Alternative focus technique for tilted lens
Simple iterative way to focus with tilt
A simple step by step procedure to set the plane of focus for your tilt/shift lens
Some time ago Keith wrote an article explaining how tilting a lens affects the plane of focus. The article includes a discussion about what happens when you focus and tilt the lens. The technique outlined in that article is Keith’s preferred method for setting up interior and architectural shots where the subject(s) are usually some distance from the lens.
What about closer?
The alternative iterative technique described here, is what Keith is more likely to use for close-up product shots where the measurements required to use the tables of settings included in the other article, may not be very easy to obtain. This technique also works with extension tubes and teleconverters added to a lens.
If you are completely new to tilt/shift lenses you might first want to read the original article about using tilt/shift lenses that Keith wrote after getting his first tilt/shift 24mm lens.
Normally the plane of sharp focus of a lens is a flat plane parallel to the sensor of your camera. Think of focusing on a point on a wall directly in front of you. It’s possible to get the entire wall in sharp focus across your frame, even though the wall off to the sides is further away from your camera than the point you have focused on.
If you’re an old hand with large format cameras, please excuse my somewhat lax terminology, this article is firmly aimed at those relatively new to lens movements, and likely with minimal film experience at all, yet alone large format.
Tilting the lens moves this plane. The problem that a lot of people experience, is that working out just how it’s tilted seems very difficult and arbitrary. One of my aims in the previous article was to dispel some of the myths about this process and show that it really wasn’t that tricky at all (and doesn’t require any maths).
Look at this photo of a X-Rite ColorChecker Passport.
I’ve taken it at f/3.5 with a 24mm lens (on a full frame Canon 1Ds Mk3).
As you might expect at f/3.5, the depth of field is fairly thin (I’ve shot this at f/3.5 to make the examples here a bit easier to show).
The camera setup shows the relationship between camera and subject.
It should be obvious that a flat plane in front of the camera needs tilting quite a bit, if the whole device (and lanyard) are to be sharp in the photo.
The lens I’m using here is the Canon TS-E24 3.5L mk2 which as well as being extremely sharp, and with almost no chromatic aberration, also focuses down to 21cm.
You may notice that I’ve an angle finder fitted to the viewfinder – this allows me to make the kind of critical focus adjustments I need when setting up a shot like this, when outdoors or on location in a busy factory.
Here, however I’m in my office, and can use the camera tethered to a computer, in conjunction with ‘liveview’ from the camera.
The software I’m using is DSLR Remote Pro, but the process works just fine with other solutions. I’ll show some screen shots that illustrate the process I’m adopting.
Before trying to focus the lens, you do need to decide how to move the lens.
In this case, with a flat plane in front of the camera, I just need to tilt the lens downwards. If I was taking a shot of the cover of a book standing up, then the plane of focus would need to be tilted sideways (also known as ‘swing’). if you’re unsure of this, I’d suggest going back to re-read the original using tilt article, since I’ve included much more in the way of explanations and examples there.
Look carefully at the scene and select two points where the plane of sharp focus needs to pass through.
These should ideally be some way apart in your camera’s field of view but do not need to be symmetrical about the centre of the frame.
I’ll refer to these two points as the near and far points.
In this example, I’m going to be using the two X-Rite ‘circular X’ logos.
Here’s the liveview display, with no tilt of the lens
Zooming to 100% shows the limited depth of field at f/3.5
The process for focusing is:
- Pick an arbitrary value for tilt
- Focus on the far point
- Look at the near point (zoom to 100% to see clearly, if using liveview)
- Decrease the lens focus distance slightly and look what happens to the near point
- If the near point becomes sharper, then you need more tilt (select a higher value for tilt and go to step 2)
- If the near point becomes softer, then you need less tilt (select a lower value for tilt and go to step 2)
- If the change in focus of the near point is difficult to see, then move to step 8
- Focus on the far point
- Look at the near point
- Increase the focus distance slightly and look what happens to the near point
- If the near point becomes sharper, you need less tilt (select a lower value for tilt and go to step 8)
- If the focus point becomes softer, you need more tilt (select a higher value for tilt and go to step 8)
Within a few iterations of this process, you should end up with a combination of focus and tilt that has both near and far points at optimal sharpness.
Note: This method of focus is based on an article by Howard Bond – “Setting Up the View Camera” in the May/June 1998 issue of Photo Techniques Magazine. However, variants of it have been used for many years before.
Have a look through these screen shots which illustrate the process.
To start with, I picked a value of 2 degrees (the lens goes to +-8 degrees). Tilting the lens downwards will bring the bottom of the focal plane towards me (look at the diagrams in the original article if you’re unsure of this).
Here’s the far point.
Here’s the near point. Obviously wrong, but do I need more or less tilt?
Moving the lens focus closer (shorter focus distance on the lens barrel) makes things sharper.
This tells me I need to increase the tilt from its 2 degree setting.
Let’s put the lens tilt up to 6 degrees.
Here’s the far point after I’ve refocussed the lens
Whilst here is the near point (this is a lot closer to the setting I need but still not quite right).
This looks better, so I focus the lens a bit closer (below)
The image becomes softer, meaning that I’ve a bit too much tilt (at 6 degrees).
OK, back to 4 degrees.
Here’s the far point.
and here is the near point
Moving the focus closer, sharpens the image, suggesting that I need more tilt than 4 degrees (but less than 6)
I’ll not show any more shots, but suffice to say, the result at 6 degrees looked better than 4 degrees, so I went for 5.5 degrees and it was pretty much spot on. (move mouse over image below to see the lens tilted)
When you have the right tilt, any change of focus will soften the near point.
Remember that the plane of focus is set by the combination of focus and tilt, and that the two interact together.
With practice it’s quite easy to get the plane just where you want it.
It’s important to note though, that the tilted plane is actually a wedge shape, so the depth of field at the near point may be extremely thin.
There is only so much you can do with the tilted plane. It won’t bend or give you any extra depth of field. Learning to make use of tilt takes practice and ideally the ability to visualise where the plane needs to go, to produce the effects you are after.
I’ve deliberately drawn out the process to show what’s going on. It took far far longer getting all these screen shots that it did to actually set the focus…
A bit of shift?
One thing you’ll notice is that when adding tilt, the framing of your view changes slightly.
If the shift and tilt axes of your lens are aligned, then a few mm of shift will correct this.
The TS-E 24 3.5L Mk2 I’m using here, includes an adjustment for this alignment, but the older version of the 24mm lens (and TS-E45 and 90) need modifying for this function. It’s actually quite easy to do, and I’ve written up the modification process, using my own TS-E90mm as an example.
Stopping down the lens to f/8 give a very sharp image, as shown by these two 100% samples from a 21MP image.
The close point.
The ‘far’ distance.
In the steps outlined above I suggested starting with an ‘arbitrary’ amount of tilt.
My initial choice of 2 degrees was always going to be too low.
How did I know this?
Well, looking at the side view of the camera and subject, I can make a quick estimate of the ‘J’ distance that’s the important measurement from the tilt tables I’ve reproduced in the first tilt focus article.
Guestimating the distance to between 20 and 30 centimetres (less than a foot) immediately tells me that the likely value is going to be between 4.5 and 7 degrees.
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Of course you don’t need the tilt tables to use the lens, or any understanding of this distance ‘J’, but knowing what they mean makes the whole thing a lot easier, even if you are using just this iterative method.
All our UK used photographic items come from MPB – check out their stock – it’s where I got our original TS-E24 and TS-E90 lenses from.
Using tilt tables or this technique?
If I’m doing architectural work, then I tend to be in a larger space with the camera several feet from the nearest area I want in the photo. The tilt tables work just fine and give a quick way of setting tilt if I know the ‘J’ distance.
For close-up work, working out a correct value of ‘J’ is more tricky, since the precise part of the lens you need to measure it from is not readily defined. For studio use, the iterative method is quick to use, especially with a tethered camera.
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Some articles that may be of interest:
- Using a tilt/shift lens - what it is they actually do
- Focus with tilted lenses - lots more information about what's going on when you tilt a lens. See also: Focusing the view camera - External link to [very] detailed coverage of camera movements
- Keith's tilt table spreadsheet (zipped file)
- Using old lenses on your DSLR - fun with adapters
- Keith's lens reviews and lens related articles
More of Keith's articles/reviews (Google's picks to match this page)
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