"Cropping factor" when using Pentax 67 lenses on a Pentax 645

[hoshisato]

<p>Hi, I have a Pentax 645NII and picked up an adapter for my Pentax 67 lenses. I was wondering what the "cropping factor" would be for the SMC Pentax 67 lenses when used on the 645. My gut feeling is that it would be something like (6x7) / (6x4.5) = 1.56 but I'm not sure if that is the right way to approach the question. <br>

<br>

Hans</p>

[dougmiles]

<p>You do have the right approach, but it would be the ratio of the diagonals. The actual format dimensions are different from the nominal sizes... and these differ slightly among camera brands. The Pentax 645 is actually 56 x 41.5mm, so its diagonal is 69.7mm. The Pentax 6x7 is actually 55 x 70mm for a diagonal of 89.0mm. So, 89 / 69.7 = 1.28 factor.<br>

FWIW, the P645D is even smaller, 33 x 44mm with a 55mm diagonal. And I use P67 lenses on my Leica S2 which has 30 x 45mm framesize and a 54.1mm diagonal.</p>

[hoshisato]

<p>Thanks Doug, now I understand! </p>

[q.g._de_bakker]

If you put an 'X' mm Pentax 67 lens on your Pentax 645, it will be exactly like a Pentax 645 'X' mm lens on your Pentax 645.<br>There is no (as in: <b>NO</b>) factor.<br><br>Let me say that again: no factor.<br>A lens of X mm focal length is a lens of X mm focal length, is a lens of X mm focal length, is a [etc.] no matter what camera you put it on.<br><br>If there were, a comparison across either of the two sides of the rectangular would be as correct as one across the diagonal.<br>The comparisons over short sides, long sides or diagonals would produce different factors, because the formats have different aspect ratios. But not one of them would be more correct than any other.

[ondebanks]

<p>Hans,</p>

<p>It boils down to a question of "what format is your mental baseline for thinking about lens choice and composition".</p>

<p>If you are much more accustomed to using the 6x7 format, then follow Doug's advice to predict what the equivalent focal length to use would be on the 645 camera. So if you want, say, to use the 645 camera to obtain a shot where you would normally use a 45mm lens on the 6x7 camera, then you need to use a 35mm lens on the 645 camera, as the closest equivalent in the lineup.</p>

<p>If on the other hand, you are more accustomed to using the 645 format, then follow Q.G.'s advice - "A lens of X mm focal length is a lens of X mm focal length". Thus the adapted 45mm lens for the Pentax 67 gives the same composition as the original 45mm lens for the Pentax 645, if you set them to the same f-stop on the same body.</p>

 

[tim_bosley1]

<p>The focal length is the focal length.</p>

[chauncey_walden]

<p>Yes and no. True, a 700 is a 700 but for the image a 700 gives on a 6x4.5 film vs the image a 700 gives on a 6x7 film there is a "cropping factor" as Doug and Ray pointed out. Think of a 90 on a 4x5, it's a wide angle. Now think of a 90 on a 35mm, it's a short telephoto. Both are 90 and both produce the same image enlargement at the same distance. If you cut a piece 24x36 out of the 4x5 you would have the same image as if you shot it on the 35mm. Likewise if you cut a 6x4.5 out of the 6x7 you would have the same image as if you had shot it on the 6x4.5 with the same lens.</p>

[q.g._de_bakker]

If the question is what a 6x7 lens would be like on a 6x4.5 camera (and Hans has a Pentax 645, and bought an adapter to put Pentax 67 lenses on that) there is no (!) factor.<br>Replacing a Pentax 645 X mm lens with a Pentax 67 X mm lens will not produce a different image. No factor.

[ondebanks]

<p>Hans said he "picked up an adapter for my Pentax 67 lenses", which I took to mean that he already has Pentax 67 lenses...and the most probable reason for that is that he uses them on a Pentax 67 film body. Coming from that starting point would be the first scenario that I outlined.</p>

[pjdilip]

<p>This is really confusing. In the 35mm world, we think in terms of full-frame lenses, so on a cropped frame sensor, we'll have the central cropped portion of the same image, same sizes, same perspective. However, if a lens is specifically labeled for the crop frame (DX in nikon), does it mean that the whole field of view is reproduced on the small frame? What's the difference between say a 35mm full-frame (FX) lens and a 35mm cropped-frame (DX) lens?<br>

<br /> Coming to 645 and 67 formats, say Pentax produces same f length lens in each format. How are we to understand this? That everything else is the same, only field of view or coverage changes? Would that hold across formats to include 35mm (FX) and cropped frame (DX) as well? Size of objects will be same across all formats for a given focal length, and only coverage of the field become wider as we go to larger format lenses? Then to get a similar overall image, we would have to use defferent focal lengths accoiding to the 'cropping factor' multiplier, eg 135 mm on 67 produces a similar image and background as 120mm on 645, except that the size of things would br different to fit them into the different sensor frames... but then that would not take into account the difference in perspective of the two lengths. If we used the same focal length, say a 135mm 645 lens on 645 and a 135 mm 67 lens on 67 format, perspective and size of objects would be same, but background coverage would be different. Using a 135 67 lens on the 645 camera would give the same result?<br>

<br /> FWIW, I did an experiment with a medium format (6x8) 100mm lens and an FX lens of 105mm on a cropped frame camera, and actually got more or less the same size image. Can't say about sharpness or resolution of ddetail, since I was basically holding the MF lens in one hand at a distance from the camera... couldn't have been good for the dust on sensor :( It helps to put a short extension tube on the camera when doing such experiments ;)<br>

<br /> If that's the case, larger formats should be sensible only for landscape photographers, whereas for detail photographers who want to isolate and focus only on the central object, larger formats will not be that useful?</p>

[pjdilip]

<p>Just to clarify my understanding a little further: a 200mm medium format lens (not so costly) is not going to substitute for a 400mm FX lens (costly) if we want a bigger image of our bird or flower or whatever.. it will give just the same size image as our 200mm FX lens. Even though they may say that there is a 2X cropping factor between 6x7 and 35 mm format. What the 200mm medium format lens will give on the FX sensor is the same as the 200mm FX lens itself would give on the same FX sensor. <br>

When people talk of the 2x cropping factor 'advantage' of cropped sensor (4/3) cameras or the 1.5x factor of DX, compared with 35mm FX full-frame, it doesn't mean that we can make do with a cheaper short telephoto lens to capture that small bird in the bush. It just means that the smaller sensor will have less of the surrounding vegetation, so the bird doesn't look so tiny and lost. Yes, that could be achieved by just cutting out the centre of the bigger format image. The only advantage would be if the smaller sensor packed an equal number of pixels in the smaller frame ... but increased noise may offset some of this advantage (not applicable to film, though).<br>

Is that a correct understanding?</p>

<p> </p>

[dougmiles]

<p>You're pretty much right, but it's not that complicated sounding. A 200mm lens is a 200mm lens no matter the film/sensor size. Some of these 200mm lenses are made to "cover" a larger sensitive area than others. Some are made for small-sensor cameras, but it's still 200mm. The small sensor will just show less of the subject than a larger sensor with the same focal length.<br /> <br />If you want your 35mm camera to image the same subject area as the 6x7, then you can figure it can do that with about 100mm instead of the 6x7 camera's 200mm. That's where we get tangled up in the so-called "crop factor".<br /> <br />Another thing to note is that lenses do not have "perspective"... Perspective is strictly a matter of the relative positions of the camera vs the subject matter.</p>

[witold_grabiec]

<p>It's refreshing to see the "there is no such thing as a crop factor". This oxymoron has existed since smaller (then film) sensors hit the market. Was never discussed prior to digital. Luckily at least some people understand it, but sadly not a single photo magazine I know of acknowledges it in simple terms, in fact the magazines and many books are the reason why this is asked daily.<br>

I don't know, if I can add anything to above comments. I'd just say that, if somebody claims to have that EYE to quickly switch camera formats and know exactly how a particular focal length would frame it within a given sensor/film size, he is ... lying. And, if there are indeed those who spent exorbitant amount of time to train their eye to actually get close to making such claims real, they are not focusing on photography at all. Those people would belong to the same category as the countless pixel peepers who cannot see an image beyond the pixels that created it.</p>

[sashakrasnov]

<p>The simple way to understand what part of FOV will be cutted -- just cut from black paper a square of 70x55mm size and than cut off inside it another square of 55x41.5mm and put this frame into Pentax 67 screen. That will be FOV of any 67 lens mounted on 645 camera. I made some examples on <a href="http://skrasnov.com/pentax-67/lens-adapter-pentax-645/">my article about it</a>. Just scroll to the "Crop" section. There re two examples available.</p>