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03-04-2010, 04:13 AM   #1
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[langtitle=fr]Optiques 6x7 sur APS-C.[/langtitle]

[lang=fr]Bonjour les copains.

Depuis peu, je possède un adaptateur K pour des optiques 6x7.



Ayant un boitier de la série K avec un capteur APS-C, je me pose la question, à propos de la focale obtenu.

Je m'explique:

Avec mon boitier, le coefficient multiplicateur est de x1.5, donc si je monte un objectif 36x24 de 100mm, j'obtiens un 150mm.

Mais avec un objectif 6x7 de 100mm, quelle sera la focale obtenue?

Sur la notice il est écrit:
QuoteQuote:
The angle of view of the takumar 6x7 20mm lens is 26° for 6x7cm format, but it becomes 12° when it is monted on the 35mm pentax camera, the same as the regular 200mm lens for the 35mm pentax.
Mais ce n'est pas très clair pour moi.

Merci d'avance, pour votre aide.

Seb.

03-04-2010, 05:26 AM   #2
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Due to the poorness of my French, I have to reply in English. :-)

QuoteQuote:
Mais avec un objectif 6x7 de 100mm, quelle sera la focale obtenue?
The focal length multiplier when going from a 135 lens to a 67 one is roughly 2. Subsequently, the multiplier would be 3 when going from APS-C to 67. For the above 100mm 67 lens, it is close to 50mm in standard full-frame 135 format and somewhere between 33mm and 35mm for APS-C.

Also see this table that contains the 135-equivalent focal lengths for all Pentax 67 lenses.

Cheers,
Tassilo
03-04-2010, 06:09 AM   #3
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Merci vparseval

Ton Anglais est meilleur que mon français.

Finalement, ce n'est pas super de mettre une optique 6x7 sur un boitier tel que les APS-C.

Encore merci, tu m'as fait économiser de l'argent, sur ce coup.
03-04-2010, 01:28 PM   #4
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I don't understand French, but the reply above isn't quite correct.

Focal length is focal length, regardless of which system a particular lens is designed for.

A 100mm APS-C lens, 100mm FF lens, and 100 mm 6x7 lens will perform exactly the same on an APS-C body.

The only difference is that a 100mm "APS-C only" lens will vignette on FF/MF cameras, and a 100mm FF lens will vignette on an MF camera.

03-04-2010, 03:38 PM   #5
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QuoteOriginally posted by Entropy Quote
I don't understand French, but the reply above isn't quite correct.

Focal length is focal length, regardless of which system a particular lens is designed for.

A 100mm APS-C lens, 100mm FF lens, and 100 mm 6x7 lens will perform exactly the same on an APS-C body.

The only difference is that a 100mm "APS-C only" lens will vignette on FF/MF cameras, and a 100mm FF lens will vignette on an MF camera.
That is correct. My 645 120mm macro is 120mm on the K-7. Just like a Pentax M* 67 400mm is 400mm on a k-7.
03-04-2010, 04:38 PM   #6
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QuoteOriginally posted by vparseval Quote
Due to the poorness of my French, I have to reply in English. :-)



The focal length multiplier when going from a 135 lens to a 67 one is roughly 2. Subsequently, the multiplier would be 3 when going from APS-C to 67. For the above 100mm 67 lens, it is close to 50mm in standard full-frame 135 format and somewhere between 33mm and 35mm for APS-C.

Also see this table that contains the 135-equivalent focal lengths for all Pentax 67 lenses.

Cheers,
Tassilo
Well, sorry, but that is entirely incorrect application of "focal length multiplier" or "crop-factor". The multiplier or factor is useful only to determine equivalency of
angle of view.

For example, *any* 200mm lens on APS-C gives the same angle of view as a 1.6x200=320mm lens on full frame, or a 1.6x2x200=640mm lens on 67 camera. This means 200mm on APS-C, 320mm on full frame, and 640mm on 67 all give the same angle of view, i.e. with identical framing of the same scene/subject.
03-04-2010, 05:24 PM   #7
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The 200mm 67 lens is going to be a whole lot heavier than an APS-C 200mm lens. That's the only difference.
03-05-2010, 12:00 AM   #8
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There is a lot of discussion of 67 lenses on APC here:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/49428-medium-f...20-sensor.html

03-05-2010, 12:28 PM   #9
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Des essais sont à venir mercredi prochain, avec un 150mm F/2.8.

à suivre...
03-11-2010, 12:26 AM   #10
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Bon! J'ai enfin la réponse!...
03-11-2010, 01:11 AM   #11
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I took my 67 165mm 2.8 and added my Tamron 2x mc7 bbar Tc. I now have a 330mm 5.6.

Image quality is as good as my Pentax SMC 300 f4, but there are advantages to the 67 330. For example, I have a close-focus distance of about 1.5 meters with the 330mm which is far better than the SMC 300. also, I get the %10 added reach, though I lose a stop.

However, since I'm shooting on the sweetest of sweet spots, the 2x TC & 165 f 2.8 produce images even to the corners--very nice. There is ca wide open, but it is not as bad as the SMC 300. Also, the 165mm and TC weigh about the same or less than the SMC 300. The 165mm has a tri-pod collar too.

In this case, the weight of the 67 lens is a non-issue.

QuoteQuote:
Seb: Bon! J'ai enfin la réponse!...
Quelle est votre réponse?
03-11-2010, 01:15 AM   #12
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QuoteQuote:
Seb: Des essais sont à venir mercredi prochain, avec un 150mm F/2.8.

à suivre...

Où sont les sentiers à partir de 150 f 2.8?
03-11-2010, 02:06 AM   #13
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Voilà, voilà!

Salut Jewelltrail, voilà mon retour d'expérience, avec finalement un 105mm F/2.4.

Alors voilà deux photos, une prise avec un 105mm F/2.4 (6x7), et une autre avec un zoom 70-300mm.
Elles ont toutes les deux été prises dans les mêmes conditions, (ouverture, balance des blancs, iso, temps expo...).

[1] 105mm, f/2.4, 6x7:


[2] 70-300 à 100mm:


On remarquera, que la balance des blancs est différente entre les deux, une perte de luminosité, peut-être dû à la bague d'adaptation.

Ensuite, un objectif 6x7 105mm cadre comme un 100mm. :cry:

Mais la photo est prise au cœur de la meule, donc vignettage inexistant et meilleur définition.

Voilou!
03-11-2010, 09:03 AM   #14
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J'ai pas résister à essayer cet objectif sur mes gouttes et là...:

[10]


[11]


[12]


Et là, plus de batterie sur l'un des flash, finalement des choses intéressantes:

[13]


[14]


Voilou!
03-11-2010, 11:27 PM   #15
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Seb, your pics will not fully load for me--not sure why. I went out with my SMC 300 f4 & the 67 165 2.8 w 2x tc. Nothing scientific, just a couple of quick snaps, from the same spot. Keep in mind, these are handheld, and there is some effort involved in dismounting the SMC 300 in order to mount a 67 165, TC, and 67 to K-Mount adapter--so the shots are not the identical. Also, the 67 setup is a 330mm, while the SMC is 300mm--so, just an informal presentation here, but to make a point. The performance is very similar. The SMC shot, @ 5.6, is stopped down 1 for this lens, whereas the 165mm 2.8 w 2x tc, @ 5.6, is actually wide open. Again, my post is not to prove anything, but just to show similar performances.

Seb, vos photos ne se charge pas complètement pour moi - pas très bien pourquoi. Je suis sorti avec mon SMC 300 F4 & les 67 165 2.8 w 2x TC. Rien de scientifique, juste un couple de Quick à soufflet, à partir du même endroit. Gardez à l'esprit, ce sont des ordinateur de poche, et il ya un certain effort impliqué dans le démontage du SMC 300, afin de monter un 67 165, TC, et 67 à K-Mount Adapter - si les plans ne sont pas identiques. Aussi, l'installation 67 est un 330mm, tandis que le SMC est 300mm - oui, juste une présentation informelle ici, mais pour faire un point. La performance est très similaire. Le coup SMC, @ 5.6, est arrêté en baisse de 1 pour cet objectif, alors que le 165mm 2.8 w 2x TC, @ 5.6, est effectivement ouverte. Encore une fois, mon poste n'est pas de prouver quoi que ce soit, mais simplement de montrer des performances similaires.


All Pics re-sized, no PPing, for forum. SMC 300 f 5.6 followed by %100 crop:





Here is the 67 165mm 2.8 w Tamron F 2xKR BBAR MC7-- wide open:



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