M Lens DOF question

[jaapv]

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That is indeed the correct way for testing in respect to focus shift. But otoh, stopping down decreases accuracy on the focussing screen and in the AF, making finding the plane of focus a bit of a guessing game. If I recall correctly Canon describes the tolerance on their professional DSLRs as "within half DOF" So if you stop down to double the DOF, you double the tolerances of the AF at the same time. The same argument applies to manual focussing.

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[lct]

I'm no techie Jaap. I simply take a FF camera and a R lens. Then i use the DoF scale. I look to the result. And i'm generally happy. Same with the 5D or the R4s. Hence the simple idea that FF DoF behaves more or less the same way in both cases. As i get the same results with DoF formulas as well, i feel reasonably comfortable about that. But i may be wrong like everybody needless to say so i ask other members here if they can share the same experience as mine with any FF stuff. No M8, R-D1 or other APS please because of the crop factor.

[jaapv]

Even the techie answer should make you happy: The theoretical DOF is the same for the same receptor format, but the different medium (sensor) makes DOF more defined, which should make you careful and mentally allow for less DOF. The same goes for large prints, the larger the print, the more shallow the DOF, if viewed from the same distance. I'm sure our FF friends will confirm this.

The crosstalk in this thread results from the fact that basically DOF is not a product of our camera and lenses, but a result of the limited resolution of our eyes, and the interpretation thereof by our brain. That means it will never be an absolute and grippable phenomen.

[ho_co]

Yes we cannot agree Howard.

1. Focus shift is indeed an RF problem, not a reflex one obviously. Choosing an M8 to compare film to digital DoF is useless because of this and of course the crop factor. The only valid comparisons are FF ones so far. See my post above.

2. Again, the 0.68x mag. is not enough given the crop factor. The proof is in the pudding i.e. the M8's inability to focus proprely some fast lenses without a magnifier. How many times have we discussed this on this good forum? Leica should have decided (a) to lengthen the base line of the rangefinder a la Zeiss and/or ( to choose a higher VF mag. with possibly a larger VF window a la Epson.

3. Do you really suggest that the M8's RF accuracy is the best of all M bodies? If so, we don't speak the same language Howard so i prefer letting you the last word on that.

 

lct, here we go again!

 

Point 1 has already been resolved. You speak of focus shift and crop factor as if they were related. "Choosing an M8 to compare film to digital DoF is useless..." Where does that come from? No one is comparing film and digital.

 

Point 2 is an entirely different issue, also inapplicable here. "Longer base length" and "greater magnification" are identical in function and expressed as "Effective measuring base length." Remember Contax's greater base length as opposed to Leica's higher magnification? Each claimed an arguing point for a short time, but the EMBL of the cameras was identical. Every rangefinder runs out of adequate focusing accuracy at some point. The .58x cameras fail with even more lenses than the M8--and who cares? The issue is a red herring.

 

Point 3 is just crazy. Nowhere have I said or implied that the M8's focusing accuracy is greater than that of other M bodies, nor compared it to theirs, but have pointed out that the question of focus shift has nothing to do with focusing accuracy. See the last paragraph of my previous post (http://www.l-camera-forum.com/leica-forum/customer-forum/72097-m-lens-dof-question-3.html#post753245).

 

Once again, you are trying to change the topic to something entirely different.

[lct]

...Once again, you are trying to change the topic to something entirely different.

Thank you much Howard. This topic relates to DoF. Lenses offering greater DoF than others, remember? Nothing to do with focus shift i'm afraid. Who compares film to digital, me if you don't mind. Comparison with 0.58x Ms is useless being cameras made for wide lenses. Again, the effective base length of the M8 is not long enough to use some fast lenses without a magnifier. Won't repeat calculation ad nauseam. My last take on this here if you don't mind Howard. Not sure if you want to ask me any other questions, otherwise please feel free to PM me.

[SJP]

1 Focus shift is not exclusively a RF problem, also with SLR's unless you stop down which no-one does unless they have to.

2 RF focussing works fine even without a magnifier, and I have glasses and -6® -7(L) eyes. Focus the lens first and then move you body to and fro (few cm). It is focussed when the RF patch becomes 'crisp' (i.e. high contrast, non-fuzzy) this is much more accurate than just overlaying the images.

3 M8 is just as easy or difficult to focus as an M2, otherwise I really wouldn't know

 

I do not see any reason for anyone to get worked up on this. And indeed this thread is about DOF etc.

[ho_co]

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Thanks, lct. Sorry if I've offended.

 

You're right that the topic is depth of field. I didn't intend to give another impression. And in trying to respond to what I thought you were saying, I may have gotten off topic.

 

You and I just approach these matters differently. I use one set of comparisons to illustrate and defend my points; and you use another to illustrate and defend yours. I'm sure you find my arguments as off-topic and confused as I do yours.

 

And somehow we end up trying over and over to explain but without making progress.

 

Where to go from here? Each admit defeat, I guess, and move on. Or (the same thing) declare victory and move on.

 

I'm sure we'll meet again soon on another topic.

[lct]

No offence taken, Howard. I guess we'll finally agree when full frame becomes the norm again at Leica.