Best Focusing Aid: Prism magnifier, or Microprism Screen?

Rob F. · August 23, 2013

Looking to improve my focusing ability, especially with wide-angle lenses. I've learned I can put a magnifier on my PME 45 prism. Sounds like a good idea. And I rather like the microprism aids on my Nikons, so it seems to me that could be an alternative, if I wanted to try just one or the other. Some have posted here they find the microprism or split image rangefinder distracting on the Hassy. Apart from that, which enhancement have you found to be the more useful or effective?

q.g._de_bakker · August 24, 2013

Focusing aids in the middle of the focusing screen are indeed distracting. But they do work. I find a microprism array not so usefull. A split image however is very acurate, but to prevent one half going dark needs your eye exactly in the right position, and the same for the lens' exit pupil (which mustn't be too small either). So most usefull with a prism or hood when used with magnifier, and with fast enough lenses (microprisms need fast enough lenses too). And you need some feature in the subject that lends itself to 'split-imaging'. The best aid is a high power magnifier on a fine, plain 'ground glass' focusing screen.

d_purdy · August 24, 2013

If by magnifier on a prism you mean a plus diopter, then I think if you need a plus diopter get it regardless of the screen type.

I have the split image focus and micro prism collar on my roll film cameras. It would be great if it wasn't necessary but otherwise I waste a bunch of time rolling my focus back and forth to be sure it is where it needs to be.

Dennis

Rob F. · August 24, 2013

Q.G.: How fast a lens would you say is needed for microprism or split-image focusing? Is f/4 or f/3.5 fast enough? Or f/5.6 possibly too slow? Sounds like you are recommending against bothering with screen enhacements, though.

D. Purdy (or anyone): That "rolling back and forth" is exactly what I'm trying to get away from, or at least minimize. For me, it involves moving the focus until the image goes soft one way; then rolling back through the point of sharp focus until it goes soft on the other side; then centering between the two extremes. The eye doesn't see the precise point best focus; the fingers remember where the end-points are, and then split the difference. It seems to be a brain-body phenomenon in addition to an eye and brain one. Maybe that's good enough, but I think it would be better to be able to visually verify the point of best focus. Like I can with a rangefinder camera.

Perhaps the eye-brain-body system is very accurate, and I don't give it enough credit. Maybe that would be worth a thread of its own.

q.g._de_bakker · August 24, 2013

Rob, It depends on the split image, but the ones in Hasselblad screens need a lens of f/4 with the pupil not too far away (not all f/4 lenses are created equal, and with a 150 mm it will do better than with a 120 mm). Trying to use a split image with the f/5.6 250 mm is already something of a task. Nearly imposible. I didn't mean to recommend against the split image rangefinder. I found that, with my eyes going, they canbe a great help. But they are indeed a bit of a distraction. An obstruction, even, when composing with a folding hood. But if you need acurate focus, and use a prism (the split image rangefinders do not work better with a prism, but the prism's fixed eyepiece position obviates moving your head in search for the correct eye position. The flip-up magnifier of a folding hood does the same, but you probably will want to fold the magnifier out of the way to compose, and then your eye will move and the split image rangefinder will be reduced to something that is in the way), they are hard to beat. But if a magnifier helps to get good focus, i would skip the split image rangefinder.

Bill C · August 24, 2013

Years ago I ran a test between two screen styles, split image vs plain ground glass. It was an industrial setup fixture, using a focus chart for a target. I had about 12-15 people individually focus, using both screens (there were two interchangeable focus tools). Evaluation was done based on the final lens barrel position.

Here's what I found. The split-image screen results were spread out nearly twice as far as the plain ground glass. Exactly the opposite of what I had expected. I figured there had been a mix up between the screens, so got the same people back and repeated the test. Same results - the plain ground glass focus positions had a much tighter spread, roughly half the total excursion of the split-image screen.

So the plain ground-glass screen was clearly better, at least in this situation. A curious thing is that everyone felt they had nailed focus with the other (split image) screen, and were uncertain about the plain screen. But results were contrary to the user's impressions.

I never understood how the split-image could have come in second place, but after this, I never underestimated the seemingly ambiguous plain ground glass again.

ps: my setup was 35mm format.

Rob F. · August 24, 2013

Well, the accuracy of a rangefinder is limited by the effective base length. I remember reading that the EBL of a split-image on an SLR is equal to the diameter of the lens (front element? rear? iris?). And that is probably only 25 to 50mm or so: less than for most 35mm rangefinder cameras. So maybe that is a limiting factor here.

david_henderson · August 24, 2013

I used a Bronica, not a Hasselblad, for about ten years. For my eyes at least the split image device was usually helpful, and the microprism not useful at all. I agree with QG on the conditions under which the split image is most useful. I'd just like to add this- and it may not be available with a Hasselblad screen. When I had a horizontal or vertical split image I always seemed to have a plethora of subjects that would suit the other better. Eventually I got a diagonal split screen device and that was much more useful.

paul_loveteck · August 25, 2013

Rob,

The EBL is a theoretical number and allows you to compare two systems when everything else would be perfect. In practice, it's a very different story. The 35mm range finders cannot compete with an Hasselblad for accuracy of focusing for multiple reasons. Some of them:

- Even in the best rangefinders (take Leica, Contax), the link between the lens and the range finder mechanism is very delicate, complex and not consistent across the full range of lenses and distances. There is no need for this mechanism in a V system, suppressing a cause of inaccuracy.

- The accuracy of focus is directly linked to the enlargement and to the brightness of what you see. Your PME45 enlarges 2.5 times and you can add a magnifier for another 2.5. On the contrary, most 35mm have an enlargement smaller than 1 (they reduce) and, in addition, the "more recent" like the Leica M6 have a reduced luminosity due to the need of light measurement system. (In fact the M3 was better for focusing accuracy).

When you say " EBL of a split-image on an SLR is equal to the diameter of the lens . . .", I believe it's a mistake as whoever said that forgot to multiply by the magnification of the prism, which must be part of the EBL calculation, almost by definition. Do you remember where you found this by any chance?

If focusing accuracy is your main goal (not clear in your post as you mention distraction from some screens, which is not related to focus accuracy), you will achieve the best results with a split image 42170 or 42217 (the same for this purpose), your PME45 and a magnifier.

If you did not already have a PME45, a PME51 with magnifier would even be slightly better.

I have used this combination with 50, 60 and 80Mpx digital backs: they are the best test possible for accuracy. When you get an image and you can enlarge it at 400% in Photoshop and see perfect details where you focused, you know that something is correct!

In fact, I am pretty certain that if something like a 50Mpx back existed for a Leica M6 it would be impossible to achieve a perfect focus. You can achieve decent focusing with an M6 only because film is much more forgiving (and lower resolution).

As mentioned by others, a micro-prism won't give you anything more.

I have used many 35mm range finders (Leica, Contax, Voigtlander, Foca, Zeiss Ikon . . .) and none come close to even an old 500C with a split image as far as accurate focusing is concerned.

david_smith35 · August 25, 2013

"you probably will want to fold the magnifier out of the way to compose," Why would you do that? It makes more sense to compose with the magnifier up, as it improves screen contrast by reducing stray light and enables you to see small details.

q.g._de_bakker · August 25, 2013

The increased magnification comes with a reduced field of view, David. Eye relief, and all that. Depending on things like brightness, eyesight i.e. having to use glasses or not, the degree of magnification, you may or will not see the entire viewfinder image when using a magnifier so you would be composing for the reduced field of view only. You could argue, by the way, that since composition is about how the elements filling the frame relate to each other and share/compete for the limited area available in the frame, it would be a good thing if you couldn't be distracted from the whole picture by fine detail. So for that reason too perhaps. ;-)

david_smith35 · August 25, 2013

'The increased magnification comes with a reduced field of view'

Well, not with my cameras Quinten. I can see the entire focussing screen with more than adequate eye relief - but I suppose it depends on your choice of dioptre lens.

I find it much easier to look around the frame when using a magnifier, which makes composition easier for me. I find viewing the screen without a magnifier, much too small for my liking as I like to check individual parts of the image before exposing. Depends on your choice of subject matter I suppose.

paul_loveteck · August 25, 2013