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Wednesday, 22 August 2007

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Amen!

Amen.

Did Friedlander make the "more sky" comment in reference to the Hasselblad SWC? Perhaps the comment was a reference to both the change in aspect ratio and focal length? I'm not nitpicking, I'm just curious!

Cheers,

Joe

Yeah, but...

Just kidding. You are correct, no matter what some of the the numbskulls over at dpreview say.

I think the bit about "aperture is not primarily a control of, or a measure of, DOF; it's primarily a measure of exposure" is what throws most people off. They're so used to hearing that f/2.8 gives you much less depth of field than f/11, which is not exactly the whole story. Maybe you should write a little treatise on that one, Mike.

I think Mike hit the nail on the head when he said this has come up because all of a sudden lots of "regular" folks are feeling the need to compare the many significantly different formats now available.

Several years ago, it was only the relatively well-versed MF and LF shooters that might have given this much thought--nearly everybody else whether it was a compact or an SLR or a rangefinder was shooting 35mm. The MF/LF community could accept and probably deal with these differences in a (relatively) intelligent manner without being compelled to standardize.

Now, from tiny digicam sensors to APS-C D-SLRs, the marketers had to make sure that their potential customers knew that their compact, shorter-focal-length lenses had similar ability to capture the same images as their old 35mm cameras. It is unfortunate that way back, 35mm didn't use something a little more versatile or portable (like Mike's somewhat imperfect but practical horizontal angle-of-view) as the primary means of describing their lenses.

All in all, 35mm equivalence isn't as useless as another term being bandied about in the digicam world--3x zoom, 10x zoom, etc. These *sort* of work only because the wide end is nearly the same (~35mm equiv.) on so many digicams. Some of us alternatively take this term to roughly determine how much the lens being described has been compromised from 'ideal' (e.g. 10x zoom is likely to have much more distortion, slower performance, vignetting issues, soft corners, etc. than a more conservative 3x zoom). This of course is only a rule of thumb and doesn't take into account actual lens design quality.

Aren't binoculars generally sold by magnification? Convenient but doesn't much address any differences in field-of-view, does it?

Amen. Thanks be to Mike.

"Did Friedlander make the 'more sky' comment in reference to the Hasselblad SWC?"

Hi Joe,
Yes. I'd have to look for the source, but that's my memory.

Mike

Well, I erred in the comment about DOF that I sent to your previous article. Good thing you didn't publish it. (grin)

Wrotniak has a good article about DOF in the digital age here:

http://www.wrotniak.net/photo/tech/dof.html#FRAME

and a handy chart for fourthirds users to calculate DOF here:

http://www.wrotniak.net/photo/tech/dof-43.html

I would argue that DOF is more than a "side effect" when considering focal lengths in the sense that photographers would like to have a handy way to predict DOF now that the old 35mm standard we all kept in the back of our heads is mostly unusable and lens makers seem to have stopped embossing those little guides on their lenses.

Of course, digital allows for lots of trial and error.

I guess the question is whether using these "35mm equivalents" makes people smarter or dumber about things like FOV and DOF. Maybe it would just make more sense to specify the actual focal length and format, and qualify the field of view descriptively--"moderate wide-moderate tele zoom," "normal," "super tele," etc. After all, no one thinks of the 15mm lens on a Minox, the 80mm lens on a Hasselblad, and the 300mm lens on an 8x10" camera all as "50mm equivalent" lenses.

Perhaps APS-C and other small-format digital shooters would be smarter, if they understood that they were shooting a smaller format than 35mm, and how that relates to the actual focal length of the lens, field of view, and depth of field.

Andrew has a good idea in using format-independent measures to describe lenses. He's also right that these would probably never catch on, but I'm interested in how to do it anyway:

Instead of using focal length, we could use angle of view. Manufacturers actually specify this for some of their lenses.

For light transmission ("brightness"), Mike is right that aperture is already format-independent.

I'm not aware of a format-independent way of describing the DOF of a lens at a given aperture. Is there a good one?

Or we can just go around saying, "my lens has the angle of view of a 82mm lens on 35mm film, an aperture range of f/2.8-f/22, and the DOF of an f/4-f/32 lens on 35mm film." (Speaking of the long end of my Nikon 17-55/2.8 DX, for example.)

"Perhaps APS-C and other small-format digital shooters would be smarter, if they understood that they were shooting a smaller format than 35mm, and how that relates to the actual focal length of the lens, field of view, and depth of field."

David,
Point taken, but it really isn't easy. The names of the "tiny formats" are opaque, and there are a number of them. It means that the people with the least serious cameras (digicams/digital point-and-shoots)--most of whom don't know what "focal length" even means--would need to be the most aware of the sensor sizes and they way focal lengths related to those sensor sizes. It seems a lot to ask.

Mike

Doesn't a 110mm lens on 2 1/4 square format translate into a 170 mm lens on a 35mm format [110x(56/36)]?

Stefan,
Given that apparent depth of field is dependent on a large number of factors and a large number of variables--not only aperture but focus distance, degree of enlargement, and even the sharpness of the in-focus image (the less sharp the plane of focus is, the more similar to it slightly out-of-d.o.f. objects will appear), I'd say that "finding a format independent way of describing DOF" would be impossible....

Mike

"Doesn't a 110mm lens on 2 1/4 square format translate into a 170 mm lens on a 35mm format [110x(56/36)]?"

Peter,
No. Written the way you have it, the correct equation would be [110x(36/56)].

Mike

I've always hated the term "crop factor" although I understand its use. I prefer "format/focal length relationship" which is more descriptive but, most likely, more confusing to those photographers more familiar with 35mm.

Mike writes:
When "translating" a focal length, all we're doing is bringing back a FOV crop to a context that is more or less readily understandable to people because it's familiar and/or standardized.

Hmmmmmm.....well --koff, koff-- actually.....
to many of us Children of the Digital Age,
"it" is neither a familiar nor an accepted standard.
Lament this as you will, but our concept of a 50mm lens
was formed by its use over a 23.7 x 15.7mm sized sensor.
So we have a different frame of reference (so-to-speak).

But that doesn't make us naive or stupid about things.
(Not, of course, that I think any of you all thought it did.)
We do "get it" -- that is, to "make the same photo"
on a larger "35mm" sensor would require us
to use some foot zoom and make other necessary adjustments.

Assuming that our 50mm lenses were not DX, of course,
and could actually be used over a larger sensor.

It's all relative, I think the man once said.


When you look at the info on different Leica lenses from the company (in pdf format), they illustrate different FOVs graphically, depending on aperture and distance to the object in focus. This isn`t perfect, but I guess it gives you pretty much of an idea of how a given lens work.

Or rather gave. Because with the 1.3X crop factor of the M8, things change. I think Leica would do the M8 users a favor, by providing separate graphical illustrations regarding that camera.

And in an ideal world, every camera- and lens maker should provide the same for their equipment, printed on paper, so people could carry it around during the first weeks of trying out a different format from what they were used to.
Also, they could add, as a standard formula, that other factors, like the above mentioned enlargement of the image and the sharpness of the plane of focus, also play a role.

At least I think it would give people an idea, and be quite helpful in all this confusion.

(I know that there are FOV calculators on the net, but this really should be provided on print with the box, like the info printed with films in the...eh film days.)

Dear Stefan,

Unfortunately, there's no clean rule that relates DoF to format size. The approximation I've given-- that the f/# scales up with the format-- is VERY crude. It fails in a lot of cases. It's a useful factoid to have in the back of your head, that you need to stop down more with larger formats to get the same DoF, but the amount is highly variable.

DoF is very complicated. Blanket rules are at best qualitative, and sometimes they're entirely wrong. For example, it's not true that 1/3 of the DoF lies in front of the subject and 2/3 behind it. At close distances, DoF is symmetric; as you approach the hyperfocal distance, the back/front ratio climbs from 1:1 to infinity. Another example-- DoF field is NOT independent of focal length (when on-film magnification is held the same). It's true for close working distances/large apertures. The further away you get and the more you stop down, the more DoF depends on actual focal length as well as magnification.

Al Blaker wrote a whole book on the subject-- Applied Depth of Field. Kinda the Bible of the field, and even he missed a number of important bits.

There's no way you can be "smart" about this, unless you can solve DoF equations in your head easily. All you can do is crunch the numbers when you need specific results.

pax / Ctein

How about a "Perspective Ratio"?:>)

Normal, i.e. 42mm on 35mm film, would be "1".

One thing I'm always noticing (on silly arguments at dpreview, or pick your favorite forums site) is that the weenies always think that shallower DOF is an unalloyed good thing. Probably because (a) it's something their SLRs have that the other guy's compact doesn't, and (b) because of the argument that you can always increase your DOF by stopping down, but you can't decrease it past the lens' maximum aperture.

What they're forgetting is that there are huge swathes of photography for which limited DoF is a curse, not a blessing. It's the limited DoF that makes very wide-aperture (f/1.2 or f/1.0) lenses not a practical solution for a lot of photographic purposes for which they'd sure be useful from a light-gathering point of view. While it can occasionally be neat to have one eye in focus while the other isn't, and the nose and ears of your subject heavily blurred, it's not if that's not the effect you need.

Macro shooters want more depth of field, not less. Long shutter speeds risk movement of the subject, either by itself or by wind movement, camera/tripod shake, etc etc.

Long-telephoto shooters would also love a bit more depth of field, too.

Quite apart from the weight and size savings, I can see that as a bonus to e.g. Four Thirds, not a disadvantage.

Oh, my -- remember when I wrote this to you just short of two years ago:

"My brain fell out today reading some horrible threads/flames/rants/insanity on DPReview about DOF and crop factors.

[...]

Anyway, can you *please* bring back the SMP, a photo blog, or something??? Really now, the world needs you."

And here we are -- back to DOF and crop factors. I guess you're right --it never goes away!

The reason I brought up DOF is this: I have long wondered why Olympus could make an f/2 constant-max-aperture zoom when nobody else seems to make one faster than f/2.8. So I was interested to realize that Oly's f/2 zoom would *not* get you less DOF than, say, Canon's f/2.8. And, of course, since f is lower for the same FOV, the aperture is not necessarily larger in absolute terms ...

"the weenies always think that shallower DOF is an unalloyed good thing."

Matthew,
It's as you say...for most photographers for most of photography's history it's been the opposite.

Either way, though, you can learn to use your tools. If they want shallower depth of field, they should just get a longer lens and/or move closer. It's not only aperture that matters. I've taken plenty of "shallow DOF" shots with a digicam.

Mike

Ben,
You're not getting this. The aperture IS greater in absolute terms. It absolutely is. An f/2 lens wide open will allow you to shoot with one stop faster shutter speed than will an f/2.8 lens wide open. Aperture is not a measure of depth of field!! It is a measure of how much light is coming through the lens. An f/2 lens is always one stop faster than an f/2.8 lens, regardless of the format.

Mike

As mentioned above, the problem lies with the internet-mixture of first-timers and experienced professionals, and what lies in between. To give an idea of the FOV of a lens, a relatively accessible standard is needed.

That said, I think the reviewer in the linked article went with an elegant solution by speaking of "Long telephoto lenses (less than 10 degree diagonal viewing angle) ...", giving the information that really matters and is not dependent on external factors such as FOV crops. Since the relationship between mm focal length and expected FOV is arbitrary anyway, perhaps it is time to change to a more universal look at lens specifications.

All you'd need is a little chart (available on the internet) in the beginning to convert the mm-degrees values for a certain format. Once it sticks in your mind, you're set to go.

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