[bull drinkwater]

let me state something I believe but you can dispute if you must.
with 75mm beng normal for 645 film cameras, and 55mm being normal for digital cameras, when you mount a 645 film lens on a 645D you get a crop factor. I received from a fellow member, 1.26 from 645 digital to 645 film and 1.62 from 645 digital to 67 film. if this be true then the 45mm would be the proper length for the 645D. it would equiv to 56mm. the 75mm would equiv to 93mm on the 645D. it would take a 15-17mm lens for real wide angle. the other option is for all the new 645D lenses being built forthe645D sensor size. when shooting from normal to telephoto it's all good. when shooting wide or extra wide you have a dilemma, my only solution has been a 35mm lens with a screw on aux lenses.

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Last edited by bull drinkwater; 07-24-2019 at 08:55 PM.

[johnha]

According to Ricoh's lens data, the 645 55mm is equivalent to 44mm in 24x36 format, the 645 33-55 is equivalent to 26-44 in 24x36 format and the 645 28-45 is equivalent to 22-35.5 in 24x46 format. How much wider do you want to go?

When you're comparing focal lengths from cropped formats to different sized uncropped formats, it all gets a bit messy. Better to translate them into a baseline format and go from there.

[bull drinkwater]

According to Ricoh's lens data, the 645 55mm is equivalent to 44mm in 24x36 format, the 645 33-55 is equivalent to 26-44 in 24x36 format and the 645 28-45 is equivalent to 22-35.5 in 24x46 format. How much wider do you want to go?

When you're comparing focal lengths from cropped formats to different sized uncropped formats, it all gets a bit messy. Better to translate them into a baseline format and go from there.

I don't see your logic. the aps-c is smaller than full frame, so that when you use a full frame on a aps-c you get a 1.5 crop factor. ie a 100mm lens from afull frame would equiv 150mm on a aps-c camera.
the 6345D is smaller than the 645 film so you should get a 1.26 crop factor. a ventage 200mm 645 lens should equiv 250mm on the 645D camera.

[stevebrot]

let me state something I believe but you can dispute if you must.
with 75mm beng normal for 645 film cameras, and 55mm being normal for digital cameras, when you mount a 645 film lens on a 645D you get a crop factor. I received from a fellow member, 1.26 from 645 digital to 645 film and 1.62 from 645 digital to 67 film. if this be true then the 45mm would be the proper length for the 645D. it would equiv to 56mm. the 75mm would equiv to 93mm on the 645D. it would take a 15-17mm lens for real wide angle. the other option is for all the new 645D lenses being built forthe645D sensor size. when shooting from normal to telephoto it's all good. when shooting wide or extra wide you have a dilemma, my only solution has been a 35mm lens with a screw on aux lenses.

Here are a few talking points:

• Normal, while a useful idea, is hard to pin down
• The convention of normal being "length of frame diagonal" sound good except when comparing field of view for so-called "normal" lenses for formats having different aspect ratios (e.g. 3:2 vs. 4:3 vs. 7:6 vs. 2:1 vs. 1:1, all of which are used for medium format capture). Better to consider field of view (FOV) on long, short, or diagonal axes, which ever suits your favor.
• Likewise, useful consideration of crop factor is also limited to formats having the same aspect ratios

Regarding actual product:

• Both the DA 25/4 and D FA 25/4 are seriously wide on the 645D (95.5° FOV diagonal). That is comparable to 20mm on 24x36mm FF.
• Neither of the two lenses above are current product. If you want a current product Pentax prime for the 645D, you are stuck at 35mm or longer.
• If you can live with a zoom, the D FA 28-45/4.5 SR might be a good alternative offering a field of view comparable to 22mm on 24x36 FF (89° diagonal).

Steve

[bull drinkwater]

Here are a few talking points:

• Normal, while a useful idea, is hard to pin down
• The convention of normal being "length of frame diagonal" sound good except when comparing field of view for so-called "normal" lenses for formats having different aspect ratios (e.g. 3:2 vs. 4:3 vs. 7:6 vs. 2:1 vs. 1:1, all of which are used for medium format capture). Better to consider field of view (FOV) on long, short, or diagonal axes, which ever suits your favor.
• Likewise, useful consideration of crop factor is also limited to formats having the same aspect ratios

Regarding actual product:

• Both the DA 25/4 and D FA 25/4 are seriously wide on the 645D (95.5° FOV diagonal). That is comparable to 20mm on 24x36mm FF.
• Neither of the two lenses above are current product. If you want a current product Pentax prime for the 645D, you are stuck at 35mm or longer.
• If you can live with a zoom, the D FA 28-45/4.5 SR might be a good alternative offering a field of view comparable to 22mm on 24x36 FF (89° diagonal).

Steve

I don't know where 24x36mm comes into the picture. I compering the equiv focal length of say a 100mm lens made for a 56x41.5 film area use on a 44x33 sensor area. the 645D could be considered the asp-c of 645 medium format.

[stevebrot]

I don't know where 24x36mm comes into the picture. I compering the equiv focal length of say a 100mm lens made for a 56x41.5 film area use on a 44x33 sensor area. the 645D could be considered the asp-c of 645 medium format.

I'm sure you will figure it out.

Addendum: The FOV for 645 on 120 film are provided with the review pages on this site, should that be relevant.


Steve

[johnha]

I don't see your logic. the aps-c is smaller than full frame, so that when you use a full frame on a aps-c you get a 1.5 crop factor. ie a 100mm lens from afull frame would equiv 150mm on a aps-c camera.
the 6345D is smaller than the 645 film so you should get a 1.26 crop factor. a ventage 200mm 645 lens should equiv 250mm on the 645D camera.

Rather than rely on logic, I'm using Ricoh's lens data and over a year's personal experience with the D and the 33-55 zoom (I'd love to be able to justify the cost of the 28-45). I've also tried various P6x7 lenses (including the 35mm fish-eye).

Any crop factor based on the 645 or 6x7 film formats is related to the field of view in those formats. A 45mm on 6x7 film is a very wide lens, a 'normal' wide angle on 645 film, slightly less wide angle on 33x44 and 'normal ish' lens on 24x36 (approx. 24x36 equivalents of 22mm, 28mm, 35mm & 45mm respectively).

[Adam]

This table might help simplify things:


The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

[Sandy Hancock]

My DFA645 35/3.5 is a fantastic wide angle lens on the 645Z - roughly equivalent to 28mm on 24x36 and 45mm on full frame 645.

If you want you much wider you'll need to spend serious bucks on a DA/DFA 25/4.0 or DA 28-45/4.5

[rdenney]

Learn lens lengths in relation to the film diameter. This is a model, and all models are false even though some are useful. This one is useful.

Those who have broad multi-format experience soon give up trying to equate focal lengths across formats, and instead learn what focal lengths are long and short for that format.

150mm is a format-diameter baseline for 4x5 large format, for example. And 75mm is half that while 300mm is twice that.

So, if 55 is the format-diameter baseline for 645 digital, then 23 is half that and 110 is twice that. Therefore, 23 on a 645D is like 75 on 4x5.

Likewise, 70 is the format-diameter baseline for 645 film, so 35 is half that and 140 is twice that.

Round to the nearest available lens. The 25 for the 645d is closest to the 35 for 645 film, if you can find (er...afford) one.

I have a 65 for 4x5, a 35 for 645 film, a 45 for 6x7, a 21 for 24x36, and a 12 for APS-C. All are half the format diameter or less. That’s why the lack of a lens in the 20-25 range for the 645 digital represents a hole. But the 28 end of the 28-45 has been useful enough.

When you change format shape, lenses feel different based on how you use that shape, but it’s more productive just to learn them through experience than to try to analyze it out.

Rick “format omnivore” Denney

[kaseki]

Two points.

(a) My understanding is that all recent Pentax 645 "digital" lens assemblies that include "FA" in their designations can fully cover the exposed 120 film plane area of the 645 film cameras. I definitely would like to know if that is not true.

(b) My second understanding is that the term "normal" was intended to yield (when printed) the same perspective of objects in a cluttered field as the human eye does. This tends to be a bit short of the focal length deemed optimal for portraits, where too close yields obvious facial distortion, and too far tends to lose depth cues. This may be due to conversational distances being shorter than nature observation distances.

[rdenney]

Two points.

(a) My understanding is that all recent Pentax 645 "digital" lens assemblies that include "FA" in their designations can fully cover the exposed 120 film plane area of the 645 film cameras. I definitely would like to know if that is not true.

(b) My second understanding is that the term "normal" was intended to yield (when printed) the same perspective of objects in a cluttered field as the human eye does. This tends to be a bit short of the focal length deemed optimal for portraits, where too close yields obvious facial distortion, and too far tends to lose depth cues. This may be due to conversational distances being shorter than nature observation distances.

A. I don’t think the DFA lenses cover the film format, but others will know those details better than I. I guess I’ve never fitted my two digital-era lenses to my 645NII.

B. “Normal” is strictly a convention. Large-format photographers describe focal length as “short”, “normal” or “long”, just as a convenience for discussion. Perspective is entirely controlled by camera position, and focal length only controls how much of the scene will be included in the image from that position. For portraits, long lenses require longer distances (if you want more than just the subject’s nose in the picture), and when viewing a small piece of a scene from a long distance, perspective will seem compressed. But the lens is merely constraining possible camera positions, and camera positions are doing the work of perspective relationships.

“Wide angle”, “normal”, and “telephoto” have to do with lens design, not focal length in relation to format. Wide-angle lenses have wider coverage relative to focal length, which is needed by shorter lenses and larger formats. For SLRs, they are also retrofocus, meaning the distance to the rear node is longer than the focal length, which makes a short focal length possible in front of a deep mirror box. A telephoto lens has a distance to the rear node shorter than the focal length, making fitting on a shorter bellows possible. Normal lenses are often wide-angle designs to accommodate camera movements.

Fora camera like the 645z, I think in terms of short and long, and normal is just a convenient label.

Rick “not normal” Denney

[texandrews]

645 DFA and FA and A lenses cover 645 film. DA does not (although in the case of the 25 it's a lens hood problem only). That's why Pentax has a latent advantage over Fuji, if they can ever use it.

[pentaxus]

Also part of the water muddying in format comparisons is the fact that Leica used a 50mm lens on their first cameras because they found it too difficult to make a wider lens. 50mm became "standard" when 43mm is the actual diagonal.

[texandrews]

Also part of the water muddying in format comparisons is the fact that Leica used a 50mm lens on their first cameras because they found it too difficult to make a wider lens. 50mm became "standard" when 43mm is the actual diagonal.

"Standard" is maybe a better term than "normal".