Generic focus issues.

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soupdragon

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Is it me or is critical focus harder to achieve with a wide angle lens than say a normal/long?
I can nail focus with a 180mm but 90mm is a different story.

I'm using a 10X loupe with a weighted dark cloth and a ground glass screen with a Fresnel but it still seems a bit hit and miss.
 
If you are not using movements, then using shorter focal lengths will give you a much longer depth of field.

Using a DoF calculator, at 180mm, f/5.6 for a subject at 10m, the DoF is 3.87m

At 90mm, for the same aperture and distance, the DoF is 39.82m

If you use the hyperfocal distance of 2.56m at f/32, everything will be acceptably sharp from 1.28m to infinity, without diffraction.
 
With wide angle lenses "critical" focus is far less important compared to a standard 150mm or longer because you also have far greater depth of field. Assuming the same subject & position you are magnifying a portion with a 180mm compared to a 90mm with a lens that has less DOF even wide open.

I actually find it easier without a loupe. Anyway focused to 15ft and f22 DOF is from 8.3ft to Infinity, with the 90mmmm.

Wide open at 15ft focus DOF is 12.4ft to 21.8ft, with the 180nn 13.93ft to 16.25ft.

Ian
 
soupdragon said:
Is it me or is critical focus harder to achieve with a wide angle lens than say a normal/long?
I can nail focus with a 180mm but 90mm is a different story.

I'm using a 10X loupe with a weighted dark cloth and a ground glass screen with a Fresnel but it still seems a bit hit and miss.
Click to expand...
I don't think it's just you.
With a wide angle lens everything seems so much smaller and further away. And it appears to me at least, that the light is more spread out, less bundled and things generally look darker especially the more you move away from the centre.
 
In my experience a 10x loupe is really hard to focus with because you are more focusing the grit of the screen than the image itself.
6x to 8x loupes seem to work better.
The other thing is using a loupe combined with the fresnel - this doesn't work well.
Trying out either the loupe without fresnel or the fresnel with a reading glass will probably give better results.
 
The fact is that because of the DOF at f5.6 with a 90mm when you first focus it still appears in focus over a few increments as you turn the focus pinion. The secret is to find the beginning and end of this range then go around midway.

It is also important to remember that many thousands of LF photographers around the world use these combinations with no issues.

Try using pre 1900 f16 wide angle lenses :D

Ian
 
Yes, trickier. The movement of the camera to change focus by the same amount is much smaller for shorter lenses. (That is, depth of FOCUS is smaller.) Also, unless you are focusing in the centre of the screen, the light gets dimmer as you move towards the edges. That’s why we use Fresnel lenses. A 10x loupe is not ideal, despite sounding as though it should be better. As Reginald says, it exaggerates both the grain of the screen and the structure of the Fresnel. Once you can discern the grain, you won’t see any more detail in the image (It will be rather like magnifying pixels.) As a precaution, in any case, you should check that the loupe is accurately focussed exactly on the rough surface of the ground glass. If your existing loupe can’t do this then I’m afraid your wallet will have to suffer - good loupes are not cheap, but they’re a lifetime investment. Four or six magnification seems to be about optimum. See other comments by other members above.
 
David M said:
Yes, trickier. The movement of the camera to change focus by the same amount is much smaller for shorter lenses. (That is, depth of FOCUS is smaller.) Also, unless you are focusing in the centre of the screen, the light gets dimmer as you move towards the edges. That’s why we use Fresnel lenses. A 10x loupe is not ideal, despite sounding as though it should be better. As Reginald says, it exaggerates both the grain of the screen and the structure of the Fresnel. Once you can discern the grain, you won’t see any more detail in the image (It will be rather like magnifying pixels.) As a precaution, in any case, you should check that the loupe is accurately focussed exactly on the rough surface of the ground glass. If your existing loupe can’t do this then I’m afraid your wallet will have to suffer - good loupes are not cheap, but they’re a lifetime investment. Four or six magnification seems to be about optimum. See other comments by other members above.
Click to expand...
My loupe has adjustment.
I (rightly or wrongly) calibrate it on a magazine page or cereal box to match my dominant eye.
Could this be a mistake?
 
It is best to focus the loupe on the actual frosted bit on the ground glass which is a bit farther away than the magazine as it is on the back side of the glass screen.
 
Only if you place the loupe directly onto the ground surface itself.
This would be most unusual. As we've seen, there are various ways that the focus screen can be sandwiched. None of them present the actual ground surface to the photographer, completely unprotected from the abrasive edge of the loupe.
The conventional, and by far the most common arrangement, is that the ground surface faces the lens directly, with no other component intervening. If there's a Fresnel (although it's by no means essential) it goes on the photographer's side of the ground glass, usually with the grooves facing the lens. Then, because Fresnel screens are made of plastic and scratch easily, it's customary to have a plain cover-glass on top of that, on the photographer's side. Thus, there are three layers between the foot of the loupe and the focussing surface – the thickness of the focus screen, the thickness of the Fresnel and the thickness of the protective cover glass. This will be several millimetres away from the foot of the loupe, which is where a magazine page would be when you focus on that.
Other members have pointed out that this is not the only possible configuration. Some cameras use proprietary screens where several functions are combined and some cameras have different arrangements of what I'm calling the sandwich.
The thing to do is to remove the camera back assembly (for greater convenience) put it onto some kind of lightbox, for better visibility, so you are facing it as though you were operating the camera, and adjust your loupe to give the clearest possible view of the gritty texture of the ground glass itself. That is, the matt surface on which the lens forms an image. You may have to rack it back and forth, narrowing down the sharpest point.
 
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Where possible always focus by moving the ground glass instead of the lens. Moving the lens changes magnification whereas moving the ground glass changes focus.
 
It’s changing the distance between lens and film surface that changes magnification and focus, however it’s achieved. On the other hand, changing the distance between lens and subject does make a difference, but only if the subject-lens distance is relatively small. For a distant subject, the effect is negligible.
Tilts and swings are a different matter and front and back have different effects.
 
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