Basic Questions on Aperture and DOF


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Tweek

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#1
Hi guys, got some really simple questions, hope someone can help me out.

1) I read that the f number is obtained from dividing the focal length of the lens by the physical aperture diameter, is that right? Does that mean that for lenses of the same focal length and same f number, the physical aperture size is the same?

2) Is DOF wholly determined by the physical size of the aperture, assuming that lens-to-subject distance and subject-to-background distances remain constant?

3) This one is related to qns 1 & 2. Say I use a 28-90 f4-5.6 lens and a 28-70 f2.8 lens, and take the same scene at 50mm f5.6 for both lenses. Will the DOF be the same for both pics?

T.I.A!
 

YSLee

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#2
1. Yes.
2. Yes.
3. Yes.

All theoretical, some design or manufacturing compromise might come into play.
 

beachbum

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can someone enlighten....

Canon can make faster lens because of larger lens mount which translate to larger rear element which means bigger apeture size?
thus manufacturers with smaller lens mount like pentax, nikon are technically restricted and can never make a 50mm f1.0 or a 85mmf1.2 or 200mmf1.8 like Canon?

this is not meant to start a manufacturer vs manufacturer war.....
 

#4
Originally posted by beachbum
can someone enlighten....

Canon can make faster lens because of larger lens mount which translate to larger rear element which means bigger apeture size?
thus manufacturers with smaller lens mount like pentax, nikon are technically restricted and can never make a 50mm f1.0 or a 85mmf1.2 or 200mmf1.8 like Canon?

this is not meant to start a manufacturer vs manufacturer war.....
Not true, that's just Canon's marketing bs (sorry). Have you seen a Leica 50/1.0? That's dunno-how-many-times smaller than the Canon. ;)

Regards
CK
 

StreetShooter

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#6
Well, never seen either the Canon or Leica 50mm f1.0, but I think beachbum has a theoretical point there.

The Leica 50mm f1.0 is probably smaller because it does not have to house the AF motor (manual focus, is it not?).

All things being equal, a larger lens mount SHOULD allow a larger aperture. Not sure if there is a Nikon 200mm f1.8?
 

YSLee

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#7
There's a 200/2 sitting at Max Photo at Centrepoint.. and again there's the 300/2.
 

#10
Originally posted by StreetShooter
Well, never seen either the Canon or Leica 50mm f1.0, but I think beachbum has a theoretical point there.

The Leica 50mm f1.0 is probably smaller because it does not have to house the AF motor (manual focus, is it not?).

All things being equal, a larger lens mount SHOULD allow a larger aperture. Not sure if there is a Nikon 200mm f1.8?
If I didn't remember wrongly, a Canon EF 50mm f/1.0 is roughly the size of a 28-70/2.8.

AF motor affects overall size of the lens, but has no effect on exit diameter of the rear element or the front element. Both the Leica and non-Leica 50/1.0s has to cover the same 24mm x 36mm film area. So it's not true that you need to have a bigger lens mount to have a faster lens.

Nikon has a 200/2, which means aperture size is 100mm. The Canon equivalent 200/1.8 has aperture size of 111mm, not a lot bigger. And the 300/2 YS mentioned would have an aperture size of 150mm. So Nikon CAN actually have a 200/1.8 if they want. Or maybe even a 200/1.4.....

And yes, there are f/0.9 lenses around, forgot which camera and which mount though I remember it's not EF or F mount. One of those RangeFinders I think. :)

Regards
CK
 

Red Dawn

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#11
Originally posted by ckiang


And yes, there are f/0.9 lenses around, forgot which camera and which mount though I remember it's not EF or F mount. One of those RangeFinders I think. :)
yes, it can be found in one of those HEGR* :devil: :D :cool: :rbounce:

* HEGR - (From the street photography mailing list) High End German Rangefinders.
 

Red Dawn

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#12
Hi

btw i honestly never knew until today that Canon's lens mount is bigger than that of other camera makers, and that they actually use it in their collaterals and advertising material. Shows how much i know about competing products :p

while i don't know about whether there will be a limitation in aperture size where size of lens mount is concerned (i am thinking that should be, but not sure), i do think that 50 f1.0 lenses are not very practical for normal use at f1.0, whether small or not.

at 50mm and at f1.0, and at close focus distance (with a 50 u will be working quite close to your subject) the DOF is counted in terms of millimetres. Movement of a few millimetres off, either of the camera, or the subject, will throw the subject out of focus. You will mostly likely end up with soft pictures. Throw in the possibility of focus error on the photographer's part, or AF error, and u get the picture.....

i know the DOF of a Leica M 75mm f1.4 lens at f1.4 and minimum focus distance is exactly 6 mm only. I think the 50 f1.0 will be less than that(?)

For portraiture and other specialised shooting, it still might work, but for rapidly moving subjects, especially stage shooting, it's pretty useless. Much better to pump up the ISO (esp with DSLRs!) and shoot with a little more DOF.

And of course, the heavy light falloff might irritate some ;)

I would guess the problem is also there for lenses like 200 f1.8 or 300 f/2. Canon or Nikon. Especially for closeups of people or animals. DOF is likely to be extremely limited.

Of course the bokeh of these fast lenses (especially the longer ones!) will be out of the world........
 

Red Dawn

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#13
Originally posted by ckiang


If I didn't remember wrongly, a Canon EF 50mm f/1.0 is roughly the size of a 28-70/2.8.
no lah....it's fatter for sure, but certainly not longer ;)

for comparison sake, i think it has 77mm filter size, same as the 28-70 f2.8L. The Leica version is only 52mm filter size. That's the same filter size as the Canon 35mm f2, which is a whole 2 stops slower.
 

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