I'm worried- should I be? 645n and film flatness

If your slides look good on the light table, they probably are good. This is one of those obsessive compulsive things that people can worry about for no logical reason. If you're enlarging by 10X+, it might become something to worry about, depending on your subject.

If the film isn't flat, then focus won't be uniform. Since, if the film isn't flat, then the film isn't all uniformly on the focal plane, and the parts that aren't on the focal plane will be less sharp than the parts that are. It is more likely that you would notice a difference between the center and the edges if you had a film flatness problem, but since lenses aren't as sharp at the edges as they are in the center, you might see some difference in focus even if your film is "flat enough." Of course there are also generally depth of field issues with many subjects, and so subject sharpness and background sharpness are also influenced by this factor.

To your question "should I be worried?", I would answer with an emphatic NO. Not if you can't see a problem by looking at your own images.

No, don't worry. If this was really a problem, the film back would have been redesigned decades ago.

I had been having problems with my 35mm lens on my Mamiya 645 Pro (sections of the frame with soft focus), so I put a test roll of 120 film in it, advanced to the second or third frame, and let it sit for two days. I opened the back and observed the first frame, i.e. the frame that had been sitting in the film gate for 48 hours. It looked flat. I then advanced the film to the next frame (the frame that had been in the standby position for 48 hours) and looked at it. I was shocked: it had a horrendous wave pattern to it and was nowhere even close to being flat.

Since then, I've been saving to buy a Mamiya 7.

In the meantime, I shoot 220 and don't use any frame that has been sitting in the standby position for more than a few minutes. The 35/3.5 has been much better behaved.

I'd recommend the same thing, i.e. shooting 220 and not using any frame that has been sitting in the standby position for more than a few minutes, with the Pentax 645 as well.

(As I understand it, wide angle lenses are much more sensitive to film flatness problems than longer lenses.)

I've seen thousands of transparencies from this camera projected up to 5 feet square on a Hasselblad projector. Their sharpness stands up to pretty much anything in medium format. You don't have much to get concerned about here.

This is an oldie, and is often academically debated, but the proof has to be in the pudding. If your images are uniformly sharp then presumably there is no problem. The frame should be unevenly sharp, eg corners soft and centre sharp, with film curling.

You shoot fairly quickly and auto bracket, so the film is being drawn across the rollers then. If in doubt with the first shot after the film has been stored for a while, then take two shots.

One feature not often considered is that depth of focus increases with stopping down, as does depth of field, so the 'problem' is presumably more likely wide open (which is often when the edges are blurred anyway with narrow depth of field on a subject).

A cynical view is that some of the issue arose with a kind of 'push marketing' by back manufactures trying to sell vacuum backs. Zeiss, though, claims (in a 2000 Camera Lens News) that it is a real problem and they investigated it in order to design lenses better. And that 220 is better than 120 and there is definitely an issue after 2 hours. But perhaps they were helping out Contax, too or have ultra sensitive measuring equipment that gives results not significant in the real world.

I take the point that if there was a huge issue, then manufacturers would have redesigned backs. Also, the theoretical issue may not be of much practical relevance, given the depth of focus situation and the like.

Flatness got dragged further into left field by Zeiss I believe, now that you mention it. Supposedly, and the reason for changing pressure plate orientation, the two sizes differ slightly in curl from rail to center. Pressure plates aren't exactly one plane - there is a slight rebate at two edges - and changing by ninety degrees allows more or less thrust through the center of the frame.

Nice-to-know stuff, but it won't keep me awake.

ROn:

As another 645n shooter landscape/architectural here is the lowdown on film flatness issue.

When the film is shot part ways throught the roll, the next frame tends to conform to the feed rollers of the film holder. Due to the path of the film, if left for a few days or more would take up the curvature of the feed rollers.

I was concerned and did a test - shot three frames and let the film sit in the camera for 1 week. when I advance to the next fram I removed the lens and open the shutter on B and saw there was a kind of a wave on the film. The next frame turned out OK.

I am now in the habit of wasting a frame if the film has been left in the camera for more than a day. Most of the time I just fire off the rest of the roll.

Hope that helps - try it for yourself

Hi Ron,

If, as you say, your slides seem very sharp when viewed with a decent loupe, I would not worry too much. The short length of time you have the roll in the camera probably prevents any problems.

The problem definitely exists on some cameras though. I would imagine those people who dismiss it simply haven't experienced it. The signs are blurred *sections* of the image, when nothing else seems to explain their presence, e.g. parts of a subject are out of focus even when they are the same distance away from the camera so cannot be explained by depth of field. I have seen first hand examples on both 35mm and medium format cameras. MF cameras with 16 frame backs appear to be more susceptible to the problem. I have read about it occurring on a 645N but I have not seen any example slides.

One poster has stated "If this was really a problem, the film back would have been redesigned decades ago" - this would be a reasonable assumption, but for some cameras it doesn't appear to be the case. For example, the worst case of film flatness I have seen was on a brand new Mamiya 645AFD owned by a friend of mine. He had the film back modified by Mamiya, which has cured the problem, but the last I heard (earlier this year), Mamiya were still selling the camera with unmodified backs. Others seem to have had this problem - see these threads:

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=003kkW

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=001GYA

As a footnote, the manual for the 645NII (which can be switched from 15 to 16 frames) states that "with 16 frames set, the film flatness of 2nd frame may be deteriorated". Pentax obviously think there is a potential issue, at least they are decent enough to admit it!

Thanks to all of you for taking the time to reply. I suspect that I don't have to worry given my shooting style. I will definitely burn a frame if I have to leave the film partly shot in the camera for more than a few hours.

And hey...I could always use an extra body! Maybe a used NII with a motivated seller will show up on ebay...

Ron Huff