[shuttertalk]

Most people say the 50mm lens is the "standard" lens because the perspective is similar to what the normal eye sees. But they said that in the old 35mm days too. So with digital, the 50mm becomes more like a 75mm, and a 35mm becomes more like a 50mm.

So is the 35mm the new "standard" lens for digital SLRs?

[Toad]

Difficult question.

In theory - if you judge the magnification factor to be the measure of a standard lens - then because of the crop factor, the 35mm becomes standard. But this is not really the true story.

The 1.5/1.6 crop factor does not actually change the perspective of the photo. Because the sensor is smaller, the frame is completely filled by the "center" of the area that the lens focusses on. The perspective and DOF is identical to what it would be without the crop factor. Thus, the perspective and DOF of the 35mm lens on a digital camera is the same as it would be on a 35mm camera - but the magnification level is greater.

So if you judge perspective rather than magnification as the measure of a standard lens (and I think this is the "correct" definiition) - then you are back to 50mm for a standard perspective. You just have to take a couple of steps back to get the same view.

[slejhamer]

Toad is correct. A 35mm lens is still a 35mm lens. Cropping by itself does not change perspective or DOF.

However, if you are shooting at the same distance to subject, then field of view (FOV) on a 50mm lens/35mm camera will be the same as it is on a 35mm lens/1.5x camera.(*) And because you moved (relative to how you'd use a 35mm lens on a 35mm camera) and are now shooting at the same distance to subject, perspective becomes the same. (**)

So if 50mm is "normal" on a 35mm camera, then 35mm can indeed be considered normal on a DSLR with APS-C sized sensor, just as 80mm is normal on some medium format cameras.

(Notice how no one talks about the "crop factor" or "multiplier" of 35mm cameras relative to 6x4.5cm medium format cameras. Yet you need to use different lenses to achieve similar perspectives, and adjust shooting position accordingly.)

DOF will still be different - at all comparable apertures, the 35mm lens on a 1.5x/1.6x camera will have more DOF than the 50mm lens on a 35mm-frame camera. Similarly, the 50mm lens / 35mm camera will have more DOF than the 80mm lens in medium format.


(*) - really you'd need a 33.33mm lens x1.5 to be the same as 50mm.

(**) - there will still be some minor differences based on where the focal plane lies. Prove this to yourself - set focus to infinity, keep everything else the same, and notice how field of view changes slightly as you turn the focusing ring away from infinity. When comparing lenses of different focal range, FOV will only be the same with both lenses set to focus at infinity after adjusting for distance to subject. (This applies even with two different zoom lenses set to the same focal length!)

[matthew]

For such a simple-sounding question, I've been amazed at the complexity of the answer. The original question brought to mind something that I've been mulling over for a while: is the 35mm film format an anachronism that digital photographers should move away from in their language and thinking?

There's a lot to be said for having common terms, and 35mm film is what we all speak, even if it's a second language. (I've never owned a 35mm film camera, not even a P&S.) Various digital formats are similar, but none are the same: Nikon likes a 1.5x conversion, Canon's at 1x, 1.3x, and 1.6x, Sigma uses 1.7 and Olympus is at 1.96x. We can all talk about a 50mm Macro lens and know how it would behave on our body of choice, but it would behave differently for every one of us.

But this is nothing new...

So if 50mm is "normal" on a 35mm camera, then 35mm can indeed be considered normal on a DSLR with APS-C sized sensor, just as 80mm is normal on some medium format cameras. (Notice how no one talks about the "crop factor" or "multiplier" of 35mm cameras relative to 6x4.5cm medium format cameras. Yet you need to use different lenses to achieve similar perspectives, and adjust shooting position accordingly.)

Small Format (135, 110, APS) film seems unusual in its standardization and dominance of a particular film size. There's a wide variety of formats in larger film, and each one will behave differently with lenses of the same focal length. I've never heard MF aficionados talk in 35mm equivalent focal lengths. Maybe they do. I suspect that they just understand each other's own native languages in a way we digital photographers haven't learned.

I suggest that, rather than continue to translate into a legacy language, we should use what we speak 'at home'.

[shuttertalk]

Good points everyone... I thought the answer would be straightforward as well! :/

Matthew - regarding standardization between MF and 35mm vs 35mm and digital... I think the difference is that MF was probably designed from the ground up as a different system (probably like that the 4/3 system is trying to do), where as digital started off as a "hey let's whack on a digital sensor onto the film plane of a 35mm camera!". And let's face it - digital SLRs share the same accessories, mounts, and up until recently - lenses as 35mm as well...

[Kombisaurus]

An interesting (and confusing) topic guys... and good to see a discussion on crop factors that doesn't just turn into a debate on sensor sizes

Reading Toad's and Slej's posts, I get the feeling that both state the facts correctly, but I'm still left thinking the perspective aspects are a bit unclear regarding ST's original question about finding an "equivalent" standard lens to the 50mm on a full-frame.

AFAIK, no matter what lens you use, if the camera and the subject and the background all remain in the same place, the perspective will *always* be the same. It doesn't matter what focal length or sensor/film size or "crop factor" you are using. Perspective is purely a property of the physical spacial relationships between the camera and elements of the photo*.

So in order to get the same perspective as a 50mm lens on a full-frame camera, you simply need to be standing in the same position when shooting. Of course if you are looking for the equivalent to the 50mm lens for APS-C cameras then you'll also want something that gives the same field of view as a 50mm lens, and in this case a 35mm lens would be the closest for people with a crop-factor around the 1.5 mark. So... yes! a 35mm lens will give you the same perspective as a 50mm lens on a full-frame!

But as the others point out, that's not the whole story. The depth-of-field will be different between the two lenses, the bokeh will be different, and there are a number of other things worth thinking about.

All lenses usually provide optically inferior results towards the edges and corners of a a photograph. Vignetting, a loss of sharpness and chromatic abberation are all common undesirable properties that can creep into photos towards the corners. By cropping the image circle down, photos from an APS-C camera use more of the "sweet spot" of the lens and less of the corners, which can greatly reduce some of these optical problems.
The flipside of this however is that the resolving power of the lens needs to be greater in order to maintain the sharpest possible image, as the APS-C camera is effectively "cropping and enlarging" the centre portion of the image circle when producing a photo relative to a full-frame camera**.

But finally... my real question is... Does a 50mm lens really represent what the human eye sees? And I'd suggest the answer is NO. Well not any more than a heap of other focal lengths anyway.

In fact, when I think about what I can see, I notice that my field of vision covers well over 90deg horizontally. Of course I can't make out sharp detail at the edges, but when looking at a spectacular landscape the entire view adds to the impact of the scene.
Even my 10mm lens doesn't go that wide (on an APS-C), and it is 5x wider than a 50mm lens or 3.5 times wider than a 35mm.
And have you ever noticed how when you take photos of the moon, it always comes out a lot smaller than the way you saw it with your eyes? Try taking a moon photo with a 50mm lens, it'll be a little white dot in a big sea of black. I think you need around a 150-200mm lens (on an APS-C camera) to re-create the feeling you get of looking up and experiencing a big moon in the sky.

But.. what's going on? Our eyes don't have zoom lenses, how come I'm suggesting we'd need a 10-200mm zoom (on an APS-C) to approximate human vision??? What's going on???
Well... I simply believe it means we see with our brains, not our eyes. Our brain can "zoom" its attention in and out in a way a camera can't. In photography we use different focal lengths to lead the viewer toward whatever it is that we had our attention on at the time. And I don't believe there is any standard level of attention, so therefore no standard focal length.

Of course... this is all just my unscientific opinion.

Cheers
Adrian


* I think this is complicated slightly by some ultra-wide angle rectilinear lenses and fisheyes which introduce some distortions to try to best represent a large 3D scene in a small 2D photo, but these are not the same as real changes in perspective (although they are often referred to as things like "exaggerated perspective").

** Its not really enlarging the image when it captures it, but when you print it out at a standard size (ie 8x10") then it will be effectively enlarged more than a full-frame image printed at the same size... because the enlargement factor is always relative to the sensor/film size.

[Toad]

Hmm. An interesting commentary.

Totally agree with your discussion about whether a 50mm lens represents a natural view.

But I disagree that the focal length of the lens has no effect on perspective if you are standing in the same spot.

I worked in 3D graphics and perspective is much more complicated than that. "Percieved" perspective is a combination of a number of factors such as distortion and light falloff, and these factors are toally characteristics of the lens. A larger distortion gives an impression of a much tighter point of view - its just a trick that our mind plays on our eye. Thus a "real" 20mm lens looks like a much tighter perspective than a digital 30mm lens.

In reality, you may be correct that the perspective remains the same, but the perceived perspective is quite different, and as photography is about the visual result, I would need to maintain that a 35mm with a 1.5 crop is quite different than a natural 50mm.

[shuttertalk]

Hm... perplexing isn't it... even though the distance is the same, a 35mm x 1.x is not the same as a 50mm. Perhaps it has something to do with the "focal length"? I guess some optical characteristics (DOF for instance) are dependent on focal length...

[Kombisaurus]

I think you're right Toad regarding perspective. On the one hand I do believe that *real* perspective will always be the same when camera, subject and background remain in the same places, regardless of focal length.
But on the other hand you are right in saying our perception of perspective can be manipulated by distorting an image (tilt/shift lenses are a clear example of this, as are the photoshop equivalent functions), and obviously it is the viewer's perception that is important to us, not what is "real" and what is "introduced" by the lens (if they want reality they should put down the photo and go to the place where it was taken! ) And its clear that different focal-length lenses *do* distort the image to a varying extent because they need to represent a 3D world on a little 2D rectangle. The wider the lens, the more "curve" it needs to flatten (think of how a map of the globe gets either distorted or chopped up when printed on a page, and take into account that lenses don't usually chop-up our photos and so must distort them instead).
But... Having said that I also think the distortion is minimal in the centre of the frame of most lenses (again, think of the map of the globe example - minimal distortion/chopping-up around the equator), as that is the part of the "curve" that is parallel to the focal plane and so needs no distortion. So by cropping the edges of a wider lens, I think you'd be removing most of the distorted areas and leaving the undistorted parts.
But I'm sure I'm still dumbing the whole thing down to just a couple of variables, whereas in reality there is a lot more to the equation as you say.

I'd be interested to perform the test though to compare the images. Apart from providing a definitive answer it would show just how obvious *all* the differences are in a real-world example (DOF, Bokeh, etc).
I don't have a 35mm lens (or a full-frame SLR), but doesn't WeddingShooter have a 5D and a 350D??? hehe.. <hint>
And it wouldn't even need to be a 35mm vs 50mm comparison... It could be 17 (or 18) vs 28, 55 vs 85, 85 vs 135 or any number of other common focal-length combinations that are approx 1.6 times apart.
hmmm.. there must be some examples floating around the web. I've seen so much discussion about crop factors and stuff like this, its surprising how few real-world sample shots there seem to be to illustrate things.

Who'd have thunk such simple things could be so complex?

Cheers
Adrian

[shuttertalk]

Hey great idea Adrian - I think it would make a great article if someone could post some real world comparisons of different focal lengths & 35mm comparisons, and also some theory and physical reasoning behind it.

At the moment, we're just debating about theoreticals...

[shuttertalk]

I found this article... now my brain is all warped!!

http://www.panoramafactory.com/equiv35/e...%20lens%20

What makes a lens "normal?"

A "normal" lens produces a field of view that is similar to human vision. The diagonal of the image plane is typically used as the reference for field of view determination. A "normal" lens has a focal length that is approximately equal to the diagonal measurement of the image plane. When the focal length is equal to the diagonal measurement it produces a diagonal field of view of approximately 53° which is similar to human vision.

For 35mm film (24mm by 36mm), the diagonal measurement is 43.27mm. 50mm is close to this measurement so it is considered a "normal" lens. A 50mm lens produces a 47° diagonal field of view which is close to 53°.

So my D50 has a sensor measuring 23.7 x 15.6 mm, which is roughly 28.373 diagonally. The article mentions that not all of the area on the CCD may be used, but for argument's sake - let's disregard that since I don't know how much is/isn't being used. The closes focal length therefore would be the 28mm lens - i.e. the "normal" focal length for my camera!


Ow, head hurts now... :/

[Kombisaurus]

Here's another article for you Jules, from Luminous Landscape.
Mike Johnston is arguing why he thinks 40mm is the real "normal" length for lenses.

http://www.luminous-landscape.com/column...y-05.shtml

As long as they're in the right hands, most common focal lengths can be used to make pictures that are excellent. So why 40mm? I'd say that the 40mm focal length is special precisely because it's not special. Purely by convention, 50mm has long been considered the "normal" focal length for 35mm photography. Early WA's were 35mm. Many photographers have made a choice between these two focal lengths as their own "normals." Many, like myself, have switched back and forth. The truth is, neither of these common focal lengths are quite "normal" for 35mm. 50mm is just a touch long, and 35mm is just a touch wide. Using the diagonal of the format as the standard, the true normal would be about 42mm (curiously, that's about exactly a 28mm lens on an APS-C digital sensor). The various oddball "intermediate" focal lengths (38mm, 40mm, 43mm, and 45mm), although much less common, are actually closer to a true normal for the format.
Why would you want a lens to be "normal," anyway? So what's so special about not being special? Glad you asked...

I notice though that nowhere does he claim that a "normal" focal length is closest to the human eye's vision, simply that the view they give is "not special" (while clearly ultra-wide and telephoto lenses will give a "special" look). This "not special" look makes the viewer look past the lens at the subject instead.

I also thought it was very interesting that he draws the same conclusion as you about the 28mm lens on a D50.