I'll ask this as clearly as I can manage!

As you may know, I am a keen user of Pentax 67II, and consider the system and format to be superb. One of my main subject matters is steam locomotives in action, sadly often in poor weather. Given the nature of the subject, I need BOTH high shutter speeds, and depth of field, which in poor light, on long lenses, on 6x7 can be very hard to achieve! I usually manage to balance the factors fairly successfully, but there are times when something has to give, and a compromise be achieved.

I am aware that, for any given angle of view, the bigger the film format, the longer the lens, and hence the less the depth of field (all other things being equal). I occasionally think that I should have a 645 system too, so that I can get greater depth of field when the light dips (while maintaining angle of view).

Then I thought - isn't that silly! If I'm going to use 645, why don't I just use my 67, put on a shorter lens, and use only the centre of the negative/transparency (equivalent to a 645 image) - getting the increased depth of filed by using the shorter lens.

Now my question! Would doing that lead to an image the same as a 645 image (in terms of perspective, proportions, depth of field, etc.)? Or would there be an advantage in having a genuine 645 image taken on a shorter lens (apart from the ease of use of the 645 camera - I mean purely optically/visually in terms of the result)?

I hope this question is clear - let me know if it's not!

-- Ed Hurst (BullMoo@hotmail.com), August 22, 2001

Answers

Ed; you could do what you say, using the same focal length as a 645 lens on your 67II and cutting down the slide, HOWEVER, it would be much easier to use the right lens on the 67II and just use the new Provia F 400 film. That way you could have the f-stop and shutter speed you want. That three stop advantage over Velvia is hard to beat. Also, the grain is surprisingly small. I'm sure it would push 1 stop if necessary.

-- Steve Rasmussen (srasmuss@flash.net), August 22, 2001.

Ed, by using the same focal length lens on the 67 as the 645, then cutting the slide down to 645 would yield the same DOF as if you had used the 645 from the start. The cut down image would be the same as the 645 in field coverage (using the same lens). There are times when a 35mm camera works better than a 67. This could be one of those.

-- Steve Rasmussen (srasmuss@flash.net), August 23, 2001.

Would it be the same in every other sense too?.............. Yes

-- Steve Rasmussen (srasmuss@flash.net), August 23, 2001.

I see your point, Steve. Unfortunately, I often have situations where I am already using 400, 800 or 1600 ASA, and still need help with depth of field! Any thoughts about the optical question?

-- Ed Hurst (BullMoo@hotmail.com), August 23, 2001.

Thanks again, Steve. You say that the image would be the same as a 645 image in terms of field coverage. Would it be the same in every other sense too (depth of field, perspective of the image [by which I mean the relative size of near objects, compared to distant objects])?

-- Ed Hurst (BullMoo@hotmail.com), August 23, 2001.

Ed, A few years ago I was doing somewhat similar things (only with ships). I found that almost any substantial 35mm tele lens was able to outperform the p67 set-up I was then using. I remember borrowing a friends Sigma SA5 and a Sigma APO 400mm lens. The body had mirror lock-up -which sort of amazed me- and the APO lens was excellent, with a very strong tripod collar. The whole set up was light, and worked well when needed with a Sigma 1.4 teleconverter - even though Sigma literature said it wasn't supposed to work with that long tele. The enlargements were superior to the ones with P67, partly or mostly because I did not have to always use fast film, and never had to crop as the 400mm in 35 adds that much more reach. For me anyway, 35mm worked better for real long stuff, and also I learned, for true macro; I have heard the same from at least 5 other uses, who nowadays always break out the 35mm stuff in these select cases.

-- Miles Stoddard (p67shooter@yahoo.co.uk), August 27, 2001.