New lens idea

[steve_rasmussen]

I like the idea of Pentax having a 35mm lens for this camera but a fisheye has very limited use. What are some opinions about Pentax producing a rectilinear(no distortion)35mm? SR

[tapas_maiti3]

Hi Steve

 

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Could they do it ? Mamiya claim that they couldn't produce the 43mm

for an RZ/RB because of the retrofocus design limitations (presumably

they mean a sufficient quality 43mm).

 

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Would a 35mm rectilinear lens could pair up with a 55mm lens, wheras

now we end up choosing between the 45mm and 55mm. I went for the 45mm

because I tend not to use wide angles as general lenses.

 

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What I think Pentax should do it improve their short telephoto lenses

in the 135mm to 300mm range. Why do they bring out 400mm and 800mm

star lenses but neglect the most used telephoto range ?

 

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Maybe the new 200mm ED IF lens for the 645 means they are going to

update some 67 lenses as well.

 

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How de we apply pressure on Pentax ? I e-mailed their comments site

for the UK but got no response.

 

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Tapas

[marcelo_p._lima]

Steve, Tapas; in response to the original post,

 

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What exactly is a rectilinear lens? Does that mean that lines are

straight and not curved?

 

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Does a lens still qualify as rectilinear if lines near the edges are

slanted, for instance a building on the right of the frame would look

sheared (slanted) to the right? Does the 45mm qualify as a

rectilinear lens?

 

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If so, I agree with you that a rectilinear 35mm would be much more

useful. Also, I hear that it has problems with light falloff at the

edges. Is there any solution for this? It seems that by the design of

the lens, attaching a neutral density filter (the ones that correct

this kind of defect) is impossible.

 

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Tapas, why do you think that the lenses in the 135-300mm range need

to be worked on? I agree that the 300mm would need some redesign

because of the difficulty of setting it up on a tripod, but aren't

all the other lenses in that range optically (and ergonomically)

excellent?

 

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regards,

Marcelo

[steve_rasmussen]

A rectilinear 35mm could be produced but because this lens would be

even more asymmetrical than their moderate wide angle lenses,

correcting distortion would be difficult. Thr easiest way to correct

distortion is to make the lens symmetrical. This of course is not

possible in SLRs with mirror boxes that need to be cleared(for wide

angle only). The 135, 165, 200 and 300mm lenses actually perform

fairly well once you get used to their limitations. Color correction

at those focal lengths is not all that hard to solve. The 500, 600 and

800 f/4 are another story. If the 300 were redesigned to be ED, it

would be very expensive. Pentax may change this lens though. As far as

contacting Pentax, try writing them via snail mail. Rectilinear means

straight lines are rendered straight. Falling back effect will still

happen with rectilinear lenses; these are two different things. The

45mm is rectilinear but does have what is called mustache distortion.

This is the attempt by the designer to correct distortion and what you

end up with is a slightly wavey, almost straight line. Have not heard

that the 35 fisheye has optical vignetting. The correction for optical

vignetting seems to be secretive among the optics companies. It is not

considered an aberration so my guess is that designers use a trial and

error method to some degree. They design several iterations for the

same focal length, all corrected for the normal aberrations and pick

the one with the least vignetting. I've heard that distortion is

treated the same way because it also is not considered an aberration.

In the LF world, center filters are used for optical vignetting. SR

[tapas_maiti3]

Marcelo

 

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My view on the 135mm to 300mm is a general point about modernisation.

I've used both 165mm lenses and I am not making a complaint about

their quality but if you try and pick lenses for an aplication the

range seems a bit peculiar.

 

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An example is portraiture, pentax promote the 165 2.8 as their

"portrait lens" yet it incapable of doing a full frame headshot. My

personal view is that it is too short as well for that. If you look

at other lens line ups, Bronica has the 180mm that focuses down to

1m, the Mamiya has the same (though it uses bellows focusing), the

blad has a 250mm focusing down to 2.5m. We have the 300mm that

focuses down to 5m, is a old design and according to all the opinions

floating around then net is one of their worst lenses.

Another example is macro, I've used the 135mm and though it is an

excellent lens it is hardly macro. The new 100mm is but is very

expensive and a very short focal length - in 35mm we tend to want

macros in the 60mm to 105mm lengths.

 

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What we end up with is 3 normal length lenses and 3 short telephotos -

is this logical ?

 

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I guess my main winge is because I want a 300mm with good wide open

performance (else just make it a f5.6 lens and lighter) and focusing

down to 2~2.5m and a 165mm-180mm focusing down to 1.0m (dreamland -

preferably a 2.8 version) to complement it and I want it for the

Pentax 67II.

 

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The longer length lenses obviously need the special glass etc more

but the short telephotos are the most purchased and generally useful

and should be where Pentax's design efforts should go IMO.

 

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Tapas

[marcelo_p._lima]

Tapas,

 

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I completely agree with you that these lenses need major redesign.

Although I'm very new to MF, the opinions I've seen around the web

aren't that positive, as you said.

I bought the 200mm/4 as a general purpose short tele, thinking about

using it as a portrait lens too. I was a bit disappointed with its

close-focusing abilities, though. It seems that to do serious

portrait work (if you're working with fashion, for example) you need

the auto extension tube set, which will allow you to crop a bit of

the model's head and get a REALLY close shot.

I find Pentax a somewhat bizarre company. It seems that there are

"Big Brothers" in Japan that control the entire affair, with no

regards to what is happening in the USA (I don't know about Europe,

you could tell us). You've probably read posts of people who called

Pentax in Colorado and it seemed like they really didn't know what

they were talking about, or they told you that Pentax in Japan "does

not release that kind of information.."

I also find it interesting that there are still no ad campaigns for

the 67II in the US..

My point is that I get the impression that it is hard to make oneself

"heard", and that in this respect, Pentax might not be a very

"friendly" company. Maybe we should all try snail-mail, as Steve

suggested.

 

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regards,

Marcelo

[steve_rasmussen]

Marcelo & Tapas:

If Pentax's short telephotos seem a bit awkward in

portrait/studio/fashion work, I feel it is because this camera system

was not primarily designed to do that. The 90 and 165 leaf shutter

lenses seemed like an afterthought for a camera that was originally

designed for nature and outdoor work. The slow flash synch is more

evidence of this. Pentax probably tried to cover too much subject

matter in order to draw more customers.

[rolland_elliott2]

Well, there is a way to get a rectilinar 35mm lens. But it requires

quite a bit of work.

 

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Get a 35mm tilt shift lens designed for 35mm cameras, like the Canon

FD 35mm TS lens. or Nikon 35mm PC lens. Remove the lens elements from

the helical assembly and fit them to a spare Pentax 67 mount. Design

the mount so it holds the mirror up after exposure.

 

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You will have to use the lens with mirror up and for the P67II you

will have to remove the lens after each exposure so the mirror can

go down all the way and can be recocked. Add to the fact that the

corners will probably vinginette and a viewfinder will have to be

used and focusing will be by hyperfocal distance it doesn't sound

that appealing.

 

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I've been thinking of doing this with my Nikkor 28mm f/3.5 PC lens,

but haven't gotten around to it yet. It only has an image cicle of

about 70mm, so the corners would vingenette, but after cropping it

would still be extremely wide.

 

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Food for thought.

 

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Peace, Rolland