Jump to content

Viewfinder magnifiers - do they help with focussing?


The Kevster

Recommended Posts

Advertisement (gone after registration)

A separate rangefinder is accurate to find the distance but other than that it won't help you. Please do realize that your goal is NOT how to find the distance. but to shoot a sharp photo.

 

It's a pedagogical rangefinder.

Link to post
Share on other sites

So anyway the Technique>Rangefinder Issues section on the Puts website is close enough to correct...four tables to illustrate the rangefinder accuracy. Tables 3-4 come from the accuracy formula given above. Tables 1-2 come from equating this to a d.o.f. formula.

 

Table 1 would be Jaap's "proof" that magnifiers are redundant for 50mm. The minimum useable f-numbers for a 50mm lens (at 0.58X, 0.72X, and 0.85X) are f/0.42, f/0.34, and f/0.30, respectively. In other words, f/0.5. So any larger f-number ensures the accuracy of the rangefinder is within the depth of field. (Kind of.) But you can't have your cake and eat it, too. In Table 3 he gives the accuracy at 50 meters as plus or minus 18.672, 15.042, and 12.741 meters, respectively. And as he says, "the figures...for 50 meters are NOT a typing error." This, because the accuracy varies with the distance squared.

 

Now where this comes from and what it means is another story (which you will not get from Puts, or the nonsense in this thread).

Link to post
Share on other sites

c = 0.03 mm; res. r = 0.0003 (Table 2-3) (1 min. of arc); r = 0.0003 / 6 = 0.00005 (Table 4); r = 0.00021 (from God knows where, Table 1).

Link to post
Share on other sites

So anyway the Technique>Rangefinder Issues section on the Puts website is close enough to correct...four tables to illustrate the rangefinder accuracy. Tables 3-4 come from the accuracy formula given above. Tables 1-2 come from equating this to a d.o.f. formula.

 

Table 1 would be Jaap's "proof" that magnifiers are redundant for 50mm. The minimum useable f-numbers for a 50mm lens (at 0.58X, 0.72X, and 0.85X) are f/0.42, f/0.34, and f/0.30, respectively. In other words, f/0.5. So any larger f-number ensures the accuracy of the rangefinder is within the depth of field. (Kind of.) But you can't have your cake and eat it, too. In Table 3 he gives the accuracy at 50 meters as plus or minus 18.672, 15.042, and 12.741 meters, respectively. And as he says, "the figures...for 50 meters are NOT a typing error." This, because the accuracy varies with the distance squared.

 

Now where this comes from and what it means is another story (which you will not get from Puts, or the nonsense in this thread).

 

Not really sure who is speaking nonsense in this thread, but before you answer me this, please show me a SINGLE lens in the Leica line that has a 50m marking. You do realize this is a camera for photography and not a telescope right?

Link to post
Share on other sites

Advertisement (gone after registration)

Yes pop, but it is not 50m where there is the accuracy problem of the M base, and then the 135mm is already the limit in lenses. The new 135 APO however stops at 20m.

 

edit: actually out of 22 lenses only the 135mm stops at 20m. Most of them stops in less than 10m and at 10m Puts gives an accuracy of around 1.5%, whereas this number improves greatly at normal distances of 1, 2, 3m.

Edited by diogenis
Link to post
Share on other sites

Not really sure who is speaking nonsense in this thread, but before you answer me this, please show me a SINGLE lens in the Leica line that has a 50m marking. You do realize this is a camera for photography and not a telescope right?

 

pick 10m then, but you are supposed to look at the whole table.

Link to post
Share on other sites

You challenged people to name a lens with a distance engraving which lies within the range where the M's RF becomes very inaccurate. 40m is only 20% less than 50m. The RF's accuracy will not be markedly better at 40m than it is at 50m, will it?

 

Yes, and you answered that there is no such a lens. There was an old 135mm lens but still not at 50m.

It should be hugely better. As Erwin claims, its distance squared.

 

Furthermore, I took the time to check on tech specs of two lenses:

the 50mm 'lux:

even at full aperture (f=1.4) this lens gives a DoF that varies from 45mm at 70cm distnace to 880mm at 5m distance.

 

and the 50mm nokti:

at f=1 DoF= 20mm at 1m distance, and 2350mm at 10m distance.

 

These numbers are very well covered from Erwin Puts tables for RF accuracy, which are for example: 1mm-100mm error. Which means that you are well inside the DoF zone and that your photo will come out sharp no matter what.

This is actually what I also experience in reality, where I have never lost focus once. I dont have the nokti but I do have the lux and I tend to shoot wide open

 

edit:

I'm pretty sure even people with 20/20 vision can still capture out OOF photos with am M9. It's just par for the course with a rangefinder it seems. If in doubt, take sone extra photos. Not always easy when you are shooting action/street type subjects. But in that case just stop down and hopefully you'll get decent enough focus. I'm thinking about a magnifier myself for my 90mm which I find unruly to say the least. But I think it's back focussing like a bast anyway. I just nudge it a smidge past focus on the focus area and it seems fine.

So, Danny, be pretty sure to trust that RF as it is VERY accurate. And you dont need 20/20 vision for that

Edited by diogenis
Link to post
Share on other sites

pick 10m then, but you are supposed to look at the whole table.

 

It's a non issue. Pick any distance you like, and the error you will get at ANY magnification will be covered by a factor of 10x, 20x or 100x from DoF. Do you realize this?

Anyway.... I quit :p

Link to post
Share on other sites

It's a non issue. Pick any distance you like, and the error you will get at ANY magnification will be covered by a factor of 10x, 20x or 100x from DoF. Do you realize this?

 

His Tables 1-2 compare the error to the d.o.f.

 

But the claim was you don't need to cover anything with depth of field. Magnifiers were supposed to be useless when the "RF accuracy...is about equal to the accuracy of the average human eye. without introducing DOF." (post 45)

 

Anyway, I have gone to some trouble to show there isn't any such magic formula for magnifiers.

Link to post
Share on other sites

Ok. I tell Erwin he is a fool in your opinion.

 

I made no comments about Erwin Puts.

 

I said you will not get the whole story (website, 1st edition), but that is literally true-- there is no derivation or formula, because it's not supposed to be a technical book.

Same with Osterloh; this is not meant to be technical, and isn't.

Link to post
Share on other sites

Here, I will give you the usual derivation. It's based on the diagram (triangle) in Osterloh. You have the distance, the physical base, and the angular uncertainty in vision (visual resolution). You can get an expression for the uncertainty in the distance in terms of those three (by the geometry, with an approximation for small angles)...this gets you a formula for the accuracy. Multiply the visual res. by the reciprocal of the magnification to account for magnification. Say you already have a formula for the d.o.f. ...neglect a small bit in the denominator and you will be able to cancel out the distance. So set double the accuracy (it was plus or minus) to be less than the d.o.f. Now you have a formula

 

r/(mb) < cn/(f^2)

 

where r is the visual res., m the magnification, b the physical base, c the c.o.c. constant, n the f-number, and f the focal length. You can rearrange this several ways to get all the usual beautiful charts and tables, including the minimum usable baselength and minimum usable f-number. You can also solve for the maximum usable focal length: let r be the usual 1/2300 for good conditions, c the usual 0.023mm, m be 0.68X, n be f/1.0, and then the maximum usable focal length is ...

 

f = 49.9mm

 

(About 50mm. So Jaap is always right, just this time he needed a little depth of field in his argument.)

Link to post
Share on other sites

I made no comments about Erwin Puts.

 

I said you will not get the whole story (website, 1st edition), but that is literally true-- there is no derivation or formula, because it's not supposed to be a technical book.

Same with Osterloh; this is not meant to be technical, and isn't.

 

I quoted his Compendium in the present version, not his website.

Link to post
Share on other sites

His Tables 1-2 compare the error to the d.o.f.

 

But the claim was you don't need to cover anything with depth of field. Magnifiers were supposed to be useless when the "RF accuracy...is about equal to the accuracy of the average human eye. without introducing DOF." (post 45)

 

Anyway, I have gone to some trouble to show there isn't any such magic formula for magnifiers.

 

That is a summary of his chapter in his book, not my claim, as you put is so quaintly.But let's get back to the original thread, which does not need al these theoretical considerations. the point is that Leica and respected experts agree that the M9 rangefinder is accurate enough to focus all Leica lenses accurately with the posible exception of 135 lenses. The may of course be individual circumstances that make the use of a magnifier desirable, but I strongly object to the view that for instance a Summicron 90 cannot be focused without.

Edited by jaapv
Link to post
Share on other sites

That is a summary of his chapter in his book, not my claim, as you put is so quaintly.But let's get back to the original thread, which does not need al these theoretical considerations. the point is that Leica and respected experts agree that the M9 rangefinder is accurate enough to focus all Leica lenses accurately with the posible exception of 135 lenses. The may of course be individual circumstances that make the use of a magnifier desirable, but I strongly object to the view that for instance a Summicron 90 cannot be focused without.

 

I'm sure you are right...I don't think the theory is going to tell you what lenses you can and cannot focus in real life. But I guess you are "mis-summarizing" Puts, 2nd ed., and I know you had Osterloh wrong. Look at the quote from Puts:

 

"As a bottom line for accuracy we must state the distance of the focusing error is at least equal or less than the DOF distance."

 

He is basing his claims on d.o.f. And I showed you how to get his tables. And the same in the quote I gave from Goldberg:

 

"...the focussing error is E=+- U^2 R/L ... Relating this to the formula for depth of field, we can find the effective baselength required for any focal length and f-stop."

 

Why would you still think you are doing something besides "depth of field" ? If you got this info from Osterloh or Puts, why can't you come up with a quote?

Link to post
Share on other sites

Yes pop, but it is not 50m where there is the accuracy problem of the M base, and then the 135mm is already the limit in lenses. The new 135 APO however stops at 20m.

 

the 50 meter "mark" for a 135mm lens with 2m close focus would be some 4% of the extension, so probably not at the stop.

 

edit: actually out of 22 lenses only the 135mm stops at 20m. Most of them stops in less than 10m and at 10m Puts gives an accuracy of around 1.5%, whereas this number improves greatly at normal distances of 1, 2, 3m.

 

that's a typo, plus the accuracy will vary with the magnification and what you use for the resolution. For the magnifications and resolution figures he used, the accuracy at 10m will vary from 2 to 15%.

 

btw, I would check how realistic this formula is by testing an actual camera.

Link to post
Share on other sites

btw, I would check how realistic this formula is by testing an actual camera.

 

I wouldn't because:

a) I trust Leica more, and if I wanted to solve equations I can always solve a Sudoku

B) I have never missed focus in my shots, with or without a magnifier.

 

But, it's a free world so, just go and you buy a magnifier to get that fealing of better accuracy:rolleyes:

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...