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01-06-2007, 04:48 PM   #1
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lens focal length equivalencies and standards

Can someone explain - or point me to an explanation of - the difference between an N mm lens used on a digital SLR and the same lens (or at least a lens of the same focal length) used on a film SLR? I gather that an N mm telephoto lens used with a digital SLR gets me "closer" (provides greater magnification or a smaller field of view) than the same lens used on a conventional film SLR, but I'm not sure why. I would also like to know if this is a constant, that is, does the Tamron 18-200mm zoom I use with my Pentax provide the same range as the same lens used with a Canon or Nikon digital SLR?

And is there a standard for lens focal length now and is it the digital value or the film value? In other words, if somebody says a photo was shot with a 300mm lens, do I have to know what kind of camera was being used in order to know what that focal length means?

If this question doesn't quite make sense, well, that would indicate that my confusion is deeper than I know, so I'd be grateful if someone would slap me and straighten me out. Thanks,

Will

01-06-2007, 05:05 PM   #2
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READ THIS
DSLR Magnification

It should clear up some of the things you are asking about.
Regards,
01-06-2007, 05:12 PM   #3
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QuoteOriginally posted by regken Quote

Excellent - thanks. With Google's help, I have now also found this article on the dpreview.com site, which says the same thing slightly differently.

This seems an important subject to know about.

Will
01-06-2007, 05:25 PM   #4
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Most 35mm-type DSLRs have a sensor smaller than a 35mm film frame, therefore, using a 35mm film SLR as a benchmark, it is like taking a photo and then cropping the frame to the size of the sensor. Therefore a particular focal length lens will have an n degree field of view in a 35mm film camera and an m degree field of view in a particular DSLR, where M is usually smaller than m.

The shorthand is to say that a particular focal length lens on a particular DSLR has an equivalent field of view to a lens of another particular focal length when used on a 35mm film camera.

That's the generality of it, now let's talk specifics. This is all approximate, because the format (aspect ratio) of most DSLR sensors is different to that of a 35mm film frame, but let's forget about that for now.

Also I will only deal with 35mm-type DSLRs, so let's leave aside Mamiya, Hasselblad, those mega-expensive medium format digital backs and the upcoming Pentax digital "645".

Current 35mm-type DSLR sensors fall into three categories...

The four-thirds system was developed by Olympus and is now also used by Panasonic/Leica. Its sensor is about half the size of a 35mm film frame, and therefore a 100mm lens has an effect equivalent to a 200mm lens on a 35mm film camera (1:2)

A few cameras (just Canon?) have a "full frame" sensor, meaning it is the same size as a 35mm frame. Therefore a 100mm lens has the effect of a 100mm lens on a 35mm film camera (that one's easy, 1:1).

All other 35mm-type DSLRs (as far as I am aware) have a so called "APS-C" size sensor (I assume that means it's the same size as an APS film frame). This type of sensor is 2/3 the size of a 35mm film frame, so a 100mm lens has the effect of a 150mm lens used on a 35mm film camera (1:1.5)

All current and past Pentax DSLRs fall into the third category.

Simon


Last edited by Simon; 01-06-2007 at 06:03 PM. Reason: Clarification
01-06-2007, 05:27 PM   #5
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QuoteOriginally posted by WMBP Quote
Can someone explain - or point me to an explanation of - the difference between an N mm lens used on a digital SLR and the same lens (or at least a lens of the same focal length) used on a film SLR? I gather that an N mm telephoto lens used with a digital SLR gets me "closer" (provides greater magnification or a smaller field of view) than the same lens used on a conventional film SLR, but I'm not sure why. I would also like to know if this is a constant, that is, does the Tamron 18-200mm zoom I use with my Pentax provide the same range as the same lens used with a Canon or Nikon digital SLR?

And is there a standard for lens focal length now and is it the digital value or the film value? In other words, if somebody says a photo was shot with a 300mm lens, do I have to know what kind of camera was being used in order to know what that focal length means?

If this question doesn't quite make sense, well, that would indicate that my confusion is deeper than I know, so I'd be grateful if someone would slap me and straighten me out. Thanks,

Will

I'm not sure I can explain anything. But you can have my point of view on it:

A lens is an isolated optical system. Let's think of a great lens; the SMCP FA31/1.8... Focal length (FL) is 31mm. That's it. The FL doesn't change when moving the lens from an Pentax film SLR (fSLR) camera to a Pentax digital SLR (dSLR). The FL remains the same when unmounting the lens, and when holding it pointing up side down in your hand.

The lens give you a certian field of view (FOV). Think of the camera seen from above. Imagine rays of light entering through the lens hitting the film in the fSLR: you get a certain FOV measured over the 24x36mm film surface. It's most common to measure diagonally over the picture. As we are looking at the system from above we measure from the left to the right edge of the picture. Ok?

Now we know Pentax (unfortunately in my opinion) use a sensor that is smaller than 24x36mm. It is just 23.5 mm wide. In the case of the fSLR the distance from the center to either the left or right edge of the picture is 18mm. In the dSLR case it is a little less than 12mm from the center of the sensor to any of the side edges. Meaning?

Everything that was captured with the fSLR camera doesn't fit on the sensor. (Remember that the distance between the lens and film/sensor is the same in both cases.) Parts of the picture is cropped away - hence the expressions "crop factor" and "cropped sensor".

So, the FOV has changed while everything else is the same. The FL remains the same and the magnification factor (how much the lens magnifies the subject) are the same. The only thing that changed is the FOV.

So, the lens when used with a "cropped sensor" doesn't bring you closer or increase the magnification. The impression you get while looking at the subject (a distant bird, perhaps) is that the bird fills more of the picture compared to the same situation/location with a fSLR camera. This is important to understand as it is the key to understand the rest of the effects following to this relatively simple point:

Side effects:
* You "get closer" with regards to take advantage of the available sensor or film size. Bird photographer like this.
* You loose some surroundings. Wide angle freaks dislikes this.
* The depth of focus (DOF) has changed a little (I won't go deeper into this subject as it should only make us look like a crcle of confusion right now)
* You get less distorsion, less vignetting and better edge resoultion thanks to the fact that we use only the central part of what the lens is made to be able to cover. It's always the edges that loose out when you push a lens to it's limits (test it). This can be of great or little advantage depending of what lens you are using.

Hmm. Good enough for a start I think. I surely forgot some basic things but I'm also sure other members will fill in.

Two links:
DPR editorial on the subject: Focal Length Multiplier: Optical: Glossary: Learn: Digital Photography Review
DOF and calculating the same:
Hyperfocal Distance and Depth of Field Calculator - DOFMaster

I'm aware there are better explanations but this is the way I like to think about it.

Some standard lenses with a fSLR (dSLR 1.5 crop) camera:
80-90mm (50-60mm): considered a short tele, good for head an shoulder portraits giving nice proportions to facial details
45-50mm (28-35mm): The standard lens since ages
35mm (22-24mm): moderate wide, good for street shooting and indoor use
20-21mm (14mm): extreme wide for perspective effects and covering a lot

regards,

EDIT: Lol, yes I type slowly...

Last edited by Jonas B; 01-06-2007 at 05:29 PM. Reason: clicking reply showed a lot of replies not there when starting this...
01-06-2007, 07:53 PM   #6
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There is a nice comparison of the two professional DSLRs from Canon and Nikon at:

Canon 5D & Nikon D2x

Consumer Canon DSLRs use cropped sensors, the high end uses full frame. The professional Nikon uses a cropped sensor.

Last edited by superfuzzy; 01-06-2007 at 08:02 PM.
01-07-2007, 10:22 AM   #7
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QuoteOriginally posted by WMBP Quote
In other words, if somebody says a photo was shot with a 300mm lens, do I have to know what kind of camera was being used in order to know what that focal length means?
Others have explained that a smaller sensor or film frame will result in a narrower field of view for a given focal length. So, the answer to your question is that you do need to know the camera format to understand whether a given focal length gives a wide or narrow field of view. For example, an 80mm lens gives a narrow field of view with a Pentax DSLR, a normal field of view with a 6x4.5mm medium format film camera, and a wide field of view with a 4x5" view camera.
01-07-2007, 10:59 AM   #8
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Thanks to everybody for these excellent responses to my question. I have a MUCH better understanding of this topic now. In fact, my understanding has changed so much that I want to respond to my own original post and - for the benefit of others who might wonder the same thing - correct the confusions implied in my own questions. So this is me, correcting myself.

QuoteOriginally posted by WMBP Quote
Can someone explain - or point me to an explanation of - the difference between an N mm lens used on a digital SLR and the same lens (or at least a lens of the same focal length) used on a film SLR?
I understand now that it's not a matter of digital SLR vs film SLR, but of the size of the sensor or capture element inside the camera. A 100mm lens on one of those high-end Canon cameras that has a sensor the same size as 35mm film's capture area will produce the same field of view.

So when trying to understand the meaning of a given lens's focal length, you have to know the size of the camera's sensor.


QuoteQuote:
I gather that an N mm telephoto lens used with a digital SLR gets me "closer" (provides greater magnification or a smaller field of view) than the same lens used on a conventional film SLR, but I'm not sure why.
I understand now that it's not a matter of getting "closer" or providing greater magnification. The true magnification is exactly the same. It's just that, if the sensor is smaller than (say) the capture area of 35mm film, then the image will fill more of the captured area, because the captured area is being cropped. It's exactly like taking scissors to an 8x11 photo of a bird and cutting off two inches of paper all around the photo. You'll end up with a 4x7 image in which the bird LOOKS larger, but actually isn't. [/quote]

I also understand now that the image captured on my K100D with a crop factor of 1.5x is not any higher resolution than the image captured with the same lens on a camera that has a sensor the same size as the capture area of a 35mm film camera. Taking the image on a smaller sensor does NOT make it possible to print the picture at any larger sizes than I could have if I'd shot to film.

The bottom line this. I've seen a lot of talk - not just in forums, but in books that I've been reading, about focal length equivalencies, you know, this 200mm lens on a K100D = a 300mm lens on a 35mm film camera. I now think this talk is very misleading. It certainly mislead me. If I read a book on photography from 20 years ago talking about using a 300mm lens for a shot, in terms of the level of detail captured (rather than the field of view), that lens will work exactly the same way on my Pentax K100D. It's not a matter of focal length equivalencies at all - it's a matter of CROP FACTORS in the relationship of sensors to lenses.

It's a bit of a bummer, because I was sort of under the impression that my 300mm lens = a 450mm lens for a film camera, but I see now that it doesn't. I simply get to save less of what the lens "sees".

Thanks again to everybody.

Will

01-07-2007, 11:40 AM   #9
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shows side by side comaparo of a full frame vs aps sized sensors

on a side note...canon's none full frame sensors have a 1.6x factor, instead of the typical sony 1.5x
01-07-2007, 12:43 PM   #10
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if we all viewed our images 1:1 and the image quality was identical between the larger film size image and the smaller digital sensor image all this would be of more relevance..

when u enlarge (as we all do) the smaller sensor bird image to the same size as the larger film (sensor) u get a bigger bird.. the difference being if the original film and digitial image quality were identical the bigger bird would suffer a degrade in quality and the dof woud be different..

but the original film quality isnt identcal to the digital sensor quality so all we get is a bigger bird with different dof..

we dont view our digital images 1:1 we view them all enlarged.. the so called digital crop would be enlarged more than the theoretical film image..

so assuming sufficient image detail present in the smaller digital sensor in the first place the real life difference comes down to dof..

its all about detail.. given enough detail we could all use a 50mm lens and simply crop and enlarge digitally.. the only difference being the dof..

apart from the dof factor i think more is being made of this than needs be.. for most intents and purposes the good old "35mm equivalent" explains the situation well enough..

its one i am happy with..

trog

ps.. its a shame we all cant forget film and learn to think "digitally"..
01-07-2007, 02:31 PM   #11
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Focal length/Field of veiw

Just remember this is just the field of view that is different not the focal length which stays the same, since it is a fixed distance no matter what size sensor or film you use.

Focal length is the distance from the focal center of the lens to the film plane or sensor.

Tom
01-07-2007, 03:23 PM   #12
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QuoteOriginally posted by WMBP Quote

It's a bit of a bummer, because I was sort of under the impression that my 300mm lens = a 450mm lens for a film camera, but I see now that it doesn't. I simply get to save less of what the lens "sees".
Did you mean to say "I was sort of under the impression that my 300mm film camera lens acts like a 450mm lens on my K100D" ? That's still true on a working basis. Your camera saves what your eye sees thru the viewfinder and what you see is what a 450mm lens would see on a film camera. Your camera is designed to capture that and provide an image that will print what you see.

Take a bird picture with the same focal length lens on a APS and FF camera. Print the full image on 12 inch paper. They should both have the same detail because an 8x12 isn't enough to differentiate the two, but the APS one will have a bigger bird. You can't make the APS bird smaller. It's been magnified. You can argue if quality of the bird suffices, but it's a bigger bird,

Sure, one can enlarge, crop, and go to bigger print sizes and soon the FF image will outdo the APS image, as it should.
01-07-2007, 03:37 PM   #13
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QuoteOriginally posted by chedoy Quote
Did you mean to say "I was sort of under the impression that my 300mm film camera lens acts like a 450mm lens on my K100D" ? That's still true on a working basis. Your camera saves what your eye sees thru the viewfinder and what you see is what a 450mm lens would see on a film camera. Your camera is designed to capture that and provide an image that will print what you see.

Take a bird picture with the same focal length lens on a APS and FF camera. Print the full image on 12 inch paper. They should both have the same detail because an 8x12 isn't enough to differentiate the two, but the APS one will have a bigger bird. You can't make the APS bird smaller. It's been magnified. You can argue if quality of the bird suffices, but it's a bigger bird,

Sure, one can enlarge, crop, and go to bigger print sizes and soon the FF image will outdo the APS image, as it should.

No, what I meant to say was "I was under the impression that the 300mm attached to my K100D gives me the telephoto power of a 450mm lens attached to a 35mm film camera." I see now that it does NOT. Moving the lens from a 35mm film body to a digital SLR body doesn't get me 1 inch closer to the bird. It just means I get less of the sky or branches or whatever is around the bird. The sharpness of the part that I do capture with my digital camera (the bird in the middle) is no better than on the film camera.

It's very helpful finally to have this sorted out. But it means that where I thought I was getting some extra telephoto power "for free" by using a digital SLR, in fact, I'm not.

By the way, I am aware that these effects reverse themselves at the low end of the focal length scale. I just don't care very often about that end of the equation. :-)

Will
01-07-2007, 09:45 PM   #14
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"The sharpness of the part that I do capture with my digital camera (the bird in the middle) is no better than on the film camera"

that depends on the relative quality of the two images.. as i said earlier they are not of indentcal quality.. they would have to be for your comment/conclusion to be true..

if u blow up the theoretical "film" bird.. the part of the image that u require.. till its the same size as the digital bird.. would or would not the quality of the two similar sized birds be the same or not..

the quality might be the same or it might not be.. it depends on several factors.. the quality of a iso fast film bird will differ from a slow film bird..

the quality of a three mega pixel digital bird wont be as good as a ten mega pixel digital bird.. a theoretical twenty mega pixel digital bird would be even better..

dont be too quick to stop thinking u are getting something for nothing cos i am pretty sure that in fact at the longer end of things something for nothing is exactly what u are getting..

of course u pay the price at the shorter end of things..

trog
01-07-2007, 10:06 PM   #15
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QuoteOriginally posted by trog100 Quote
the quality of a three mega pixel digital bird wont be as good as a ten mega pixel digital bird.. a theoretical twenty mega pixel digital bird would be even better..
Understood. But throughout this thread I've been assuming that other things were equal or as equal as they can be.

Now, I see that when I try to compare film to digital, I'm comparing apples and oranges, as film's "resolution" is not measured in megapixels. So let me compare digital to digital, that is, let me instead compare the effect of a lens used at 300 mm focal length with my K100D to the effect of the same lens used with one of those digital SLRs that has a large sensor the size of the capture area in 35mm film. And let's assume further that the two cameras have the same resolution in terms of megapixels. THEN I can see that the camera with the smaller sensor would, effectively, be getting "closer" to the bird, because, if it could zoom in tight on the bird, it would capture the bird alone in 6 MP of detail, while the camera with the larger sensor would have to give some of those megapixels to the extra sky that it's also capturing.

On the other hand, if larger sensor were 1.5x the size of the smaller sensor, and the larger sensor also had 1.5x as many pixels, then it seems to me that the smaller camera would have no advantage at all, at least not with respect to magnification.

Will
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