Midground unsharp - why?

[Eric James]

My favorite image from a recent trip perplexes me. (I wish I could post a shot but it was just sent in for scanning.) The background (at "infinity") is sharp. The foreground (from three to six feet) is sharp. But the midground (immobile rocks about ten to fifteen feet out) is less than sharp. The lens was a Fuji A 240mm at f22ish; the only movement was rear tilt and front rise (front and rear swing and shift were locked tight at the null settings); the rig is a 4X5; the film holder was a Fuji Quickload.

Film flatness comes to mind as the potential culprit, but I have found no other shots from this trip that show the same abnormality - in fact a few shots from the same trip that were taken after the image in question were very sharp in the midground/mid image.

Can you suggest any other "potential culprits"?

Do some Fuji Quickloads buckle upon loading in an otherwise functional holder?

Your comment are appreciated

Eric

[Brian Ellis]

That kind of thing is often the result of the midground (the rocks in this case) protruding up above the plane of focus after a tilt, combined with an aperture that wasn't small enough to make it appear sharp. However without seeing the photograph it's hard to tell for sure.

[Eric James]

Thanks Brian,

Looking at another exposure from the same motif, the midground rocks are below (i.e. lower than) the foreground rocks. It is conceivable that your suggestion, in principle, also applies to subject matter "below" the plane of focus. In the image I'm looking at now I was worried about movement (of a four-legged foreground subject), and for that exposure the aperture was f16ish. In this shot the soft midground is worse, so I suspect that you are on to something!!! Damn my defraction concerns. I should have stopped down!

The scan is probably 2 weeks away I'll post a shot when it becomes available.

Cheers

[QT Luong]

It's almost certainly the problem Brian mentioned if your midground is lower than your foreground and background. With a 240, you can shoot at f16 only if everything on the image is in perfect focus.

[Eric James]

Dang.

Well, I'm learning!

I've just ordered a 4x loupe, so that should help. On the bright side - the Quickload holder is no longer suspect!

Thanks

[scott_6029]

Depending on base tilt or axis....focus far, tilt near and then Hunt for the middle. Focus on middle. Check and see how many mm on the rail you went when you focused from far to middle. Bring it back half way....and then stop down (as a safe rule at least f 22 /32 - I am not going to get into diffraction issues here and I wouldn't worry too much at f 22 and probably not too bad at f32. It's certainly better than not getting things in focus). Then check behind glass when stopped down to see if everything is in focus. The key is hunting for the middle and focusing and then bringing it back and then stopping down, or your middle won't be in focus.

[Eric James]

A wise person recently recommended that I affix a mm scale to the bed rail(s). The suggested source for a free remedy was a company that I have not done business with, so I shied away from the request. Can anyone suggest a commercial source for a durable mm scale, with an adhesive back and miniscule thickness?

[Eric Leppanen]

This is a common error: even after applying movements, one still needs to check for objects or areas above or below the plane of focus and adjust the focus point and f-stop accordingly. I use the focus-near/focus-far/focus-in-the-middle-and-stop-down technique as well (selecting the f-stop using the technique described at http://www.largeformatphotography.info/fstop.html), which works really well for me in all my LF formats.

Near-to-far compositions with a 240mm lens in 4x5 usually requires stopping down to around f/32 to get everything in focus in a typical scene. Also, most LF lenses are optimized for around f/22 and exhibit little resolution benefit when opened up further. If you look up the 240A in Chris Perez' resolution test chart (http://www.hevanet.com/cperez/testin...0mm_thru_270mm), you'll see that resolution peaks at f/22; the center gets a bit sharper at f/16 but the corners get softer.

[Eric Leppanen]

A wise person recently recommended that I affix a mm scale to the bed rail(s)...Can anyone suggest a commercial source for a durable mm scale, with an adhesive back and miniscule thickness?

I created a home-made millimeter scale in Microsoft Excel, printed it on adhesive paper using my laser printer, then cut out the scale with an X Acto knife and stuck it to the rail of my camera. Works well for me! One of my cameras has had such a scale for four years, and it is still holding up well.

I similarly created small f-stop tables which I stuck to the sides of my cameras (so that the millimeter scale and f-stop table can be referenced simultaneously).

[Eric James]

Thanks for the link to the Christopher Perez lens test results - I've heard much of their testing but never discovered this site. Intra-design variance seems to be a big issue; what is up with that second 110XL - dropped from cliff?.

I think I'll contact the recommended source for the adhesive tape and offer up some $$$ - if that doesn't work I'll look into adhesive laser papers. Four years and still going sounds adequate