How small do your tilt angles get?

[Eugene van der Merwe]

Like most i also use the groundglass as a guide. For some reason i find the difference (in sharpness, ignoring perspective changes) easier to spot when applying a rear tilt than when doing a lens tilt, so in those cases tilts can be as little as 2 degrees. For lenses shorter than 90mm on 4 x 5, i'll often rather use rear tilts since, particularly when shift/rise is also used cutting the corners of the image circle becomes more likely.

[Leonard Evens]

First, let me report the results of some experiments. I put my camera at various heights over some horizontal plane to see if I could detect any focus shift between a near point and a far point. Using a 150 mm lens, at height about 15 meters, the associated tilt would be about 1/2 degree. I couldn't detect any focus shift. At height about 5 meters, where the associated tilt would be slightly under 2 degrees, I could detect a shift of about 1 mm. Given that I just used my 2 X glasses, where my normal focusing error (due to depth of focus) could be 0.5 mm or larger, that was consistent with the expected tilt angle. I could have got closer by using a more powerful loupe.

Those experiments, together with my past experince and what several of you reported, suggest that in practice, tilts less than 2 degrees are rare, and tilts smaller than 1 degree are not detectable on the gg.

A couple of other points.

I've already mentioned rare cases in which I would use a tilt despite not being able to set it based on what I see on the gg. I gather from the responses that most people never do that, but some occasionally do so.

Finally, let me remark that while what you see on the gg is a good guide, it is not that simple. What you see depends on the power of your loupe, the quality of the focusing screen, how good your vision is, and other factors. I am not usually surprised when looking at the developed negative or a print, but I usually stop down a bit extra, when I can, in order to account for inevitable errors in focusing, etc. But sometimes I find that despite my best efforts the result is softer in some areas than I would like. I've seen the same thing in prints on exhibition. I really doubt that anyone always gets exactly what he sees on the gg in the final print.

One point I keep making is the following. If you have to stop down beyond f/22 to get the depth of field you want, it will often be difficult if not impossible to do that by just looking at the gg at the taking aperture. In most cases the image will be too dim. You can improve that by using a bright focusing screen, but there is a limit to how far that will go. I have a Maxwell screen which is exceptionally bright, and I can't see much of anything past f/16. A younger person with better dark vision will probably do somewhat better, but when I polled this forum a few years ago on this subject, I don't think anyone claimed to be able to see well enough beyond f/22 to judge DOF.

One way to deal with the dimness is to use a higher power loupe. I'm not sure why that works but it appears to do so. The trouble is that a loupe in effect decreases your CoC. One can make the argument that a 2 X loupe shows approximately what you would see in a 2 X enlargement at normal viewing distance (or a larger print from proportionately further away.) If you use a 4 X loupe, you divide the viewing CoC in half, which means you in effect double the f-number you need to have everything you want in focus. Of course, if you stop donw something like two stops more than absolutely necessary, then you will get everythi8ng you want in focus. But you may also have to use too long an exposure problem. I am particularly sensitive to that because even a slight wind can blur foliage if the exposure time is too long, and that is often a problem for the kinds of pictures I take. I frequently find myself debating whether I should stop down just a bit more to get the DOF I want to to worry more about leaves blowing in the wind.

So what I do is the following. I focus using the near point far point method by seeing where on the rail those points are in focus and focusing halfway in between. Of course, I also look at the gg and make adjustments based on what I see. I set the tilt by a method based on what I see on the ground glass, though occasionally I use Wheeler's Rule, which requires some measurments and calculation. I calculate the needed f-stop based on the focus spread. (My rule of thumb is multiply the focus spread by ten and divide the result by two.) If that tells me to stop down to smaller than f/32, I also sometimes check what Paul Hansma's table, which tries to take diffraction into account, suggests. I then stop down to see if my choices seem consistent with what I see, in so far as I still can see anything. I also have a rule which allows me to estimate what I would see say at f/32 based on what I can see at f/16, and that helps.

This may seem much too complicated, but it doesn't really take much time to do. I spend much more time thinking about the location of the camera and the framing of the scene, which are by far the most important things. I don't carry a calculator with me since I can do all the necessary calculations in my head.