Unexpected soft focus with lots of camera movement

[Ian_5357]

Hi

Over the weekend I took some shots of a series of lampposts. To get the framing I wanted whilst keeping them vertical, I had to raise the camera slightly, tilt the back back to the vertical and then employ as much rising front as I could before the bellows stopped me.

I used a 6x7 roll-film back on a 4x5 back, so the framing was not at the absolute edge of the lens' (Rodenstock APO Sironar-S 150mm, if it helps) coverage.

Before stopping down, I checked (and, I'm 99% sure, re-checked) the focus with the lens wide-open on the front lamppost. I then stopped down to f45 so would have assumed that the focusing would have been safe anyway. But when I developed the film, the same lamppost was distinctly fuzzy at the very top of the frame.

I thought of three possible reasons...

1. I cannot focus properly. Whilst always a possibility, I would've thought that f45 would cover most sins in that department.
2. The roll film back (a Horseman) had buckled the film slightly. But I took a few frames of the lampposts (for filters etc) and all of them were iffy. Yet I used the back for other shots and there didn't seem any problem at all with them.
3. There's an issue with "extreme" camera movements and stopping down that I haven't considered... when stopped down I could not really check the focus properly as it was a little dark, so this seems the most likely to me.

Answer / suggestions / comments to help me go back this weekend and get a printable negative are greatly appreciated!

Many thanks

Ian

[Bob Salomon]

At f45 you were in diffraction. You will get the best results at f22.

[Paul Fitzgerald]

Ian,

there is a difference between what a lens can 'illuminate' and what the lens can 'cover', you may have over-run what the lens can cover.

[Eric Wagner]

With my camera pressure from the bellows, when in the configuration you describe, can cause the front standard to slowly creep out of vertical position after I have focused. I have to remember to tighten the knobs that control lens tilt a little more than usual when using a lot of rising front.

[Ole Tjugen]

You tilted the camera, then tilted the back to vertical. But you didn't tilt the lens back to vertical?

Did you check the focus over the whole of the post?

No wonder some of the lamppost was fuzzy - even at f:45 the DoF is reduced a lot when using non-parallell standards.

[Craig Wactor]

It sounds like to me it was just a scheimpflug thing. If you pointed the camera up, and then the back standard back to vertical, you would have something like this (except pointed diagonally upwards):

l i

with l= rear standard, = front standard and i= lamppost. I think you may have needed to return the front standard to vertical to get both standards parallel with the lamppost. It is a little bit of a closeup, so DOF is going to be limited.

In addition, you put enough rise to possibly get past the sharp coverage of your lens as paul mentioned.

One thing that may help is a loupe that you can tilt in the corners of the groundglass.

I hope I did not misunderstand your post and now am sounding condescending.

[Sam Crater]

According th the comparison charts on this website, your lens allows 45mm of rise in Portrait mode. Based on your description of your technique it sounds likely that you exceeded that and ran into the area where as the other poster said there is illumination but not acceptable sharpness.

[Brian Ellis]

I have to respectfully disagree with Bob, I think it's unlikely that your soft area at the top of the image is a result of diffraction. A loss of definition due to diffraction would show up over the entire image, not just at the very top. Also, with 6x7 film at f45 you'd have to make a huge enlargement before the effect of diffraction would be obvious. I'm also not so sure about the idea that your rise exceeded the circle of good definition. I'm not a lens guru but it's my understanding that with modern, top quality lenses like the one you were using there usually is little if any difference between the image circle and the circle of good definition. Also, as you point out, you were using a lens with 6x7 film that has more than adequate coverage for 4x5 film.

While I certainly don't claim to know the answer to your question for certain, I think a more likely explanation is that you didn't get the back standard and the front standard in correct alighnment with each other and/or with the subject when you tilted them (I assume you tilted the front as well as the back) after raising the camera. While a small aperture can cure some problems of this kind it can't necessarily cure them all. If you don't already do so you might find it helpful to use a level on the front and the back to bring them parallel with each other and with a vertical subject after they've been tilted as you describe.

[Eric Wagner]

I agree with Brian. If diffraction was the problem it would not be confined to one area of the negative. I use a 180 Sironar-S on 5x7 and it is sharp almost all the way to the vignette point. If you had exceeded the circle of good definition I'm sure the corners would be vignetted. Also, to reach the edge of the circle of good definition with your 150 would require about 3 inches of rising front when you are using a 6x7 back. That is a lot of rising front! Neither of the cameras I have regularly used for 4x5 are capable of that much rising front with a 150. So my best guess remains that you suffered some kind of an alignment problem.

[Ian_5357]

Thanks to all. I suspect that not tilting the front standard back to vertical is the problem here, as now you mention it I cannot recall doing it. I'll give it another go this weekend. Thanks very much. Ian