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01-12-2012, 06:19 PM   #106
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bossa, lens specs are as they are regardless of the camera they're mounted on.
Now lets move on from this - it's a frequent catching point for many, but a search of this forum will give you a lot more explanation of the concept. It's also featured in the articles section of the forum under Photography articles.

01-12-2012, 06:22 PM   #107
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QuoteOriginally posted by bossa Quote
On a camera the exit pupil has something to do with the F ratio. On a telescope it's focal length divided by aperture. I assume then that the 'new' f stop size is only related to DOF and not speed of the lens?
I think you mean pupil magnification.
en.wikipedia.org/wiki/Pupil_magnification
01-12-2012, 06:55 PM   #108
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QuoteOriginally posted by Ash Quote
bossa, lens specs are as they are regardless of the camera they're mounted on.
Now lets move on from this - it's a frequent catching point for many, but a search of this forum will give you a lot more explanation of the concept. It's also featured in the articles section of the forum under Photography articles.
I wasn't aware that you were policing the forum and determining what and when things were to be discussed. Thanks for setting me straight.
01-12-2012, 07:16 PM   #109
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QuoteOriginally posted by Anvh Quote
I think you mean pupil magnification.
en.wikipedia.org/wiki/Pupil_magnification
I'm really only going on my experience with telescopes.

The Exit pupil of a lens would determine it's image circle and registration distance. That would indeed have a magnification factor to it as the angle of projection over a set distance combined with sensor size and pixel size/density would all contribute to the final result. The 'same' basic lens design could have a different rear element design with a different exit pupil (FF) and thus have a different magnification factor. I.E. The same lens could project differing image circles depending upon the rear element design. Surely the same lens projecting all it's light onto a smaller circle would be brighter/faster?

For some reason I wasn't only thinking that the total light passing through a lens was the actual speed of that lens, but that a partial collection of that light (APS-C) would actually affect image brightness (speed). If you are only collecting 50% of the total light from an optical system then surely that affects the speed of the lens? The DOF would stay the same until some magnification was added by the rear element group or output to print etc.

If the exit pupil of an eyepiece is smaller than your own pupil you get an image dancing around in a small image circle on your retina. It's like looking through a keyhole in a way. Both pupil sizes have to match so the FOV of the scope is fully realized by the eye of the viewer. I was curious if the exit pupil of a lens (DA or FA) affected the speed of the optical system as a whole and not necessarily the lens alone.

01-12-2012, 07:46 PM   #110
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QuoteOriginally posted by bossa Quote
I know that.. I was just asking about the speed of the lens...
I was more or less asking for a clarification of what people are saying re' F stop equivalence.
Sorry. When I see "XXX IS JUST LIKE YYY ON APS-C" I just go rabid. Pass the Thorazine, please.

For DOF equivalence, I'll mount an M42 lens on my APS-C cam and set the aperture to f/11, I read the DOF scale just inside the f/8 marks. That extra fractional stop is just for paranoia; I could probably get away with a simple 1-stop adjustment. I can see the DOF equivalence on the different formats.

So 300/4.5 on APS-C should have similar DOF as 450/6.3 on 135/FF. Easy way to find out: Plug numbers into the Online DOF Calculator.

Last edited by RioRico; 01-12-2012 at 07:52 PM.
01-12-2012, 08:02 PM   #111
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QuoteOriginally posted by bossa Quote
Does this mean that my FA*300 F/4.5 is really an 450 F/6 (?) on APS-C?
No it doesn't. The optical properties of a lens do not change if you put it on a different camera. It's still a 300mm lens.
01-12-2012, 08:03 PM   #112
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Thanks. Actually I forgot about my iPhone DOF app.. it's pretty good as it adjusts per format and also shows HFD. I'm also having a bad day I think.
01-12-2012, 08:04 PM   #113
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QuoteOriginally posted by alohadave Quote
No it doesn't. The optical properties of a lens do not change if you put it on a different camera. It's still a 300mm lens.
I should have put a smiley on that post.. too late now.

01-12-2012, 08:52 PM   #114
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Answering the real question

QuoteOriginally posted by bossa Quote
Does this mean that my FA*300 F/4.5 is really an 450 F/6 (?) on APS-C?
I just can't help myself. Sorry, Ash

The lens doesn't magically change properties, but effectively - read, effectively - it is like shooting a 450 f/6.3 on FF, as far as you, the shooter are concerned.

I think when people say "a 300mm lens is a 300mm lens, period!" and leave it at that, it actually confuses the situation more, and is not helpful, because it doesn't really answer the question the person is probably asking. It's almost always really a question about equivalence, and how that 300mm lens will behave, for them, the shooter, on either format.

So, yes, if you shoot that 300 4.5 on FF, then move it to an aps-c camera, it will effectively (in FOV/DOF) be like shooting a 450 f/6.3 lens on FF. Or, if you're shooting it on aps-c, and get a chance to shoot it on a FF camera, that 300 f4.5 will have, on FF, the FOV/DOF equivalent of about a 200mm f/2.8 would on aps-c.

Not with regards to actual light gathering - your shutter speeds won't be affected, it's still exposing at f/4.5. We're talking FOV/DOF.

Now, if you're shooting a FF camera with enough pixels, you can simply crop that image to cover the same FOV as the aps-c image, and it will basically be the exact same image. The reason that this isn't universally done is because you'd lose IQ over simply shooting with an aps-c camera in most cases - you'd need about a 38MP FF camera to match the K-5's resolution after cropping, for example. I sometimes shoot aps-c lenses on my 12MP D700 in 'DX mode', which superimposes an aps-c sized frame in the viewfinder and crops the image to aps-c FOV for you, and it only gives me about 5MP on the target. (still good enough for 11x16 prints.)

So - I don't know, it's just a pet peeve of mine when the real question being asked here is answered with "No - lens doesn't change. Full stop." It's almost never helpful, almost never answers the real question.



.

Last edited by jsherman999; 01-12-2012 at 09:07 PM.
01-12-2012, 09:20 PM   #115
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QuoteOriginally posted by jsherman999 Quote
So - I don't know, it's just a pet peeve of mine when the real question being asked here is answered with "No - lens doesn't change. Full stop." It's almost never helpful, almost never answers the real question.
Eh. Maybe, maybe not. You're making an assumption about the "real question" but I think those that ask probably don't know what they are asking really -- they've just heard this and that about "crop factor". (Anyone who has actually used both formats is probably sophisticated enough to know what's what already.) So more explanation is better -- I'm with you there -- cause it is usually a newbie (at least to SLRs) that is asking. That said, since they probably haven't used both formats to talk about "equivalence" at all is just confusing.

And now people are talking about changes in compression when you blow up an image -- another piece of confusion with no real meaning. When you blow up an image, it is just the same as when it is smaller -- just bigger. (Optical qualities don't change. OOF areas don't change. All of the relative relations and ratios remain the same.)
01-12-2012, 09:21 PM   #116
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The hair-splitter in me has got to point out it's f/6.8, not f/6.3. That fraction of a stop makes all the difference

I think equivalence is kind of important to understand for new users, not just for those that shot film. This is because there are now many different sensor sizes in the same price bracket, and one of the important ways to compare apples and apples is converting lenses and apertures to a common base.

What the marketters try to get away with is giving FL equiv but not aperture equiv.
01-12-2012, 09:29 PM   #117
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QuoteOriginally posted by jsherman999 Quote
I just can't help myself. Sorry, Ash

The lens doesn't magically change properties, but effectively - read, effectively - it is like shooting a 450 f/6.3 on FF, as far as you, the shooter are concerned.

I think when people say "a 300mm lens is a 300mm lens, period!" and leave it at that, it actually confuses the situation more, and is not helpful, because it doesn't really answer the question the person is probably asking. It's almost always really a question about equivalence, and how that 300mm lens will behave, for them, the shooter, on either format.

So, yes, if you shoot that 300 4.5 on FF, then move it to an aps-c camera, it will effectively (in FOV/DOF) be like shooting a 450 f/6.3 lens on FF. Or, if you're shooting it on aps-c, and get a chance to shoot it on a FF camera, that 300 f4.5 will have, on FF, the FOV/DOF equivalent of about a 200mm f/2.8 would on aps-c.

Not with regards to actual light gathering - your shutter speeds won't be affected, it's still exposing at f/4.5. We're talking FOV/DOF.

Now, if you're shooting a FF camera with enough pixels, you can simply crop that image to cover the same FOV as the aps-c image, and it will basically be the exact same image. The reason that this isn't universally done is because you'd lose IQ over simply shooting with an aps-c camera in most cases - you'd need about a 38MP FF camera to match the K-5's resolution after cropping, for example. I sometimes shoot aps-c lenses on my 12MP D700 in 'DX mode', which superimposes an aps-c sized frame in the viewfinder and crops the image to aps-c FOV for you, and it only gives me about 5MP on the target. (still good enough for 11x16 prints.)

So - I don't know, it's just a pet peeve of mine when the real question being asked here is answered with "No - lens doesn't change. Full stop." It's almost never helpful, almost never answers the real question.



.
I really wish I'd put a smiley on that post now... LOL. I think I'll go and lay down in front of a truck. ... ouch!
01-12-2012, 09:37 PM   #118
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QuoteOriginally posted by vonBaloney Quote
Eh. Maybe, maybe not. You're making an assumption about the "real question" but I think those that ask probably don't know what they are asking really -- they've just heard this and that about "crop factor".
In all the years I've seen that question asked in forums, I don't recall one time when it wasn't actually a question about format equivalence. Usually it's asked because the poster has seen discussions about the subject, but is unaware of the concept of equivalence.

QuoteQuote:
(Anyone who has actually used both formats is probably sophisticated enough to know what's what already.)
Exactly. If the question comes up it's because they are seeking further explanation, and "a lens doesn't change properties, period" just doesn't help them understand anything better when it's not accompanied by some very basic info about format equivalence. I've seen it actually mislead folks, giving them the impression that lenses bring exactly the same image no matter what format camera they put them on.

.

Last edited by jsherman999; 01-12-2012 at 09:48 PM.
01-12-2012, 09:38 PM   #119
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QuoteOriginally posted by bossa Quote
I really wish I'd put a smiley on that post now... LOL. I think I'll go and lay down in front of a truck. ... ouch!
Go back and edit it
01-12-2012, 09:48 PM   #120
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QuoteOriginally posted by jsherman999 Quote
Go back and edit it
I was going to leave it but if I do I'll keep getting good advice.. lol
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