Hasselblad focus discrepancy between finders

[Pál_K]

This is something of an odd problem.

First, I have a 500C/M with a groundglass having a split-image focusing aid. Using the WLF, I can use the built-in magnifier for fine focusing. Second, if I replace the WLF with a 45-degree prism, the prism provides a magnified view of the same groundglass (which stays in place).

The problem is that when I replace the WLF finder with the 45-degree prism finder and then focus on objects at known distances, the prism finder shows the object coming into focus at a more distant point. For example I’ll focus on something that’s verifiably 6’ by measurement, then, when using the WLF, the lens focus ring shows 6’. But after replacing the WLF with the prism, when the object comes into focus the focus ring is at 8’.

Other distances:

3’ with WLF is 4’ with prism
10’ with WLF is 15’ with prism
Infinity is the same with both

Being that the prism is looking at the exact same split-image as the magnifier in the WLF, why the difference?

In the past I’ve always used the WLF. I bought the prism a year or so ago (used, from a photo shop) and checked it at infinity and at a few distances that seemed reasonable. I didn’t notice the wide discrepancy until today.

I suppose I could create a conversion chart for close-ups, as most of my Hasselblad photography is cityscapes and landscapes.

 

[Sarcophilus Harrisii]

This is something of an odd problem.

First, I have a 500C/M with a groundglass having a split-image focusing aid. Using the WLF, I can use the built-in magnifier for fine focusing. Second, if I replace the WLF with a 45-degree prism, the prism provides a magnified view of the same groundglass (which stays in place).

The problem is that when I replace the WLF finder with the 45-degree prism finder and then focus on objects at known distances, the prism finder shows the object coming into focus at a more distant point. For example I’ll focus on something that’s verifiably 6’ by measurement, then, when using the WLF, the lens focus ring shows 6’. But after replacing the WLF with the prism, when the object comes into focus the focus ring is at 8’.

Other distances:

3’ with WLF is 4’ with prism
10’ with WLF is 15’ with prism
Infinity is the same with both

Being that the prism is looking at the exact same split-image as the magnifier in the WLF, why the difference?

In the past I’ve always used the WLF. I bought the prism a year or so ago (used, from a photo shop) and checked it at infinity and at a few distances that seemed reasonable. I didn’t notice the wide discrepancy until today.

I suppose I could create a conversion chart for close-ups, as most of my Hasselblad photography is cityscapes and landscapes.

I'd be interested in knowing what if any discrepancy exists between the two finders if you ignore the split wedge rangefinder and compare focus using the adjacent ground glass exclusively. Split wedge rangefinders are very useful, but not 100% infallible. Perhaps finder coverage has an effect. Comment back please after conducting a comparison as outlined above.
Cheers
Brett

 

[Vince Lupo]

Is it possible that the 45-degree finder is specifically meant to be used with an Acute Matte D-screen? If so, that might account for the differences in focus.

 

[Rob-F]

I'd be interested in exactly which 45 degree prism you have. Is it a PME, or PM, or some other? Also, which lens does this problem occur with? Is it a normal 80mm, a longer focal length that is easy to focus with, or a wide angle lens that doesn't snap into focus decisively?

 

[Sarcophilus Harrisii]

I'd be interested in exactly which 45 degree prism you have. Is it a PME, or PM, or some other? Also, which lens does this problem occur with? Is it a normal 80mm, a longer focal length that is easy to focus with, or a wide angle lens that doesn't snap into focus decisively?

That's a good point, Rob. Also would be interested to know if it's a problem with a certain lens or lenses, but not with others.

 

[Pál_K]

Thank you for the responses - later today I will try different lenses and I will also check for focus on the groundglass itself in addition to the split image circle.

Yesterday, I was using:

- old style architectural grid groundglass with split image circle
- older 45-degree non-metered prism with cold shoe
- 50/4 CF FLE lens, with floating element ring set appropriately

More tests later today.

 

[Pál_K]

So, after truly exhaustive tests with two bodies (500C/M, 501C), two groundglasses (Acute Matte and the architectural grid with split image circle), the WLF and PM prism, plus various lenses (50, 80, 120, 150), my conclusion after many swaps and repeated tests is: there’s something about the combination of the 50mm lens and the split image groundglass that results in a more distant focus point.

Some elaboration: using the 50mm lens with the PM prism and the split image groundglass yields a hard to focus image for me. With that lens, the focus area is small and although it’s magnified, focusing is much easier with the WLF and built in magnifier. For these tests I had to use a black vertical bar against a white background to ensure proper critical focus at 6 feet. So, actually contrary to what started this all yesterday, I’ve confirmed that using either the WLF and the PM prism will indeed result in the same focus distance for that split image groundglass. However, yesterday I did know that distance to be wrong.

In my tests, when I use a combination of any lens, either body, WLF or prism, or a different groundglass, the focused distance is correct. It is only the 50mm lens with the split image groundglass where the distance is considerably off.

Not being an optical scientist, I can’t explain it but my guess is that the angle of the light coming through the 50mm in combination with that split image circle causes a problem.

Since I know this indicates a serious flaw which Hasselblad or Zeiss would be eager to correct quickly, I will graciously accept an expenses-paid invitation to their facilities to resolve the issue.

 

[Sarcophilus Harrisii]

As I mentioned in my previous post, split wedge rangefinders are very useful things but they are not 100% infallible. Rather than outline the reasons why this is the case, myself, I think you'll find this 1965 Modern Photography article by Herb Keppler relevant and informative.
Cheers
Brett

 

[Pál_K]

As I mentioned in my previous post, split wedge rangefinders are very useful things but they are not 100% infallible. Rather than outline the reasons why this is the case, myself, I think you'll find this 1965 Modern Photography article by Herb Keppler relevant and informative.
Cheers
Brett

Thank you; the article indeed provided information about the split image prism that I did not know.