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Has anyone compared the Pentax 150mm 3.5 "M" and &
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PostPosted: Fri Oct 14, 2022 7:57 am    Post subject: Has anyone compared the Pentax 150mm 3.5 "M" and & Reply with quote
Recently I picked up a Pentax-M 150mm 3.5 (PK mount) and I am really impressed with this lens. I see that Pentax also made a 150mm 3.5 for medium format 645, with a different optical formula by the looks of it. The M is 5 elements in 5 groups, the 645 is 4/4. So not the same.

Has anyone had the opportunity to compare these two 150mm lenses? If yes I would be curious as to your findings.

Regards, Christine


PostPosted: Fri Oct 14, 2022 8:18 am    Post subject: Reply with quote
A simpler optical design Typical of many 135mm lenses. The M-series was designed to be more compact, so it added a double concave second element to achieve short physical length.(seen in the 120,135,150 and 200mm) Not necessary for 645 since that has a much larger flange focal distance. Lenses in the portrait range are usually shorter than their 35mm counterparts.



PostPosted: Fri Oct 14, 2022 7:32 pm    Post subject: Re: Has anyone compared the Pentax 150mm 3.5 "M" a Reply with quote
connloyalist wrote:
Recently I picked up a Pentax-M 150mm 3.5 (PK mount) and I am really impressed with this lens. I see that Pentax also made a 150mm 3.5 for medium format 645, with a different optical formula by the looks of it. The M is 5 elements in 5 groups, the 645 is 4/4. So not the same.


A 150 on 645 format has a greater angle of view than a 150mm lens on 35mm, so they will never be the same optical computation. Maybe on rare cases they can have the same configuration, but never the same computation (i.e. curvature, glass type, position of each element, glass diameter)

On the other hand, if the angle of view and aperture is similar, they could have the same configuration.

For example the 35/3.5 SMC Takumar (for 35mm) and the 75/4.5 Pentax 6x7 SMC Takumar have the same optical configuration. The latter is more or less an "enlarged" version of the former. The optical diagrams are very similar. Both have similar angle of view and aperture.


PostPosted: Sat Oct 15, 2022 4:20 pm    Post subject: Reply with quote
D1N0 wrote:
A simpler optical design Typical of many 135mm lenses. The M-series was designed to be more compact, so it added a double concave second element to achieve short physical length.(seen in the 120,135,150 and 200mm) Not necessary for 645 since that has a much larger flange focal distance. Lenses in the portrait range are usually shorter than their 35mm counterparts.



Fwiw that is the Pentax 645 150mm f3.5.

This in full frame terms is equivalent to the look of a 93mm f/2.17, where with 35mm lenses you see Double Gauss and Sonnar types intermingle between 90-135mm.

I went looking at other 645 format 150mm lenses, and a lot of them are double-gauss at similar speeds.

What I think is happening here: scaling-up double-gauss designs scales up everything, including flange distance, which at 150mm focal length which be quite long, ideal for a medium format SLR. Having seen in design software and confirmed with some lenses I own.


PostPosted: Sat Oct 15, 2022 6:51 pm    Post subject: Reply with quote
I think classes of lens designs are mainly relative to field of view. Focal distance is longer on medium format so normal fov is around 80mm where you see a double gauss design and often a short tele design in 35mm which has normal around 50mm. In medium format you will see wide angle lens designs around 50mm.


PostPosted: Sat Oct 15, 2022 7:27 pm    Post subject: Reply with quote
D1N0 wrote:
I think classes of lens designs are mainly relative to field of view.


True, but certain conveniences appear under certain circumstances. But I suppose I was just describing my experience with how Double-Gauss lenses can work when simply 'scaled up'.

So I suppose the benefit for a manufacturer: a quasi-symmetrical lens like this gives more consistent performance when focused at close distances, compared to a Sonnar-like one.

And the long flange distance no longer becomes an issue with a medium format SLR, which has a long one.



A constraint on back focal distance will play a big role in lens design of any format, in some way. But I think you said as much anyway Razz
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