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05-02-2012, 11:38 AM   #16
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QuoteOriginally posted by sinus007 Quote
Hi,
As other posters said on this and zillion other threads a focal length is a focal length regardless of the sensor size.
However, if you don't factor in the sensor size, knowing this fact is worthless. Worse, really.

05-02-2012, 12:09 PM   #17
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To be perfeclty clear: all lenses state their actual focal lengths. Not the focal length that would yield a similar FOV if mounted to some other camera besides the one you are actually mounting it to, but the actual focal length of the lens.

Thus, if you have two lenses marked 50mm, one designed for film and the other for digital both have an actual focal of 50mm. That means they will have exactly the same field of view when mounted to the same camera, or to two different cameras with similar sensor sizes. The fact hat one happened to be designed for film and the other for digitla is no more relevant than the fact that one is metal and the other plastic, or one AF and the other MF, or one black and the other white, or one a macro and the other a non-macro. 50mm is 50mm, period.

So a 50mm lens mounted to your Canon "cropped" body will have roughly the same field of view as a 50mm lens mounted to a Pentax "cropped" body, except to the extend Canon's crop factor is ever so slightly different than Pentax'.
05-02-2012, 01:25 PM   #18
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QuoteOriginally posted by twitch Quote
The prime kit to build is DA15, FA31, FA77. With that kit there is no compromise on iq, and whatever Pentax do or don't release down the track you're OK.
No love for the 43?
05-02-2012, 02:09 PM   #19
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QuoteOriginally posted by jk333 Quote
However, if you don't factor in the sensor size, knowing this fact is worthless. Worse, really.
Read original question before making such statements.

05-02-2012, 03:02 PM   #20
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QuoteOriginally posted by Sage97 Quote
No love for the 43?
The focal length spacing makes more sense with 15/31/77. A 21/43/77 would work too, but 21 is not wide enough for my liking. But then i like any kit that includes the 31.
05-02-2012, 03:13 PM   #21
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QuoteOriginally posted by jk333 Quote
According to the other thread the DA 15 will not work on full frame. Do you have other information?
The DA15 won't work on full frame. But with a 15/31/77 kit you only have to sell off 1 lens if you buy a FF Pentax DSLR (if one is ever released), and in the meantime you have 3 of the best lens Pentax make today. Buy 3 DA limiteds and you may be selling off all 3.

You could buy a kit like FA*24/FA50/DFA100 all of which are 100% FF capable. However 24 is not that wide on APS-C and only available secondhand. Depends on your focal length preferences and your budget.
05-02-2012, 04:18 PM   #22
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QuoteOriginally posted by twitch Quote
The focal length spacing makes more sense with 15/31/77. A 21/43/77 would work too, but 21 is not wide enough for my liking. But then i like any kit that includes the 31.
I have all "three amigos" and, I must confess, I very rarely use 43mm... It's either 31mm or 77mm for me. 43mm is quite odd focal length on APS-C. It's neither normal, nor tele. I bet that proportion of use of my lenses would change with FF and 43mm would become my main prime for walk around.
05-03-2012, 01:29 PM   #23
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QuoteOriginally posted by jk333 Quote
However, if you don't factor in the sensor size, knowing this fact is worthless. Worse, really.
No. Sorry, but no.

What you are asking is in fact that people start using field of view instead of focal length to describe the properties of lenses. You're free to wish that, and it would make comparisons between sensor sizes easier.

The reality won'T change, however. Focal length is a property of the lens. The focal length is related to the f-stop (f-stop is a ratio of aperture and focal length). It is also necessary to know the REAL focal length to properly set SR systems. The real focal length will also influence the depth of field (which is why getting a blurry background is difficult with a compact).

When you try to work around actual, measurable physical properties in order to make comparisons easier, you run around newer, much worse problems.

The best thing is for people to understand how their sensor size influences field of view. It should actually be easier than to change how we measure focal length

05-03-2012, 01:40 PM   #24
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QuoteOriginally posted by Sage97 Quote
No love for the 43?
Are you kidding?
I too have all three amigos, and the 43 is my most favoured for portraits, as it permits some intimacy between the camera and subject without any significant perspective distortions and at the same time is sharp and very small. I have the same affinity to the 31 for group or couple portraits, and the 77 for more distant, background separating effects created particularly in outdoor portraiture.
05-03-2012, 05:55 PM   #25
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QuoteOriginally posted by bdery Quote
No. Sorry, but no.
Focal length is a property of the lens. The focal length is related to the f-stop (f-stop is a ratio of aperture and focal length). It is also necessary to know the REAL focal length to properly set SR systems. The real focal length will also influence the depth of field (which is why getting a blurry background is difficult with a compact).

When you try to work around actual, measurable physical properties in order to make comparisons easier, you run around newer, much worse problems.The best thing is for people to understand how their sensor size influences field of view. It should actually be easier than to change how we measure focal length
I think FOV is much more usable. DOF is relative, its dependent on the sensor size and shooting distance in addition to the ratio.

The camera/engineer can set the SR system; engineers can do that. Its not a necessary item for the photographer.

QuoteQuote:
The real focal length will also influence the depth of field (which is why getting a blurry background is difficult with a compact).
Because of the sensor size. No?

Its not correct to compare APS-C to FF with just a correction factor. A 77mm/f1.8 lens on FF doesn't become a 115mm/ f1.8 lens. Its becomes a 115mm f/2.5 lens with a f1.8 light gathering ability. BUT!, I think its preferable to know the FOV and light gathering ability; to me, 115mm f1.8 is more useful. Then, I remember that APS-C has ~ a stop less DOF than FF (from what I have read).

I'm not trying to confuse the OP. He's gotten his answer, the lens is 21mm, especially compared to his old Canon system. But its FOV (in 35mm terms), is 32mm, its speed is f/3.2. And its DOF is less than a 32mm f/3.2 would be. (due to framing, etc) Hopefully I don't have this wrong.
05-03-2012, 06:06 PM   #26
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Luckily the mental gymnastics are quite a bit easier than that. As has been covered many times on this forum; 77mm f/1.8 on 1.5 crop APS-C camera is the equivalent to shooting with a 116mm f/2.7 on FF assuming the sensor tech (QE) is the same (eg D7000 & D800). You really do just multiply the focal length and aperture to get an equivalent lens.

There's no light gathering advantage on APS-C because the FF sensor has a 1 stop ISO advantage owing to it's larger sensor suface area.
05-03-2012, 07:30 PM   #27
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QuoteOriginally posted by jk333 Quote
I think FOV is much more usable. DOF is relative, its dependent on the sensor size and shooting distance in addition to the ratio.
Well, FOV isn't characteristic of the lens itself. It's characteristing of the lens mounted on specific camera.
Focal length is a characteristic of the lens itself.

I prefer to see 50mm on the lens and I will know what this lens is going to be on my camera.
Instead of seeing:
  • 3130' on camera A,B,C,D
  • 46 on camera E,F,G,H
05-04-2012, 05:20 AM   #28
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QuoteOriginally posted by jk333 Quote
I think FOV is much more usable. DOF is relative,
That's exactly what I keep saying

QuoteOriginally posted by jk333 Quote
The camera/engineer can set the SR system; engineers can do that. Its not a necessary item for the photographer.
Except with, say, manual lenses on a Pentax body?

QuoteOriginally posted by jk333 Quote
Quote:
The real focal length will also influence the depth of field (which is why getting a blurry background is difficult with a compact).
QuoteOriginally posted by jk333 Quote
Because of the sensor size. No?
No.

QuoteOriginally posted by jk333 Quote
A 77mm/f1.8 lens on FF doesn't become a 115mm/ f1.8 lens. Its becomes a 115mm f/2.5 lens with a f1.8 light gathering ability
No.




I'll say it again : no.


A 77mm lens is a 77mm lens. It doesn't become anything. It doesn't care for the sensor. The sensor records the image that the lens creates. They do not interact. You can try to turn that fact any way you want, it won't change the truth.

I'll try to explain it one last way, then I'm out. The sensor records the image that is created by the lens. But the lens will create an image, whatever the sensor behind it. It will create an image even if there is no sensor. The size of the sensor means that the (circular) image created by the lens is cropped more or less importantly. Even a FF sensor crops the image. But whatever the crop factor you use, the lens doesn't give a tiny rat's bottom. The image that is created by the lens already has all its characteristics (DOF, bokeh, etc) by the time it reaches the sensor.

I'm out.
05-04-2012, 07:47 AM   #29
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QuoteOriginally posted by bdery Quote
That's exactly what I keep saying
But whatever the crop factor you use, the lens doesn't give a tiny rat's bottom. The image that is created by the lens already has all its characteristics (DOF, bokeh, etc) by the time it reaches the sensor.
But the DOF and bokeh [of a a lens] are still a function of the crop factor (and therefore sensor size) because they are impacted by the distance you can frame a photo with a focal length from.

In any event, I understand you're larger point. Thanks.
05-06-2012, 05:01 AM   #30
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Focal length by definition is defined as focal length. There does not exist anything like an equivalent.
There have always been lenses for 35mm format, medium format (several 6x... formats), large format.... It is on you to figure out what angle of view may be equivalent between systems. It would be fraud to print an equivalent focal length in mm on a product that does not match the real focal length. Kodak did that for some time and all of us got used to the term of equiv. focal length which is completely meaningless. It appears that Pentax DA lenses are full frame lenses with the xception of wide angle lenses - here a design for the smaller image circle of APS-C cameras has large advantage in size and image quality over full frame designs.
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