Focal length - digital vs film

Forgive a question that is probably elementary to most of you, but I'm just getting started with digital.

 

If I'm correct, lens focal lengths are not equal in film versus digital (35mm) photography. In other words,

a 50mm lens for film is really a slightly tele lens in digital since there is a multiplier (1.5) due to digital

sensors being smaller than the size of a 35mm negative. So, having said that, to get a slightly wide lens

for the D80 I plan to purchase I would need a 24mm or 28mm lens (equivalent to 36mm or 42mm,

respectively, in film photography), correct?

 

If this is correct, do you then see through the D80's viewfinder in the focal length of the digital lens, or is

it "filtered" because of the multiplying factor of the smaller digital sensor. For example, if the len's true

focal length is 24mm, does the smaller sensor mean that what I see through the viewfinder is really 36mm

(if considered in terms of 35mm film photography) Does that make sense?

 

Can you tell why it's taken me so long to get into digital... Thanks for the help and the patience!

The focal length of the lens is determined by the lens and not the camera. So a 50mm lens is still a 50mm on film or digital. Because the DX sensor is smaller the angle of view is smaller which causes an effective increase in focal length-but the DOF will be that determined by the lens and not the camera. So while a 50mm lens on a D80 will have the effective focal length of 75mm, the DOF will be that of a 50mm. What you see in the viewfinder is essentially what the camera is taking a picture of, the effective focal length. Remember that most SLR viewfinders don't show 100% but say 95% of the image depending on the camera. (I'm pretty confident that's all correct-if not someone smarter will straighten me and you out)

a given focal lenght is the same, whatever you mount it on. it's the size of the sensor (film) that gets to say what will be the field of view from that given focal length. for example, a 50mm is considered standard lens for a 35mm camera, it is is considerably wider on a 6x7 camera (given it has enough coverage for the format), because the film area is much larger. on a small sensored digicam otoh, it's tighter, as the sensor is small. but it's still a fifty... but to simplify, you can multiply by 1.5 (in nikons case) and get the "equivalent" (how i've come to hate that word) field of view.

First of all, a 50mm lens is a 50mm lens, no matter what sensor or film size it is in front of, as the 50mm refers to the focal length, which remains the same. So does the relative compression or decompression of perspective.<p>

Angle of view, however, changes, depending on what size film or digital sensor is used, where an APS-C size sensor, being about half as big as 35mm film, catches a smaller part of the image you would get with the same lens on a 35mm camera. This is also why many call it the crop factor, which is a more correct way of looking at it, since the image is cropped relative to a 35mm frame, usually by a factor of 1.5 or 1.6.<p>

If you are used to thinking in 35mm, so to speak, then a simplification to get your head around what angles of view you would like to use for your photography is to multiply the focal length of lenses by the crop factor to see what the equivalent angle of view would be in 35mm terms. So, as per your calculations, a 28mm lens would have the same angle of view on a 1.5 crop camera as a 36mm lens on a 35mm camera.<p>

The viewfinder on a digital SLR would naturally be dimensioned for sensor size, and show somewhere between 90 and 100%, usually, of the image that you will capture when you release the shutter. Actually, it would have been fun if SLR viewfinders showed a larger image with lines marking the limit of what is captured, since that makes composing easier, and for many shooting situations, action in particular, knowing what is immediately outside of the frame can be useful. But that is one of the advantages of rangefinders and it will probably remain that way.

Imagine you are an average USA height of 5ft 9 inches; ie 1.75 meters. On a pro basketball team you are considered "short" . If you move to country "X, Y or Z" you will be considered "tall". If you play on that pro team, or watch from the crowd, or move to country "X" you are still 1.75 meters tall. The same goes for a 50mm lens. On the basketball team they might call you "runt"; or 0.8x. In country X they might call you 1.2x or giant.

It is not that complicated if you don't try to be too accurate.

 

20mm = 28mm,

24mm = 35mm,

28mm = 45mm,

50mm = 75mm,

58mm = 85mm,

85mm = 135mm,

135mm = 200mm,

200mm = 300mm

 

What you see in the view finder is what you get. If you use 24mm or wider in film, buy a 12-24 Tokina or Nikon.

It is simple.

 

A 50mm lens on your smaller digital sensor is, as you suggested, slightly long. Period. A

nominal-normal lens is the diagonal of the sensor (or when comparing perspectives of

lenses between dissimilar aspect ratios, the long side of each format is used in the

diagonal calc.)

 

What you see in the finder is what you get. No worries. No filtering. Nothing special.

 

Mr. Flannigan's comments notwithstanding, if you were to go to Ireland, your physical

dimensions wouldn't matter one bit after a few pints of Guinness. Highly recommended.

The focal length is as listed on the lens, no matter what. The EFFECTIVE focal length is the actual focal length times 1.5. So, the effective focal length of your 28mm lens would be 42mm (1.5 x 28)

 

What you see through the viewfinder is very close to what the sensor sees. My D70s viewfinder sees slightly less than what the sensor sees. Your D80 probably does the same thing.

 

It'd be silly to have a DSLR viewfinder 'see' the same thing as a film SLR, eh? You'd have about 1/3 of your image lopped off on every frame.

<i>The EFFECTIVE focal length is the actual focal length times 1.5. So, the effective focal

length of your 28mm lens would be 42mm (1.5 x 28)</i><p>

Nonsense. You are making the standard error of comparing it to a 35mm format which has

just got to stop.<p>

If digital has won, then why do manufacturers continue to reference their focal lenghts to a

dead technology with a different format? <p>

50mm is 50mm. The EFFECTIVE focal length is still 50mm!<p>

In movies there is little confusion; one thinks in "ANGLE". Thus one has a scene that requires a certain angular coverage; and the lens focal length is chosen, which varies whether one is using 70mm, 35mm, 16mm film format. Thus a 50mm lens is still a 50mm lens no matter wahyt format; it just "covers less angle" when on a 16mm camera. "effective focal length is just for folks who dont want to deal in angles; typically marketing chaps! :)

Look at my post below (w/nw old Nikkors on new Nikons) to see what a 28mm looks like on a digial camera. The useful 28-50mm zoom becomes a rather unappealing 42-75mm zoom, sort of like a super normal. Remember when Nikon made the popular 43-86mm zoom for film? I usually use a film camera 24-120mm zoom on my D80 which is much more useful range but miss a true wide angle.

It's funny - this question comes up all the time, but there's still a lot of confusion around it.

 

Let me explain this way: FOCAL LENGTH is a PHYSICAL PROPERTY (you can search "focal length" in wikipedia if you want the technical definition).

 

Since focal length is a physical property, there is NO SUCH THING as EFFECTIVE FOCAL LENGTH.

 

As a physical property, it is like your height -- it is the same no matter what kind of body it is mounted on. Just like, if your height is 6 feet, you will have the sam eheight whether you are standing, sitting or prone.

 

The difference between mounting a lens, say a 50mm lens, on a full frame SLR vs. a Nikon DSLR is the ANGLE OF VIEW (AOV).

 

1) A 50mm lens, mounted on an F100, has an AOV of 46.79 degrees (diagonal).

 

2) The same 50mm lens, when mounted on a D80, has a narrower AOV of 32.18 degrees (diagonal).

 

3) A 75mm lens, when mounted on an F100, has an AOV of 32.18 degrees (diagonal).

 

So, the AOV, or WHAT YOU SEE is the same in 2) and 3).

 

The FOCAL LENGTH of 1) and 2) is THE SAME = 50mm, even though they would have different AOV's.

 

Hope that makes it easier.

 

KL

KL IX

 

Well said and clearly explained. It really is time for people who call themselves photographers to graduate from this kindergarden nonsense of "equivalent focal length" and start thinking in terms of angle of view. In the final analysis that specification is a damn site more informative.

 

What makes it even more rediculous is that today the vast majority of those shooting DSLRs with cropped sensors have never shot a 35mm. So where's their frame of reference?

The "frame of reference" is plastered in EXIF camera data; such as from this CS3 Metadata screen shot I just captured on my laptop. The Browse is of an image from a new Nikon L6 camera. Thus since CS3 is not obsolete, or my new Nikon L6. Folks see this "equalvalent focal length stuff" and often ask daily questions!<BR><BR><img src="http://i4.photobucket.com/albums/y148/ektar/PNdesktop/FocalLengh35mmfilm64.gif?t=1169367189">

Lots of technically sound and correct posts about how focal length is an attribute of the lens and how it's the angle of view that changes with (APS) digital sensors.

 

All of which probably just confuse the average person who keeps asking this question on photo.net so they get the right lens for a vacation landscape or a portrait of their kid.

 

*Practically* speaking...

 

* Yes, multiply by 1.5 to get the equivalent focal length in 35mm.

 

* Yes, you will see the "multiplied" view when you look through the camera.

 

* DoF differences between 35mm and the typical DSLR are small enough that 99% of people won't care. So don't even worry about it.

 

Simplifying things for the vast majority of people who will never need to know or care about focal length vs. angle of view, and who will never have their pictures affected in the least by this knowledge, is not "kindergarden nonsense". It's helping them out.

I have a better idea.

 

Especially for those of us shooting digital only now...

 

Let's stop all this multiplying nonsense and simply start thinking differently with the

numbers. 17mm is wide. 24mm is slightly wide. 35mm is "normal" (whatever "normal"

is...). 50mm is slightly long, etc...

 

The thing is, what's been said above is dead on. it's all about angle of view or field of view,

NOT equivalent focal length, which is a myth...

As most people can agree on, focal length is an invariant property of the lens. So it never changes and the very idea of "effective" length is pure nonsense. These days, where budding photographers flock to digital without a previous relationship to 35mm film, the reference value is dwindling towards zero anyway. Why the camera producers perpetuate this nonsense in the EXIF headers is beyond me.<p>

 

The real problem with the concept of a multiplier or "crop" factor is as follows,<p>

 

1. people think a 50 mm lens "is" a 75 mm<p>

2. Everyone "knows" a 75 mm lens has different DOF from a 50 mm lens<p>

3. Ergo, digital DSLR has different DOF from film<p>

 

a conclusion which is fallacious and causes endless and silly debates.

<p>

I've been shooting the same scene with 6x9cm, 6x12cm, 4x5", or 8x10" formats using the same lenses, so am rather familiar with the fact that nothing except for the field of captured view changes. In fact, the angle of view of the lens itself doesn't change, it will always project the same image, but when the recording format gets smaller relative to the image circle you get proportionally more stray light and increased risk of flare. The latter fact is rarely discussed, but can be a real nuisance especially when you do close-ups.

> 3. Ergo, digital DSLR has different DOF from film

 

> a conclusion which is fallacious and causes endless and silly debates.

 

> I've been shooting the same scene with 6x9cm, 6x12cm, 4x5", or 8x10" formats using the same lenses, so am rather familiar with the fact that nothing except for the field of captured view changes

 

Depends on the frame of reference. If you crop differently, but enlarge the cropped picture to the same output size, you change your acceptable circle of confusion (because of different enlargement ratio) and thereby the DOF, all else being equal. If you crop your output, you obviously only change the crop ;-)

 

So, considering equal output size an 1,5 crop will give about 1,5x

less DOF at equal focal length, but 1,5x more at equal angle of view...

 

Andreas

I beg to disagree. DOF is just an illusion and you can perceive it with your mind, but treating DOF as a physical entity and using it in calculations is a waste of time.

> DOF is just an illusion and you can perceive it with your mind, but treating DOF as a physical entity and using it in calculations is a waste of time.

 

Ah, yes. There is no systematic connection between f-stop, enlargement ratio, some other parameters and DOF, DOF just happens to you accidentally. And the earth is flat and your next door neighbour is a witch...

 

Andreas

Silly as silly can be. A model is just a model, it is not reality.

"Nonsense. You are making the standard error of comparing it to a 35mm format which has just got to stop."

 

Pico, please re-read the question in the original post, and try responding to it.

The 24x36mm sensor/film area for 35mm film is not the holy grail. For several decades starting from the 1960's to just a few years ago, that was the most popular film format so that a lot of us are familiar with the relationship between angle of view (tele, wide, super wide, etc.) and focal lengths for that particular format.

 

Unfortunately, now that era is over, most of us continue to use those focal lengths to describe the angle of view. If we learn to describe lenses with their angle of view in terms of degrees instead of focal lengths, there will be no confusion.

> A model is just a model, it is not reality.

 

Indeed. A pysical model depicts and allows predictions about reality - within certain limits. But dismissing geometric optics because there are areas where quantum electrodynamics (Just another model!) are necessary? "Silly as silly can be"...

 

Everyone else shares the same "illusion" about the DOF in my pictures , an "illusion" which is precisely depicted by the "DOF model", that's good enough for me.

 

This discussion leads nowhere, I'm out.

 

Andreas

Hang a big picture on the wall, look at it up close. Then go back a metre or two, does DOF change? Basically that is the gist of what you state.

 

I agree you should go elsewhere with your sarcasms.