[Class A]

Sorry, but I wholeheartedly disagree with your conclusions.

Ben, as Marc already said: We are not presenting our private views on the matter. Have you checked out the "Low noise benefit of FF vs APS-C equals ... zero" thread? It contains a lot of "aperture conversions" from people who know their math (e.g., falconeye) and may provide further assurance that this isn't a folly of Marc and myself.

Viewed from that point, it is clear, that - following your arguments – any print or projected slide or projected digital image would change DOF, according to the size.

You are right, that's the implication. But the implication is true. A proper DOF calculator will ask you for the output/print size.

I see your idea of looking at DOF only at the sensor/film but that would make it impossible to talk about equivalent images for different formats. Also, you don't print all your APS-C shots 1.5x smaller each side than your FF shots, do you? Because you don't, the same lens will show less DOF on your APS-C prints (due to the higher magnification).

My opinion is, that it is not helpful in the least to mix hard facts (aka physical lens properties) with assumptions.

We don't create such a dangerous mix. If a parameter (such as viewing distance, print size) isn't explicitly mentioned, it is assumed to be the same for both APS-C and FF format shots. It's always "everything else being equal".

P.s.: a personal note: I just thought, I am getting as grumpy in my answers as [Wheatfield] sometimes have been lately! Haha…

Exercise, Ben, exercise. You've got a long way to go.

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Last edited by Class A; 12-22-2009 at 02:41 AM.

[Class A]

... it is simply a numbers game that I still don't see the practical use for.

For anyone staying in APS-C land only, you are right.

However, most have either got an FF history or read books/articles that assume the FF format.

Don't you somewhat associate "50mm" with a normal focal length (one that is neither wide nor long)? Haven't you come across portrait recommendation that state a range of 85-135mm is useful/flattering?

All these numbers need to be converted into APS-C lens to make sense on your Pentax DSLR.

I'm fine with anyone shooting away without concern for number games, and doing 25 test shots rather than posting on a forum will probably be more useful for telling you which focal length you like for portraits than converting the classic range, but there are people who feel that they benefit from being able to do the conversions.

[wallyb]

Don't you somewhat associate "50mm" with a normal focal length (one that is neither wide nor long)? Haven't you come across portrait recommendation that state a range of 85-135mm is useful/flattering?

As I wrote before, I'm saying that this aperture-equivalence for purposes of DOF is *not* purposeful like how the crop factor affects the perceived focal length. That's really the only thing people with FF experience need to know when using APS-C, or any other sized format. The information about what aperture will be needed in order to create the same DOF is useless in my opinion. It has no application. It may "comfort" someone, but it seems like it would comfort the same people who think of their computer as a typewriter connected to a television.

[Ben_Edict]

Obviously. But the point of equivalence is, it is not the actual physical aperture (or even aperture ratio!) that we care about in photography. It is the *effect* this has on DOF and on what kind of shutter speed we can get for a given level of noise. And that's the point of "equivalence" - to tell us what focal length and aperture will give the same DOF/shutter/speed/noise characteristics betwene two different formats.

This is not something that "Class A" or I just pulled out of our hats for the purpose of this discussion. It's a widely reported and well-researched topic. there are any number of web sites out that will walk you through the calculation if you need to see the mathematical proof of this. But the proof *is* there, and I can tell you as a person with a degree in mathematics, while I've bothered to work through it all myself, it looks absolutely 100% rock solid.

But Marc and Class A, aperture is more than just the DOF determining factor. And the simple , basic and all important function the diaphragm serves as controling the amount of ligh falling through the lens, gets not only lost in your argument, it will also be falsified! Because f/2.8 is f/2.8, completely independent of sensor or print format. The basic function of the aperture does very obviously not adhere to your calculations.

And the whole DOF discussion concentrated on the on sensor format is completely pointless. Because as we agree, the final output determines "visible" DOF. So, what about the typical screen printed ad poster at the next bus stop. How will you define DOF, when dealing with a print resolution of 10dpi? In that case, all the fine sensor-based DOF calculations are completely off. And that is not even an exotic example.

Ben

[cousinsane]

A F2.8 lens will always be a F2.8 lens, I think we can all agree on that.

The "equivalent aperture" thing only applies to DOF.

What's there to argue about?

[rparmar]

And I am just being an Asswipe

No way, it was an entertaining post. I was just about to ask what you put in the eggnog.

Because I want some.

[rparmar]

But this does not explain, why you suddenly transform the 40/2.8 into a f/4.3 lens - not at all.

I might be a bit late to suggest this article. But I warn you: many would prefer to instead take photographs.

Remember Magic Cube flash...

God yes. Thank you so much for reminding me.

I do not even know why I bother pullin out the DSLR anymore...

Because it makes you happy?

The only problem is: why? Why would you want to think that? It seems like an exercise in playing with numbers for a mathematical purpose, rather than a photographical purpose.

Say I love the look of my wide open FA 75/2.8 on my 645 and want to know how it might work mounted to my K1000 (not that I have one right now) or my K20D? Or maybe I'm interested in optics and camera theory as well as practice? Or maybe I like mathematics -- some do, you know.

Or perhaps, like Joseph James, simply to set the record straight. "The motivation behind this essay was to dispel common myths about different formats which all sprang from one central fallacy: to compare systems at the same f-ratio."

[wallyb]

Say I love the look of my wide open FA 75/2.8 on my 645 and want to know how it might work mounted to my K1000 (not that I have one right now) or my K20D? Or maybe I'm interested in optics and camera theory as well as practice? Or maybe I like mathematics -- some do, you know.

I love to explore different mathematical subjects, especially optics. There's no need for condescension. But that still isn't an explanation of its usefulness. And I don't buy your other explanation about using your 75/2.8 645 elsewhere -- there's nothing special about a certain amount of DOF. Like I said, there's no point to it, like there is in calculating a crop factor and how it effects the apparent focal length. Show someone two different images, one taken at f/5.6 and one at f/6.3, and ask them to tell which is which. Or one at f/19, and a different one at f/22. That's the amount of usefulness as has this calculation for DOF.

[Class A]

It may "comfort" someone, but it seems like it would comfort the same people who think of their computer as a typewriter connected to a television.

You are entitled to your opinion whether it is useful to you or not but I don't see the point of abusing other people.

How will you define DOF, when dealing with a print resolution of 10dpi? In that case, all the fine sensor-based DOF calculations are completely off.

I'm not sure what you mean by "fine sensor-based DOF calculations"? Obviously these must assume some common output format because it is irrelevant what the circle of confusion is on the sensor because people don't look at the sensor or at images the size of the sensor. DOF is always dependent on output size and viewing distance (and viewing acuity).

Ben, I value your posts. I have learned a lot from them. I don't want to convince you of anything. What Marc and I have written about equivalent f-ratios is as true as the point about equivalent focal lengths. You'll find it confirmed from other sources. If you have a question, I'd be more than happy to answer it, but I don't want to convince you against your will.

[rparmar]

I love to explore different mathematical subjects, especially optics. There's no need for condescension.

I meant no condescension. In fact I was proposing hypothetical reasons, not ones I personally imbue with truth value. To be clear, I am not fascinated by optics and find the overlong content of the article I cited tedious. But it is obvious that there are those who would disagree. I am willing to grant them respect for their interests. However, I feel you have become like a dog with a bone on this issue.

And I don't buy your other explanation about using your 75/2.8 645 elsewhere -- there's nothing special about a certain amount of DOF.

If you hold that there is nothing special about certain lenses or certain DOF then you are missing a great amount of joy in photography. But I don't believe you anyway; otherwise you'd shoot everything at f/8.

Furthermore, some people share lenses between systems so they don't need to spend more money and so they can cut down on the size of their load. These are practical reasons, not fanciful obsessions with particular apertures. Knowing how the lens will work on a different system can be key to determining if one needs a different lens or a larger camera bag. How will the 75mm focal length act on my digital sensor? Will the DOF control afforded by that maximum aperture that I am used to on 645 be good enough for me on APS-C? I would have thought that these are very real questions of importance to anyone serious about photography.

As for dismissing the three stops of difference needed to properly compare 645 and APS-C, I find that rather unenlightened (pun intended). I believe that there is a difference between f/1.4 and f/4. I can show it to you plain and simple. Dismissing this as the difference between f/19 and f/22 is wrong both numerically and qualitatively.

[wallyb]

You are entitled to your opinion whether it is useful to you or not but I don't see the point of abusing other people.

How on earth did you get "abuse" out of anything there?

[wallyb]

As for dismissing the three stops of difference needed to properly compare 645 and APS-C, I find that rather unenlightened (pun intended). I believe that there is a difference between f/1.4 and f/4. I can show it to you plain and simple. Dismissing this as the difference between f/19 and f/22 is wrong both numerically and qualitatively.

I never said there wasn't a difference between f/1.4 and f/4. There is a greater visible difference between that gap than the gap between f/19 and f/22 -- and you of all people should know that, so writing what you just did above only serves to weaken your argument and make you look like you're grasping at straws.

[rparmar]

Oh, for those keeping score, I would need a 30mm f/1.1 lens at ISO 50 to be equivalent to my 75mm f/2.8 shooting Neopan 400. Which just goes to show that there are simply some things medium format does that APS-C can never do.

[rparmar]

I never said there wasn't a difference between f/1.4 and f/4. There is a greater visible difference between that gap than the gap between f/19 and f/22 -- and you of all people should know that, so writing what you just did above only serves to weaken your argument and make you look like you're grasping at straws.

Or not, since the f/19 and f/22 quoted was part of your argument, not mine. I hardly need straws, rather a shotgun for this fish I see here in the barrel.

Would you care to explain why you chose those numbers, how they form a 3 stop interval, and how they relate in any way to the point under discussion?

[wallyb]

Or not, since the f/19 and f/22 quoted was part of your argument, not mine. I hardly need straws, rather a shotgun for this fish I see here in the barrel.

Would you care to explain why you chose those numbers, how they form a 3 stop interval, and how they relate in any way to the point under discussion?

You're not even reading the posts being made, so there's no need to spend time formulating an argument until you do.