DIY Fresnel for 5x4"

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The ground glass in my Shen Hao has been bugging me for a while. It's fine for focussing, but the image becomes markedly dull away from the centre and composition tends to be a bit hit or miss. I did some reading on Fresnel lenses, saw the prices, and decided to have a go at a DIY version. I went down to the local shops and picked up a cheapie Fresnel magnifier thing for a fiver, and set about fitting it to the camera.

First, get the camera back off and remove the ground glass...

DIy Fresnel 01.jpg

I had been thinking of ways to fit this such that it didn't push the GG backwards, causing a focus shift. I was musing on a possible latch/catch setup at the corners, when I noticed the detail of the corners of the little ledge that the glass sits on...

DIY Fresnel 02.jpg

The GG is only supported along the short edges. The two long edges are recessed by about 1.5mm. Having recently been looking at the Shen Hao web site, and noticing that they do an add-on Fresnel, I put 2 and 2 together and realised that these lower edges are for supporting that. The chaepie Fresnel is thicker at about 2.3mm, but I knew I could add rebates, so I got on with marking out for cutting...

DIY Fresnel 03.jpg

The try square is set just off centre to allow for the width of the line of the fine marker pen. Four centre lines are marked at the edges. I took measurements of the space the Fresnel needed to occupy and then placed additional lines to indicate this size (half of each dimension from the centres)...

DIY Fresnel 04.jpg

...and joined up the cut lines...

DIY Fresnel 05.jpg
 
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I then put some bits of double sided tape outside the central area, flipped the Fresnel over and stuck it down onto the thin card that was underneath...

DIY Fresnel 06.jpg

This is now shiny side down. The card is to protect the surface while it slides across the table on my bandsaw. After trimming the protruding bits of card so that I'd have the edges of the Fresnel to run against the fence on the saw, I did a test cut, realised that this stuff is pretty nasty (non-expanded polystyrene) and was never going to look great, I did the rest of the cuts and ended up with...

DIY Fresnel 07.jpg

After a bit of sanding and fettling at the edges, it fitted the recess in the camera back...

DIY Fresnel 08.jpg

I did some measuring and cut a couple of rebates on my milling machine...

DIY Fresnel 09.jpg

These are done on the Fresnel side, which goes towards the lens, and are about 0.8 to 0.85mm deep. Here's an end view...

DIY Fresnel 10.jpg

I had actually practiced on an offcut first to get the depth set.
 
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This was then given a clean and popped into the camera back...

DIY Fresnel 11.jpg

Here's a close-up of a corner...

DIY Fresnel 12.jpg

The GG sits on the left-hand ledge, and the rebate in the Fresnel is face-down on the right-hand ledge. This places the shiny side of the Fresnel just about flush with the GG.

And, stick it all back together...

DIY Fresnel 13.jpg

I went out for a play, and was rather impressed with the difference. It's still a bit dull with no dark cloth, but enough to just see an image and do a very coarse focus and compose. Under the dark cloth, it's transformed - it's a nice, clear picture with even brightness across the frame.

I didn't take any photos of the GG in the field, but I did a couple indoors using my LED light panel thingy to fill the frame. Curiously, the centre hotspot seems much more emphasised here than it appears in the field. The following two photos, before and after, were taken with the camera on manual settings, same for both. I also did exactly the same post processing, which was to lighten shadows by 25% and darken highlights by 25%. Nothing else, other than crop and resize.

Before - ground glass only...
DIY Fresnel 14.jpg

After - with the Fresnel fitted...
DIY Fresnel 15.jpg

Even though the digicam seems to emphasise the hotspot unduly, I think the difference in the outer areas is easy to see. In the before picture, aside from the very bright centre, the general fall-off to the corners isn't all that far from what it's like in the field - there's stuff there, but it's by no means clear. With the Fresnel fitted, the difference is night and day - can easily everything that's in the frame. I used both 90mm and 150mm and didn't really notice any difference in brightness at the corners. There should be some, though - a given Fresnel will bend light a certain amount, meaning that the angle of incidence ( and hence the focal length) is a factor.

Focussing wasn't an issue - I was able to home in on tiny details with the loupe and get them as crisp as possible. The concentric rings of the Fresnel are visible as very thin dark lines, but they don't really get in the way. I'd rather they weren't there, but their presence is a tiny price to pay for the improved view.

I have yet to see if there is any focus shift. Although the GG should still be in the same position, there's a possibility that refraction through the Fresnel could have an effect. Whether or not this will be noticeable remains to be seen. I took some shots with the lenses wide open (both f/5.6), and know what I focussed on (details of the Forth Bridge), so I'll see how things look when they're developed. The shots were taken at a fairly oblique angle to the bridge, so it should be possible to work out what direction any focus shift has gone in, which will mean either a modification to the camera, or a shim under the GG. I'm hopeful that focus in the centre will be unaffected or negligible (I did try a rough trial indoors, just holding the parts in the back, and I really struggled to see if there was a difference - if there was any, it was so small that it's within my own range of error.

So far, so good. Even if it gets abandoned or replaced with something better, it has been a cheap experiment to see what difference a Fresnel can make.
 
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Happy to be proven wrong here, but I'm pretty sure that the developed photos will either be soft / out of focus, or have the focus shifted if you're dealing with close up, narrow DOF scenes.

What you've done here in the first instance is project an image onto the ground glass. Then you've added another optical element between the ground glass and the lens, thereby changing the optical system. What you are not then doing is replicating this change to the optical system when replacing the lens/GG with the film. As such the image (for the same physical object distance) projected by the lens is now coming to a focus (likely) behind the film plane and hence will produce a soft negative at the originally intended distance. I would instead do what the Ebony Fresnel/GG focusing screens do and place the Fresnel lens between the GG and yourself.

Whether you notice this in the final image is a different matter; that is a matter of perception and if it's acceptably sharp that's all good.
 
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Unless you are moving the ground glass position to compensate, the Fresnel should be on the outside.



Steve.
 
I fitted a fresnel screen below the ground glass on my Polaroid 110 build and didn't see any focusing issues as a result. The Fresnel screen just aligns the light path coming in which results in a brighter overall image, particularly at the edges, so I'm not sure how it would impact the focusing distance itself?

Added a fresnel screen to the rear of the ground glass and the difference in brightness is like night and day! by Steve Lloyd, on Flickr

Added a fresnel screen to the rear of the ground glass and the difference in brightness is like night and day! by Steve Lloyd, on Flickr
 
stevelmx5 said:
The Fresnel screen just aligns the light path coming in ... ?
Click to expand...

While this is essentially true, it's not the whole story.

Take a single point source of light which emits in all directions. If you image this with a lens onto a viewing plane, the lens is collecting the light that is entering it, and (ideally) directing it all towards a single point on the image plane. This makes sense on an intuitive level because if you photograph a city scene from far away, for example, small street lights (say) are reproduced as proportionately small points on the sensor/film. It can also be proven mathematically. This position and distribution is representative of the object position with respect to the central axis of the lens. The result of this is that light which reproduces the image on the imaging plane arrive at many different angles, as projected by the back of the lens / lens system. In uniform media (air, in our case) the direction of travel of this light is unperturbed, and so for a correctly focused image, the distance between the lens and the focal plane is specific to the properties of the scene being recreated and the lens which is recreating it. By adding a Fresnel lens, or even just a piece of uniform media, between the lens and focal plane, you're altering the optical path length in a way which is not recreated when this new system (including the focal plane) is replaced with the film. In more precise terms, by adding media of refractive index greater than 1 between the lens and imaging plane, you force a situation when the optical path length is the same, but the physical distance between the lens and imaging plane has changed in order to create a correctly focused image. By not keeping the media in place when the focal plane is replaced by the film, the focus distance required to generate a correct focus is now different to what has been set for the original object/scene. In a real scene, this will result in a different part of the scene being in focus.

To put it yet another way, you're adding a lens after an initial lens and using that new system to generate a focused image on the ground glass. One is then taking that lens out when replacing the ground glass with the film. How can the focus of the projected image possibly be the same on the imaging plane?

I'm not at all saying you can't generate a sharp image on the ground glass when composing, not at all. I'm simply saying that the resulting image you record will not be what you imaged on the ground glass if what is between the lens and ground glass is not the same when the film replaces the ground glass.
 
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Woodsy said:
While this is essentially true, it's not the whole story.

Take a single point source of light which emits in all directions. If you image this with a lens onto a viewing plane, the lens is collecting the light that is entering it, and (ideally) directing it all towards a single point on the image plane. This makes sense on an intuitive level because if you photograph a city scene from far away, for example, small street lights (say) are reproduced as proportionately small points on the sensor/film. It can also be proven mathematically. This position and distribution is representative of the object position with respect to the central axis of the lens. The result of this is that light which reproduces the image on the imaging plane arrive at many different angles, as projected by the back of the lens / lens system. In uniform media (air, in our case) the direction of travel of this light is unperturbed, and so for a correctly focused image, the distance between the lens and the focal plane is specific to the properties of the scene being recreated and the lens which is recreating it. By adding a Fresnel lens, or even just a piece of uniform media, between the lens and focal plane, you're altering the optical path length in a way which is not recreated when this new system (including the focal plane) is replaced with the film. In more precise terms, by adding media of refractive index greater than 1 between the lens and imaging plane, you force a situation when the optical path length is the same, but the physical distance between the lens and imaging plane has changed in order to create a correctly focused image. By not keeping the media in place when the focal plane is replaced by the film, the focus distance required to generate a correct focus is now different to what has been set for the original object/scene. In a real scene, this will result in a different part of the scene being in focus.

To put it yet another way, you're adding a lens after an initial lens and using that new system to generate a focused image on the ground glass. One is then taking that lens out when replacing the ground glass with the film. How can the focus of the projected image possibly be the same on the imaging plane?

I'm not at all saying you can't generate a sharp image on the ground glass when composing, not at all. I'm simply saying that the resulting image you record will not be what you imaged on the ground glass if what is between the lens and ground glass is not the same when the film replaces the ground glass.
Click to expand...

As ever Woodsy, your explanation is considerably more technically correct than mine ;0)

I agree that in adding another layer between the lens and glass you're increasing the risk of missed focus but, in my experience anyway, that didn't make enough difference in the focus accuracy to negate the improvement in brightness.

If I was shooting at F1.4 I'd agree that there was a higher risk of focus inaccuracy but as I shot at F16 (at least) and above, the increased dof covers the slight change in flange depth.

Whilst on paper you're obviously correct from a scientific point of view, the reality in the field is that it doesn't make enough of a difference to lose sleep over :0)
 
Woodsy said:
Whether you notice this in the final image is a different matter; that is a matter of perception and if it's acceptably sharp that's all good.
Click to expand...

stevelmx5 said:
the reality in the field is that it doesn't make enough of a difference to lose sleep over :0)
Click to expand...

Oh totally, I've no doubt the magnitude of the effect will differ on a scene to scene basis - DoF, specific lens used, magnification ratio, optical thickness of the Fresnel etc, all have their say. That being said, where is the boundary between a satisfactory and unsatisfactory result? Is it not surely safer to stick the Fresnel between the GG and viewer?

Also, in fairness, the OP said he was shooting on LF, wide open at 5.6?
 
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