Equivalent focal length?

[ECam]

Calculating the "equivalent focal length" of a lens originally designed for a 35mm film camera, as I understand it, is required because the area of the APS-C size sensor used in digital cameras is smaller than the area of a 35mm film frame. Thus, lenses designed for film cameras that are subsequently used with digital cameras and the smaller APS-C size sensor must have their focal length multiplied by a factor of 1.5 to convert the focal length to a "35mm film camera lens equivalent". A lens designed for a film camera that has a 20-35mm focal length becomes a 30-52.5mm "equivalent" lens when used with a digital camera having an APS-C size sensor. Why all this??? I am looking for a wide angle zoom lens for a Sony a350 that is of better quality than the Sony 18-70mm kit lens. The kit lens is made for the Sony digital camera, so it seems to me that no focal length "conversion" would be necessary; like the "equivalent" of the 18-70mm kit lens being 27-105mm. I have also seen, on some camera equipment web sites, the Minolta AF 11-18mm DT lens being described as being the equivalent of 16.5-27mm lens. But if the Minolta AF 11-18mm DT lens was originally designed for the Minolta digital camera that uses an APS-C size sensor, why would there be any need for converting the lens focal length, unless the lens were going to be used on a film camera. As I understand it, the only reason to do a "conversion" is if you are using a lens designed for a film camera that is being used on a digital camera. And if you really want to see what "vignetting" is, just put a lens designed for a digital camera on a film camera body. (This can be done with Minolta lenses on Sony Alpha bodies, as everyone knows, and maybe with some others that I don't know about.) I don't think I am confused about this focal length thing, but then again, maybe I am. Maybe my confusion comes from reading descriptions about lenses for digital cameras that are inaccurate with respect to this topic. Any ideas??

[Javelin]

the sensor size doesn't change the focal length of the lens. a 70-210 is still a 70-210 no matter what sensor is behind it. it's because the calculation (measurement really) that makes the focal length acounts for the film/sensor plane and not the size of the sensor. The only reason people need a "crop factor" is to help people with more film than digital experience have a better idea of what the scene will look like though it.

The 11-18 lens is still going to be 11-18 on a full frame sensor camera. It's made for APS-C which only means they don't care about the area of the image that exceeds the sensor size. so is there are defects towards the edge of the glass they won;t matter at all.

The 18-70 is the same. the pictures look like they came from a 27-105 lens on an APS-C camera but writing 27-105 on it would be a lie and would be a number that has no meaning beyond marketing.

[jcoffin]

The "intent" of the lens doesn't really change anything. A given focal length projecting on a given size of film/sensor plane will cover a given angle of view. If you increase the size of the plane it's projecting onto, that focal length will have to cover a wider angle of view to fill the plane. If the plane gets smaller, a smaller angle of view fills it.

The conversion is equally necessary for "DT" lenses as those designed for 35mm because in the end there are really only two factors to consider: the focal length and the size of plane it's filling. Between them, these determine the angle of view that that lens will cover.

In the end, the "conversion factor" comes from the fact that the angle of view isn't a property of the focal length alone, but of the focal length coupled with the size of focal plane being covered. The 35mm format, however, has been popular enough for long enough that quite a few people think of a given focal length primarily (if not exclusively) in terms of the angle of view that focal length would cover on the 35mm format.

[David Kilpatrick]

This is no different from the issue with depth of field. DoF is quoted for a certain size circle of confusion (equivalent to 300 lines per inch) at a print size of 10 x 8 inches viewed at arm's length, which is considered to be 18 inches. Lenses once used to be engraved with scales based on this. Of course, if you made a 5 x 4 inch print or a 20 x 16 inch print, or blew up a small part of a negative, or viewed a print from six feet away on the wall, or through a magnifying glass - the scales and tables for the 'standard' are all wrong.

But we know more or less what that standard means, therefore having it around is useful.

Similarly, we know more or less what a '500mm lens' or a '17mm lens' means in terms of a full 35mm frame, just as rollfilm users know what a 150mm or a 40mm means. Therefore it makes sense, still, to mention full-frame angle of view equivalents when describing or discussing APS-C lenses. It also makes sense for APS-C users to get familiar with the native (real) focal length and to know what 11mm means or 250mm means, in terms of what they can do with it. Use of both sets of terms - 18-250mm lens (28-375mm equivalent) is a reasonable and clear way to describe that particular lens.

The pedantic thing about 'no it's not really this it's that' only serves to confuse anyone who fails to understand and irritate anyone who does (and is seeing it for the hundredth time). Life would be fun it we all started saying what figures were not really what they seem to be:

"Sorry, but f/4.5 is not really the focal length divided by 4.5, as the maker has adjusted the physical diameter to compensate for light loss and create a T-stop of 4.5 - the real aperture is f/4.32, and therefore the depth of field is..."

"No, at 1/2000th your shutter is not open for only that time - it consists of a slit which takes 1/250th of a second to travel from top to bottom of the frame, narrow enough to expose each pixel for only 1/8000th, but the total duration of the shutter action is 1/250th..."

"No, your viewfinder does not have a 0.95X view of the scene in front of you - you are using a 24mm lens, it actually has a X0.456 view because the 0.95X is stated for a 50mm lens..."

"Your flash duration is not actually 1/1000th because the maker has quoted it at T=0.5, which means that the period when the light output is equal to 50% of the total energy is 1/1000th - in fact it has a short build up and a long decay making the total duration of usable flash 1/300th..."

"No, your GN58 flashgun does not have a GN of 58 ISO100/metres despite claiming this. GNs are supposed to be given for an angle of coverage of 60 degrees horizontal and vertical, or sufficient to cover a 35mm focal length used on 35mm full frame, or a 75mm lens used on 6 x 6cm rollfilm. However, makers all started quoted GNs for the longest zoom setting of their flash, which exaggerates the figure because the light is concentrated over a smaller angle. The HVL-F58AM is only GN58 at a coverage for a 105mm lens on full frame, and has about one-sixth of the output of a true GN58 flash.."

"Mind you, 6 x 6cm rollfilm isn't 6 x 6cm either - it is 56 x 56mm..."

The photographic world is full off stuff like this, and the 'equivalent focal length' is just another - a convenient simplification to help people use their gear without scratching their heads to understand it all. So for that matter is 'ISO' on a digital camera.

David

[KevinBarrett]

The focal length multiplier, or FOV crop factor, or whatever it is called, allows users of smaller formats a reference back to a larger global standard. By knowing the crop factor, somebody considering a Sony a350 or Panasonic G1 can know the difference between the lenses supporting each system. For somebody already married to a system, the crop factor can help them understand where they lie with their lenses and what kinds of options would be open, or closed, to them should they move from APS-C to FF or vice-versa.

[InTheSky]

This is no different ... matter is 'ISO' on a digital camera.

David

Wow ... at the end of this answer you should say : Amen

Or it is almost like in the Matrix, when Neo learn that the world where he is living is just an illusion .

I think this kind of specifications are the same in electronic, mechanic or everywhere. (remember trying to explain why my Stereo system was just a 50Watts ... but the sound was louder than my friend having bought a 200 watts cheap system ).

[ECam]

My sincere thanks to Javelin, jcoffin, David Kilpatric, Kevin Barrett and InTheSky, for taking the time to answer my questions and clear up my confusion over what "equivalent focal length" really means. Having just begun to delve into the world of digital photography (that is probably obvious), I find this site, and the members here very helpful. Thanks again.

[Vidgamer]

... - a convenient simplification to help people use their gear without scratching their heads to understand it all. So for that matter is 'ISO' on a digital camera.

David

I recently researched this... I didn't realize that the ISO is set to whatever the manufacturer wants-- they just change the sensor amplifier gain. I was having a lot of trouble figuring out how a small-sensor P&S can have the same exposure as a DSLR!

I like the pedantic-David. I learn a lot of things I never needed to know that way!

As for using a 35mm equivalent, it's also helpful for using different P&S cameras, all of which seem to have different designs....

[01af]

The pedantic thing about 'no it's not really this it's that' only serves to confuse anyone who fails to understand ...

Actually it's just the other way around. These ubiquitous 'simplifications' confuse the hell out of those who did not yet understand the basic underlying principles. Pedantic-mode doesn't help out in the field when trying to figure out the best focal length and the optimal settings for the subject at hand. But it does help when trying to comprehend the basic coherence of things.

Simplification makes life easier only for those who understand the simplification.

"Sorry, but f/4.5 is not really the focal length divided by 4.5, as the maker has adjusted the physical diameter to compensate for light loss and create a T-stop of 4.5 - the real aperture is f/4.32, and therefore the depth of field is ..."

And don't let us get pedantic about this one ...

-- Olaf

[David Kilpatrick]

I think the best comment here has been the one which refers to sub-DSLR formats - compact digicams can have a huge range of possible sensor sizes, and confusing close 'real' focal length markings like 4.2mm or 4.5mm or whatever. With so many possible sizes (not to mention format ratios from 16:9 to 4:5, and not that many matching the 2:3 35mm shape) the use of '35mm equivalent' focal lengths really does help.

The most logical approach would be simply to state the angle of view - 60, 80, 45 or whatever degrees. But that would mean retraining the thinking of all camera users. Another way would be to take the angle of the standard lens (call it 45 degrees) as '1', and then describe lenses in factors of this - 0.5 to 3X. But that of course begins to sound like magnification at macro level. So ultimately the historic accident of 35mm format focal lengths ending up being used as a way of describing view angle is probably as neat as we will ever get.

David

[KevinBarrett]

I wondered why we haven't gone to an angle-of-view standard--at least for P&S cameras--such a measure would encourage more manufacturers to get competitive on the wide angle.

Well, they seem to be getting competitive there now, thanks to Panasonic and Samsung.