Loupe focussing issue

[patrik_LF]

Hi there,
I have an "issue" when focussing with a loupe, which I would like your input on.

When I focus (using this loupe, https://www.bhphotovideo.com/c/produ...for_Canon.html) I get acceptable focus on the ground glass. But if I retract the loup a distance from the glass (so that there are air between the loupe and the glass, around 1-2 cm) the focus is always sharper.

Is this a problem in getting the sharpest possible negative or is this as it should be? Should I not worry about this or is a new loupe the way to go? Is it ok to focus the camera with the loupe some distance from the ground glass?

I "think" I get fairly decent negatives out of the camera, but as I'm new to this I'd like to nail the process/technical stuff as soon as possible so that i can focus (sorry..) on the creative side of things.

I have attached a typical scan fwiw, maybe you can get some clues from this.



Best regards,
Patrik

[Leigh]

When I focus (using this loupe, I get acceptable focus on the ground glass. But if I retract the loup[e] a distance from the glass (so that there are air between the loupe and the glass, around 1-2 cm) the focus is always sharper.

What camera are you focusing?

That loupe is a dedicated accessory for a specific DSLR camera model with a specific LCD. As such I would expect its default focal plane to be some distance in front of the loupe's base, allowing for the thickness of the cover glass and protector.

I would not consider that product appropriate for focusing a view camera.
Its magnification is only 2.2x. I expect you'd be happier with 5x to 7x for most view cameras.
Look at products from Silvestri or Wista for dedicated view camera magnifiers.

- Leigh

[Doremus Scudder]

What camera are you focusing?

That loupe is a dedicated accessory for a specific DSLR camera model with a specific LCD. As such I would expect its default focal plane to be some distance in front of the loupe's base, allowing for the thickness of the cover glass and protector.

I would not consider that product appropriate for focusing a view camera.
Its magnification is only 2.2x. I expect you'd be happier with 5x to 7x for most view cameras.
Look at products from Silvestri or Wista for dedicated view camera magnifiers.

- Leigh

Leigh correctly points out that your loupe is not designed to be used resting on the ground glass of a view camera. As you've noticed, you have to hold it away from the surface a bit to get it the right distance from the ground-glass image to focus. Also noted is the rather low magnification of that particular loupe, which is not particularly suited to fine focusing...

So, a couple of comments:
Most LFers like loupes in the 4x-8x range to focus with. 2x is really not enough to fine focus with. In fact, you can get to 2x pretty easily with reading glasses. (I use 4-diopter reading glasses to view the ground glass image with and an 8x loupe for fine focusing).

Also, most like loupes with skirts that rest on the ground glass at the proper focusing distance so they don't have to find focus by moving the magnifier back and forth all the time. Many (and most good) loupes have an adjustment that lets you set the exact length of the skirt, i.e., distance from focus plane to your eye, to match your eyesight. These type of loupes are de rigueur for view-camera focusing. Top-of-the-line loupes like this have skirts that tilt so you can adjust the viewing angle to see better in the corners of the image.

Some theory:
Any magnifier in an appropriate strength can do the job focusing on the ground glass. There are, however, inherent problems using just any old magnifier that need to be considered and dealt with if you decide not to go the more traditional route and get a loupe with a skirt. First, as you have discovered, you need to move the magnifier back and forth to find the right position for focusing. Finding this position is not as easy as it may seem, since it's easy to get distracted by things like Fresnel lines, grid lines, or anything else that is in the wrong plane that you might accidentally focus on. That said, if you spend some time figuring this out for your particular applications, it is doable.

An common myth is that an out-of-focus image from the loupe equals an out-of-focus image on the negative. Not true. You have also discovered that if you have a loupe sitting at the wrong distance from the ground glass, you will never get a really sharp image viewing through the loupe. It will be "more sharp" and "less sharp," just never really correct. In cases like these, however, the "sharpest unsharp image" is the correct focus for the camera. The problem here is that it is not so easy to make fine distinctions between "pretty sharp" and "a bit less sharp" and focusing errors are easy to make. It's always easier to have the loupe at the right distance from the ground glass; a dedicated loupe with a skirt ensures that this is always the case.

Now for the heresy:
I use loupes that do not rest on the ground glass (mostly because I can't justify the expense of the loupe I'd really like). I do a lot of work with shorter-focal-length lenses and this facilitates focusing in the corners, since I can find the correct angle to hold the loupe fairly quickly and don't have to rely on a tilting skirt. The downside to this is that I have to find the correct loupe-to-ground-glass position every time I focus. I've gotten really good at this, and it is now second nature to me. Others I have talked with find this approach more trouble than it's worth and prefer the loupes with skirts. My personal preference is for an 5x-8x loupe for fine focusing. I have used many different kinds of magnifiers, but now use only a couple types: a high-quality loupe designed for slide viewing (mine is a Peak 8x) and a compact magnifier like the one Sherlock uses in the TV series (see here: http://www.sherlockology.com/props/sherlocks-magnifier ).

FWIW, I've been drooling over this Silvestri loupe for some time now... http://www.silvestricamera.com/ita/c...ng.asp?ID=5050

Best,

Doremus

[Leigh]

You have also discovered that if you have a loupe sitting at the wrong distance from the ground glass, you will never get a really sharp image viewing through the loupe. It will be "more sharp" and "less sharp," just never really correct.

The surface on which you want to focus is on the ground side of the ground glass. That's the side toward the lens.

The best option I've found is to use a ground glass with a grid on its ground side.
A variable-focus loupe can be adjusted by focusing on that grid.
If you have no grid, stick a piece of electrical tape on the ground side and focus on its rear edge.

You won't achieve a sharply-focused image at any point because the image is on a bumpy ("ground") surface.

The easiest solution is to adjust the focus one direction, going through "best", and on to a noticeable distortion.
Note the focus adjustment at that point, then go back the other way through "best" to a similar point on the other side.
Once you've noted those two adjustment points, set the focus adjustment midway between them.

- Leigh

[patrik_LF]

Thanks for your replies.
I have a Sinar P1 and, judging from your insights, I guess I'll have to get myself a proper loupe.
Thanks again,
Patrik

[Jac@stafford.net]

[...]
FWIW, I've been drooling over this Silvestri loupe for some time now... http://www.silvestricamera.com/ita/c...ng.asp?ID=5050

Best,

Doremus

Okay! I hope Santa is reading this.
...and this - that loupe needs an extension for those who use a ground glass hood.

[Mick Fagan]

FWIW, I've been drooling over this Silvestri loupe for some time now... http://www.silvestricamera.com/ita/c...ng.asp?ID=5050

Best,

Doremus

Oh my goodness, now that I have seen this, the appeal is heightened by the possibility of actually seeing the corners of my 65mm lens on my 4x5 cameras to a degree that will allow focusing check, among other things. Seeing something so obviously well thought out and with some original thinking in the design, makes one wonder just what will some people around the world think of next that we don't know we will lust after.

Thanks for that link.

Mick.

[Jac@stafford.net]

Oh my goodness, now that I have seen this, the appeal is heightened by the possibility of actually seeing the corners of my 65mm lens on my 4x5 cameras to a degree that will allow focusing check, among other things. Seeing something so obviously well thought out and with some original thinking in the design, makes one wonder just what will some people around the world think of next that we don't know we will lust after.

Please excuse my skepticism: it was born into me. The angle view of the Silvestri loupe is a gimmick. If you understand your lens, and you should, then focusing within the 50% central area should suffice. You know the rest of the focus area.

[Mick Fagan]

Please excuse my skepticism: it was born into me. The angle view of the Silvestri loupe is a gimmick. If you understand your lens, and you should, then focusing within the 50% central area should suffice. You know the rest of the focus area.

Your scepticism is noted, and yes, you are correct

However with a centre filter on and photographing an engine bay in an automobile, there is a very fine line between acceptable focus, which happens around 2 stops down, and very good focus which more or less, really gets underway around 2½ stops down. But, as the camera is sometimes not at 90º to the engine, due mainly to a non removable bonnet getting in the way; compromise is the order of the day.

I currently use a linen counter loupe, as I can hold it loosely above the GG at any angle to ascertain if in fact, I have acceptable, close to acceptable, or, borderline acceptable on the corner that must be in very good focus.

As strange as it may sound, and shortly after purchasing my 65mm lens for 4x5" I have been photographing some of the engine bays of a private collection of interesting automobiles. Something I never in my wildest dreams I thought I would be doing. I am using a column stand with an extending arm that is capable of holding 10kg of camera 1.4m from the column. This was built expressly for this job by the in-house mechanic after I initially declined the invitation to photograph the engine bays, as I didn't have a suitable camera mount. Using sand bags, the 10Kg limit can easily be exceeded; bit of a once in a life time job.

I picked this exercise up when the owner of the collection saw me photographing our car with my camera about 45cm off the ground, and about 40cm away from one of the front corners of the car. He looked at what I was getting on the GG; the rest is history, as they say.

That said, I do not ever see me picking up one of those loupes. I would love to have one, it isn't going to happen, but one can lust after one, can't one?

Mick.

[Doremus Scudder]

Please excuse my skepticism: it was born into me. The angle view of the Silvestri loupe is a gimmick. If you understand your lens, and you should, then focusing within the 50% central area should suffice. You know the rest of the focus area.

Jac,

I don't know about the Silvestri loupe, but changing the angle of view of the loupe for viewing the corners of the ground glass, especially with shorter lenses and when using extreme movements, is a real help. As mentioned, I use smaller 8x loupes and use them inverted, that is, with the skirt toward my eye. This allows me to view at any angle to the gg. I find I have to hold the loupe significantly off of perpendicular for many situations in order to find the bright spot for best focusing. If the Silvestri loupe can do this with the base resting on the gg, then it should be a great tool. For many of my shots, there are near objects at the edge of the image that I really need to focus on; using just the center 50% is a no-go.

Best,

Doremus