Saturday, August 12, 2017

Something different

Most of my posts are about achieving the best exposure or focus that you can: so I decided a while ago to 'try something different'.

Pinhole photography!

The other day my new lens arrived: a pinhole lens from thingyfy


Unlike other pinhole lens, or one you make yourself, the Thingyfy lens has variable apertures. From 0.1 to 0.8mm.

The lens has a focal length of 50mm, which can be changed by using extension tubes.

The ability to dial-an-aperture is the real selling point for me, as the Prober-Wellman equation in the visible bands shows the optimum pinhole size varies with focal length and M, the magnification, which is simply the focal length divided by the subject distance from the aperture:
Others have plotted the above for various focal lengths and magnifications:
For landscape work, where M approaches 0, for a 50mm focal length the optimum pinhole diameter is about 0.25mm.

So what does an image look like with my new lens?

Here are a few test images taken this afternoon. Both typical scenes around us at the moment: crops and cricket (note I shot when the bowler was running).

Both images benefited from being ETTRed via Magic Lantern, of course; and were post processed in Lightroom and Photoshop. At ISO100 the ETTR exposure was 5s on one and 3.2s on the other.

One of the downsides of pinhole photography is that you are shooting at very high f/stops, ie in the above images at F/200. At such F/stops you will see every dust particle and therefore you will need to carry out a bit of post processing. I'll be writing about post processing in another post.

Obviously pinhole photography is a statement: it creates ethereal images that are the antitheses of every thing I tried to do in photography, up until now. I'm looking forward to exploring the new lens, including IR pinhole photography.

 
 

Sunday, August 6, 2017

Macro is difficult enough, with out this...

As we know, macro photography is difficult, ie even with closed down apertures we still have very narrow depths of field.

On top of that, macro photography can take the environment out of the image: leading to rather subject-fixated images.

A while ago I bought a rather unusual lens: the Laowa 15mm f/4 Wide Angle Macro:

Billed as the world’s widest 1:1 Macro Lens, it features an ultra-wide 110 degrees angle of view of with 1:1 maximum magnification. Thus achieving focus very close to the subject but at the same time, able to include background details, ie to show where and how the subject lives. Rather unusually, it also has with a +/- 4mm shift feature.

I also have the Macro Twin Flash KX-800 from the same company: https://www.venuslens.net/
To complement the set up, as I like getting low, I also have a PlatyPod Max and the ReallyRight Stuff  BC-18 Micro Ball:



Finally, I need to add in my Varavon Multifinder, which allows me to access LV from above the camera:
Pulling all this together you end up with a 5D3 that now looks like this on my kitchen work surface:

As for what all the above can do: all I can offer at the moment is a test image from the garden and a row of mushrooms:
This image was taken at not a very large magnification, with the manual aperture of the lens closed down to F/32 and a downward shift of a few millimeters. 

Clearly this is not an award-winning image: just a test capture; and I have much more practicing to do with this rather unique set up. Look out for more reporting :-)


Auto Landscape Bracketing Script Update

Just a quick post to say I've updated my Auto Landscape Bracketing Script: V5 on the right.

This update fixes a problem in ML's depth of field calculation.

...and what about diffraction?

In this post I'll carry on talking about focusing with the focus bar script and introduce the diffraction blur.

Without any equations, the diffraction may be considered a fixed blur that degrades focus across the image and into the scene. The only camera variable that impacts diffraction is the aperture. The smaller the aperture, the more diffraction blur will occur.

Graphically it looks like this:
We now see two blurs that are degrading the image and reducing the depth of field: the defocus blur (still showing the defocus only depth of field), which varies with focus, aperture, focal length and the CoC; and the diffraction blur, which only varies with aperture.

As mentioned in previous posts, these two blurs are added in quadrature, that is the root mean square or: Total_Blur = SQRT(Defocus_Blur^2 + Diffraction_Blur^2).

Illustratively the total blur curve looks like this, showing that even at the point of focus there is a degradation in focus because of diffraction:
Finally, the impact on the depth of field looks like this:
In other words the depth of field has 'shrunk' and the blur at the point of focus will not be less than the diffraction blur.

Bottom line: The last two posts have discussed the two contributions to depth of field and achieving tact sharp images. The focus bar manages both diffraction and defocus, and allows you to find the optimum focus.
 

Saturday, August 5, 2017

Understanding focus blurs

As readers will know from a previous post, I'm aware that some find my focus bar too complex and too confusing. Hopefully this post will help those struggling to understand the focus bar.

First, let's talk about focus and depth of field. 

The depth of field is simply the zone where our brain and eyes see things 'equally' sharp, ie in focus. Outside this zone the focus falls away. The following illustrates a simplified sensor-lens arrangement:
In the real world, what is called object space, we recognise three distances. The point of focus (d2), the near depth of field distance (d1) and the far depth of field distance (d3).

On the sensor, in the image space, there is a so-called circle of confusion, ie a blur, that if we are inside this circle, then we perceive things as 'in-focus'.

For a full frame DSLR you will typically see this CoC stated as 29 or 30 microns, or 0.03mm. For a crop sensor camera, this CoC is reduced by the crop.

We won't complicate things here, but we will note that a CoC of, say, 30 microns, is only just acceptable, ie good for digital projection, but not necessarily for high quality (close scrutiny) print viewing, where a CoC of, say, 15 microns would be considered a better criterion.

So far we haven't mentioned diffraction, which is an additional blur that 'adds' to the defocus blur from the lens. Without proof, it is normal practice to 'add' the defocus and diffraction blurs in quadrature, ie Total_Blur = SQRT(Defocus_Blur^2 + Diffraction_Blur^2).

Handling defocus and diffraction blur are the two 'secrets' to getting a tact sharp image.

Sticking with just defocus blur for now, it is important to understand how the lens defocus blur varies through the image. This next cartoon is illustrative:

 
Here we see the classical defocus curve. At the point of focus the blur is zero. As we move away from the point of focus the blur increases, but the near and far curves are not the same. That is the defocus towards the camera is different than towards infinity.

Let's now add in the CoC (or blur) criterion that we mentioned above, which then allows us to see the 'in-focus' zone:
Without any equations, we can see how the focus zone appears to us. The curves illustrate that focus is not a 'black and white' affair, and thus the CoC blur criterion part of a continuum of defocus from the point of focus.

Let's now start using this knowledge of defocus: in this post let's continue to ignore diffraction. 

Let's focus at the hyperfocal distance, which is simply where the infinity depth of field's blur is at the CoC blur criterion:
We now see that the depth of field goes from a near field distance, that is HFD/2, to infinity. But we also see one of the limitations of HFD focusing. That is, for a large part of the image, all the way to and at infinity, the blur is close to the barely acceptable CoC blur criterion.

Of course if we were to focus at infinity, then at infinity the blur would be zero. But as we know, if we do this we loose a lot of depth of field in the near field. Clearly there must be a better place to be, ie between the HFD and inifinity. 

This is where the focus bar helps you decide where that optimum focus is. This next cartoon illustrate that optimum focus point:
But how far should I focus away from the HFD and towards infinity?

Fortunately we have a very easy way to know when to stop focusing. Without proof, we stop focusing when the defocus blur becomes less than twice the sensor's pixel pitch. Thus on my 5D3, with its 6.3 micron sensor pitch, I will not seek out defocus blurs less than, say, 13 microns. This last cartoon shows the sensor limit, which the focus bar alerts you to. Thus, if you use the focus bar, you will always be able to set the optimum focus, ie an infinity blur between the HFD (CoC criterion) and the sensor limit.
The focus bar tells you what the infinity defocus blur is and provides you information on the  defocus blur, the diffraction blur and the total blur of the defocus and diffraction blurs combined in quadrature. 

In future posts I will talk about diffraction blur in more detail. For now, I hope this post has helped those struggling with the focus bar, and the concept of infinity blurs, understand the difference between the CoC blur criterion and the defocus blur that varies as you adjust focus.

Friday, August 4, 2017

EOSM Infrared Conversion Test

Finally the UK's weather changed today and I managed to get a test image with my 'new' Infrared converted EOSM. 

The conversion was done by Alan Burch at http://www.infraredcameraconversions.co.uk. The service was great: personal, fast and reasonably priced. Based on my experience with Alan I can recommend him.

The test image is of one of our local churches. Here is the OOC image - the wind was blowing, so there is movement in some of the trees:


The image was captured at 11mm and an aperture of F/6.3. I used Magic Lantern to set the ETTR shutter at 1/100s. 

I choice the rather open aperture, as IR diffraction is more than in the visible region, by a factor of around 850/550, say about 50% more. I will be looking to see how far I can close down the aperture, to gain greater depth of field and get closer to the lens MTF sweet spot.

As for focus, I used my focus bar script and optimised the infinity focus, ie between the HFD and infinity.

I had already used the Adobe IR Profiler to create a custom profile for Lightroom. One that recovered the highlights via a linear gamma and reduced the red cast. This profile is my starting position for post processing.

I then used Lightroom and luminosity masks in Photoshop to get a reasonable look to the image: which is still a little 'muddy' for my taste, ie more post processing practicing required

This is the resultant image. From a camera that will now become part of my 'travel light' bag: together with its brother, the non-IR converted EOSM.

 

Monday, July 31, 2017

For goodness sake Garry: KISS



As we know, KISS means Keep It Simple Stupid. Well, from feedback, it is clear my focus bar fails the KISS test :-(

As I’m introducing a ‘new’ way of (landscape) focusing Canon cameras with the focus bar, ie optimising focus between the hyperfocal distance and infinity, I clearly need to do a better job of explaining what is going on: that is in KISS language.

Having said that, if you are using a DSLR, then you are using a complex piece of hardware and software, ie you are not a point & shoot person. In addition, if you are using Magic Lantern you most probably are aware of depth of field and want to get more out of your camera.

As we know the defocus, that occurs as you move away from the point of focus, is a function of the focus distance, the focal length, the aperture and a blur criterion, usually called the circle of confusion (CoC).

Simply put, the circle of confusion is the defocus blur that ‘just’ looks sharp on the sensor, ie a larger blur would look out of focus.

Thus to get the best out of the focus bar it is essential to understand the basics of depth of field and the concept of blurs.

For a full frame DSLR producing images for digital display, ie on the web, a CoC or blur of 29 or 30 microns is considered (just about) OK. For more exacting work, ie a judged print, a CoC of about 15 microns might be a better choice. This is a simplification and in reality you should choose the blur to fit the viewing conditions, the size of the print and the distance that the viewer is away from the print. But let’s keep it simple for now.

For the full frame landscape photographer, simply set the Magic Lantern CoC to 29 or 30 microns. For a crop shooter reduce this by the crop factor. That’s it: don’t touch this again :-)

So let’s look at an example: a FF camera and a 16mm lens, at F/8 and focused short of the HFD, the defocus field (without diffraction) looks like this:


The left hand axis shows the blur, with a unity blur being the set CoC criterion, ie 30 microns. The plot clearly shows a zero blur at the point of focus and the focus field defocusing either side. By definition the depth of field is between the curve where it crosses the unity blur line. In this zone, our eyes think everything is tact sharp (relative to our criterion of 30 microns). Out side this zone the image gets progressively out of focus. Note that the near and far fields behave radically differently, ie the focus field is not symmetrical about the point of focus.

Let’s now move the focus to the hyperfocal distance. Now we see the classical HFD focus field, ie the near field depth of field (unity blur) is just short of the focus point, i.e. at HFD/2, and the far field (unity blur) is at infinity (although it is not shown here you can see the far field curve approaching unity blur, i.e. 30 microns in this illustration, where it will at infinity):


Finally, let’s now focus beyond the HFD and short of infinity, ie use the focus bar. This is where the focus bar ‘magic’ starts. As you can see the far field curve now asymptotes to a blur that is half of that at the HFD. Remember, in all the charts the unity (CoC) blur remains constant at 30 microns.


Finally, to emphasise what has happened, let’s show the depth of field curves for the HFD case and for the focus bar, together:


Here we see the advantage of using the focus bar to tell us when to stop focusing short of infinity. In other words, by focusing slightly beyond the HFD, ie at about 2m in this illustrative case, we have created an infinity defocus blur, ie no diffraction effect yet, of half of the ML set 30 microns; ie half of the HFD blur. 

Put another way, we have focused for a high quality print, with a near depth of field of about 1 meter at a blur of 0.5 of the CoC, ie about 15 microns, or a slightly larger depth of field if you accept the 30 microns blur for the near field.

Finally, for defocus blur you really can’t do much better than two sensor pixels; and on a 5D3, for example, that means a defocus blur at infinity of about 13 microns. Therefore the full frame guidance is to aim for a reported (defocus) focus bar blur at infinity of, say, 29 or 30 microns, for digital projection; and about 15 for high quality print work.

Note, the ML set CoC stays at 30 throughout. All we need to do is act on the reported focus bar blurs.

I truly hope this post has helped those that are struggling with using the focus bar; as I believe for landscape photography, the focus bar is about the best you will get from a focusing perspective.