the flipping mirror, so the front glass can be quite large, but the actual exit aperture (look through the back of the lens) is small.
Apertures like f3.5 are half values. The full sequence is 1.4, 1.7, 2, 2.3, 2.8, 3.5, 4, 4.5, 5.6, 6.3, 8, 9.5, 11, 12.5, 16........etc. Some photographers (and Canon) will "simplify" things by calling apertures things like "f 8 and a half" but that doesn't really help to clarify!
T stops are transmission stops, sometimes found on old cameras. A four element uncoated lens might lose 35% of its light to internal reflection, so a physically sized f2.8 lens might have an actual transmmission of T3.5 or T4. This isn't a problem with SMC lenses, but even 0.2% per surface loss can mount up in a 15 element lens. Auto exposure takes care of any slight error because of light loss. So T stops are now obsolete.
I should have made this an article....
Best regards, John
Posted 09/03/2009 - 10:22
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Thanks for clearing that up John.
I think
So is there any way to know from an F stop what my DoF would be at any given distance? Or does that come from experience?
I think
So is there any way to know from an F stop what my DoF would be at any given distance? Or does that come from experience?
Getting there! Thanks to you guys
Pentax K3ii, Pentax K10d, Kit lens ( 18-55mm ), 50mm f1.7 lens, Tamron 70-300mm lens, Prinzflex 70-162 manual lens, Various old flashes.
Pentax K3ii, Pentax K10d, Kit lens ( 18-55mm ), 50mm f1.7 lens, Tamron 70-300mm lens, Prinzflex 70-162 manual lens, Various old flashes.
Posted 09/03/2009 - 10:48 - Helpful Comment
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Quote:
So is there any way to know from an F stop what my DoF would be at any given distance?
Yes. Look up 'hyperfocal distance calculator' on google.So is there any way to know from an F stop what my DoF would be at any given distance?
Although the Hyperfocal distance is geared for determining how to get your entire image "in focus", the calculators will often give you the DOF for any general focus and aperture configuration.
A good example is here:
e.g. http://www.dofmaster.com/dofjs.html
Posted 09/03/2009 - 11:40
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Thanks
Getting there! Thanks to you guys
Pentax K3ii, Pentax K10d, Kit lens ( 18-55mm ), 50mm f1.7 lens, Tamron 70-300mm lens, Prinzflex 70-162 manual lens, Various old flashes.
Pentax K3ii, Pentax K10d, Kit lens ( 18-55mm ), 50mm f1.7 lens, Tamron 70-300mm lens, Prinzflex 70-162 manual lens, Various old flashes.
Posted 09/03/2009 - 11:55 - Helpful Comment
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Every lens will have a Depth of Field table and someone recently gave us a link to the Pentax website which apparently has them all.
Older instruction manuals used to contain DOF tables.
Older instruction manuals used to contain DOF tables.
Best regards, John
Posted 09/03/2009 - 12:13
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Thank you John .. You really are a mine of information.
At the risk of sounding pedantic (which I'm not being) Why would you need to know all of that information (which I can't get my head around any how ) And what sort of situations would it be used in please.
At the risk of sounding pedantic (which I'm not being) Why would you need to know all of that information (which I can't get my head around any how ) And what sort of situations would it be used in please.
Pentax K7 and K5 and several black tubey things with magnifying glass in that twist around and move in and out - to be honest I am not sure exactly what I have. Will list the proper names one day..
Posted 09/03/2009 - 12:14
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Sorry to be picky, John, but T stops are far from osolete. They are used on motion picture camera lenses. The T stop for setting aperture and the F stop for calulating DOF. There can be quite a difference the CZ 25mm f1 (yes f1) Distagon for Super 16 has an f stop of 1 and a T stop of 1.2 The lens has indices for both values.
Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -
Posted 09/03/2009 - 12:17
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At the risk of sounding pedantic (which I'm not being) Why would you need to know all of that information
It's like all knowledge, it's interesting. But as to why in practice, well I've never referred to a DOF table yet. If I had a very specific DOF requirement that might be different, but I don't.At the risk of sounding pedantic (which I'm not being) Why would you need to know all of that information
The rule of thumb to use a small aperture and focus one third of the way into a scene is as much as I ever use. I even use that for small product shots - look at my eBay page for over 100 current examples!
Best regards, John
Posted 09/03/2009 - 13:23
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Sorry to be picky, John, but T stops are far from osolete. They are used on motion picture camera lenses. The T stop for setting aperture and the F stop for calulating DOF. There can be quite a difference the CZ 25mm f1 (yes f1) Distagon for Super 16 has an f stop of 1 and a T stop of 1.2 The lens has indices for both values.
No problem, but I thought I'd covered enough without bringing in Movie lenses as well...as far as amateurs and still cameras go they are obsolete.Sorry to be picky, John, but T stops are far from osolete. They are used on motion picture camera lenses. The T stop for setting aperture and the F stop for calulating DOF. There can be quite a difference the CZ 25mm f1 (yes f1) Distagon for Super 16 has an f stop of 1 and a T stop of 1.2 The lens has indices for both values.
I suppose some collectors will use cameras with T stops though, and some will use cameras with non-standard aperture values and shutter speeds. It gets more and more complicated the more you think about it.
Best regards, John
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18 years
Plymouth UK