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07-17-2010, 12:27 PM   #16
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QuoteOriginally posted by Clinton Quote
When we're talking about different mounts, I think this makes sense, however, there is no difference in the DoF between your full frame film and APS-C. My 85/1.4 has the same amount of DoF in each, just the APS-C is cropped.
Nat Geo disagrees:
Jim Richardson on Taking Out-of-Focus Pictures -- National Geographic

07-17-2010, 12:30 PM   #17
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QuoteOriginally posted by CarbonR Quote
You mixed a bit (or misunderstood) what I said. I'll try to be clearer.
"Enlarge factors" :
- 6x7 format : 2
- 6x6 format : 1,8
Crop factor of APS-C format : 1,5

It means that you need a lens 2 times longer on 6x7 (or 1,8 times on 6x6) than on 2'x36 to get the same angle of view (and the lens only needs to be 2/1,8 times slower to get the same dof, for the same reproduction ratio obviously). As the APS-C is smaller than 24x36, you need a lens 1,5 times shorter than on 24x36 to get the same angle of view (and 1,5 times faster to get the same dof). These figures can be combinated : crop factor between APS-C and 6x6 is 1,8x1,5=2,7, which means your 85mm lens on APS-C will act as a 85x2,7=230mm lens would to on 6x6 format. And to equal (in terms on angle of view) a 80mm lens on 6x6 with an APS-C body, you need a 80/2,7=29mm lens (it is normal : 80mm is the stand focal lengh on 6x6 while 29mm is around normal focal lengh on APS-C too).

Edit : to answer about the 180/2.8. As we said, it will still transmit f/2.8. It will remain a 180mm lens (so with a DA 50-200 @180mm, you would have the same angle of view), but you will have the same angle of view of a 180x1,5=270mm lens would have on 24x36, or the same angle of view a 180x2,7=486mm lens (almost 500mm) would have on 6x6 format.
Don't bother with focal lengh and formats. If you know that your 85mm Takumar is enough for you in terms of angle of you, any ~85ish lens, whatever format it is designed for, would do it.

Bonus : A Pentacon 500/5.6 on 6x6 would have approximatively the same angle of view than a 180mm lens would have on APS-C. But in matter of dof, the lens on 6x6 format would be an equivalent of f/5.6/2,7= f/2. So if you use a 500/5.6 @f/5.6 on 6x6, you have a narrower dof than with a 180/2.8 @f/2.8 lens on APS-C, despite you have the same angle of view and despite one is 2 times slower than the other
Sorry but you've completely and utterly lost me there... This is clearly not one of my strong suits....

I'm going to make this as simple as I can. I want the same view as my Takumar 85mm on my Pentax Kx with a Pentacon 6 lens. Which Pentacon 6 lens should I buy? 50mm? 80mm?

Last edited by hangu; 07-17-2010 at 12:37 PM.
07-17-2010, 01:02 PM   #18
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I said : don't bother with the format. Whatever format the lens is designed for, the important thing is the format on what you use it. Even a large format 85mm lens will act like your 85mm Takumar if the lenses are used on the same format (assuming the format is covered by the two lenses). So the most close lenses will be the standard 80/2.8 lens, wich is much slower than the Takumar, and probably not sharper, because both 85mm Takumar are sharp stopped down to f/2.8 (the S-M-C f/1.8 is already sharp at f/1.8)
07-17-2010, 01:11 PM   #19
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QuoteOriginally posted by hangu Quote
Sorry but you've completely and utterly lost me there... This is clearly not one of my strong suits....

I'm going to make this as simple as I can. I want the same view as my Takumar 85mm on my Pentax Kx with a Pentacon 6 lens. Which Pentacon 6 lens should I buy? 50mm? 80mm?
If you want the Tak 85mm look, you need another 85mm lens. Whether its medium format or large format is irrelevant.

The only difference you'll encounter is, that most medium format lenses are slower. But in the case of a Takumar 85 equivalent you are lucky as Mamiya maes the fine 80/1.9 lens, which will even proved the fast max. aperture we are used to get from our small format lenses.

Ben

07-17-2010, 01:18 PM   #20
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I'm hoping to use Pentacon 6 lenses on my Pentax K-x. I have no interest in medium format with film.

There's no way a CZ Biometar 80mm mounted on a Pentax Kx will look anything like the SMC Tak 85mm.

edit:

QuoteOriginally posted by Ben_Edict Quote
If you want the Tak 85mm look, you need another 85mm lens. Whether its medium format or large format is irrelevant.

The only difference you'll encounter is, that most medium format lenses are slower. But in the case of a Takumar 85 equivalent you are lucky as Mamiya maes the fine 80/1.9 lens, which will even proved the fast max. aperture we are used to get from our small format lenses.

Ben
Goddamnit. Had I just been fed wrong information previously? The Mamiya 80mm f1.9 mounted on a Pentax Kx will be the same as the Takumar 85mm mounted on a Pentax DSLR?!
07-17-2010, 01:25 PM   #21
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A bit wider (80 vs 85mm, but approximatively the same). An other thing : a 80mm lens from medium format is built like a standard lens, generally the typical double gauss. A 85mm for 24x36 is rather a telephoto lens, for example, the S-M-C Takumar 85/1.8 is a planar, the Auto Takumar 85/1.8/Super Takumar 85/1.9 are Ernostars, the Helios 40 is a Biotar... They wont have the same bokeh at all (I've already done a comparison of the two Takumar and the H40 all at f/1.8 Here )
07-17-2010, 01:38 PM   #22
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QuoteOriginally posted by CarbonR Quote
A bit wider (80 vs 85mm, but approximatively the same). An other thing : a 80mm lens from medium format is built like a standard lens, generally the typical double gauss. A 85mm for 24x36 is rather a telephoto lens, for example, the S-M-C Takumar 85/1.8 is a planar, the Auto Takumar 85/1.8/Super Takumar 85/1.9 are Ernostars, the Helios 40 is a Biotar... They wont have the same bokeh at all (I've already done a comparison of the two Takumar and the H40 all at f/1.8 Here )
Wow, very, very interesting! So planar lenses typically have stronger contrast and better sharpness while Biotar lenses will give the swirling bokeh while Ernostars have the smoothest bokehs.

Can't wait to try an 80mm medium format Pentacon lens on my Pentax K-x....
07-17-2010, 04:35 PM   #23
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forget all the dam conversion factors for different formats

every one is getting tied up in knots about conversions between formats.

lenses are really simple things.

Focal lenght is focal length, it is measured by (for a simple lens) the fistance from the focusing plane to reproduce a point source at infinite distance as a point. It is totally constant and independant of format

Aperture is also totally independant of format. Again for a simple lens it is simply the focal length of the lens divided by diameter of the lens .

Fstops change in a square ratio simply because when you magnify a subject, doubling the magnification (i.e. doubling the focal length) but you keep the physical diameter of the aperture the same the lens spreads the light coming through the aperture over 4 times the area, (i.e. 2x length and 2x width). This is why Fstops are in the sequence 1.2, 1.4, 2. 2.8 4....... each stop doubles or halves the light compared to the next.

Depth of field changes with aperture, but for any lens, regardless of format, an image focused on either film or sensor, and printed to the same size (i.e. a 6' tall person making an image 1 inch high on a sensor (rougly full vertical format on an ASP-C sensor) if that person is printed to the same size on paper, the depth of field will be the same. Nothing changes.

Now, here is where the change happens. Imagine an image in 4 inch by 5 inch film. Print the entire image to 8 x 10, (2 x enlargement) it will have pretty good depth of field. cut the middle .66 inch by 1 inch out og the middle of th enegative, and print it to 8 x 10, the depth of field will appear to be really bad, yet it is the exact same negative as the first print. Depth of field is, for any lens, a function of the enlargement of the final image relitive to the sensor or negative size.

The same issue occures with focal length and shutter speed for hand held shots. The bigger you print, the fuzzier the image looks,

Depth of field and shutter speed are generally based upon enlarging yoour image up to 8 x 10 and what with that enlargement from the origonal format appears acceptably sharp to the unaided eye. That is all

There are formulas to define all this, but what really changes the depth of fiels for any lens at any aperture is how big you make the final print because it is all relitive to the enlargement ratio,

07-17-2010, 04:56 PM   #24
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QuoteOriginally posted by alohadave Quote
F actually refers to the focal length. The proper notation is f/4. Focal length divided by 4 will tell you the diameter of the aperture opening.

This notation is used because it is consistent across all focal lengths and lens formats. It always works. f/8 on a 100mm lens and f/8 on a 25mm lens will be the same relative size even though the apertures are not the same physical sizes.

Thank you Dave for injecting the correct answer to the OP's question and also that answer that makes the most sense.


Steve
07-17-2010, 05:01 PM   #25
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QuoteOriginally posted by hangu Quote
Goddamnit. Had I just been fed wrong information previously? The Mamiya 80mm f1.9 mounted on a Pentax Kx will be the same as the Takumar 85mm mounted on a Pentax DSLR?!
Similar FOV with both lenses on APS-C. (Pentax KX is a film camera You have a K-x)


Steve


(Owns a KX, may yet buy a K-x)
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